DEPARTAMENTO DE BIOLOXÍA VEXETAL E CIENCIA …D. Manuel Arias Estévez e D. Juan Carlos Nóvoa...

DEPARTAMENTO DE BIOLOXÍA VEXETAL E CIENCIA DO SOLO

ÁREA DE EDAFOLOXÍA E QUÍMICA AGRÍCOLA

FACULTADE DE CIENCIAS DE OURENSE

DINÁMICA DE CARBOFURANO E METALAXYL EN SOLOS DEDICADOS Á AGRICULTURA INTENSIVA

TESE DE DOUTORAMENTO PRESENTADA POR:

Alipio Bermúdez Couso

Ourense, Decembro 2012

D. Manuel Arias Estévez e D. Juan Carlos Nóvoa Muñoz, profesores titulares

de Universidade da Área de Edafoloxía e Química Agrícola,

Informan:

Que o Licenciado en Ciencia e Tecnoloxía dos Alimentos D. Alipio

Bermúdez Couso realizou baixo a nosa dirección nos laboratorios de Edafoloxía e

Química Agrícola da Facultade de Ciencias de Ourense o presente traballo que

constitúe a memoria titulada ·”Dinámica de carbofurano e metalaxyl en solos

dedicados a agricultura intensiva” presentada para optar o grao de Doutor pola

Universidade de Vigo

Que este traballo resulta válido para a súa presentación como Tese de

Doutoramento, polo que autorizamos a súa presentación na Universidade de Vigo.

E para que así conste aos efectos oportunos, asinamos o presente, en Ourense

a 13 de Decembro de 2012.

Manuel Arias Estévez Juan Carlos Nóvoa Muñoz

ÍNDICE

ÍNDICE XERAL

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ÍNDICE

1. INTRODUCIÓN.......................................................................................................................3

1.1. PESTICIDAS...........................................................................................................................3

1.1.1. Clasificación xeral ......................................................................................................3

1.2. IMPACTO MEDIOAMBIENTAL DOS PESTICIDAS. ......................................................................7

1.2.1. Incidencia dos pesticidas no medio ambiente.............................................................7

1.2.2. Mecanismos que rexen o comportamento dos pesticidas no solo. ..............................7

1.3. PESTICIDAS OBXECTO DE ESTUDIO......................................................................................12

1.3.1. Carbofurano..............................................................................................................12

1.3.2. Metalaxyl. .................................................................................................................13

1.3.3. Lexislación. ...............................................................................................................14

1.4. ZONA DE ESTUDO................................................................................................................15

1.4.1. Cultivos. ....................................................................................................................16

1.4.2. Climatoloxía..............................................................................................................16

1.4.3. Xeoloxía. ...................................................................................................................17

1.4.4. Tipos de solo. ............................................................................................................17

1.4.5. Hidroloxía.................................................................................................................18

2. REVISIÓN BIBLIOGRÁFICA ............................................................................................21

2.1. EFECTOS DE CARBOFURANO E METALAXYL NOS ORGANISMOS DO SOLO. ...........................22

2.2. DETERMINACIÓN DE CARBOFURANO E METALAXYL NO MEDIO AMBIENTE. ........................25

2.3. ADSORCIÓN-DESORCIÓN DE CARBOFURANO E METALAXYL. ..............................................27

2.4. DEGRADACIÓN DE CARBOFURANO E METALAXYL. .............................................................38

3. XUSTIFICACIÓN E OBXECTIVOS ..................................................................................55

4. RESULTADOS E DISCUSIÓN ............................................................................................59

4.1. CINÉTICA E ADSORCIÓN-DESORCIÓN DE CARBOFURANO E METALAXYL EN SOLOS ÁCIDOS.59

4.2. COMPARACIÓN DE EXPERIMENTOS BATCH, CÁMARA DE FLUXO AXITADO E COLUMNAS NA

ADSORCIÓN, DESORCIÓN E TRANSPORTE DE CARBOFURANO E METALAXYL EN DOUS SOLOS

ÁCIDOS. .....................................................................................................................................69

4.3. FACTORES QUE INFLÚEN NA DISIPACIÓN DE CARBOFURANO E METALAXYL ........................78

4.4. CONTAMINACIÓN POR METALAXYL E NITRATOS EN AUGAS SUPERFICIAIS NA COMARCA DA

LIMIA . .......................................................................................................................................85

5. CONCLUSIÓNS.....................................................................................................................95

6. BIBLIOGRAFÍA..................................................................................................................101

6.1. BIBLIOGRAFÍA DA INTRODUCIÓN. .....................................................................................101

6.2. BIBLIOGRAFÍA DA REVISIÓN BIBLIOGRÁFICA. ...................................................................103

6.3. BIBLIOGRAFÍA DA XUSTIFICACIÓN E OBXECTIVOS. ...........................................................123

ÍNDICE XERAL

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6.4. BIBLIOGRAFÍA DOS RESULTADOS E DISCUSIÓN. ................................................................126

7. ANEXOS .................................................................................................................131

7.1. ANEXO I. ADSORPTION AND DESORPTION KINETICS OF CARBOFURAN IN ACID

SOILS......................................................................................................................... 133

7.2. ANEXO II. ADSORPTION AND DESORPTION BEHAVIOR OF METALAXYL IN

INTENSIVELY CULTIVATED ACID SOILS..................................................................... 145

7.3. ANEXO III. COMPARISON OF BATCH, STIRRED FLOW CHAMBER, AND COLUMN

EXPERIMENTS TO STUDY ADSORPTION, DESORPTION AND TRANSPORT OF

CARBOFURAN WITHIN TWO ACIDIC SOILS................................................................. 155

7.4. ANEXO IV. METALAXYL DYNAMICS IN ACID SOILS : EVALUATION USING

DIFFERENT METHODS................................................................................................ 165

7.5. ANEXO V. INFLUENCE OF DIFFERENT ABIOTIC AND BIOTIC FACTOR ON THE

METALAXYL AND CARBOFURAN DISSIPATION .......................................................... 177

7.6. ANEXO VI. POLLUTION OF SURFACE WATERS BY METALAXYL AND NITRATES

FROM NON-POINT SOURCES....................................................................................... 187

7.7. ANEXO VII. INFORMACIÓN E CRITERIOS DE CALIDADE DAS PUBLICACIÓNS... 199

ÍNDICE DE TÁBOAS

iii

ÍNDICE DE TÁBOAS Táboa nº 1 Clasificación dos pesticidas segundo a súa toxicidade (OMS,

2012).

6

Táboa nº 2 Principais características do carbofurano (Tomlin, 2003). 13

Táboa nº 3 Principais características do metalaxyl (Tomlin, 2003). 14

Táboa nº 4 Número de artigos científicos onde, segundo a base de datos

SCOPUS, a adsorción e degradación de carbofurano e metalaxyl

son obxecto de estudo tendo en conta a súa presenza no resumen,

titulo ou palabras clave.

21

ÍNDICE DE FIGURAS

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ÍNDICE DE FIGURAS Figura nº 1 Mecanismos que rexen a distribución dos pesticidas no medio

ambiente. 7

Figura nº 2 Adsorción de carbofurano en función do tempo en dous dos solos empregados nos experimentos batch (S2 e S14). 59

Figura nº 3 Curvas de adsorción de carbofurano para catro dos solos empregados nos experimentos batch (S2, S5, S14 e S15). 60

Figura nº 4 Descrición da montaxe empregada nos experimentos de cámara de fluxo axitado. 62

Figura nº 5 Adsorción acumulada de carbofurano para os solos estudiados nos experimentos en cámara de fluxo axitado (S2, S5, S14 e S15). 62

Figura nº 6 Relación entre a adsorción relativa de carbofurano (q/qmax) e a raíz cadrada do tempo (t1/2) para os solos estudados nos experimentos de cámara de fluxo axitado (S2, S5, S14 e S15). 63

Figura nº 7 Adsorción de metalaxyl en función do tempo en dous dos solos empregados nos experimentos batch (S2 e S14). 65

Figura nº 8 Curvas de adsorción de metalaxyl para 4 dos 16 solos estudados (S2, S5, S14 e S15). 66

Figura nº 9 Adsorción acumulada de metalaxyl para os solos estudiados nos experimentos en cámara de fluxo axitado. 67

Figura nº 10 Relación entre a adsorción relativa de metalaxyl (q/qmax) e a raíz cadrada do tempo (t1/2) para dous dos solos estudados nos experimentos de cámara de fluxo axitado (S5 e S14). 68

Figura nº 11 Cinética de carbofurano. Solo S1 (círculos abertos), solo S2 (círculos pechados). 70

Figura nº 12 Adsorción en equilibrio de carbofurano para as dúas mostras de solo estudadas (S1 e S2). 71

Figura nº 13 Adsorción acumulada de carbofurano nos experimentos de cámara de fluxo axitado para as dúas mostras de solo estudadas (S1 e S2). 72

Figura nº 14 Curvas de avance de Br-(círculos) e carbofurano (triángulos) para os solos estudados (S1 e S2). 73

Figura nº 15 Adsorción acumulada de carbofurano nos experimentos en columnas en función do tempo. 73

ÍNDICE DE FIGURAS

vi

Figura nº 16 Cinética de metalaxyl. Solo S1 (círculos abertos), solo S2 (círculos pechados). 74

Figura nº 17 Adsorción en equilibrio de metalaxyl para as dúas mostras de solo estudadas. 75

Figura nº 18 Adsorción acumulada de metalaxyl nos experimentos de cámara de fluxo axitado. 76

Figura nº 19 Curvas de avance de Br-(círculos) e metalaxyl (triángulos) para os solos S1 e S2. 76

Figura nº 20 Adsorción acumulada de metalaxyl nos experimentos en columnas en función do tempo. 77

Figura nº 21 Crecemento bacteriano en función da concentración de pesticidas. Solo 1 (triángulos); Solo 2 (círculos). 79

Figura nº 22 Disipación de metalaxyl (A) e carbofurano (B) en disolucións filtradas en presenza (círculos) e ausencia (triángulos) de caldo de cultivo (12h luz/12h escuridade). 81

Figura nº 23 Disipación de metalaxyl (A) e carbofurano (B) en disolucións con microorganismos extraídos do solo (S1) en presenza (círculos) e ausencia (triángulos) de caldo de cultivo (12h luz/12h escuridade). 82

Figura nº 24 Disipación de metalaxyl e carbofurano nos experimentos con solo (S1 e S2). 83

Figura nº 25 Puntos de mostraxe de augas na area de estudo. 86

Figura nº 26 Niveis de metalaxyl na zona 2 durante o período de mostraxe. 88

Figura nº 27 Relación entre a concentración de nitratos e o caudal no punto 2 da mostraxe. 89

Figura nº 28 Concentración de nitratos nas augas en diferentes puntos de mostraxe e a precipitación mensual. 90

INTRODUCIÓN

INTRODUCIÓN

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1. INTRODUCIÓN

1.1. Pesticidas.

Denomínase pesticida a calquera sustancia ou mestura de sustancias destinadas a

previr, destruír ou controlar calquera praga, incluíndo os vectores de enfermidades

humanas ou dos animais, as especies non desexadas de plantas ou animais que causen

prexuízo ou que interfiran de calquera forman na produción, elaboración,

almacenamento, transporte ou comercialización de alimentos, produtos agrícolas,

madeira e produtos de madeira ou alimentos para animais, ou que podan administrarse

aos animais para combater insectos, arácnidos ou outras pragas nos seus corpos ou

sobre eles (FAO, 2012).

1.1.1. Clasificación xeral

Os pesticidas pódense clasificar pola natureza do axente patóxeno, a súa

estrutura química, o seu modo de actuación, a súa toxicidade ou ben polo momento da

súa aplicación. Seguidamente detállanse os diferentes grupos existentes dentro de cada

clasificación (FAO, 2012):

1.1.1.1. Pola natureza do patóxeno:

• Insecticidas: compostos destinados a matar insectos e parasitos.

• Acaricidas: compostos que atacan matando aos ácaros.

• Nematicidas: sustancias que matan aos nematodos das partes aéreas dos

vexetais que están en contacto co solo ou das partes subterráneas.

• Funxicidas: compostos que atacan aos fungos.

• Antibióticos: compostos químicos que inhiben o crecemento dos

microorganismos.

• Herbicidas: compostos que destrúen os vexetais leñosos ou herbáceos que

limitan o desenvolvemento de plantas de interese.

• Rodenticidas: compostos tóxicos para os roedores.

• Avicidas: compostos tóxicos para as aves.

• Molusquicidas: compostos con fin de eliminar os moluscos (caracois).

• Larvicidas: compostos que perturban o desenvolvemento de larvas de insectos.

• Atraentes e repelentes: compostos que atraen ou repelen aos insectos coa

finalidade de eliminalos.

INTRODUCIÓN

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• Defoliantes: compostos que atacan as partes indesexables dunha planta.

• Reguladores: compostos que modifican o normal funcionamento das plantas.

• Aficidas: compostos destinados a matar pulgas.

• Ovicidas: sustancias que impiden a eclosión dos ovos dos insectos.

• Antitranspirantes: compostos que protexen as follas da perdida de auga.

• Desinfectantes: matan ou desactivan ós microorganismos produtores de

enfermidades (bacteria, virus, etc.)

1.1.1.2. Pola súa estrutura química.

• Organoclorados.

Estes pesticidas posúen átomos de cloro na súa composición e peso

molecular de 291 e 545. Pola súa estrutura cíclica e gran peso molecular, presentan

unha gran semellanza aos hidrocarburos clorados empregados como disolventes.

Entre os mais coñecidos atópanse o DDT (Dicloro Difenil Tricloroetano) e o

Endosulfan.

• Organofosforados.

Trátanse de ésteres derivados dos ácidos fosfórico, tiofosfórico,

ditiofosfórico, fosfónico e fosfínico. Os compostos mais característicos son o

malation, o paration e o diazinon.

• Carbamatos.

Insecticidas derivados do ácido carbámico. Neste grupo hai que destacar o

carbofurano e o carbaril.

As principais vantaxes en comparación cos organofosforados son a súa

eficacia contra insectos resistentes a estes últimos, e maior seguridade no seu

manexo xa que son menos tóxicos o degradarse facilmente no medio ambiente.

Entre as súa desvantaxes atópanse a súa difícil produción, o elevado custo, maior

toxicidade para os insectos polinizadores e ademais, tanto os carbamatos como os

seus produtos de degradación, son potencialmente contaminantes do medio

ambiente.

INTRODUCIÓN

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• Tío e ditiocarbamatos.

Son unha clase especial de carbamatos, xa que posúen átomos de xofre

substituíndo a un átomo de osíxeno (tiocarbamatos) coma o tiocarbazil; ou ben os

dous átomos de osíxeno (ditiocarbamatos) coma o dimetilditiocarbamato.

Normalmente utilízanse como herbicidas, inda que nalgúns casos actúan

como funxicidas de colleitas de campo e na prevención de enfermidades en produtos

cultivados.

• Piretroides.

Son insecticidas sintéticos, quimicamente similares as piretrinas naturais

pero mais estables no medio ambiente. Os mais representativos son o rotenone, o

piretrin e o alletrin. Trátanse de compostos pouco solubles na auga, polo que é

pouco probable que acaden as augas subterráneas. Presentan baixa toxicidade para

os mamíferos e son nocivos para a organismos non considerados praga.

• Triazinas.

Son herbicidas de estructura hexagonal que teñen tres átomos de nitróxeno e

tres de carbono, dos cales pódense dispor de forma alternada, simétrica ou

asimétrica. Os mais utilizados son a atrazina e a propazina. A concentracións moi

baixas diminúe a fixación do CO2 e inhibe a síntese de glicosa, provocando como

consecuencia o bloqueo da fotosíntese.

• Derivados da urea.

Tratase de compostos que teñen na súa estructura diferentes grupos

funcionais que substituíron a un ou mais átomos de hidroxeno da estructura inicial

da urea. Algúns dos mais característicos son o diuron, o linuron e o bensulfuron.

Utilízanse fundamentalmente como herbicidas, e a súa principal desvantaxe é que

son persistentes no medio.

1.1.1.3. Polo seu modo de actuación.

• Protectantes: o seu obxectivo é previr os danos causados polos axentes

nocivos.

• Esterilizantes: impide a reprodución das pragas.

INTRODUCIÓN

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• De contacto: acaban coas plantas sobre as que caen. A súa acción tóxica

é de pouca duración.

• Sistémicos: desprazase polo sistema circulatorio da planta ou do animal

para combater a pragas en danar ao hóspede.

• Traslocables: absórbense na zona da planta onde son aplicados sen

exercer a súa acción noutra parte da planta.

• Fumigantes: destrúen aos organismos patóxenos cando son inhalados.

• Selectivos: son mais tóxicos para unhas especies vexetais e animais que

para outras.

• Non selectivos ou totais: son tóxicos para a maioría das especies.

• Feromonas: modifica no comportamento das pragas.

• Sinérxicos: o seu efecto é maior en presenza de determinadas sustancias.

1.1.1.4. Pola súa toxicidade.

A toxicidade dos pesticidas expresase mediante o parámetro dose letal 50

(DL50), definido como a cantidade de pesticida capaz de causar a morte do 50% dos

individuos sometidos a estudo. En función desta definición os pesticidas clasifícanse

como:

Táboa nº 1: Clasificación dos pesticidas segundo a súa toxicidade (OMS, 2012).

Clasificación DL50 en ratas (mg kg-1) Vía oral Vía tópica Sólidos Líquidos Sólidos Líquidos

Extremadamente tóxico ≤ 5 ≤ 20 ≤ 10 ≤ 40 Altamente tóxico 5-50 20-200 10-100 40-400

Moderadamente tóxico 50-500 200-2000 100-1000 400-4000 Lixeiramente tóxico < 500 > 2000 > 1000 > 4000

1.1.1.5. Polo momento da aplicación.

• Presementeira e replantación: aplícase unha vez preparado o solo

para o cultivo, pero antes da sementeira.

• Preemerxencia: aplícase despois da sementeira, pero antes de que

naza a planta.

• Postemerxencia: aplícase despois de que aparezan as malas herbas e

a planta cultivada.

INTRODUCIÓN

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1.2. Impacto medioambiental dos pesticidas.

1.2.1. Incidencia dos pesticidas no medio ambiente.

A evolución experimentada pola agricultura nas últimas décadas trouxo como

consecuencia un gran desenvolvemento no uso de sustancias químicas orgánicas

sintéticas para o control de malas herbas, insectos e outras pragas, co cal contribuíu a

aumentar de forma significativa as producións agrícolas e o desenvolvemento de

sectores socioeconómicos implicados. O aumento do consumo de pesticidas puxo de

manifesto o risco potencial que entrañan os praguicidas para a saúde humana e o medio

ambiente. A agricultura intensiva combinada coa escorrentia superficial e a lixiviación

pode levar a un aumento do contido de nutrientes e pesticidas nos ríos e acuíferos,

provocando unha aceleración dos procesos de degradación ambiental tales coma a

contaminación do sistema solo-auga, problemas de eutrofización e descenso na calidade

das augas.

1.2.2. Mecanismos que rexen o comportamento dos pesticidas no solo.

Na Figura 1 amosase de maneira detallada os mecanismos que rexen a

distribución dos pesticidas no medio ambiente. Pódese observar como os pesticidas,

unha vez introducidos no medio ambiente pódense distribuír na atmosfera, no solo e na

auga, podendo trocarse entre eles, polo que a aplicación dun pesticida sobre algún

destes medios repercute sempre, antes ou despois, sobre os demais (Canter, 1986).

AUG AS

SUPERFICIAIS

ATM ÓSFERA SOLO

PESTICID AS

CULTIVOS

AUG AS

SUBTERRÁNEAS

SERES VIVOS

CAPACIDADE DE

INTERCAM BIO

CATIÓNICO DO SOLO

Volatilización Arrastre

Lixiviación Adsorción

Erosión

Deposición

Figura nº 1: Mecanismos que rexen a distribución dos pesticidas no medio ambiente.

INTRODUCIÓN

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O comportamento dos pesticidas no solo é controlado por unha gran variedade

de procesos físicos, químicos e biolóxicos, os cales, con frecuencia son moi complexos.

Os principais procesos que afectan a evolución dos pesticidas nos solos

agrúpanse en: a) retención, b) degradación e transformación, c) absorción polas plantas

e outros organismos e d) procesos de transporte.

1.2.2.1. Procesos de retención.

Os procesos de retención son a clave na evolución dos pesticidas no medio solo-

auga. A retención refírese a capacidade do solo de inmobilizar un pesticida e evitar o

seu transporte tanto dentro como fora da matriz do solo. Así, a retención refírese

principalmente ao proceso de adsorción, pero tamén inclúe a absorción dentro da matriz

do solo e os organismos deste, tanto plantas como microorganismos.

Defínese como adsorción á acumulación dun pesticida tanto na interfase solo-

auga como na interfase solo-aire. A adsorción empregase a miúdo para referirse a un

proceso reversible no que inclúese a atracción entre unha sustancia e a superficie do

solo durante un tempo determinado, que depende da afinidade entre o pesticida e

superficie (Koskinen e Harper, 1990).

Como consecuencia do fenómeno de adsorción, as moléculas de pesticida

atópanse retidas sobre a superficie dos coloides minerais e orgánicos, atopándose

nunhas condicións fisicoquímicas particulares, que son as do estado adsorbido, e

adquirindo un comportamento diferente o das moléculas en disolución. A adsorción dós

pesticidas pola fracción coloidal do solo actúa modificando o proceso de degradación e

de transporte destes compostos no solo, así como a súa funcionalidade para atacar os

organismos aos que son destinados. Estas consideracións teñen repercusións

agronómicas importantes que deben ser tidos en conta á hora de empregalos.

Comprobouse que a materia orgánica e as arxilas son os principais substratos do

solo para a adsorción de pesticidas (Koskinen e Harper, 1990). Polo tanto, a densidade

de carga de arxilas así como a cantidade e a natureza da materia orgánica determinan a

extensión dos procesos de adsorción e desorción. Os principais constituíntes da materia

orgánica que interveñen na adsorción do pesticida son os ácidos húmicos e os ácidos

fúlvicos (Bollag et al., 1992; Walker e Crawford, 1968). Estes presentan unha

capacidade de adsorción diferente, sendo maior para os ácidos fúlvicos; sen embargo, a

proporción de ácidos húmicos nos solos sempre e maior que a de ácidos fúlvicos.

INTRODUCIÓN

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Outros factores que afectan ós procesos de adsorción e desorción son o pH, a

composición da disolución do solo e a presenza de ións competitivos (Kookana et al.,

1998; Mingelgrin e Prost, 1989).

1.2.2.2. Procesos de degradación e transformación.

A degradación dos pesticidas nos solos pode suceder a través de procesos

bióticos ou abióticos. No caso da degradación biótica, considerada a ruta principal de

degradación, os pesticidas pódense transformar directamente a través de procesos

metabólicos (como mineralización, polimerización, acumulación, etc.) ou ben

indirectamente polos efectos secundarios da actividade microbiana, alterando o pH do

solo e as condicións redox (Bollag e Liu, 1990). Esta degradación vai estar

condicionada polo tipo e contido de materia orgánica, polas arxilas presentes no solo, o

réxime de humidade predominante no solo e a dispoñibilidade de nutrientes para o

desenvolvemento dos microorganismos degradadores.

A degradación abiótica inclúe procesos de hidrólise, oxidación-reducción e

fotólises (Wolfe et al., 1990).

1.2.2.3. Absorción polas plantas e outros microorganismos.

Os organismos terrestres e acuáticos xogan un papel importante na fixación do

pesticida, especialmente se o pesticida é altamente hidrófobo ou lipofílico, xa que estes

tenden a acumularse na cadea trófica (Cunnigham e Lee, 1995).

1.2.2.4. Proceso de transporte.

Os pesticidas aplicados no solo pódense mobilizar cara as augas superficiais

mediante arrastre e erosión, cara as augas subterráneas a través do proceso de

lixiviación e liberarse á atmosfera a través da volatilización. Son varios os principais

procesos de transporte dos pesticidas que hai no solo.

A difusión, movemento ao chou de moléculas a causa dun gradiente de

concentración, promove o fluxo da materia dende as zonas máis concentradas ás menos

concentradas. O fluxo da materia depende do coeficiente de difusión que, a súa vez,

depende tanto das características da molécula como do medio no que se difunde. No

solo a difusión intervén na redistribución, diminuíndo os gradientes de concentración

das sustancias aplicadas.

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A lixiviación, un dos procesos mais importantes no movemento dunha sustancia

no solo, está ligado a dinámica da auga, ás propiedades hidráulicas do solo e das

características fisicoquímicas do pesticida. Os compostos aplicados ao solo tenden a

desprazarse coa auga e lixiviar a través do perfil, alcanzando as capas mais profundas e

o acuífero que, en consecuencia, resulta contaminado.

A evaporación é responsable da taxa de perdas de praguicidas por volatilización,

proceso que depende da súa presión de vapor, da temperatura, da súa volatilidade

intrínseca e da velocidade de difusión cara a superficie de evaporación.

Son varios os factores que afectan os procesos de transporte do pesticida no solo:

1. Propiedades físicas e químicas do solo:

• O contido en arxilas. A súa presenza dificulta o transporte dos

pesticidas por mor da súa elevada capacidade de adsorción.

• O contido en materia orgánica. A materia orgánica actúa como

substrato para a adsorción do pesticida, co cal, canto maior

contido en materia orgánica menor é o transporte de pesticidas o

solo.

• A textura. Os solos de textura areosa ou franca polo normal

permiten un movemento rápido da auga dificultando así a

adsorción dos pesticidas. Pola contra, os solos de textura fina

tenden a unha baixada na velocidade de fluxo da auga, permitindo

unha maior adsorción dos pesticidas nas arxilas.

• A permeabilidade. O lavado é mais sinxelo nos solos permeables,

elevando así o risco de contaminación das augas subterráneas.

• A vexetación. A existencia de cuberta vexetal proporciona unha

protección fronte o proceso de erosión do solo, evitando desta

maneira o transporte de pesticidas adheridos ó material arrastrado

(sedimentos). Por outra banda, a vexetación diminúe a enerxía

luminosa na superficie do solo co que dificúltanse os procesos de

degradación dos pesticidas por fotólises.

• A humidade. Cando a humidade é baixa, os compostos

cristalízanse dificultando a súa retención por adsorción. Para

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contidos de humidade elevados, o medio pasa a ter condicións

anaerobias, co cal prodúcese unha diminución na degradación de

compostos.

• O pH. O seu efecto sobre o transporte de pesticidas depende das

propiedades fisicoquímicas dos compostos. Ademais, a variación

de pH inflúe sobre a actividade microbiana coa conseguinte

influencia sobre a degradación de pesticidas.

2. Condicións meteorolóxicas:

• A temperatura. As baixas temperaturas favorecen o transporte de

pesticidas por mor dunha menor degradación dos compostos

químicos.

• O vento. Favorece a erosión do solo e o conseguinte transporte

dos produtos químicos.

3. Características físicas e químicas dos pesticidas:

• A formulación química. Os pesticidas non iónicos e os

compostos orgánicos teñen unha gran afinidade pola materia

orgánica, e polo tanto a erosión e o transporte da materia

orgánica condicionan en gran parte o seu movemento.

• A solubilidade na auga. Canto maior sexa o seu valor, maior é a

cantidade de pesticida que poderá transportarse en disolución.

• A volatilidade. Depende da presión de vapor do pesticida e está

influenciada polas condicións meteorolóxicas.

• A persistencia. Depende de moitos factores entre os que destacan

o tipo de produto químico, o método e as condicións de

aplicación, o tipo de cultivos e os procesos de descomposición,

entre outros.

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1.3. Pesticidas obxecto de estudio.

1.3.1. Carbofurano.

O carbofurano, (2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-methylcarbamate),

é un pesticida sistémico pertencente o grupo dos carbamatos.

Ao ser un pesticida sistémico presenta un longo efecto residual e

extremadamente perigoso. O carbofurano atópase presente en formulacións líquidas e

sólidas. Pode actuar como insecticida, acaricida ou nematicida en cultivos de millo,

arroz, alfalfa, tubérculos, froita e outros vexetais, mediante aplicación directa no solo ou

sobre a planta. O seu mecanismo de acción baséase na inhibición da actividade da

enzima acetil-colinesterasa en animais e humanos de forma rápida e reversible,

afectando así ó sistema nervioso central.

O carbofurano como insecticida polivalente sistémico desenvolve a súa

actividade por contacto e inxestión; controla elatéridos (verme do arame) e outros

vermes do solo, especies do xénero Chaetocnema (Coleóptera: Chrysomelidae). Como

nematicida actúa sobre Ditylenchus spp. e Meloidogyione spp. entre outros nematodos.

Absórbese facilmente por vía radicular e transportase á parte aérea da planta

(Tomlin, 2003). Actúa por contacto sobre nematodos e vermes do solo, e por inxestión

sobre insectos mastigadores e chupadores da parte aérea, en especial durante os

primeiros estadios do desenvolvemento do cultivo.

Trátase dun composto moderadamente persistente no solo e cunha vida media de

30 a 120 días (Tomlin, 2003) dependendo do contido en materia orgánica, humidade e

pH do solo. A súa degradación ten lugar mediante procesos de hidrólises química baixo

condicións alcalinas, hidrólises microbiana, así como por fotólises, dando lugar ó 2-

hidroxifuradan e furadan fenol (Howard, 1991).

O carbofurano presenta unha elevada mobilidade e distribución nos solos, é

altamente soluble en auga e non é volátil.

Trátase dun composto altamente tóxico por inhalación e inxestión, e

moderadamente tóxico por absorción a través da pel.

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Táboa nº 2: Principais características do carbofurano (Tomlin, 2003)

CARBOFURANO. NºCAS:[1563-66-2]

Peso molecular 221,26 g/mol

Punto de fusión 151 ºC

Clasificación Carbamato

Estructura química

Fórmula química C

12H

15NO

3

Solubilidade en

auga 3.2 g L-1 (20ºC)

Solubilidade

noutros disolventes

Benceno (40 g kg-1), ciclohexano (90 g kg-1), acetonitrilo (140 g kg-1), acetona (150 g kg-1),

dimetil sulfóxido (250 g kg-1), dimetilformaldehido(270 g kg-1)

Estado físico Sólido cristalino, de cor branco e inodoro

Presión de vapor 0.08 mPa (25 ºC)

LogKOW: 2.32

Usos e modo de acción

Usos Insecticida sistémico

Modo de acción Actúa por contacto sobre nematodos e insectos do solo, e por inxestión sobre insectos

chupadores e mastigadores da parte aérea. O efecto de control é prolongado

Toxicidade

Efectos

canceríxenos Non se atopou efectos canceríxenos en humanos.

Efectos ambientais

Non se adsorbe en sólidos en suspensión nin en sedimentos, non se volatiliza dende a superficie

da auga. A bioconcentración en organismos acuáticos non é un proceso importante para o

destino destes compostos.

1.3.2. Metalaxyl.

O metalaxyl (methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate) é un

funxicida sistémico do grupo das fenilamidas.

Empregase no control preventivo e curativo de enfermidades provocadas por

Phytophtora infestans (mildeu en diversos cultivos como a vide ou a pataca) e outras

podredumbres. O seu modo de actuación consiste en suprimir a formación de

esporanxios, o crecemento do micelio e suprime o establecemento de novas infeccións.

Presenta unha baixa adsorción no solo en xeral, dependendo do contido en

materia orgánica, arxila e do pH do solo (Arias et al., 2006; Fernandes et al., 2003). A

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causa desta baixa retención, o metalaxil, é un composto cunha alta mobilidade o que

favorece a súa presenza en masas de auga. A súa vida media no solo varía entre 10 e 40

días (Tomlin, 2003), este tempo pode variar coas condicións do solo. Provoca irritación

nos ollos e na pel. Estudos levados a cabo en animais demostran que poden causar

danos no fígado e no ril, e acumulase en músculos e grasas.

Táboa nº 3: Principais características do metalaxyl (Tomlin, 2003).

METALAXYL. NºCAS:[ 57837-19-1] Peso molecular 279.33 g/mol

Punto de fusión 67.9 ºC

Clasificación Fenilamidas

Estructura química

Fórmula química C15H21NO4

Solubilidade en auga 7.1 g/L (20 ºC) Solubilidade noutros

disolventes Benceno (550g L-1), hexano (90.1g L-1), metanol (650g L-1), Cloruro de metilo (750g L-1)

Estado físico Cristais incoloros.

Presión de vapor 0.75 mPa (25 ºC)

LogKOW: 1.65

Usos e modo de acción

Usos insecticida sistémico

Modo de acción Sistémico con acción curativa e protectora, suprimindo a formación de esporanxios, o crecemento do micelio e o establecemento de novas infeccións.

Toxicidade

Efectos canceríxenos Non se atopou efectos canceríxenos en humanos.

Efectos ambientais Non presenta toxicidade para aves, peixes e abellas. Pode bioacumularse en peixes pero eliminase facilmente por depuración. Presenta unha baixa adsorción ao solo.

1.3.3. Lexislación.

O perigo potencial destas sustancias de síntese orgánico e o seu amplo uso deu

lugar a unha gran preocupación pola contaminación de ecosistemas. Con respecto a

saúde humana, a directiva europea relativa á calidade das augas destinadas ao consumo

humano, establece como límite máximo dun praguicida individual en 0.1 µg L-1 e o

limite máximo da suma total de praguicidas en 0.5 µg L-1 (Council Directive 98/83/CE).

Posteriormente, o 13 de xuño de 2007 unha decisión da Unión Europea acordou a

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prohibición do uso e comercialización de carbofurano en tódolos estados membros

debido o seu perigo para a saúde humana (Commission decision 2007/415/CE).

1.4. Zona de estudo.

A comarca da Limia é unha das unidades xeográficas cunha personalidade máis

definida de Galicia. Atopase na provincia de Ourense e ten unha extensión de 800 km2

onde viven aproximadamente uns 25.500 habitantes.

A comarca reúne os seguintes concellos: Vilar de Santos, Sandiás e Vilar de

Barrio ao Norte; Rairiz de Veiga e a Porqueira ao Oeste; Sarreaus e Trasmiras ao Leste;

Calvos de Randín e Baltar ao Sur; e Os Blancos e Xinzo de Limia no centro.

Aínda que baixo o punto de vista xeográfico non corresponde estritamente cos

lindes administrativos dos concellos que a integran xa que ó norte queda fora unha

ampla zona do concello de Xunqueira de Ambía que forma parte da depresión, pero que

inclúe terras do concello de Vilar de Barrio que xeograficamente pertencen a Serra de

San Mamede. No sur acontece algo similar coas terras pertencentes a Serra de Larouco

e da Pena (acadando cotas superior ós 1.300 m).

A Limia é unha ampla depresión tectónica de fondo chan, que ten unha altitude

media aproximada de 600 m sobre o nivel do mar, e aparece encadrada por unha serie

de relevos que poden chegar a supera-los 1.600 m de altitude, como por exemplo en San

Mamede ó norte.

A depresión pertence a finais do terciario e durante moito tempo estivo ocupada

pola Lagoa de Antela. O feito de que se trate dunha conca case cerrada polo suroeste, na

que o río sofre un notable encaixamento, levou a un proceso de colmatación da lagoa

durante o cuaternario coa achega de materiais procedentes dos relevos veciños

(principalmente de natureza granítica), ata que o fin a primitiva lagoa quedou reducida a

unha ampla zona pantanosa de escasa profundidade e reducida extensión.

Nesta zona atopábase un dos humidais de maior extensión do país con 7 km de

longo, 6 km de ancho e unha profundidade de auga que variaba entre 60 cm e 3 m. A

mediados do século XX procedeuse á desecación do mesmo coa finalidade do seu

aproveitamento agrícola e gandeiro mediante a construción de canles de drenaxe que

conflúen nunha canle principal, vertendo as súas augas nun río próximo (Río Limia), o

cal desemboca no Océano Atlántico pasando por Portugal.

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1.4.1. Cultivos.

A superficie fixada coa desecación foi de 3243 ha (Decreto 2336/1962), as cales

foron repartidas para o seu uso agrícola. Os cultivos máis estendidos na zona teñen sido

o cereal e a pataca, sendo este último o de maior importancia económica. As

necesidades hídricas do cultivo da pataca e a sequidade estival da zona, fixo que se

construísen unhas comportas na canle para manter a altura da auga e así poder

empregala para regar, a consecuencia disto o nivel freático esta máis elevado o que dá

lugar a grandes zonas encharcadas en períodos chuviosos.

En toda a comarca hai declaradas 3084 ha dedicadas ao cultivo de pataca cun

rendemento de 30 Tm ha-1 (IGE, 2009) o que supón unha produción de máis de 90000

Tm de patacas ao ano. Este cultivo realizase en rotación co trigo, onde se distinguen

dous tipos de rotacións en función do tamaño das empresas produtoras; por unha banda

os grandes produtores alternan un ano trigo e outra pataca, mentres que os pequenos

produtores combinan 2 ou 3 anos trigo e un pataca.

Habitualmente este cultivo leva a aplicación de grandes cantidades de

fertilizante; utilízanse abonos orgánicos na presembra (esterco de polo) e fertilizantes

inorgánicos. As cantidades aplicadas de fertilizantes inorgánicos varían en función do

tipo de produtor, nesta comarca aplícanse 100-200 kg N ha-1, 100-200 kg P2O5 ha-1 e

100-150 kg K2O ha-1 (López-Mateo, 2007). A sementeira de pataca realízase dende

marzo ata maio, dependendo das condicións climáticas do ano. Antes da sementeira e

durante a mesma aplícase o nematicida (carbofurano) en gránulos quedando enterrado.

Os compostos que se usan conteñen entre un 2 e un 5 % de carbofurano e aplícanse a

razón de 15-25 kg ha-1. A aplicación do funxicida (metalaxyl) realízase dende que a

planta comeza o período de crecemento ata 21 días antes da colleita. O número de

aplicacións depende das condicións climáticas que teñan lugar durante o

desenvolvemento da planta. A aplicación realízase pulverizando o produto sobre a

planta. Os produtos comerciais conteñen entre un 5 e un 25 % de metalaxyl e a dose de

aplicación é de 0.50 a 0.08 % (5-0.8 kg ha-1) respectivamente.

1.4.2. Climatoloxía.

O clima da zona é oceánico de transición (Gómez Nieto, 1996), con certo

carácter mediterráneo e trazos de continentalidade. A temperatura media anual é de 11

ºC e a precipitación media acumulada ao longo do ano é de 881 mm. O mes máis frío é

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xaneiro (5.1 ºC de media) e os máis cálidos xullo e agosto (18.4 e 18.0 ºC de media

respectivamente). O mes máis chuvioso é febreiro con 120 mm e os máis secos xullo e

agosto cuns 20 mm.

1.4.3. Xeoloxía.

Na zona de estudo preséntanse sedimentos cuaternarios, cuxa potencia pode

alcanzar os 200 m nas zonas centrais da cunca. Na zona superficial os sedimentos son

grosos, de textura areosa ou franco areosa. A profundidade variable (entre 0.5 m e

varios metros) aparecen sedimentos arxilosos. Segundo o (IGME, 1974), o tipo xeral de

depósitos corresponde a unha depresión continental semiendorreica, sen grandes arterias

de achegue. A colmatación seguiu un proceso de achegue intermitente, desenvolvéndose

mesmo episodios palustres con formación de turbas. O marco da depresión está

constituído por materiais ígneos e metamórficos (IGME, 1974). As rochas ígneas son

rochas graníticas, incluíndo granitos de dúas micas e granitos moscovíticos. Entre os

materiais metamórficos distínguense rochas paleozoicas e rochas precámbrico-

cámbricas. Dentro das rochas paleozoicas poden distinguirse: esquistos de cuarzo e

mica, filitas, micacitas e micaesquistos andalucíticos. As rochas precámbrico-cámbricas

son porfiroides e gneises glandulares.

1.4.4. Tipos de solo.

Os solos da depresión están desenvolvidos sobre materiais sedimentarios.

Xeralmente presentan unha textura grosa nos horizontes superficiais, aparecendo unha

capa arxilosa a profundidade variable. Os pH son ácidos (<6) mesmo nos solos de

cultivo. O contido de materia orgánica é alto nalgúns solos onde as condicións de

hidromorfía favorecen a acumulación de materia orgánica, e relativamente baixo

(próximo ao 3%) en moitos solos de cultivo. Os grupos de solos de referencia (IUSS

Grupo de Trabajo WRB, 2007) máis representados na zona segundo López-Mateo,

(2007) son os seguintes:

- Leptosols: son solos escasamente desenvolvidos que abundan nas ladeiras de

maior pendente que conforman o marco da depresión da Limia. Os principais

calificadores de Grupo I para os Leptosols serían Lítico, Úmbrico, Cámbico e Háplico.

- Umbrisols: Desenvolvidos tamén sobre rocha consolidada, son solos que

aparecen en zonas de menor pendente que se sitúan entre o fondo da cunca e as ladeiras

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de maior pendente. Dentro dos Umbrisols, os calificadores de Grupo I mais probables

son Léptico, Cámbico e Háplico .

- Cambisols: Son solos que se espallan por toda a zona de estudo,

desenvolvéndose sobre rochas ígneas, metamórficas ou sobre sedimentos. Neste último

caso presentan con frecuencia propiedades gleicas. Os calificadores de Grupo I mais

probables para os Cambisols son Léptico, Endogleyico y Háplico.

- Gleysols: Localizados na antiga lagoa de Antela, os labores de drenaxe

permitiron a súa posta en cultivo, aínda que o nivel freático aparece a pouca

profundidade e pode chegar á superficie na época invernal. Neste caso, os principais

calificadores de Grupo I para este grupo de solos serían Fólico, Hístico, Úmbrico e

Háplico.

- Histosols: Aparecen no centro da antiga lagoa e presentan unha profundidade

variable. Tamén se puxo en cultivo tras a drenaxe da lagoa. Os calificadores de Grupo I

mais probables para os Histosols nesta área serían Fólico, Límnico, Sáprico e Rheico.

1.4.5. Hidroloxía.

O río Limia atravesa a depresión en dirección E-SO, e recibe a auga das serras

que rodean esta depresión (Serra de San Mamede, Serra de Larouco e Serra da Pena).

Unha canle achégalle a auga drenada das terras que estiveron ocupadas pola antiga

lagoa. O curso do río ten 108 km dos cales 41 km están situados na provincia de

Ourense e 67 km discorren por Portugal, onde desemboca no Océano Atlántico na

cidade de Viana do Castelo. O río presenta o maior caudal nos meses de xaneiro,

febreiro e maio e o menor en agosto e setembro (Gómez Nieto, 1996).

O acuífero da Limia, de carácter non confinado ou semiconfinado (Díaz-Fierros,

2000), ten unhas reservas de 400 hm3 e un volume renovable de 90 hm3 ano-1. O

elevado nivel freático contribúe á mobilización de elementos nutritivos e praguicidas

das terras de labor.

REVISIÓN BIBLIOGRÁFICA


21

2. REVISIÓN BIBLIOGRÁFICA

Nas últimas décadas prestouse un especial interese polos problemas ambientais e

de sustentabilidade do sistema agrario que poden xerar os produtos fitosanitarios. Isto é

consecuencia do incremento destes compostos no solo debido a súa constante aplicación

co fin de aumentar a produtividade da agricultura e cubrir a demanda de materia prima

que favoreza o desenvolvemento da industria.

Un dos aspectos fundamentais a estudar en relación cos potenciais problemas

ambientais que derivan do emprego intensivo de pesticidas son os mecanismos que

rexen o seu comportamento nos solos, sendo os máis importantes os procesos de

adsorción-desorción e os procesos de degradación. Nos últimos 40 anos, o número de

traballos sobre metalaxyl e carbofurano incrementouse notablemente. Na táboa 4

amósanse o número de traballos publicados dende ano 1970.

Táboa nº 4: Número de artigos científicos onde, segundo a base de datos SCOPUS, a adsorción e degradación de carbofurano e metalaxyl son obxecto de estudo tendo en conta a súa presenza no resumen, titulo ou palabras clave.

Anos Adsorción de

metalaxyl

Adsorción de

carbofurano

Degradación

de metalaxyl

Degradación

de carbofurano

1970-1975 0 0 0 3 1976-1980 0 3 0 18 1981-1985 1 4 2 25 1986-1990 0 12 2 45 1991-1995 2 14 4 34 1996-2000 3 23 7 40 2001-2005 12 12 25 36 2006-2010 31 24 26 56

Tendo en conta o número de referencias, as citas bibliográficas foron agrupadas

en catro grandes bloques: os efectos de carbofurano e metalaxyl nos organismos do

solo; a determinación e carbofurano e metalaxyl no medio ambiente; a adsorción-

desorción de carbofurano e metalaxyl e, finalmente, a degradación de carbofurano e

metalaxyl.


22

2.1. Efectos de carbofurano e metalaxyl nos organismos do solo.

Ahemad e Khan (2012) estudaron o efecto da adición de metalaxyl a

Rhizobacterium Pseudomonas putida, observando que a doses recomendadas do

pesticida, os efectos eran menores que en doses máis altas.

Zhou et al. (2012) investigaron o efecto da adición de carbofurano ao solo

carbofurano na poboación microbiana. Os resultados amosan que a poboación

microbiana do solo e a actividade metanóxena foi lixeiramente estimulada o engadir

carbofurano, pero foi inhibida cando se engadiron 0.5 mg kg-1 de carbofurano. Esta

inhibición durou 7 días e despois diminuíu ata valores de control.

Ahemad e Khan (2011) estudaron o efecto do metalaxyl na actividade de

Pseudomonas aeruginosa, observando que a concentracións superiores as

recomendadas inhibía a súa actividade.

Teixeira et al. (2011) estudaron a fitoremediación potencial do metalaxyl con

plantas de Solanum nigrum L. Observaron que estas plantas acumulaban grandes

cantidades de funxicida nos seus tecidos superficiais completando o seu ciclo de vida

sen sufrir maior estres. A tolerancia ao metalaxyl é debido a unha adecuada resposta

antioxidante que reduce os danos oxidativos nos órganos da planta.

Xu et al. (2011a) observaron que o metalaxyl era rapidamente tomado polas

lombrigas (Eisenia foetida) e que a súa acumulación era enantiomeroselectiva,

acumulandose preferentemente o S-enantiómero.

Lo (2010) observou que a presenza de carbofurano no solo estimula o

crecemento da poboación de Azospirillum e outras bacterias anaerobias fixadoras de

nitróxeno, tanto en sistemas inundados como non inundados.

De Silva et al. (2010) estudaron o efecto de pesticidas en lombrigas tropicais,

Perionyx excavatus. Observaron que, dos tres pesticidas avaliados, o carbofurano era o

máis tóxico e que as formulacións eran máis tóxicas que os ingredientes activos.

Tripathi et al. (2010) estudaron o efecto da presenza de carbofurano a distintas

concentracións en tres tipos de lombrigas. Observaron que o carbofurano incrementaba

a respiración das lombrigas. A máis afectada foi a lombriga epixea, que habita na capa

máis superficial do solo pólo que pode estar sometida a un maior contacto co pesticida.


23

As especies menos afectadas foron a lombriga endóxena e a anécica, as cales viven no

solo pero a maior profundidade co que evitan un contacto tan prolongado co pesticida.

De Silva e van Gestel (2009a) empregaron dúas especies de lombrigas Perionyx

escavatus (tropical) e Eisenia andrei para observar a sensibilidade a adición de

carbofurano en solos. As condicións de incubación foron 26ºC e concentracións de

carbofurano de 1-32 mg kg-1 de solo. Observaron que Eisenia andrei foi máis sensible

ao pesticida cun EC50 2-3 veces máis baixo que para Perionyx escavatus.

De Silva e van Gestel (2009b) observaron que a EC50 do carbofurano, para

efectos na reprodución de lombrigas, foi máis baixa en solos modificados con cáscara

de arroz que en solos modificados con compost de turba de coco.

De Silva et al. (2009) observaron que a temperatura inflúe na toxicidade do

carbofurano en lombrigas, debido probablemente a que aumenta a actividade da

lombriga.

Neelamegam et al. (2006) observaron que o carbofurano inhibía a xerminación,

crecemento e produtividade de leguminosa negra (Phaseolus mungo Roxb). O engadir

biofertilizantes este efecto diminuíu.

Das et al. (2005) estudaron a influencia do carbofurano no crecemento de

microorganismos e a súa velocidade de disipación en solos lateríticos e aluviais. O

carbofurano estimula o crecemento de fungos e bacterias fixadoras de nitróxeno en

solos lateríticos, e en solos aluviais estimula o crecemento de actinomicetos. En solos

aluviais a poboación bacteriana foi inhibida pola presenza de carbofurano. A vida media

do carbofurano foi de 16.9 días en solos lateríticos e 8.8 días en solos aluviais.

Das et al. (2003) estudaron o efecto dos pesticidas nos microorganismos do

solo. Observaron que o carbofurano favorecía o crecemento da poboación bacteriana,

actinomicetos e fungos. Incrementouse o crecemento de Bacillus, Corynebacterium,

Flavobacterium, Aspergillus e Phytophthora. Por outra banda inhibiuse o número de

Pseudomonas, Staphylococcus, Micrococcus, Klebsiella, Fusarium, Humicola e

Rhizopus. A vida media do carbofurano no solo foi de 9.1 días.

Mosleh et al. (2003) estudaron o efecto da adición de pesticidas en lombrigas

(Aporrectodea caliginosa). Observaron que se reduciu a velocidade de crecemento nos

vermes, diminuíu a proteína soluble e incrementaronse as transaminasas e fosfatasas.


24

Monkiedje et al. (2002) observaron que o metalaxyl e o mefenoxam foron

tóxicos para os microorganismos do solo a doses elevadas. O grado de toxicidade foi o

seguinte: mefenoxam > metalaxyl > ácido metabolito.

Das e Mukherjee (2000) observaron que a presenza de carbofurano no solo

estimula a proliferación e a actividade de bacterias fixadoras de N e de

microorganismos solubilizadores de fosfato, co cal provoca unha maior liberación de N

e P no solo. A vida media do carbofurano foi de 16.9 días e seguiu unha cinética de

disipación de primeiro orde.

Das e Mukherjee (1998) observaron a baixa persistencia do carbofurano no

solo, e a estimulación deste na mineralización microbiana dos elementos nutritivos

favorecendo unha maior liberación de elementos nutritivos dispoñibles no solo.

Das et al. (1995) observaron que o carbofurano acentúa a preponderancia de

actinomicetos e estimula o crecemento de Serratia, Corynebacterium, Klebsiella,

Escherichia, Rhizopus e Humicola e a inhibición de Pseudomonas, Micrococcus e

Penicillium

Simpson et al. (1993) estudaron o efecto do carbofurano na poboación de

oligoquetos no solo. Unha primeira aplicación de carbofurano ao solo parece inhibir a

actividade dos oliquetos, pero non se observaron efectos en aplicacións posteriores.

Benson e Long (1991) estudaron a influencia das sustancias húmicas na

toxicidade do carbofurano, observando que a toxicidade diminuía co contido de ácidos

húmicos.

Nayak e Rajaramamohan Rao (1982) estudaron a influencia da aplicación de

carbofurano na fixación de nitróxeno, observando que a presenza do pesticida estimula

significativamente a fixación de nitróxeno.

Ramakrishna et al. (1978) observaron que o carbofurano tiña un pequeno

efecto na mellora da nitrificación en solos non inundados cando se aplicaba a

concentracións entre 5 e 500 ppm.

Gilman e Vardanis (1974) estudaron como lle afectaba o carbofurano a dúas

espécies de lombrigas (Eisenia foetida e Lumbricus terrestris). Observaron que a

primeira era a máis afectada polo pesticida que a segunda.


25

Stenersen et al. (1973) estudaron a toxicidade de carbofurano en Lumbricus

terrestris. A LD50 foi de 1.3 mg kg-1, e cando se mesturaba o pesticida co solo, a LD50

foi 12.2 mg kg-1. A toxicidade pode ser debida a inhibición da colinesterasa. Os vermes

excretan carbofurano sen cambios.

2.2. Determinación de carbofurano e metalaxyl no medio ambiente.

Devasis e Madhu (2011) determinaron o contido de carbofurano en solos de

plantación de banana. Recuperou dos solos contidos en carbofurano de 11.7-43.7 mg L-

1. O contido nos solos foi variable e estivo influído polo contido en materia orgánica,

polo abonado, pola temperatura e a humidade.

Devasis et al. (2011) estudaron a presenza de carbofurano en augas superficiais

da India, observando que a concentración de carbofurano en augas era de 0.186-0.262

mg L-1. Atoparon unha relación entre o pH do solo e o contido de carbofurano nas

augas.

Kumar et al. (2011) estudaron o comportamento das formulacións de

carbofurano en cultivos de patacas e observaron que as formulacións de carbofurano de

liberación controlada proporcionan unha protección dos cultivos igual ou superior as

formulacións comerciais. Non atoparon residuos de carbofurano nas patacas nin nos

solos.

Latif et al. (2011) determinaron o contido de pesticidas en vexetais (Pakistan).

Observaron que no 61 % das mostras superábase o límite máximo establecido en varios

pesticidas. O carbofurano atopouse por enriba deste límite con concentracións dende

0.01 a 0.39 mg kg-1.

Otieno et al. (2011) determinaron o contido de pesticidas en solos, augas e

plantas en Kenya atopando residuos de carbofurano. Esto indica que o composto foi

amplamente empregado na zona de estudio, onde se relacionou a súa presenza coa

morte de voitres.

Pinheiro et al. (2011) determinaron o contido de herbicidas e funxicidas

(metalaxyl) en solos a 3 profundidades distintas. Os ingredientes activos foron atopados

nas tres profundidades avaliadas. Os funxicidas atoparonse máis frecuentemente na capa

superficial. As concentracións variaron entre as zonas e as profundidades. Estos


26

resulatdos suxiren que a existencia de camiños preferenciais no perfil do solo é o factor

responsable da aparición irregular de altas concentracións en profundidade.

Khuntong et al. (2010) realizaron unha extracción de carbofurano dos solos con

éter de petróleo:acetona (1:1). A porcentaxe de recuperación estivo entre o 84 e 77%

para concentracións de 10-40 mg L-1. O contido de carbofurano no solo decrece ano tras

ano, debido as propiedades do pesticida e a súa degradación no solo. Atopouse máis

carbofurano nas mostras de solo con máis contido en materia orgánica. Os experimentos

de adsorción amosaron unha porcentaxe de adsorción entre o 30 e o 80 %. A vida media

do carbofurano nos solos foi de 8.9 días. Presentou un gran potencial de lixiviación e

era rapidamente desorbido (velocidade de desorción de 0.0228 mg kg-1 día-1).

Nishina et al. (2010) estudaron a presenza de pesticidas (metalaxyl) en solos,

sedimentos e plantas en Vietnan. Observaron que nos solos agrícolas había residuos de

metalaxyl, aínda que por debaixo dos limites establecidos polo goberno dese país.

Otieno et al. (2010) determinaron o contido de carbofurano en explotacións

agrícolas dunha zona de Kenya onde se aplicou este pesticida. O contido de carbofurano

atopado nos solos foi de 0.010-1.009 mg kg-1, en augas dos dous ríos próximos foi de

0.005-0.495 mg L-1, en augas de estanques e balsas situadas nas explotacións foi de

<LD-2.301 mg L-1 e en plantas de millo foi de 0.040-1.328 mg kg-1.

Romero et al. (2010) estudaron o comportamento do carbofurano en augas de

río e de pozos dunha zona de Brasil. O carbofurano foi un 60% inferior nas augas de río

(0.339 ± 0.087 mg L-1 ) que nas augas de pozo (0.130 ± 0.050 mg L-1).

Pose-Juan et al. (2009) determinou o contido de metalaxyl en formulacións

comerciais (Ridomil® Gold Plus e Ridomil® Gold MZ, con 25 e 40 g kg-1

respectivamente). As concenracións determinadas foron 26.5 e 41 g kg-1. Activadores e

adyuvantes foron detectados en ambos productos, tamén se atopou un tensioactivo

basado en metil-ester.

Yu et al. (2009) estudaron a influencia das emendas (cinzas de eucalipto) no

comportamento do carbofurano no solo. Observaron que en solos emendados con cinzas

había unha menor biodispoñibilidade do pesticida, reducindo a súa potencial absorción

pola planta.

Bermúdez-Couso et al. (2007) estudaron a presenza de funxicidas en solos e

sedimentos nunha zona dedicada ao cultivo de viñedo en Galicia (España). A máxima


27

concentración de metalaxyl atopada en solos foi de 1000 µg kg-1. A frecuencia de

detección dos funxicidas puido estar relacionada coas aplicacións na viña e co efecto de

lavado provocado polas chuvias.

López-Pérez et al. (2006) detectaron metalaxyl en patacas sen exceder a metade

do límite máximo de residuo, incluso cando se aplicou doses máis altas da

recomendada. Tamén atopou metalaxyl no lavado das plantas, o cal pode ser debido a

alta solubilidade do pesticida.

Haarstad e Braskerud (2005) estudaron a presenza de pesticidas (metalaxyl) en

solos, observando que o 96% do pesticida aplicado nunha conca de estudo desaparecia

dentro da conca en pouco tempo. O 4 % dos pesticidas que foron que foron aplicados

dan un pico de concentración inmediatamente despois da pulverización, alcanzando

niveis que poden ser peligrosos para a vida acuática.

Papadopoulou-Mourkidou et al. (2004) estudaron a presenza de carbofurano,

atrazina, alaclor, prometrina y propanil en solos de cultivo dunha zona de Grecia onde

se cultivaba arroz. Obtiveron concentracións de carbofurano superiores a 1 mg L-1 en

auga do solo e na auga freática dos campos de cultivo.

Kladivko et al. (1991) estudaron o movemento do carbofurano no solo en

experimentos de campo, observando que este movemento seguía os conceptos de fluxo

preferencial. As perdas anuais de carbofurano oscilaron entre un 0.05 e un 0.94% do

previamente aplicado.

2.3. Adsorción-desorción de carbofurano e metalaxyl.

Marín-Benito et al. (2012) observaron que a adición de emendas ricas en

materia orgánica incrementaban a capacidade de adsorción do metalaxyl polo solo. A

adsorción de metalaxyl atopase favorecida por un baixo contido en carbono orgánico

disolto e por un elevado grao de humificación do carbono orgánico nos solos con

substrato compostado en comparación cos solos con substrato fresco. O contido en

carbono orgánico é máis importante na adsorción que os cambios na súa natureza ao

longo do tempo de incubación. A adición de substrato tamén favorece a desorción de

metalaxyl.

Singh et al. (2012) determinaron a adsorción e a mobilidade de carbofurano en

solos sen tensioactivos acuosos e con tensioactivos catiónicos (CPC, cetyl pyridinium


28

chloride), non iónico (Brig 35) e aniónicos (DBSS, sodium dodecyl benezene

sulphonate). A adsorción de carbofurano con tensioactivo seguiu a seguinte orde;

catiónico > aniónico > non-iónico.

Baglieri et al. (2011) estudaron a adsorción de metalaxyl en residuos.

Observaron que a adsorción de metalaxyl en compost de residuos sólidos urbáns foi dun

37%, a cal viuse aumentada pola adición de esmectita. A adición de nitrato amónico no

compost favorece a degradación de metalaxyl pasando dunha vida media de 84 a 15

días.

Barriuso et al. (2011) investigaron a liberación de pesticidas dos solos. A

desorción dos pesticidas previamente adsorbidos nos solos levouse acabo con auga e

materia orgánica disolta. No caso do metalaxyl a adsorción foi baixa (KOC≤50) e a auga

foi máis eficiente na desorción que a materia organica disolta. Nos pesticidas de maior

adsorción, a materia orgánica disolta mostrou un pequeno efecto.

Chun et al. (2011) observaron que a adsorción de metalaxyl está influenciada

pola aplicación de ferro zerovalente, o que fixo que a concentración de metalaxyl se

reducira un 40-45% en 28 días. O ferro zerovalente adsorbeu máis metalaxyl en

sistemas acuosos que en sistemas semellantes á disolución do solo. Os mecanismos de

adsorción dos pesticidas nos sistemas similares a disolución do solo mostraronse como

unha adsorción de sitios múltiples.

Rodríguez-Cruz et al. (2011) amosaron a eficacia de barreiras de madeira na

diminución da lixiviación de pesticidas en fontes puntuais de contaminación. A madeira

de pino foi máis efectiva que a de carballo. O pesticida menos hidrófobo, metalaxyl, foi

limitante para a eficacia destas barreiras.

Singh et al. (2011) estudaron a adsorción de carbofurano en solos con auga

destilada e con 0.01 M de β-ciclodextrin. Nestes últimos experimentos a adsorción de

carbofurano foi menor que nos experimentos con auga destilada. A desorción nos

experimentos con auga destila presentou histéreses, ó contrario que nos experimentos

con β -ciclodextrin onde non houbo histéreses.

Yang (2011) empregou cinzas obtidas de distintas temperaturas (250, 450 e 850

ºC) como emenda do solo para avaliar a adsorción de carbofurano en experimentos

batch. Observaron que a adsorción de carbofurano aumentaba co contido en cinzas,


29

sendo a maior para o solo que contiña cinzas obtidas a 850 ºC. Nos experimentos de

desorción observouse histéreses.

De Wilde et al. (2010a) estudaron a adsorción de metalaxyl en columnas de de

solo baixo diferentes fluxos. Os estudios en macrocosmos mostran unhas BTCs moi

limitadas, onde as concentracións medidas no efluente foron moi baixas ou non

detectables. Os cambios no fluxo en macrocosmos afectan minimamente a degradación.

De Wilde et al. (2010b) observaron que os procesos de adsorción son

dependentes do tempo. Un aumento de fluxo provoca unha diminución da adsorción e

tamén unha menor degradación. Isto pode ser debido a que diminúe a oportunidade de

biodegradación ao diminuír o tempo, ou tamén pode ser debido a que ao haber un

aumento da cantidade de pesticida na entrada pode xerar unha maior toxicidade para os

microorganismos encargados da degradación. Tamén, ao aumentar o fluxo pode

provocar unha distribución da biomasa que leva a cabo a degradación. A presenza de

pesticidas no solo foi beneficioso para a degradación de metalaxyl.

De Wilde et al. (2010c) estudaron o tratamento das matrices orgánicas

empregadas como sistema de biopurificación, as cales poden conter restos de pesticidas

e hai que emprazalas. O tratamento consistiu nunha compostaxe das matrices,

observándose unha diminución da concentración de metalaxyl.

Karanasios et al. (2010) observaron que a adsorción de metalaxyl foi maior en

solos mesturados con substratos e palla que nos solos só. O compost de folla de oliveira

mostrou unha gran capacidade de degradación do metalaxyl. A desorción de metalaxyl

realizada nas mesturas foi menor que nos solos.

Leistra e Boesten (2010) estudaron a mobilidade en cristas e sucos de

carbofurano. As sustancias viaxan máis profundamente polos sucos, xa que a auga das

cristas tamén vai para os sucos. Aos 19 días de aplicación mediuse o pico de

distribución do Br- a 0.1-0.2 m nas cristas mentres que nos sucos estaba a 0.3-0.5 m.

Despois de 65 días o pico de distribución do carbofurano estaba a 0.1 m nas cristas

mentres que nos sucos o pesticida distribuíuse uniformemente por riba dos 0.6 m. O

fluxo preferencial e o transporte de sustancias aconteceu no solo areoso.

Nikologianni et al. (2010) observaron que a lixiviación de metalaxyl viuse

afectada pola presenza de turba, diminuíndo a medida que se aumentaba a cantidade de

turba.


30

Pose-Juan et al. (2010) compararon a retención no solo de metalaxyl en grao

analitico e en formulacións comerciais. O metalaxyl empregado nas formulas

comerciais presenta unha maior retención nos solos que o pesticida de grao analítico.

De Wilde et al. (2009a) mostran a retención de varios pesticidas en mesturas de

substratos orgánicos. Observaron que a retención dos pesticidas foi moi similar e

nalgúns casos a retención foi lixeiramente inferior en macrocosmos en comparación con

microcosmos.

De Wilde et al. (2009b) suxiren que segundo a capacidade de adsorción, os

substratos pódense clasificar do seguinte xeito: mesturas de turba > compost, palla >

esterco de vaca >solo franco areoso. A capacidade de adsorción está correlacionada co

contido de carbono orgánico, co CaO e a capacidade de intercambio catiónico efectiva.

Os coeficientes de adsorción (Kd) poden ser empregados para calcular os coeficientes de

adsorción das mesturas de substratos. A adsorción de metalaxyl e linuron foron

significativamente máis baixas en combinación con bentazon e isoproturon.

Kazemi et al. (2009) investigaron os antecedentes do contido en auga do solo no

transporte e degradación de carbofurano e alicarb. Aplicaron o pesticida en parcelas a

capacidade de campo (húmidas) e en pacelas próximas o punto de marchitez. A metade

das parcelas foron regadas inmediatamente trala aplicación, mentres que a outra metade

regouse as 24 h. No tratamento en condicións iniciais secas, os pesticidas atopáronse 10

cm máis profundos que en condicións húmidas. A velocidade da auga de poro foi máis

rápida en condicións iniciais secas e a degradación foi un 49% máis baixa en condicións

secas.

Marín-Benito et al. (2009a) realizaron experimentos con mostras de solos baixo

condicións de fluxo non saturado. Observaron que en solos emendados incrementase a

retención de matalaxyl. A degradación do metalaxyl encontrouse en mostras non

incubadas e aumentaba trala incubación.

Marín-Benito et al. (2009b) observaron que os efectos da adición de substratos

frescos e compostados ao solo provoca cambios nos procesos de adsorción-desorción do

metalaxyl, os cales poden ser debidos a unha diminución da mobilidade do metalaxyl.

Rodríguez-Cruz et al. (2009) realizaron estudios de adsorción de pesticidas en

lignina e celulosa, xa que ámbalas dúas presentan unha gran capacidade de adsorción de

pesticidas. A adsorción neses materiais incrementouse co carácter hidrófobo dos


31

pesticidas, sendo o metalaxyl o menos hidrófobo e por conseguinte o que menos

adsorción presentou.

Shiareef e Hamadamin (2009) estudaron a adsorción de metalaxyl e glifosato

en solos mediante experimentos tipo batch, descubrindo que seguían unha ecuación de

primeiro orde. A adsorción do metalaxyl foi inferior que a do glifosato. Polos valores de

Kd amósase que o metalaxyl ten unha moderada ou forte adsorción nas mostras

estudadas (5.96 mL g-1). Tamén atoparon unha correlación lineal entre os coeficientes

de adsorción de cada pesticida e o contido en carbono orgánico do solo.

Singh e Srivastava (2009) estudaron a adsorción de carbofurano mediante

experimentos batch en solos franco limosos, francos e franco areosos. As isotermas nos

solos franco limosos e francos foron tipo L, no solo franco areoso foi tipo S, sendo este

solo o que presentaba menor adsorción. O valor de ∆G negativo confirma a

espontaneidade da adsorción. O índice de lixiviación amosa un alto potencial de

lixiviación do carbofurano. O contido en arxila e limo favoreceron a adsorción de

carbofurano.

De Wilde et al. (2008) estudan a adsorción de pesticidas en substratos

comunmente empregados en sistemas de biopurificación (esterco, palla..etc). Os datos

foron axustados a modelos de cinéticas de adsorción de primeiro orde.

Rodríguez-Cruz et al. (2008) estudaron a adsorción de metalaxyl en mostras de

madeiras. Empregaron mostras de madeiras sen tratar e tratadas con NaOH, HCl, auga e

ODTMA (octadecyltrimethylammonium bromide). O metalaxyl adsorbeuse un 40%

cunha relación adsorbente/disolución 1:10. A adsorción foi significativamente máis alta

en pino, con maior contido en lignina, que no carballo. Nestes experimentos mostrouse

unha gran irreversibilidade da adsorción de pesticidas en madeiras tratadas en

comparación con madeiras non tratadas.

Maurya et al. (2007) estudaron a lixiviación de carbofurano en columnas en tres

tipos de solos: solos cun alto contido en materia orgánica, lateríticos, e salinos alcalinos.

A maior lixiviación de carbofurano tivo lugar no solo laterítico, e a menor no solo

salino que non liberou carbofurano ata as 18 h.

Rodríguez-Cruz et al. (2007) estudaron o efecto de solos arxilosos modificados

con tensioactivos (ODTMA, octadecyltrimethylammonium bromide) na retención de

pesticidas (metalaxyl). Os experimentos foron realizados en columnas. As curvas de


32

avance amosan unha diminución da lixiviación do metalaxyl nos solos modificados. Os

resultados amosan que os solos arxilosos modificados con tensioactivos poden ser

interesantes na inmobilización de pesticidas.

Arias et al. (2006) estudaron o efecto do Cu e do pH na adsorción de metalaxyl

nunha serie de solos. O Cu apenas afectou a adsorción de metalaxyl, pero os pHs baixos

incrementaron substancialmente a adsorción deste pesticida aumentando o valor de KF

2-3 veces. A pHs baixos, unha significantiva proporción do funxicida está cargado

positivamente e é atraído polas cargas negativas dos coloides do solo.

Arias-Estévez et al. (2006) estudaron a influencia do pH e a presenza de Cu e

Zn na adsorción de carbofurano en solos dedicados o cultivo de millo. A adsorción de

carbofurano foi lixeiramente máis baixa a pH altos. O Cu provoca un mínimo efecto

favorecendo a adsorción de carbofurano, mentres que o Zn no afecta a adsorción deste

pesticida.

Choudhary et al. (2006) obtiveron unha formulación de liberación controlada

de carbofurano, preparada con resina, sodio carboxymetilcelulosa e arxila. A adición de

arxila reduce a velocidade de lixiviación en comparación coas formulacións comerciais.

O tempo medio de liberación de carbofurano foi de 5-25 días e o período de

dispoñibilidade óptima foi de 15-44 días. A EC50 para Meloidogyne incognita sometida

ás formulacións comerciais é bastante elevada en comparación cás da liberación

controlada. A duración efectiva foi de 0.7 días para a comercial e 17.8 días para a de

liberación controlada.

De Wilde et al. (2006) estudaron a adsorción de metalaxyl en distintos

substratos. A adsorción de metalaxyl seguíu a seguinte secuencia: virutas de coco >

palla> compost > salgueiro triturado > solo franco areoso.

Elmanfe et al. (2006) observaron que o carbofurano pode inducir a coadsorción

de sales metálicas como nitrato de chumbo na interfase auga/silice.

Fernandes et al. (2006) estudaron o efecto de 3 emendas: sólida (SF), líquida

(LF) e residuo de aceite de oliva (alperuxo, OW) no comportamento dos funxicidas nos

solos. A adsorción de pesticidas incrementouse especialmente con SF, mentres que a

emenda con LF apenas afectou a adsorción ou diminuíu, especialmente no solo cun

70% de arxila. Neste solo, emendado con OW e cun contido moi alto de materia

orgánica soluble, a adsorción de metalaxyl diminuíu. En xeral a adición de emendas


33

favorece a retención de pesticidas nos solos e diminúe a desorción. Nalgúns casos a

adsorción diminúe ou non lle afecta cando hai unha competición entre a materia

orgánica disolta da emenda e o funxicida polos sitios de adsorción.

Gupta et al. (2006) estudaron mediante experimentos de tipo batch a adsorción

de carbofurano en residuos da industria de fertilizantes (suspensións de carbón) e

residuos da industria siderúrxica (cinzas, lodos, escoura de fornos). Observaron que a

maior adsorción de carbofurano presentábaa a suspensión de carbono, seguida por

lodos, cinsas e escouras. A cantidade máxima de carbofurano adsorbida na suspensión

de carbón foi 208 mg g-1, a 25 ºC e pH 7.5. Tamén observaron que a adsorción de

carbofurano diminuía co aumento do pH e coa temperatura.

Liyanage et al. (2006) estudaron a adsorción de carbofurano en solos de Sri

Lanka de zonas secas e húmidas. Os valores de Kd oscilaron de 0.11 a 4.10 L kg-1. Os

valores de Koc foron de 7.3-120.6. Estes valores foron máis altos en zonas secas que en

zonas húmidas.

Rodríguez-Cruz et al. (2006) observaron que a adsorción de metalaxyl en solos

con tensioactivo (ODTMA) foi maior que en solos naturais. A materia orgánica

derivada do tensioactivo ten unha maior capacidade de adsorción que a materia orgánica

do solo natural.

Sanchez-Martin et al. (2006) estudaron a eficiencia dunha serie de arxilas

modificadas con tensioactivo catiónico (ODTMA) na adsorción de pesticidas. O uso de

tensioactivos con solos e arxilas pode ser usado como unha barreira para previr a

mobilidade de pesticidas.

Bansal e Gupta (2005) observaron que a adsorción de carbofurano aumentou en

solos saturados con Fe, Mn e Cu ata pH 6 e despois diminuíu. A adsorción de

carbofurano foi maior en solos cun alto contido en materia orgánica > vermellos >

aluviais, tamén foi maior na presenza de Fe > Mn >Cu. A adsorción de carbofurano

aumenta co radio iónico efectivo dos catións intercambiables e diminúe co aumento da

temperatura. Tamén a adición de lodos incrementa a adsorción de carbofurano.

Yazgan et al. (2005) compararon a adsorción de carbofurano en solos mediante

métodos batch e métodos de centrifugación. Suxire que as técnicas batch sobreestiman a

adsorción de carbofurano.


34

Andrades et al. (2004a) estudaron o uso de residuos de vinificación como

emendas do solo na adsorción de pesticidas. Observaron que cando se lavaban as

columnas de solo con residuos líquidos liberábase máis metalaxyl que cando se lavaban

con auga.

Andrades et al. (2004b) observaron que a adsorción de metalaxyl e penconazol

en diversas arxilas viuse incrementada por a adición de HDPY (organic cation

hexadecylpyridinium) sendo os valores máis altos para montmorillonita, ilita e

atapulgita.

Cox et al. (2004) observaron que a adsorción de metalaxyl en solos con

emendas que teñan materia orgánica disolta diminúe, debido a competencia de esta

polos sitios de adsorción.

Tariq et al. (2004) estudaron a adsorción de carbofurano en solos franco arxilo

areosos e franco areosos, sendo maior nos primeiros. Os datos de Kd correlacionaronse

significativamente co contido en carbono orgánico do solo, pero non se atopou

correlación de Kd co contido en arxila.

Fernandes et al. (2003) observaron unha elevada adsorción de metalaxyl en

solos arxilosos (con ilita), pero cando se excluían estes solos do análise de regresión, a

materia orgánica era a propiedade que mais influenciaba a adsorción de metalaxyl a

baixas concentracións. Experimentos en columnas con fluxos saturados e non saturados

amosan unha relación inversa entre liberación e adsorción de metalaxyl. A vida media

do metalaxyl incrementouse coa adsorción e co contido de materia orgánica, sobre todo

nos solos emendados con materia orgánica.

El M'Rabet et al. (2002) relacionaron a adsorción de carbofurano co contido de

materia orgánica do solo.

Fernandas et al. (2002) estudaron a influencia de emendas orgánicas na

adsorción de metalaxyl mediante experimentos en columnas baixo condicións de fluxo

saturadas e non saturadas. A adición de residuos sólidos de agricultura e residuos da

industria do aceite incrementaron a adsorción de metalaxyl. Sen embargo, a adición de

emendas orgánicas líquidas case non tivo efecto e nalgúns casos manifestouse unha

pequena diminución da adsorción do pesticida. Esto pode ser debido a competencia da

materia orgánica disolta polos sitios de adsorción.


35

Monkiedje e Spiteller (2002) estudaron a adsorción de metalaxyl en dous solos:

un solo forestal de Camerún (franco arxilo areoso, pH 4.8 and 3.0% carbono orgánico) e

un solo de Alemania (franco areoso, pH 7.2, 1.7% carbono orgánico). A adsorción de

metalaxyl foi maior no solo de Camerún. A adsorción foi altamente irreversible nos

dous solos, tendo un porcentaxe de desorción maior no solo de Alemania.

Andrades et al. (2001) estudaron o efecto das propiedades do solo na adsorción

de metalaxyl. Os resultados amosan que o contido en materia orgánica e arxila son os

factores máis influentes na adsorción de metalaxyl nos solos empregados no estudo.

Worrall et al. (2001) observaron que a adsorción de carbofurano esta

influenciada polo contido en materia orgánica do solo. A degradación do carbofurano é

pH dependente, incrementándose ó aumentar o pH. A materia orgánica do solo protexe

o carbofurano da súa degradación.

Fernández-Pérez et al. (2000) estudaron a liberación de carbofurano en grans

formados a partir de alxinato e a modificación destas formulacións con bentonita natural

e bentonita con tratamento ácido. O tempo no que o 50 % (t50) do principio activo foi

liberado en auga para os grans foi de 4.7 horas. Os grans con bentonita natural tiveron

un t50 de 6.1 horas e con bentonita en tratamento ácido 9 horas (0.5M H2SO4) e 11.7 h

(1M H2SO4).

Marei et al. (2000) observaron que en formulacións de liberación controlada

con alxinato, a liberación de carbofurano era nove veces menor que en formulacións

granuladas.

Singh (2000) estudou a adsorción de carbofurano en solos empregando mesturas

metanol:auga. Observou que a maior adsorción foi nun solo franco arxiloso e que a

adsorción de carbofurano diminuía a medida que se aumentaba a concentración de

metanol na fase líquida.

Singh e Kumar (2000) estudaron o efecto dos tensioactivos na mobilidade de

pesticidas. A penetrabilidade dos pesticidas foi maior para os solos areosos, seguidos

dos limosos e dos arxilosos. A mobilidade dos pesticidas foi directamente proporcional

a solubilidade en auga, a polaridade e o número de carbonos da molécula, e foi

inversamente proporcional ó peso molecular da molécula. A adsorción ao solo foi un

factor moi importante.


36

Sukul e Spiteller (2000) estudaron a adsorción-desorción do metalaxyl en solos.

Observaron que a adsorción está correlacionada positivamente coa materia orgánica e

coa capacidade de intercambio catiónico efectiva, pero non co contido de arxila e de Fe.

A desorción foi moi elevada.

Hsieh e Kao (1998) observaron que a adsorción de carbofurano en solos

aumenta co pH, coa materia orgánica e coa arxila.

Jana e Das (1998) estudaron a adsorción de carbofurano en solos. Con 3 h de

contacto alcanzase o máximo de adsorción, a cal foi correlacionada coa materia

orgánica do solo e coa area superficial.

Petrovic et al. (1998) estudaron a aplicación de residuos de cervexería

compostados, compost de lodos de depuradora, turba ou zeolita para mitigar a

migración de metalaxyl de solos areosos. O metalaxyl mediuse no lixiviado,

incrementandose dende non detectable ata obter un pico de concentración entre 14 e 21

días despois da primeira irrigación, decrecendo finalmente. As concentracións máximas

de metalaxyl foron no seguinte orde; residuos de cervexería> turba> lodos> solo franco

areoso.

Stolpe et al. (1998) observaron que a mobilidade dos pesticidas no solo está

máis afectada pola solubilidade do pesticida e a Koc que polo tipo de solo.

Sharma e Awasthi (1997) estudaron a adsorción de metalaxyl en 3 solos: un

franco areoso e dous arxilosos. O valor da KF foi maior para os solos arxilosos, e a

mobilidade de metalxyl foi inversamente proporcional o valor de KF. O metalaxyl

persistiu nos solos máis de 60 días e non se moveu máis ala dos 17.5 cm de

profundidade en calquera dos solos baixo condicións de fluxo non saturado. A

mobilidade foi máis alta no solo franco areoso.

Mear et al. (1996) estudaron a adsorción de carbofurano en silica. Observaron

que a solubilidade do pesticida en auga incrementábase coa temperatura. A adsorción

tamén se incrementou coa temperatura. As curvas de adsorción foron tipo S.

Petrovic et al. (1996) observaron que a presenza de turba en solos areosos

retrasa a lixiviación de metalaxyl.

Singh et al. (1994) observaron que a adsorción de carbofurano polos solos está

correlacionada co contido en materia orgánica, contido en arxila, contido en CaCO3,


37

area superficial e capacidad de intercambio catiónico efectiva. O carbofurano presenta

unha elevada afinidade polos solos saturados de H, seguido polos solos saturados de

sodio, solos naturais a 25 ºC, solos autoclavados, solos os que se lle eliminou a materia

orgánica, solos con tensioactivo catiónico, solos con tensioactivo aniónico e solos

naturais a 50 ºC.

Levanon et al. (1993) observaron unha maior mobilidade de agroquímicos

(carbofurano, atrazina, diazinon, metolachlor e nitratos) en solos arados que en cultivos

sen arar.

Perrin-Ganier et al. (1993) observaron a adsorción de carbofurano as paredes

dun envase cerámico. O 80% da cantidade engadida quedaba retido nos primeiros 10

mm da parede do envase. A desorción con auga destilada do carbofurano das paredes

foi elevada e rápida.

Sukop and Cogger (1992) estudaron a adsoción de carbofurano, metalaxyl e

simazine en solos. Observaron que a adsorción de carbofurano o solo foi alta e estibo

fortemente relacionada co contido en materia orgánica. O metalaxyl presentou unha

adsorción máis baixa o solo e correlacionouse cás superficies minerais.

Achik et al. (1991a) estudaron como influía a estrutura do solo na lixiviación de

carbofurano mediante experimentos en columnas. Observaron que os solos arxilosos e

con materia orgánica eran os sistemas que presentaban unha maior adsorción do

pesticida.

Achik et al. (1991b) observaron que a adsorción de carbofurano ao solo

producíase nunha fase de adsorción rápida seguida por unha fase de adsorción lenta.

Empregaron tres modelos matemáticos para describir os procesos de adsorción o solo,

modelo hiperbolico e modelos de 2 o 3 compartimentos. O modelo de tres

compartimentos foi o que amosou os valores máis altos de Qmax. O solo arxiloso

presentou unha maior capacidade de adsorción de carbofurano que o solo franco

arxiloso. Tamén observaron a tendencia do carbofurano a desorberse.

Crepeau et al. (1991) estudaron a retención de pesticidas (carbofurano) en

columnas de solo mesturadas con carbón, obtendo que a retención dos pesticidas era

moi elevada. O contido en humidade do carbón correlacionouse positivamente coa

capacidade do carbón de reter os pesticidas.


38

Kumari et al. (1988) estudaron a adsorción de carbofurano en solos mediante o

uso de columnas. Observaron que un solo franco arxiloso presentaba unha maior

adsorción en comparación cun solo franco limoso. Unha grande cantidade de auga foi

necesaria para lixiviar o carbofurano ata 152 cm de profundidade no solo franco

arxiloso.

2.4. Degradación de carbofurano e metalaxyl.

Karanasios et al. (2012) estudaron a degradación de pesticidas en

biomesturadores. Os autores observaron que a palla era esencial para manter unha alta

capacidade de degradación, mentres que o abono mesturado co solo atrasa a

degradación dos pesticidas. As taxas máis elevadas de degradación dos pesticidas

obtivéronse cando se mesturaba solo, esterco e palla. Un aumento da temperatura de 15

a 25 ºC reduce a vida media dos pesticidas.

Morales et al. (2012) estudaron o efecto das microemulsións de sodium bis(2-

ethylhexyl) sulfosuccinate/isooctane/water na estabilidade de carbofurano, observando

que este medio favoreceu a hidrólise do pesticida.

Plangklang e Reungsang (2012) illaron unha bacteria capaz de degradar o

carbofurano como única fonte de carbono (Burkholderia sp.). A pH 7.5 e 35 ºC foron as

condicións óptimas para a maior taxa de degradación.

Plangklang et al. (2012) estudaron a mobilidade do carbofurano en columnas de

solo baixo condicións de saturación. Observaron que en experimentos abióticos e con

microorganismos indíxenas recuperábase un 52 e 22% de carbofurano. O tratar estas

columnas con degradadores específicos de carbofurano (PCL3) e con palla a

recuperación foi menor 14 e 15%.

Rodríguez-Cruz et al. (2012) describen a degradación do metalaxyl e do

alachlor como lenta, seguindo o orde: solo emendado con madeira de piñeiro < solo

emendado con madeira de carballo < solo non emendado. A degradación máis rápida do

metalaxyl no solo non emendado debese a alta adsorción dos pesticidas nos solos

emendados con madeiras. As emendas con madeiras de piñeiro e carballo aumentan a

inmobilización de pesticidas pero tamén diminúen a velocidade da súa degradación.

Baglieri et al. (2011) amosan que a adsorción de metalaxyl en residuos sólidos

urbáns (RSU) foi dun 37%. Cando se lle engade esmectita os RSU, a adsorción de


39

metalxyl incrementase notablemente. Ó engadirlle nitrato amonico, a vida media do

metalaxyl pasa de 84 a 15 dias.

Chen et al. (2011) investigaron o contido de carbofurano e os seus metabolitos

en plantas de millo e solos. A vida media do carbofurano foi de 4.7-5.3 días nas plantas

de millo e de 7.6-8.5 días nos solos. Non se atoparon residuos finais de carbofurano nin

dos seus metabolitos nas plantas de millo nin nos solos.

Coppola et al. (2011) estudaron a degradación de pesticidas en biomesturadores

con palla e compost. Os autores observaron que o final do experimento (112 días) a

maior parte dos pesticidas estaban completamente degradados. Ao aplicar os pesticidas,

observaron unha modificación na diversidade microbiana. Ao fin do proceso de

degradación dos pesticidas, non se observaron cambios significativos na composición

da comunidade microbiana.

Fernández-Gómez et al. (2011) observaron que o vermicompost podese

empregar como un mitigador da contaminación ambiental por pesticidas, xa que

incrementa a súa disipación nos solos.

Kadakol et al. (2011) empregou bacterias degradadoras de carbofurano

extraídas dos solos e inmobilizadas en diversos medios. Observaron que as bacterias

que tiñan unha maior capacidade degradadora do carbofurano eran as que estaban

inmobilizadas en espuma de poliuretano.

Krishna e Philip (2011) investigaron a degradación de varios pesticidas

(carbofurano) en condicións de inundación e non inundación en distintos solos: nun

areoso, nun arxiloso e nun solo vermello, ademais de nun compost. No compost

presentouse a maior adsorción de pesticidas e a menor degradación. No solos areosos

houbo a maior disipación dos pesticidas. A degradación dos pesticidas foi máis alta nos

solos inundados que nos non inundados.

Lu et al. (2011) estudaron a degradación fotoquímica do carbofurano con H2O2

e Fe+3. O carbofurano, nunha concentración de 50 mg L-1, foi degradado completamente

en 30 min, a pH 3 e con H2O2 e Fe+3.

Plangklang e Reungsang (2011) en experimentos a pequena escala observaron

que a degradación de carbofurano cos microorganismos propios do solo era lenta, cunha

vida media (t1/2) de 127 días. O aplicarlle o solo PCL3 inmobilizadas (células

degradadoras especificas de carbofurano) a vida media do pesticida pasaba a 16 días. Se


40

se usaban PCL3 libres, o t1/2 diminuia ata 28 días, observando que estas últimas non

sobrevivían durante a operación. Os autores suxiren o uso de PCL3 inmobilizadas para

empregar a técnica en condicións de campo.

Xu et al. (2011b) estudaron a degradación de cabofurano con CFDS-1,

identificada como Sphingomonas sp. Observaron que a degradación completa do

carbofurano ocurría en 48 h, a unha concentración de 100 mg L-1 de pesticida. As

condicións idóneas foron pH 8-9, 100-250 mL de disolución e 25-42 ºC.

Yang et al. (2011) observaron como un lévedo novo, Pichia anomala, degradou

o carbofurano un 95 %, cando este foi usado como única fonte de carbono nun medio de

sales minerais durante un periodo de cultivo de 48 h. As condicións idóneas para a

degradación foron unha concentración de carbofurano de 50 mg L-1, pH 7.5 e 30 ºC.

Ahmed et al. (2010) estudaron a disipación de metalaxyl en solos esterilizados e

non esterilizados. A disipación foi maior nos solos non esterilizados (64.2-79.9%) que

nos solos esterilizados (22.0-29.2%) despois dun periodo de 6 días. A vida media do

metalaxyl dependeu da concentración engadida, diminuíndo a medida que se aumentaba

a concentración de metalaxyl aportada.

Baker et al. (2010) observaron que a velocidade de degradación dos pesticidas

depende da composición da comunidade microbiana e das características fisico-

quimicas do solo. Neste traballo mostrase que as caracteristicas do solo influiron máis

na degradación dos pesticidas que a composición da comunidade microbiana.

Bansal e Gupta (2010) estudaron o efecto do pH, a adición de lodos e a adición

de fertilizantes nitroxenados na degradación e mobilidade de carbofurano en distintos

solos. Observaron que a degradación de carbofurano seguía unha cinética de primeiro

orde. A degradación foi máis elevada nun solo aluvial, seguida polo solo vermello e

finalmente o solo cun elevado contido en materia orgánica. A degradación do

carbofurano foi máis alta a pH alcalino. A aplicación de lodos e de nitróxeno ao solo

diminúe a degradación e mobilidade do carbofurano. A mobilidade foi maior no solo

non emendado, seguida polo solo emendado con nitróxeno, mentres que o solo

emendado con lodo de depuradora foi o que presentou unha menor mobilidade de

carbofurano. Conclúen os autores que a degradación e mobilidade do carbofurano esta

influenciada polo pH e o contido en materia orgánica do solo.


41

Plangklang e Reungsang (2010) estudaron a degradación de carbofurano nun

reactor con Burkholderia cepacia PCL3·inmobilizada en mazaroca de millo. Obtivose

que a disipación no reactor foi do 97%. En experimentos onde non se usou a PCL3, os

resulatdos amosan que o solo coa mazaroca adsorbe máis carbofurano que o solo só, co

cal o solo e a mazaroca actúan como adsorbentes diminuíndo o contido de carbofurano.

Shalaby e Abdou (2010) estudaron a influencia dos microorganismos do solo,

biofertilizantes e fertilizantes orgánicos na persistencia do carbofurano. A

determinación do carbofurano realizouse semanalmente ata as 6 semanas. Os resultados

amosan que en solos cultivados a disipación do carbofurano foi máis alta (93.4%) que

en solos non cultivados (85.2%). En solos con biofertilizante, a disipación foi maior

(96.1%) que en solos esterilizados (82.9%). Nos solos con fertilizante orgánico tamén se

atopou unha disipación maior (97%) que en solos sen este tipo de fertilización (94.4%).

Zeng et al. (2010) estudaron a degradación de carbofurano con nanopartículas

de TiO2 e observaron que a degradación foi dun 54 % en 4 h cando a dose de TiO2 foi

0.4 g L-1. A degradación do carbofurano no solo foi aumentada entre 15 e 20 veces e foi

mellor nos experimentos coas concentracións máis baixas do pesticida.

Abd El-Aleem et al. (2009) estudaron a persistencia de carbosulfan e

carbofurano en solos dedicados o cultivo de patacas. A disipación do carbosulfan foi

maior ca do carbofurano. A vida media do carbofurano foi de 6.7 días nos solos. A

persistencia do carbofurano foi de 30 días a partir da formación dos tubérculos. A

fotodescomposición do carbofurano tamén foi estudada, obtendo unha vida media para

o carbofurano de 20.4 horas cando se sometía a luz solar e de 89.4 horas cando se

sometía a lonxitudes de onda no ultravioleta.

Bansal (2009) estudou a influencia das propiedades do solo na persistencia de

carbofurano. A degradación deste pesticida viuse favorecida por un aumento de

temperatura, un aumento da humidade e un aumento no contido de esterco ou nitróxeno.

A degradación decrece co aumento da concentración de pesticida e é máis rápida a pH

alcalinos. Esta degradación do carbofurano foi debida as actividades microbianas e,

probablemente, as encimáticas.

Plangklang e Reungsang (2009) estudaron a degradación de carbofurano con

Burkhoderi ceparia (PCL3) libre e inmobilizadas en mazarocas. A vida media do

carbofurano foi de 3-4 días. As células inmobilizadas sobreviven 3 veces máis que as


42

libres (30 días), polo que o seu uso nos solos presenta unha vantaxe respecto as células

libres.

Farahani et al. (2008) estudaron a degradación de carbofurano en dous solos

con distinto contido en materia orgánica e a tres niveis de humidade diferentes: 100, 90

e 60% da capacidade de campo. O carbofurano no solo con baixo contido en materia

orgánica tivo unha vida media de 57.3, 38.5 e 115.5 días para os tres diferentes niveis

de humidade, respectivamente. No solo con alto contido en materia orgánica, o

carbofurano tivo unha vida media de 192.,5, 141,5 e 203.9 días respectivamente. En

condicións esterilizadas, a 100% da capacidade de campo e a 30º C, no solo cun baixo

contido en materia orgánica o carbofurano tivo unha vida media de 301.4 días mentres

que no solo cun alto contido de materia orgánica, a vida media do carbofurano foi de

147.5 días. Neste estudio observouse que o contido en humidade, en microorganismos e

en materia orgánica afectan a degradación de carbofurano no solo.

Peng et al. (2008) empregaron Paracocus sp. YM3 para a degradación de

carbofurano. Esta bacteria metaboliza o carbofurano en carbofurano-7-fenol nun medio

minimamente salino nos 6 días que o pesticida é a única fonte de carbono. A adición

doutra fonte de carbono acelerou a biodegradación.

Sukul et al. (2008) investigaron o efecto do metalaxyl sobre o solo aplicando

unha única dose elevada e unha dobre dose. Tamén estudiaron a persistencia do

metalaxyl no solo, incluíndo o seu maior metabolito o ácido metalaxyl carboxílico. En

60 días, un 22-29% do metalaxyl converteuse no metabolito. A vida media do metalaxyl

foi de 58-64 días. O metalaxyl ten un efecto directo sobre a poboación microbiana e a

súa actividade no solo, co que pode influír no crecemento e desenvolvemento de

cultivos.

Jiang et al. (2007) usaron microorganismos xenéticamente modificados para a

degradación de methyl parathion e carbofurano. A modificación consistiu na

introdución o azar dun xene methyl parathion hidrolasa nun cromosoma de

Sphingomonas sp. CDS-1. Os autores observaron que os pesticidas se degradaban

eficientemente nun rango de temperatura 20-30ºC e cun pH inicial de 6-9. As

condicións óptimas para a degradación foron 30 ºC e pH 7. Probouse este procedemento

en solos esterilizados e non esterilizados. Nestes últimos, resultou que 25 mg kg-1 de

carbofurano podían ser degradados ata niveis non detectables en 25 días.


43

Monkiedje et al. (2007) investigaron a degradación de metalaxyl en solos e

observaron que os enantiómeros de metalaxyl teñen diferentes velocidades de

degradación, sendo a degradación do R-enantiómero máis lenta ca do S-enantiómero. A

eliminación de metalaxyl foi máis lenta nun solo que contiña Cu.

Shalby e Abdalla (2006) observaron que a aplicación de microorganismos,

compost ou abono o solo tratado con carbofurano reduce a vida media deste pesticida de

8.9 días a 3.4, 5.7 ou 4.8 días respectivamente.

Sukul (2006) estudou o efecto do metalaxyl na biomasa microbiana e nas

actividades encimáticas en solos. O autor atoparou unha disipación do 52.5-56.8%.

durante 60 días de incubación así como unha pequena diminución da biomasa. Tamén

houbo un efecto moi significativo na diminución do nitróxeno total e do carbono

orgánico. As actividades encimáticas aumentaron ao principio do experimento e despois

diminuíron. A ureasa diminuíu ao longo do experimento. Os resultados indicaron que o

metalaxyl pode influír no desenvolvemento e crecemento dos cultivos xa que ten un

efecto directo sobre a reciclaxe dos nutrientes e sobre o fluxo de enerxía nos solos.

Tariq et al. (2006) observaron que o contido en humidade e temperaturas

elevadas favoreceron a degradación de carbofurano en solos franco areosos de Pakistán.

A actividade microbiana case non tivo efecto nesa degradación, o que puido ser debido

o baixo contido en carbono orgánico do solo.

De Melo Plese et al. (2005) observaron que o carbosulfan e o carbofurano teñen

unha baixa persistencia en auga e unha persistencia media na disolución de solos de

campos tropìcais de arroz en regadío.

Kamboj et al. (2005) empregaron unha cepa bacteriana chamada N-2 para

degradar o carbofurno a carbofurano fenol. As 24 horas, a porcentaxe de carbofurano e

carbofurano fenol foron 18% e 82% respectivamente, cando a tempo 0 estas

porcentaxes eran 82 e 18% respectivamente.

Monkiedje e Spiteller (2005) determinaron a degradación de metalaxyl en solos

observando que a degradación seguía unha cinética de primeiro orde. Atoparon

concentracións máis altas de metabolito ácido metalaxyl nos solos de Alemania en

comparación cós solos de Camerún. A degradación foi diferente para cada enantiómero

con vidas medias de 17 a 38 días. Todas as formas de metalaxyl tiñan menores taxas de

degradación no solo de Camerún. No solo de Alemania a degradación do R enantiómero


44

foi máis rápida que a do S enentiómero, no solo de Camerún aconteceu o contrario. Isto

foi xustificado pola diferencia das poboacións microbianas, as cales poden usar encimas

diferentes e teñen diferentes preferencias de degradación. A incorporación de metalaxyl

o solo provoca cambios no estado ecofisiolóxico da comunidade microbiana. A

degradación foi correlacionada positivamente coa actividade da ácido fosfatasa.

Graebing e Chib (2004) estudaron a fotodegradación do carbofurano en

mostras húmidas e secas de solo obtendo que a vida media do pesticida era 2.2 veces

máis elevada en mostras secas que en mostras húmidas.

Hanumantharaju e Awasthi (2004) observaron que a degradación de metalaxyl

amosou unha cinética de primeiro orde. A degradación foi predominantemente de

natureza biolóxica, pero tamén química. O metalaxyl presentou valores de vida media

entre 41 e 165 días.

Pesaro et al. (2004) estudaron a influencia do almacenamento nas características

microbiolóxicas do solo cando se lle aplica un ciclo de humectado e secado. A

degradación de metalaxyl foi 1.5 veces maior en solos almacenados que en solos non

modificados. A biomasa microbiana diminuíu e as actividades microbianas aumentaron

despois do rehumectado.

Vischetti et al. (2004) empregaron reactores para reter pesticidas. O substrato

máis eficiente na retención foi o constituido por cáscaras de cítricos e residuos sólidos

urbáns compostados. A vida media do metalaxyl foi de 14 días, moi inferior a vida

media do pesticida en solos (60-70 días).

Buerge et al. (2003) observaron que en solos aerobios a pH superior a 5, o R-

enantiómero do metalaxyl degradouse máis rápido que S-enantiómero. En solos con pH

4-5, a degradación do metalxyl foi similar para os dous enantiómeros. A solo de pH 4 e

na maioría dos solos en condicións anaerobias, a degradación foi máis rápida para o S-

enantiómero do metalaxyl.

Buser et al. (2002) suxiren que a degradación de metalaxyl é

enatiomeroselectiva, degradándose máis rápido o R-enantiómero que o S-enantiómero.

Esta degradación do metalaxyl foi maioritariamente biolóxica e seguiu unha cinética de

primeiro orde.

Marucchini e Zadra (2002) observaron que a degradación do metalaxyl foi

enantiomeroselectiva e depende do medio. Así, o (+)-(S)-enantiómero amosa unha


45

degradación máis rápida nas plantas mentres que o (-)-(R)-enantiómero degradase máis

rápido no solo.

Ogiyama et al. (2002) recolleron columnas estucturadas de solos onde se

aplicaba metaxyl. Estas columnas foron divididas en seccións de 10 cm de espesor,

centrifugáronse a 11000 rpm durante 60 min a 4 ºC para separar a auga do solo onde se

mediu o contido de pesticidas. O metalaxyl non se detectou nas mostras de auga a partir

dos 30 cm de profundidade da columna.

Webb e Aylmore (2002) observaron que aos niveis máis superficiais dos solos

cun alto contido en materia orgánica e humidade amosan grandes velocidades de

degradación de metalaxyl, concordando con cinéticas de degradación de primeiro orde.

Para nivéis subsuperficiais dos solos, aparentemente propicias para a descomposición

dos pesticidas, esta non sempre se realiza e os datos cambian o punto de vista de que a

degradación de primeiro orde do metalaxyl pode ser usada para os perfís do solo

completos.

Wu et al. (2002) estudaron a persistencia de metalaxyl no solo plantado con

céspede. Os autores atoparon residuos de metalaxyl na capa superficial do solo (10 cm)

debido ao alto contido de materia orgánica. A vida media para o mealaxyl foi de 1.4

días. Este estudio amosa que baixo boas practicas agrícolas, o transporte de pesticidas

fóra do seu punto de aplicación, é mínimo.

Doddamani e Ninnekar (2001) estudaron a degradación de carbofurano con

Micrococus sp. a cal usa o carbofurano como única fonte de carbono.

Lalah et al. (2001) observaron que a disipación do carbofurano no solo está

influenciada por diversos factores ambientais: vento, choiva, radiación solar e humidade

do solo. Tamén inflúen as características dos solos como o pH, a materia orgánica, a

textura e a actividade encimática. A vida media do carbofurano nos solos de Kenya

variou entre 66 e 115 días.

Sukul e Spiteller (2001) estudaron a degradación de metalaxyl polo efecto da

luz en solos esterilizados e non esterilizados, amosando unha vida media de 36-73 días.

En experimentos con cámara de simulación de luz, a vida media do metalaxyl foi de

188-502 horas.

Wei et al. (2001) estudaron a hidrólise de carbosulfan a carbofurano en arxilas.

As suspensións de arxilas realizaronas con acetonitrilo 40% e 1%. O diminuír o


46

carbosulfan aumenta o carbofurano. A vida media máis baixa do carbosulfan foi nunha

suspensión con montmorillonita a pH 5-6. A velocidade de hidrólise do carbosulfan está

influenciada polo contido en acetonitrilo de xeito que, a maior contido de acetonitrilo,

menor é a velocidade de hidrólise. A presenza de Al non afectou a hidrólise do

carbosulfan, sen embargo o Cu mellorou a velocidade de reacción en comparación co

Na. O carbosulfan non se disolveu completamente en acetonitrilo ó 1%. As suspensións

de montmorillonita e beilita foron as que presentaron unha maior catálise. Á hidrólise

do carbosulfan depende do pH, a pH 3 non houbo efecto. A aplicación de

montmorillonita con Na, Al e Cu mellora a catálise do carbosulfan presentando vidas

medias máis cortas. A presenza de fosfato e fluoruro inhibe a hidrólise do carbosulfan

sendo maior o efecto no primeiro. A presenza de ácidos húmicos non influíu na

hidrólise de carbosulfan, sen embargo a presenza de etilendiamina inhibiu a catálise. Os

experimentos con carbofurano foron realizados a pH 7, 8 e 10. Observaronse que as

suspensións coas arxilas non presentaban diferenzas significativas cás suspensión

acuosas deste pesticida, co cal as arxilas non inflúen na catálise do carbofurano. Sen

embargo, o pH influíu na hidrólise do carbofurano, observando que esta aumentaba a

medida que aumentaba o pH.

Bosch et al. (2000) estudaron a persistencia de carbofurano nun campo de

cultivo en varios anos. As concentracións máis elevadas de carbofurano atopáronse nos

25 cm superiores do solo e nos primeiros 30 días despois do tratamento. Pasados 30 días

da aplicación, o carbofurano degradarase un 96%. Ós 44 días, a degradación do

carbofurano foi completa nos 25 cm superficiais do solo. A vida media do carbofurano

foi de 6 días.

Bachman e Patterson (1999) estudaron a fotodescomposición do carbofurano

observando que seguía unha cinética de primeiro orde. A presenza de materia orgánica

disolta inhibe a fotólise, debido a forte interacción entre carbofurano e a materia

orgánica disolta. Os autores describiron a fotodegradación do carbofurano en tres

etapas: na primeira a molécula rompese en 2 partes, na segunda etapa no furano

prodúcese unha substitución do catecol por un grupo tert-butyl alcohol, e na terceira esta

molécula sofre unha deshidratación para formar un grupo alqueno.

Di et al. (1998) estudaron a degradación de 8 pesticidas (entre eles o metalaxyl)

en mostras superficiais de solo (0-25cm), subsuperficiais (25-50cm) e nun solo areoso.

Observaron que a degradación era distinta para cada pesticida, vendo que a metade dos


47

pesticidas degradábanse máis lentamente na mostra de solo máis superficial que na

mostra subsuperficial, mentres para a outra metade acontecía o contrario. Os resultados

suxiren que os cambios nas velocidades de degradación dos pesticidas son debidos a

cambios na actividade microbiana, e no contido en materia orgánica (adsorción) ó longo

dos perfís dos solos.

Murthy et al. (1998) estudaron a fotoestabilidade de funxicidas (metalaxyl entre

eles) en solos con experimentos de simulación de luz. O metalaxyl presentou unha

fotodegradación menor do 2% nun periodo de 72 h.

Mansour et al. (1997) estudaron a fotodegradación de carbofurano e observaron

que esta viuse favorecida pola presenza de TiO2, H2O2 e O3.

Mohapatra e Awasthi (1997) observaron que as suspensións de solos tratados

repetidamente con carbofurano melloraba a degradación deste nun medio de sales

minerais. O 97% do carbofurano aplicado a un solo degradouse cando foi inoculado con

suspensións tratadas con carbofurano, mentres que nos solos non inoculados a

degradación do pesticida foi dun 15% no mesmo tempo. Dentro dos sete cultivos

bacterianos aislados do cultivo de enriquecemento, dous foron os máis salientables,

Pseudomonas stutzeri e Bacillus Pumilis, os cales promoveron unha degradación de

carbofurano superior o 98% en 30 dias.

Murray et al. (1997) estudaron a degradación de carbofurano con

microorganismos indíxenas do solo e cunha asociación de cianobacterias e bacterias.

Observaron que cos microorganismos indíxenas, o carbofurano degradábase un 65%

cando se aplicaba el só e un 58% cando se aplicaba nunha mestura de pesticidas. En

experimentos en campo observaron que a concentración de carbofurano reducíase un 82

% cando o solo era inoculado coa asociación de cianobacterias e bacterias.

Yen et al. (1997) observaron que a disipación de carbofurano en dous solos

viuse incrementada pola temperatura e o contido en humidade. A disipación seguiu

unha cinética de primeiro orde.

Ambrosoli et al. (1996) estudaron a biodegradación de carbofurano en solos

tratados durante anos con este pesticida e en solos nunca tratados. Os autores

observaron que a degradación do carbofurano era máis rápida no primeiro caso.

Mabury et al. (1996) observaron que a degradación de carbofurano acelerase a

pH elevado, tendo unha vida media de 18-26 días. Estudos amosan que as mellores


48

disipacións do carbofurano prodúcense en aplicacións repetidas do pesticida. O

microorganismo que leva a cabo a mellor disipación do carbofurano é Pseudomona sp.

A degradación deste pesticida é rápida tanto en condicións aerobias como anaerobias. O

valor de LC50 varía dende uns poucos µg L-1 para invertebrados acuáticos a moitos µg

L-1 para peixes.

Mojašević et al. (1996) observaron que a degradación de carbofurano

incrementaba en gran medida coa humidade do solo.

Mora et al. (1996) suxiren que a disipación do carbofurano no solo prodúcese

en tres etapas: adsorción, degradación rápida e degradación lenta. As etapas de

degradación seguen unha cinética de primeiro orde. O proceso principal para determinar

a desaparición do carbofurano en suspensións no solo é a degradación abiótica por

hidrólise do carbofurano a carbofurano fenol. A vida media do carbofurano foi de 1-2

días.

Suett et al. (1996) observaron que a degradación de carbofurano aumentaba a

pHs do solo elevados.

Johnson e Lavy (1995) realizaron experimentos en parcelas de arroz

determinando o carbofurano en augas e solos. Nas augas, 28 días despois da aplicación

non se detectou carbofurano. Nos solos, a disipación do carbofurano foi rápida nos

primeiros 11 días despois da aplicación. A vida media do carbofurano foi de 10 días.

Müller e Buser (1995) Observaron que a degradación de metalaxyl é

enatiomeroselectiva, degradándose o +S-enantiómero máis rápido en augas residuais e

lodos e o –R-enantiómero o fai máis rápido en solos.

Thapar et al. (1995) amosan que a degradación de carbofurano nos solos vese

favorecida por elevados pH, humidades moderadas e altos contidos en CaCO3. Un alto

contido en materia orgánica aumentan a persistencia do carbofurano.

Levanon et al. (1994) estudaron como lle afectaba o laboreo a actividade

microbiana e o destino dos pesticidas. En parcelas aradas, a lixiviación dos pesticidas

foi elevada. En parcelas sen laboreo observouse unha alta actividade da población

microbiana e unha elevada mineralización dos pesticidas, o que favoreceu a unha

diminución da lixiviación. Os procesos de adsorción/retención tamén xogou un papel

importante na lixiviación dos pesticidas.


49

Parekh et al. (1994) observaron que nos solos que foron tratados na súa historia

con carbofurano, este degradase máis rapidamente que en solos nos que non se tratou.

Levanon (1993) estudou o papel dos fungos e das bacterias na mineralización de

pesticidas. A mineralización de carbofurano foi debida principalmente a actividade

bacteriana.

Scheunert et al. (1993) estudaron a degradación de carbofurano sometido a luz

ultravioleta en auga e en suspensións auga/solo. A fotodegradación viuse incrementada

coa presenza de TiO2, H2O2 e O3.

Shelton et al. (1993) suxiren que a perda da eficacia do insecticida era debido a

lixiviación/disolución dos gránulos e posterior degradación antes de que fixera o efecto

para o cal estaba destinado. Nos cultivos onde se aplicaba carbofurano ano tras ano,

creábase unha poboación de microorganismos que degradaban máis rápido o pesticida.

As formulacións de liberación controlada mitigaron este efecto.

Talebi e Walker (1993) observaron que a velocidade de degradación do

carbofurano era maior en solos que foran tratados anteriormente con carbofurano que en

solos nos que non recibiran aplicacións deste pesticida. Cando os solos pretratados con

carbofurano foron esterilizados, a degradación do pesticida diminuíu drasticamente o

que indica que foi debida maioritariamente a acción microbiana.

Parkin e Shelton (1992) estudaron a variabilidade espacial e temporal da

velocidade de degradación do carbofurano en condicións de campo, xa que unha

elevada velocidade de degradación dos pesticidas diminúe a súa eficacia. Os resultados

amosaron que a degradación era máis rápida no sucos que nas cristas. A variabilidade

temporal debeuse principalmente o contido en auga do solo.

Singh e Sethunathan (1992) estudaron a degradación de carbofurano absorbido

aos solos pola inoculación dun cultivo enriquecido. Usou 3 solos, un aluvial (pH

elevado, baixo contido en materia orgánica) e outros dous (pH baixo e alto contido en

materia orgánica). No solo aluvial, os tres días da inoculación non se recuperou

carbofurano, sen embargo nos outros dous solos recuperouse entre o 68 e o 83% do

carbofurano ós 6 días.

Droby e Coffe (1991) estudaron a biodegradación de metalaxyl en solos

cultivados con tabaco, cítricos, aguacate e millo. A vida media máis corta (6 días)

obtívose no solo de cultivo de tabaco.


50

Shelton (1991) estudou o efecto da humidade do solo na adsorción e

degradación de carbofurano. A degradación do pesticida diminúe a medida que diminúe

a humidade. O carbofurano soluble diminuíu o principio, pasando a ser adsorbido, e

despois houbo perdas tanto do carbofurano soluble como do adsorbido debido a

biodegradación. A Kd incrementouse durante a incubación, pero fixoo máis lentamente

a medida que ia diminuíndo a humidade do solo. A velocidade de degradación do

carbofurano excedeu a velocidade de desorción a humidades altas pero non as baixas.

Trotter et al. (1991) observaron que o carbufarano permanece estable en augas

ácidas, estando suxeito a hidrólise química a medida que as augas son máis alcalinas.

Arunachalam e Lakshmanan (1990) estudaron a importancia dos

microorganismos na degradación do carbofurano. Para isto empregaron solos

esterilizados e non esterilizados, en condicións de saturación e non saturación en auga.

Nos solos esterilizados, máis do 75% do pesticida engadido recuperouse os 60 días,

mentres que nos solos non esterilizados, máis do 75% do pesticida engadido

metabolizouse no mesmo tempo.

Getzin e Shanks Jr (1990) observaron que a degradación de carbofurano foi

máis rápida naqueles solos que recibiron tratamento co insecticida anteriormente que

nos solos control. En solos esterilizados, a degradación do carbofurano foi máis lenta

que nos solos naturais. Os solos que recibiron varias aplicacións de carbofurano no

pasado, desenvolven unha maior capacidade de degradación do pesticida.

Turco e Konopka (1990) observaron que a degradación de carbofurano en solos

que foron expostos ao pesticida foi maior que nos solos que non foran expostos

anteriormente. A degradación relacionouse cunha diminución da biomasa microbiana.

Suxire que a degradación ten lugar en dúas etapas ou procesos: hidrólise do carbofurano

seguida pola adsorción o solo do metabolito primario.

Chaudhry e Ali (1988) empregaron a inoculación de 15 especies de bacterias

capaces de degradar o carbofurano nos solos. Todos os inoculos pertencían os xéneros

Pseudomonas e Flavobacterium. Separounas en 3 grupos segundo o seu emprego do

carbofurano: Grupo I, usan o carbofurano como fonte de nitróxeno; Grupo II, usan o

carbofurano como fonte de carbono e Grupo III tamén usan o carbofurano como fonte

de carbono e degrádano a CO2.


51

Harris et al. (1988) observaron que os solos desenvolven poboacións

microbianas capaces de mellorar a degradación do carbofurano, circunstancia que inflúe

na eficacia do insecticida.

Pussemier (1988) incubou mostras de solo con carbofurano só e con

carbofurano máis Manate (maneb), Apron (metalaxil + mancozeb), Ridomil Special

(metalaxil + maneb) and Pomarsol (TMTD). Observaron que a vida media do

carbofurano nas mostras de solos que tiñan funxicidas foi 1.2-3 veces máis alta que nas

mostras de solo con carbofurano só.

Racke e Coats (1988) observaron que os solos que tiveron aplicacións de

carbofurano no pasado, conteñen unha poboación microbiana adaptable capaz de

degradar rapidamente o pesticida.

Ramanand et al. (1988) inocularon Arthrobacter sp. nun solo saturado en auga

a 35 ºC e despois engadiron carbofurano. Observaron que esta bacteria mineralizaba o

carbofurano completamente en 72-120 horas en condicións aerobias nun medio de sales

minerais, usando o pesticida como única fonte de carbono e nitróxeno. En condicións

anaerobias, non se produciu mineralización. A temperatura favoreceu a mineralización

do carbofurano.

Brahmaprakash e Sethunathan (1985) observaron que a desaparición do

carbofurano foi máis rápida en solos con plantas de arroz que en solos sen plantas, tanto

baixo condicións de encharcamento como de non encharcamento.

Rajagopal et al. (1984) obsevaron que as poboacións microbianas obtidas de

solos encharcados nos que se aplicou carbofurano, teñen un efecto maior na

degradación de carbofurano nun medio de sales minerais que as poboacións

microbianas de solos que nunca foron expostas ó carbofurano. A degradación do

carbofurano foi máis rápida en ausencia dunha fonte de nitróxeno.

Siddaramappa e Seibert (1979) determinaron o contido de carbofurano en

solos e augas. A degradación foi rápida, sendo nas augas debido sobre todo a procesos

non biolóxicos, mentres que nos solos foi principalmente resultado da actividade

microbiana. A aplicación de fertilizantes nitroxenados inflúe na duración da actividade

do insecticida no solo e na auga.

Siddaramappa et al. (1978) estudaron a degradación de carbofurano en arrozais

observando que, en 5 días, o carbofurano hidrolizabase a carbofurano fenol nos solos


52

inundados. A degradación do carbofurano foi principalmente química mentres que a do

carbofurano fenol foi biolóxica. O carbofurano degradouse máis rápido en solos nos que

se fixera un pretratamento co pesticida e non observaron efecto da luz na súa

degradación.

Venkateswarlu et al. (1977) observaron que a degradación de carbofurano é

maior en solos inundados que en solos non inundados. A degradación foi rápida en

condicións de inundación (20-40 días), agás en solos ácidos (pH 4.2) onde o

carbofurano persistiu varias semanas máis. Os autores tamén observaron que o

tratamento térmico das mostras antes da incubación aumentaban a persistencia de

carbofurano, suxerindo que a degradación era debida os microorganismos.

Caro et al. (1976) estudaron o comportamento do carbofurano no campo,

observando que un 95 % deste desaparecía en 145-434 días e dependía do pH do solo,

da humidade e da temperatura.

Caro et al. (1973) estudaron a disipación do carbofurano nos solos polo efecto

da escorrentia e do cultivo de millo. A disipación seguiu unha cinética de primeiro orde,

cunha vida media de 46-117 días. A persistencia de carbofurano aumentou no solo

cando se aplicou nos sucos, en solos de pH ácidos e con temperaturas baixas. A

degradación do pesticida aumentou co contido en auga do solo e con pH elevados. As

perdas de carbofurano por escorrentía foron de 0.5-2 % da cantidade aplicada. O millo

acumulou un 0.14 % do carbofurano aplicado nas follas durante o verán, valor que se

reduxo drasticamente na colleita.

XUSTIFICACIÓN E OBXECTIVOS

XUSTIFICACIÓN E OBXECTIVOS

55

3. XUSTIFICACIÓN E OBXECTIVOS

A agricultura que se ten levado a cabo nas últimas décadas trouxo consigo un

incremento notable na cantidade e calidade das colleitas debido ao emprego de novos

cultivos de alto rendemento, aos avances na maquinaria agrícola e ao uso de praguicidas

en xeral e de herbicidas e funxicidas en particular (FAO, 2011). O uso dos praguicidas

foi a evolución máis significativa na agricultura contemporánea, xa que reduciron

drasticamente os efectos adversos de enfermidades en plantas e cultivos, o que deu lugar

a un aumento no consumo destes compostos en poucos anos (Dalvie et al., 2009) a pesar

de ser ben coñecido o seu perigo para o medio ambiente e para a saúde humana (Everett

e Matheson, 2010; McKinlay et al., 2008). De feito o uso continuado destes produtos

aumentou a súa concentración no solo (Hildebrandt et al., 2008) ata poñer en perigo a

sustentabilidade dos sistemas agrarios.

Unha vez no solo, estes compostos están sometidos a unha serie de procesos

físicos, químicos e biolóxicos, os cales dependen da natureza química do pesticida e das

propiedades do solo, sendo os procesos de degradación e os de adsorción-desorción os

máis importantes.(Linn et al., 1993). De feito, o transporte dos pesticidas, a súa

transformación, persistencia e bioacumulación nos solos está influenciado estes

procesos de adsorción-desorción (De Jonge et al., 1996). Estes procesos poden ter un

dobre efecto, por unha banda, a adsorción dos pesticidas aos coloides do solo evita a súa

presenza en augas circundantes, facendo máis difícil a súa volatilización e

biodegradación (Krishna e Philip, 2011); polo contrario, a fácil desorción dos pesticidas

adsorbidos ás partículas do solo facilita o lavado e drenaxe destes compostos cara ao

subsolo ou cara a augas superficiais, facendo ao pesticida máis accesible para os

microorganismos do solo e facilitando a súa biodegradación.

O perigo potencial dos pesticidas para as plantas e microorganismos, ou a súa

presenza en altas concentracións nas augas, depende da concentración do pesticida na

disolución do solo. Unha mellor comprensión dos mecanismos que rexen o

comportamento dos pesticidas no solo debería facilitar a avaliación dos posibles efectos

adversos dos praguicidas para a agricultura e o medio ambiente.

Numerosos traballos estudaron os procesos de adsorción-desorción de

carbofurano (Arias-Estévez et al., 2006; Liyanage et al., 2006; Rama Krishna e Philip,

2008; Yazgan et al., 2005) e metalaxyl (Andrades et al., 2001; Arias et al., 2006;


56

Baglieri et al., 2011; Fernandes et al., 2003) en solos, os cales amosan que a disipación

dos pesticidas na disolución do solo depende do contido en carbono orgánico e arxila,

fundamentalmente. Outros procesos de disipación como a fotodegradación e a

degradación microbiana tamén foron estudados por varios autores (Baker et al., 2010;

Lu et al., 2011; Massoud et al., 2008; Plangklang e Reungsang, 2012; Trabue et al.,

2001). Estes procesos poden ter un efecto antagónico, a adsorción dos pesticidas aos

coloides do solo evitan a súa presenza na disolución do solo e dificultan a súa

degradación, co cal o seu efecto tería que ser maior e máis duradeiro. Sen embargo, os

procesos de adsorción fan que o seu efecto sobre os organismos obxetivo sexa menor ou

teña menos duración.

A vista do comentado anteriormente expuxéronse os seguintes obxectivos xerais:

1. Proporcionar unha descrición detallada dos procesos de adsorción e desorción

de carbofurano e metalaxyl en solos ácidos e relacionar as variables de adsorción

coas características dos solos.

2. Estudar as cinéticas de adsorción e desorción de carbofurano e metalaxyl en

solos mediante experimentos en cámara de fluxo axitado.

3. Estudar o transporte de carbofurano e metalaxyl en solos mediante

experimentos en columnas.

4. Comparar os resultados de adsorción-desorción obtidos polos diferentes

métodos (experimentos tipo batch, experimentos en columnas e experimentos en

cámara de fluxo axitado).

5. Avaliar os efectos da adsorción, fotodegradación e degradación microbiana e

as súas interaccións na disipación de carbofurano e metalaxyl.

6. Estudar a influencia da agricultura intensiva da comarca da Limia na posible

contaminación por produtos fitosanitarios das augas superficiais.

RESULTADOS E DISCUSIÓN


59

4. RESULTADOS E DISCUSIÓN

4.1. Cinética e adsorción-desorción de carbofurano e metalaxyl en

solos ácidos.

A. Carbofurano.

Estudouse a adsorción-desorción de carbofurano en 16 mostras superficiais (S1-

S16), co fin de coñecer o comportamento do pesticida nos distintos solos e relacionar a

adsorción coas características edáficas.

En primeiro lugar leváronse a cabo experimentos tipo batch para pescudar o

tempo necesario para alcanzar o equilibrio; para isto 1 g de solo foi axitado con 10 mL

dunha disolución que contiña carbofurano 2.3 µM e CaCl2 0.005 M como electrólito de

fondo, durante distintos tempos. Posteriormente centrifugouse a 2000 rpm e filtrouse

con filtros dun tamaño de poro de 0.45 µm, para despois determinar a concentración do

pesticida por HPLC-UV.

0

1

2

3

4

5

0 10 20 30 40 50t (h)

qs (

µm

ol k

g-1)

S 2

0

2

4

6

8

10

0 10 20 30 40 50t (h)

qs (

µm

ol k

g-1)

S 14

Figura nº 2: Adsorción de carbofurano en función do tempo en dous dos solos empregados nos experimentos batch (S2 e S14).

Na figura nº 2 pódese observar a adsorción de carbofurano en función do tempo

en mostras seleccionadas, alcanzándose o equilibrio despois das 8-16 h, en función do

tipo de solo.

A adsorción de carbofurano en experimentos batch en función da

concentración levouse a cabo do mesmo xeito, con distintas disolucións as cales

contiñan carbofurano (2-18 µM) e CaCl2 0.005 M, cun tempo de axitación de 24h.

Inmediatamente despois dos experimentos de adsorción realizouse a desorción,


60

engadindo 10 mL de CaCl2 0.005 M e seguindo o mesmo procedemento que tivo lugar

nos experimentos de adsorción. A medida do pesticida, como se comentou

anteriormente levouse a cabo mediante HPLC-UV.

Os resultados dos experimento de adsorción de carbofurano tipo batch pódense

observar na figura nº 3. Observase que a maior parte das curvas de adsorción son de tipo

L, cunha pendente que decrece a medida que se aumenta a concentración engadida; sen

embargo algunhas outras tenden a linealidade.

0

5

10

15

20

25

0 5 10 15 20

C (µmol L-1)

qs (

µm

ol k

g-1)

S 2

0

5

10

15

20

25

0 5 10 15 20

C (µmol L-1)

qs (

µm

ol k

g-1)

S 5

0

15

30

45

60

0 5 10 15 20

C (µmol L-1)

qs (

µm

ol k

g-1)

S 14

0

5

10

15

20

25

0 5 10 15 20

C (µmol L-1)

qs (

µm

ol k

g-1)

S 15

Figura nº 3: Curvas de adsorción de carbofurano para catro dos solos empregados nos experimentos batch (S2, S5, S14 e S15).

Os datos experimentais foron axustados aos modelos matemáticos de Freundlich

e Langmuir. Os axustes ao modelo de Langmuir non foron satisfactorios debido a que

levaban asociados grandes erros polo que se utilizou a ecuación de Freundlich (na forma

que aparece baixo estas liñas) para discutir os resultados:

neqFCKX =


61

onde KF (constante de adsorción de Freundlich) e n son constantes empíricas, X

é a cantidade de pesticida adsorbido en µmol kg-1 e Ceq é a concentración de pesticida

na disolución de equilibrio (µmol L-1).

O nivel de axuste dos datos experimentais a unha ecuación deste tipo pódese

avaliar a partir do valor do coeficiente de determinación (R2). A xuzgar polos valores de

R2, os axustes foron satisfactorios con valores de 0.797-0.975. Os valores do parámetro

n oscilaron entre 0.40 e 0.95 cun valor medio de 0.68; o cal indícanos unha baixa

heteroxeneidade dos sitios de adsorción. Os valores de KF para o carbofurano oscilaron

entre 0.6 e 8.7 Ln kg-1 µmol(1-n) e foron significativamente correlacionados con varias

características dos solos, pero especialmente co contido de carbono orgánico (r=0.927,

P<0.01). Estes resultados coinciden cos de outros autores (Achik et al., 1991; Farahani

et al., 2008; Singh et al., 1994) onde confirman a influencia positiva da materia orgánica

na adsorción de carbofurano aos solos.

Nos experimentos de desorción tipo batch prodúcese un incremento da

cantidade desorbida canto maior é a cantidade engadida, o cal indica que a medida que

nos achegamos á saturación dos sitios de adsorción, o pesticida vaise unindo a sitios

dunha menor especificidade. As cantidades de pesticida desorbido foron dende valores

indetectables (<0.04 µmol kg-1) ata 6.4 µmol kg-1. Estas cantidades foron dependentes

das características do solo e das cantidades engadidas. A desorción é menor canto maior

é capacidade dos solos de adsorber carbofurano, pero a partir dun nivel de adsorción

(aproximadamente 10 µmoles kg-1) a desorción xa depende doutras variables. Neste

sentido as porcentaxes de desosorción están negativamente correlacionadas coa CICe

(r=-0.732, P<0.01, n=13) e co contido de arxila (r=-0.569, P<0.05, n=13).

Por outra banda, realizáronse experimentos en cámara de fluxo axitado (Figura

nº4). Para isto 0.2 g de solo introducíronse nun microreactor de polipropileno (1.3 cm3)

cun filtro de teflón na saída e outro na entrada de 10 mm de diámetro e cun tamaño de

poro de 0.45 µm para reter o solo no interior da cámara. Para alimentar o micro-reactor

coa disolución correspondente (CaCl2 0.005M e carbofurano 4,5 µM para a adsorción e

CaCl2 0.005M sin pesticida para a desorción), conectamos a entrada unha bomba

peristáltica para manter unha velocidade de fluxo constante durante o tempo que dure o

proceso experimental. Mediante probas previas encontramos que a velocidade de fluxo

óptima foi de 0.2 mL min-1. As diferentes alicuotas recóllense cun colector de fraccións

(60 mostras en 60 minutos) en viais de polipropileno “eppendorf”. O reactor colocouse


62

no interior dunha cámara termorregulada, cunha temperatura de 25±0.1ºC. As mostras

mantéñense en axitación a 400 rpm durante todo o experimento.

Figura nº 4: Descrición da montaxe empregada nos experimentos de cámara de fluxo axitado.

Na figura nº 5 preséntanse os resultados dos experimentos en cámara de fluxo

axitado (Solos S2, S5, S14 e S15). A cantidade de carbofurano retida foi pequena en

tódolos casos, variando entre 7.1-31.9 µmol kg−1. No solo que máis carbofurano

adsorbe (S14), nos primeiros 13 minutos do experimento adsorbese o 50%, e aos 45 min

adsorbeuse o 95% da máxima cantidade adsorbida. Na mostra que menos carbofurano

adsorbe, aos 13 minutos prácticamente se adsorbe o 100% do total (solo S5).

0

10

20

30

40

0 10 20 30 40 50 60

t (min)

qs

(µm

ol k

g-1

)

S14

S5

0

10

20

30

40

0 10 20 30 40 50 60

t (min)

qs

(µm

ol k

g-1

)

S2

S15

Figura nº 5: Adsorción acumulada de carbofurano para os solos estudiados nos experimentos en cámara de fluxo axitado (S2, S5, S14 e S15).

En xeral, durante o proceso de adsorción na cámara de fluxo axitado pódense

observar dous tipos distintos de reaccións: unha adsorción rápida nos primeiros


63

instantes seguida de reaccións de adsorción lenta, o cal e coincidente cos datos de Felsot

e Wilson, (1980). No noso caso as cinéticas de adsorción de carbofurano foron máis

rápidas nos experimentos de fluxo axitado cá nos experimentos batch, nos cales

precisan máis de 8-16 horas para alcanzar o equilibrio. Os procesos de adsorción rápida

están en xeral limitados por difusión (Fernández-Calviño et al., 2010; Pérez-Novo et al.,

2011) e unha característica destes procesos é a relación lineal entre a adsorción relativa

(q/qmax) e a raíz do tempo (t1/2). Esta relación amosase na figura nº 6 onde podemos

observar que o 80% da adsorción de carbofurano no solo con maior adsorción é sobre

sitios de adsorción rápidos (solo S14, figura 6). Nos demais solos (S2, S5 e S15), con

menor adsorción de carbofurano, a porcentaxe retida mediante reaccións de adsorción

rápida é maior do 90%. Este tipo de reaccións son maioritariamente unións

electrostáticas ou ben ligazóns nas que se forman complexos de esfera externa. Doutra

banda, as reaccións de adsorción lenta están caracterizadas por unións máis fortes,

debidas a mecanismos tales como precipitación, e/ou adsorción secundaria en sitios

cunha maior enerxía de activación (Strawn e Sparks, 2000).

R² = 0.999

0,0

0,2

0,4

0,6

0,8

1,0

0 2 4 6 8

t1/2

qs/

qm

ax

S2

R2 = 0,991

0,0

0,2

0,4

0,6

0,8

1,0

0 2 4 6 8

t1/2

qs/

qm

ax

S5

R2 = 0,998

0,0

0,2

0,4

0,6

0,8

1,0

0 2 4 6 8

t1/2

qs/

qm

ax

S14

R2 = 0,998

0,0

0,2

0,4

0,6

0,8

1,0

0 2 4 6 8

t1/2

qs/

qm

ax

S15

Figura nº 6: Relación entre a adsorción relativa de carbofurano (q/qmax) e a raíz cadrada do tempo (t1/2) para os solos estudados nos experimentos de cámara de fluxo axitado (S2, S5, S14 e S15).


64

Para facilitar a análise das cinéticas de retención do pesticida, os datos de

adsorción foron axustados a un modelo matemático de pseudo primeira orde que

considera un sitio de adsorción (Aharoni e Sparks, 1991):

dqs/dt = ks (qmax-qs) (1)

onde dqs/dt (µmol kg-1 min-1) é a velocidade de adsorción do pesticida, ks é unha

constante cinética (min-1), qmax (µmol kg-1) é a capacidade máxima de adsorción do

pesticida, e qs (µmol kg-1) é a cantidade de pesticida retido no solo.

A constante cinética para o carbofurano (ks) varía entre 0.047 e 0.195 min-1,

correspondéndose o maior valor co solo de menor contido en materia orgánica (S5). O

valor máis baixo de ks foi obtido nun solo con valores intermedios de materia orgánica

(4.3%), polo que a diminución da velocidade de adsorción de carbofurano en función da

cantidade de pesticida retido non está claramente relacionada coa materia orgánica do

solo. Tampouco se puido observar ningunha relación entre ks e outras características do

solo. Ao comparar os valores de ks cos parámetros da ecuación de Freundlich

observouse que ks está significativamente correlacionada con n (r= 0.965; P<0.05).

Os datos das cinéticas de liberación (desorción) do pesticida axustáronse ao

modelo cinético de primeira orde expresada na seguinte ecuación:

dqd/dt = kd (q0-qd) (2)

onde dqd/dt (µmol kg-1 min-1) é a velocidade de desorción do pesticida, kd é a

constante cinética de desorción (min-1), q0 (µmol kg-1) é a cantidade máxima de

pesticida que pode ser desorbida nas condicións experimentais, e qd (µmol kg-1) é a

cantidade de pesticida desorbida polo solo.

A cantidade de carbofurano que pode ser desorbida (q0) varía entre 7.1 y 8.0

µmol kg-1, representando entre o 21 e 100% da capacidade máxima de retención (qmax).

Estas dúas variables (q0 e qmax) non están relacionadas, o que nos indica que a cantidade

de carbofurano desorbida durante o experimento é independente da cantidade

previamente adsorbida, sendo claramente dependente do contido en carbono dos solos.

Esto é aparentemente contradictorio co obtido nos experimentos batch, o cal indica que

a técnica de estudio da adsorción/desorción influe de maneira moi importante nas

conclusións as que se poden chegar. A constante cinética de desorción (kd) varía entre

0.086 e 0.195 min-1, sendo en xeral maiores que as constantes cinéticas de adsorción


65

(ks). Isto significa que a liberación de carbofurano é un proceso máis rápido que a

adsorción.

B. Metalaxyl.

Estudamos a adsorción-desorción de metalaxyl en 16 mostras superficiais (S1-

S16), co fin de coñecer o comportamento do pesticida nos distintos solos e relacionar a

adsorción-desorción coas características edáficas.

O procedemento fixose do mesmo xeito que no caso do carbofurano. En primero

lugar levaronse a cabo experimentos tipo batch para coñecer o tempo necesario para

acadar o equilibrio e para isto empregouse unha disolución de CaCl2 0.005 M e

metalaxyl 7.2 µM. Na figura nº 7 podemos observar que 24 h de contacto son

suficientes para acadar o equilibrio.

0

5

10

15

20

0 10 20 30 40 50t (h)

qs (

µm

ol k

g-1)

S2

05

10152025303540

0 10 20 30 40 50

t (h)

qs (

µm

ol k

g-1)

S14

Figura nº 7: Adsorción de metalaxyl en función do tempo en dous dos solos empregados nos experimentos batch (S2 e S14).

A adsorción de metalaxyl en experimentos batch en función da concentración

levouse a cabo do mesmo xeito que no caso do carbofurano, so que neste caso usáronse

concentracións iniciais diferentes do pesticida (3-100 µM). Na figura nº 8 amósanse as

curvas de adsorción de 4 dos 16 solos estudados. As curvas de adsorción son de tipo C e

decir, a relación de metalaxyl adsorbido/metalxyl en disolución permanece constante

para as diferentes cantidades iniciales engadidas.

Os datos experimentais foron axustados ao modelo matemático de Freundlich.

Os axustes ao modelo de Langmuir non foron satisfactorios debido a que levaban

asociados grandes erros. Os axustes ao modelo de Freundlich foron satisfactorios con

valores de R2 de 0.865-0.998. Os valores do parametro n oscilaron entre 0.18 e 1.42,


66

cun valor medio de 0.78. Estes valores de n son elevados o que indica unha baixa

heteroxeneidade dos sitios de adsorción. Os valores de KF oscilaron de 0.2 a 6.3 Ln kg-1

µmol(1-n) e correlacionáronse significativamente co contido de carbono total (r=0.652,

P<0.01), co contido de nitróxeno total (r=0.668, P<0.01) e co contido de arxila

(r=0.561, P<0.05), datos concordantes cos de outros autores (Andrades et al., 2001;

Fernandes et al., 2003; Marín-Benito et al., 2009a).

0

40

80

120

160

200

0 20 40 60 80 100

C (µmol L-1)

qs (

µm

ol k

g-1)

S 2

0

10

20

30

40

0 20 40 60 80 100

C (µmol L-1)

qs (

µm

ol k

g-1)

S 5

0

60

120

180

240

300

0 20 40 60 80 100

C (µmol L-1)

qs (

µm

ol k

g-1)

S 14

0

30

60

90

120

0 20 40 60 80 100

C (µmol L-1)

qs (

µm

ol k

g-1)

S 15

Figura nº 8: Curvas de adsorción de metalaxyl para 4 dos 16 solos estudados (S2, S5, S14 e S15).

Nos experimentos de desorción tipo batch prodúcese un incremento da

cantidade desorbida de metalaxyl canto maior é a cantidade engadida, o cal indica que a

medida que nos achegamos á saturación dos sitios de adsorción, o pesticida vaise

unindo a sitios dunha menor especificidade. As cantidades de pesticida desorbido foron

dende valores indetectables (<0.1 µmol kg-1) ata 74.9 µmol kg-1. A desorción de

metalaxyl foi elevada o cal pode ser debido a súa alta solublidade. Estes resultados son

similares os obtidos por outros autores (Marín-Benito et al., 2009b; Monkiedje e

Spiteller, 2002), os cales concluíron que a adsorción de metalaxyl ao solo era altamente

reversible.


67

Na figura nº 9 podemos observar os resultados dos experimentos en cámara de

fluxo axitado para os cales empregouse unha disolución que contiña CaCl2 0.005M e

metalaxyl 7.2 µM para os experimentos de adsorción, e CaCl2 0.005M sen pesticida

para a desorción. A cantidade retida foi baixa (entre 2.5 e 56.5 µmol kg−1) datos que

concordan cos obtido nos experimentos batch. A adsorción de metalaxyl polos

compoñentes do solo é rápida, alcanzando o 100 % da cantidade máxima adsorbida en 3

minutos (Figura nº 9) no solo que menos adsorbe (S4) e 40 minutos no solo que máis

adsorbe (S14).

0

10

20

30

40

50

0 10 20 30 40 50 60

t (min)

qs (

µm

ol k

g-1)

S2

S4

S1

0

10

20

30

40

0 10 20 30 40 50 60

t (min)

qs (

µm

ol k

g-1)

S10

S5

S8

0

10

20

30

40

50

60

0 10 20 30 40 50 60

t (min)qs

(µ

mol

kg-1

)

S15

S14

S13

Figura nº 9: Adsorción acumulada de metalaxyl para os solos estudiados nos experimentos en cámara de fluxo axitado.

As cinéticas de adsorción de metalaxyl foron máis rápidas nos experimentos de

fluxo axitado cá nos experimentos batch. Como pasaba no caso do carbofurano, a

adsorción de metalaxyl prodúcese fundamentalmente sobre sitios de adsorción rápida.

Isto pode comprobarse por medio da relación lineal entre a adsorción relativa (q/qmax) e

a raíz do tempo (t1/2). Esta relación amósase na figura nº 10, onde podemos observar que

a maior parte da adsorción de metalaxyl (>90%) prodúcese mediante reaccións de

adsorción rápida.

Os datos foron axustados a ecuación de primeiro orde (1), amosando que a

adsorción máxima (qmax) varía entre 2.6 e 60.4 µmol kg-1 para o metalaxyl. O solo cun

maior contido de materia orgánica (S14) é o que presenta a maior adsorción.

Os valores da constante cinética (ks) variaron entre 0.057 e 0.609 min-1

correspondendose cos solos que presentan a maior e a menor adsorción de metalaxyl

respectivamente.


68

R2 = 0.999R

2 = 0.990

0,0

0,2

0,4

0,6

0,8

1,0

1,2

0,0 2,0 4,0 6,0 8,0

t1/2

qs/q

smax

S5

S14

Figura nº 10: Relación entre a adsorción relativa de metalaxyl (q/qmax) e a raíz cadrada do tempo (t1/2) para dous dos solos estudados nos experimentos de cámara de fluxo axitado (S5 e S14).

Os datos das cinéticas de liberación de metalaxyl axustáronse ao modelo cinético

de primeira orde (2). Os valores de q0 para o metalaxyl variaron entre 2.6 e 37.9 µmol

kg-1 representando entre o 30 e o 100% da capacidade máxima de retención (qmax). Os

valores da constante cinética (kd) amosan un intervalo entre 0.132 e 0.956, valores máis

elevados que no caso da adsorción (kd).

Os métodos, os resultados esperimentais e a discusión están recollidos con maior

detalle nos seguientes artigos:

A. Adsorption and desorption kinetics of carbofuran in acid soils publicado no

ano 2011 na revista JOURNAL OF HAZARDOUS MATERIALS (Volume 190, 159-

167) que se presenta como anexo I nesta memoria

B. Adsorption and Desorption Behavior of Metalaxyl in Intensively Cultivated

Acid Soils publicado no ano 2011 na revista JOURNAL OF AGRICULTURAL AND

FOOD CHEMISTRY (Volume 59, 7286-7293) que se presenta como anexo II nesta

memoria.


69

4.2. Comparación de experimentos batch, cámara de fluxo axitado e

columnas na adsorción, desorción e transporte de carbofurano e

metalaxyl en dous solos ácidos.

Neste estudio tratase de comparar os resultados obtidos na adsorción-desorción

de carbofurano e metalaxyl dos diferentes métodos empregados. Para isto usáronse dous

solos recollidos na comarca da Limia cun contido en carbono orgánico moi distinto;

22.7% (S1) e 2.7% (S2). Realizáronse tres tipos de experimentos:

1. Experimentos batch: cinética e adsorción en equilibrio.

2. Experimentos en cámara de fluxo axitado.

3. Experimentos en columnas.

A. Carbofurano

Nas cinéticas batch empregouse unha disolución cunha concentración de 4.5

µM de carbofurano máis CaCl2 0.005 M como electrolito de fondo. Para a adsorción en

equilibrio empregáronse disolucións con concentracións de carbofurano de 2 a 18 µM.

Para os experimentos en cámara de fluxo axitado a concentración de carbofurano na

disolución foi de 4.5 µM máis CaCl2 0.005M. Os procedementos, tanto para os

experimentos batch como para os experimentos en cámara de fluxo axitado, foron os

mesmos cós descritos no apartado anterior (apartado 4.1).

Para os experimentos en columnas empregouse unha montaxe similar aos

experimentos en cámara de fluxo axitado, substituíndo a cámara por unha columna de

vidro de 8 cm de lonxitude e 2.5 cm de diámetro. O solo foi engadido na columna a

incrementos de 0.5 cm e golpeando lixeiramente as paredes da columna para consolidar

o solo e eliminar as posibles burbullas de aire (Chotpantarat et al., 2011). Despois

uniuse a entrada a unha bomba peristáltica, a cal mantiña un fluxo de 0.3 mL min-1, e a

saída a un colector de fraccións. Unha vez colocada a columna, fíxose pasar unha

disolución de CaCl2 0.005 M durante 24 h co fin de saturar a columna e eliminar os

ocos ocupados por aire. Unha vez saturada a columna introduciuse unha disolución de

10 mg L-1 de trazador (Br-) durante 400 min, recollendo 40 mostras de 3 mL e despois

substituíuse a disolución por unha de CaCl2 0.005M durante 400 min, recollendo outras

40 mostras de 3 mL.


70

A realización dos experimentos de adsorción/desorción realizáronse do mesmo

xeito que para o Br-, substituíndo a disolución do trazador por unha disolución de

carbofurano 4.5 µM e CaCl2 0.005 M como electrolito de fondo. A determinación do

pesticida realizouse por HPLC-UV.

Na figura nº 11 podemos ver a adsorción nos experimentos batch de

carbofurano en función do tempo, onde se pode observar que para estes solos as

cinéticas son relativamente lentas, alcanzándose o máximo de adsorción despois de 16-

24 horas de contacto, sendo o proceso máis lento para a mostra que máis carbofurano

adsorbe (mostra S1).

Os datos das cinéticas de adsorción foron axustados a ecuación de pseudo

primeira orde:

dqs/dt = ks (qmax-qs) (1)

onde dqs/dt (µmol kg-1 min-1) é a velocidade de adsorción do pesticida, ks é unha

constante cinética (min-1), qmax (µmol kg-1) é a capacidade máxima de adsorción do

pesticida, e qs (µmol kg-1) é a cantidade de pesticida retido no solo. Esta ecuación (1)

pode ser linealizada quedando da seguinte forma:

Ln (qs) = Ln (qmax – qs) + ks t (3)

onde t é o tempo.

0

5

10

15

20

25

0 500 1000 1500 2000 2500 3000 3500

t (min)

qs (µ

mol

kg

-1)

Figura nº 11: Cinética de carbofurano. S1 (círculos abertos), S2 (círculos pechados).

A capacidade máxima de adsorción (qmax) para o carbofurano foi 19.8 e 4.9

µmol kg-1 para os solos 1 e 2 respectivamente, observándose que o solo cun maior

contido en carbono orgánico (S1) presenta o valor máis elevado. A velocidade de


71

adsorción (ks) do pesticida foi similar para os dous solos (0.0022 min-1 e 0.0025 min-1

para os solos 1 e 2 respectivamente).

Na figura nº 12 amósanse os resultados da adsorción en equilibrio nos

experimentos batch. As curvas de adsorción foron tipo C. Isto reflexa a constante de

partición entre a cantidade de pesticida adsorbido e a cantidade de pesticida presente na

disolución (KD). Os axustes lineais foron satisfactorios con valores de R2> 0.95. Os

valores de KD foron de 8.9 L kg-1 e 1.0 L kg-1 para os solos 1 e 2, respectivamente.

S 1

0

20

40

60

80

100

0 5 10 15 20

C (µmol L -1)

qS (

µmol

kg

-1)

S 2

0

10

20

30

40

50

0 5 10 15 20

C (µmol L -1)

qS (

µmol

kg

-1)

Figura nº 12: Adsorción en equilibrio de carbofurano para as dúas mostras de solo estudadas (S1 e S2).

Os valores do parámetro KF da ecuación de Freundlich foron de 9.2 e 1.1 Ln kg-1

µmol(1-n) para os solos 1 e 2 respectivamente. Os valores do parámetro n foron cercanos

a 1 nos dous solos, obtendo valores de 0.99 e 0.95 para os solos 1 e 2 respectivamente.

Estes valores amosan unha baixa heteroxeneidade dos sitios de adsorción.

A porcentaxe de desorción de carbofurano variou entre 14.5 e 29.0 %, pero non

se observou unha relación lineal entre a porcentaxe de desorción e a cantidade de

carbofurano engadida.

Os resultados dos experimentos en cámara de fluxo axitado amósanse na figura

nº 13. Como se pode observar a adsorción de carbofurano foi maior no solo 1 (22.7 %

de carbono orgánico) que no solo 2, no cal a saturación alcanzouse nos primeiros 20

minutos.


72

S 1

0

20

40

60

80

0 20 40 60

t (min)

qs (

µmol

kg

-1)

S 2

0

20

40

60

80

0 20 40 60

t (min)

qs (

µmol

kg

-1)

Figura nº 13: Adsorción acumulada de carbofurano nos experimentos de cámara de fluxo axitado para as dúas mostras de solo estudadas (S1 e S2).

A cantidade de carbofurano adsorbido ao final do experimento foi de 71.6 µmol

kg-1 para o solo 1 e 12.9 µmol kg-1 para o solo 2. Os datos experimentais foron

axustados a ecuación (1) obtendo valores de capacidade máxima de adsorción (qmax) de

107.1 e 13.1 µmol kg-1 para os solos 1 e 2 respectivamente. A constante de velocidade

de adsorción (ks) obtivo valores de 0.018 e 0.129 min-1 para os solos 1 e 2

respectivamente. A velocidade de adsorción de carbofurano foi maior no solo 2, onde a

adsorción de pesticida foi menor. En comparación cás cinéticas nos experimentos

batch, as velocidades de adsorción de carbofurano foron moito maiores nos

experimentos de cámara de fluxo axitado.

A desorción de carbofurano foi moito máis rápida que a adsorción, con valores

de kd (constante de velocidade de desorción) de 0.106 e 0.262 min-1 para os solos 1 e 2,

respectivamente. A porcentaxe de desorción de carbofurano foi do 46 % para o solo 1 e

do 90 % para o solo 2. Podemos observar que a materia orgánica ten un efecto notable

na desorción de carbofurano, xa que o solo 1 desorbe a metade do pesticida previamente

adsorbido mentres que o solo 2 desorbe un 90% do previamente adsorbido.

Na figura nº 14 amósanse as curvas de avance do trazador (Br-) e do carbofurano

nos dous solos estudados para os experimentos en columnas. O comportamento do

trazador é moi similar nos dous solos, acadando o valor máximo de concentración

relativa a 2.5 volumes de poro.

En canto o carbofurano, no solo 2 podemos observar que alcanza a saturación a

7 volumes de poro, comportamento que tamén observamos na segunda parte da curva


73

onde se acada o valor 0 de concentración relativa no mesmo tempo. No solo 1, o

carbofurano nunca chegou a saturación; só acadou o 70% de concentración relativa

(C/C0) durante o tempo que durou o experimento (400 minutos).

S 1

0

0,2

0,4

0,6

0,8

1

0 5 10 15 20 25 30

PV

C/C

0

S 2

0

0,2

0,4

0,6

0,8

1

0 5 10 15 20 25 30

PV

C/C

0

Figura nº 14: Curvas de avance de Br-(círculos) e carbofurano(triángulos) para os solos estudados (S1 e S2).

A cantidade adsorbida de carbofurano nos experimentos en columnas (figura nº

15) seguiu a mesma tendencia que nos experimentos anteriores, é decir, foi maior no

solo 1 (36.8 µmol kg-1) cá no solo 2 (4.4 µmol kg-1).

S1

0

10

20

30

40

50

0 100 200 300 400

t (min)

qA (

µmol

kg

-1)

S2

0

5

10

15

20

0 100 200 300 400

t (min)

qA (

µmol

kg

-1)

Figura nº 15: Adsorción acumulada de carbofurano nos experimentos en columnas en función do tempo.

Os valores de ks obtidos despois de aplicar a ecuación (1) foron de 0.003 e 0.024 min-1

para os solos 1 e 2, respectivamente. Do mesmo xeito que nos experimentos de cámara

de fluxo axitado, a adsorción de carbofurano foi máis rápida no solo que menos

adsorbe (solo 2). A constante de velocidade de adsorción de carbofurano foi máis baixa

nos experimentos en columnas que nos experimentos en cámara de fluxo axitado,

pero máis alta que nos experimentos batch.

A cantidade de carbofurano desorbida nos experimentos en columnas foi de

14.4 e 3.8 µmol kg-1 para os solos 1 e 2, respectivamente. Os valores da constante de

velocidade de desorción (kd) foron 0.007 min-1 no solo 1 e 0.029 min-1 no solo 2. Estas


74

velocidades de desorción de carbofurano son prácticamente idénticas que as velocidades

de adsorción deste pesticida.

B. Metalaxyl.

Neste traballo aplicamos as mesmas metodoloxías que no caso anterior para o

carbofurano, empregando tamén as mesmas dúas mostras de solo cun contrastado

contido en carbono orgánico (S1 e S2). Só cambía a concentración de metaxyl nas

disolucións empregadas que foi de 7.2 µM para os tres tipos de experimentos. Na

adsorción en equilibrio empregáronse concentracións de metalaxyl dende 9 a 90 µM.

Na figura nº 16 podemos ver a adsorción de metalaxyl nos experimentos batch

en función do tempo para os dous solos. Os resultados son similares os obtidos para o

caso do carbofurano, sendo o tempo de equilibrio maior para o solo con máis materia

orgánica (S1), que tamén é o solo que maior cantidade de metalaxyl adsorbe.

0

5

10

15

20

25

30

35

0 500 1000 1500 2000 2500 3000 3500

t (min)

qs (µ

mol

kg

-1)

Figura nº 16: Cinética de metalaxyl. S1 (círculos abertos), S2 (círculos pechados).

A capacidade máxima de adsorción (qmax) do metalaxyl foi de 32.6 e 6.4 para os

solos 1 e 2, respectivamente. Os valores de (ks) para o metalaxyl foron de 0.0037 min-1

para o solo 1 e 0.0045 min-1 para o solo 2.

Na figura nº 17 amósanse os resultados da adsorción en equilibrio nos

experimentos batch. As curvas de adsorción de metalaxyl foron de tipo C sendo os

axustes lineais satisfactorios para os dous solos estudiados, con valores de R2> 0.95. Os

valores de KD foron 5.9 L kg-1 e 0.6 L kg-1 para os solos 1 e 2, respectivamente.


75

S 1

0

100

200

300

400

0 20 40 60 80 100

C (µmol L -1)

qS (

µmol

kg

-1)

S 2

0

20

40

60

80

100

0 20 40 60 80 100

C (µmol L -1)

qS (

µmol

kg

-1)

Figura nº 17: Adsorción en equilibrio de metalaxyl para as dúas mostras de solo estudadas.

Os valores do parámetro KF da ecuación de Freundlich foron de 12.15 e 0.88 Ln

kg-1 µmol(1-n) para os solos 1 e 2, respectivamente. Os valores do parámetro n foron

altos nos dous casos, 0.82 e 0.90 para o solo 1 e 2, respectivamente. Estes valores

amosan unha baixa heteroxeneidade polos sitios de adsorción.

A desorción de metalaxyl nos experimentos batch foi elevada, cunhas

porcentaxes de desorción entre 9.2 e 7.8% no solo 1 e entre 21.7 e 42.6% no solo 2. A

porcentaxe de desorción foi maior no solo 2 que no solo 1, o que indica que a

irreversibilidade da adsorción de metalaxyl incrementase ao incrementar o contido en

materia orgánica, o cal coincide cos resultados obtidos por Abd El-Aleem et al., (2009)

e Fernandes et al., (2006).

Os resultados dos experimentos en cámara de fluxo axitado amósanse na figura

nº 18. Como se pode observar a adsorción de metalaxyl foi maior no solo 1 cun 22.7 %

de carbono orgánico (que non alcanzou a saturación) que no solo 2 cun 2.7% de carbono

orgánico, no cal a saturación alcanzouse nos primeiros 7 minutos.

A cantidade adsorbida de metalaxyl ao final do experimento foi de 49.4 µmol

kg-1 e 10.1 µmol kg-1 para os solos 1 e 2, respectivamente. Os datos experimentais foron

axustados a ecuación (1) obtendo valores de capacidade máxima de adsorción (qmax) de

54.7 µmol kg-1 para o solo 1 e 10.1 µmol kg-1 para o solo 2. A constante de velocidade

de adsorción (ks) do metalaxyl foi de 0.032 min-1 no solo 1 e 0.275 min-1 no solo 2. A

velocidade de adsorción foi maior no solo 2, onde a adsorción de pesticida foi menor.

En comparación cás cinéticas nos experimentos batch, as velocidades de adsorción

foron moito maiores nos experimentos de cámara de fluxo axitado.


76

S 1

0

20

40

60

80

0 20 40 60

t (min)

qs (

µmol

kg

-1)

S 2

0

20

40

60

80

0 20 40 60

t (min)

qs (

µmol

kg

-1)

Figura nº 18: Adsorción acumulada de metalaxyl nos experimentos de cámara de fluxo axitado.

A velocidade de desorción de metalaxyl foi maior que a de adsorción (0.118 e

0.337 min-1 para os solos 1 e 2 respectivamente) e a porcentaxe de desorción foi dun 88

% no solo 1 e un 91 % no solo 2. Como se pode observar a desorción de metalaxyl foi

alta para os dous solos.

En canto os experimentos en columnas, na figura nº 19 amósanse as curvas de

avance do trazador (Br-) e do metalaxyl nos dous solos estudados. No solo 2 alcanzouse

a saturación a 7 volumes de poro, comportamento que tamén observamos na segunda

parte da curva onde se acada o valor 0 de concentración relativa nos mesmos volumes

de poro. No solo 1, non se acadou o 100% de concentración relativa durante o tempo

que durou o experimento, chegando o 85 %.

S 1

0

0,2

0,4

0,6

0,8

1

0 5 10 15 20 25 30

PV

C/C

0

S 2

0

0,2

0,4

0,6

0,8

1

0 5 10 15 20 25 30

PV

C/C

0

Figura nº 19: Curvas de avance de Br-(círculos) e metalaxyl (triángulos) para os solos S1 e S2.

A cantidade adsorbida de metalaxyl nos experimentos en columnas (figura nº

20) foi, do mesmo xeito que nos experimentos anteriores, maior no solo 1 (43.9 µmol

kg-1) cá no solo 2 (7.3 µmol kg-1).


77

S1

0

10

20

30

40

50

0 100 200 300 400

t (min)

qA (

µmol

kg

-1)

S2

0

5

10

15

20

0 100 200 300 400

t (min)

qA (

µmol

kg

-1)

Figura nº 20: Adsorción acumulada de metalaxyl nos experimentos en columnas en función do tempo.

Os valores de ks foron de 0.006 min-1 para o solo 1 e 0.031 min-1 para o solo 2.

De forma similar o que acontecía nos experimentos de cámara de fluxo axitado, a

adsorción de metalaxyl foi máis rápida no solo que menos adsorbe (solo 2). A constante

de velocidade de adsorción foi máis baixa nos experimentos en columnas que nos

experimentos en cámara de fluxo axitado, pero máis alta que nos experimentos batch.

A cantidade de metalaxyl desorbida foi de 30.9 µmol kg-1 no solo 1 e 6.7 µmol

kg-1 no solo 2. Os valores da constante de velocidade de desorción (kd) foron 0.007 e

0.026 min-1 para os solos 1 e 2 respectivamente. Estas velocidades de desorción son

prácticamente idénticas que as velocidades de adsorción.

Os resultados obtidos cos diferentes métodos dannos conclusións similares

cando se comparan os dous solos e os dous pesticidas estudados: carbofurano e

metalaxyl. A capacidade de adsorción de carbofurano e metalaxyl foi significativamente

máis alta no solo que tiña un maior contido en carbono orgánico, mentres que a

velocidade de adsorción foi maior no solo que tiña un baixo contido en carbono

orgánico. Sen embargo, os diferentes métodos proporcionan resultados distintos cando

se trata do mesmo solo. A adsorción de carbofurano e metalaxyl foi sempre máis alta

nos experimentos de fluxo axitado, seguida polos experimentos en columnas e en

último lugar polos experimentos batch. A axitación continua e a concentración

constante da disolución de entrada nos experimentos de fluxo axitado facilitan a

adsorción dos pesticidas en comparación cós experimentos en columnas, onde non hai

axitación do solo, e cós experimentos tipo batch onde a adsorción está limitada pola

concentración do pesticida na disolución (diminúe a medida que avanza o proceso). Os


78

procesos de desorción foron máis rápidos cós de adsorción nos experimentos de

cámara de fluxo axitado, mentres que nos experimentos en columnas foron

semellantes.


detalle nos seguientes artigos:

A. Comparison of batch, stirred flow chamber, and column experiments to study

adsorption, desorption and transport of carbofuran within two acidic soils publicado no

ano 2012 na revista CHEMOSPHERE (Volume 88, 106-112) que se presenta como

anexo III nesta memoria

B. Metalaxyl dynamics in acid soils: evaluation using different methods enviado

a revista PEST MANAGEMENT SCIENCE en Xullo de 2012 que se presenta como

anexo IV nesta memoria.

4.3. Factores que inflúen na disipación de carbofurano e metalaxyl.

Neste traballo avaliáronse os efectos da adsorción, fotodegradación e

degradación microbiana na disipación de carbofurano e metalaxyl en períodos curtos de

tempo. Para isto empregáronse 2 solos, S1 (22.7% de carbono orgánico) e S2 (2.7 % de

carbono orgánico).

En primeiro lugar levouse a cabo un experimento para avaliar a toxicidade dos

pesticidas na comunidade bacteriana dos solos empregados, usando a técnica de

incorporación de Leucina (Kirchman et al., 1985). Para isto levouse a cabo unha

extracción da comunidade bacteriana descrita por Bååth (1994) e Bååth et al. (2001).

Unha vez obtidos os extractos engadíronselle distintas concentracións de carbofurano e

metalaxyl e mediuse o crecemento bacteriano. Na figura nº 21 pódese observar os

resultados deste experimento.


79

0,0

0,5

1,0

1,5

2,0

2,5

3,0

0,1 1 10 100 1000

metalaxyl concentration (mg L-1)

Re

lati

ve b

acte

rial

gro

wth

rat

e

0,0

0,5

1,0

1,5

2,0

0,1 1 10 100 1000

carbofuran concentration (mg L-1)

Re

lati

ve b

acte

rial

gro

wth

rat

e

Figura nº 21: Crecemento bacteriano en función da concentración de pesticidas. Solo 1 (triángulos); Solo 2 (círculos).

Os tests de toxicidade amosan que os incrementos na concentración de metalaxyl

na disolución do solo non tiveron efectos negativos na taxa de crecemento bacteriano.

Polo contrario, a presenza de metalaxyl na disolución estimula o crecemento bacteriano

nalgúns casos, onde a comunidade bacteriana podería usar o metalaxyl como fonte de

carbono nos procesos de degradación (Baker et al., 2010). No caso do carbofurano

observase que a comunidade bacteriana do solo 2 non se ve afectada pola presenza do

pesticida, sen embargo no solo 1 a comunidade bacteriana viuse inhibida por

concentracións de carbofurano superiores a 20 mg L-1. Isto indícanos que a comunidade

bacteriana do solo 1 é máis sensible a presenza de carbofurano que a do solo 2, o que

pode ser explicado pola aplicación histórica do pesticida. O solo 1 ten un maior contido

en carbono orgánico que o solo 2, polo tanto a adsorción de carbofurano tamén é

superior no solo 1 (Bermúdez-Couso et al., 2012). A baixa capacidade de adsorción

carbofurano polo solo 2 supón unha maior presenza deste pesticida na disolución do

solo, co cal a comunidade bacteriana deste solo estivo máis exposta ao pesticida que a

do solo 1 e, polo tanto, a comunidade bacteriana do solo 1 é menos tolerante que a do

solo 2.

Os experimentos de disipación leváronse a cabo en viais transparentes EPA de

40 mL, os cales foron introducidos nunha cámara de simulación de luz (Atlas Suntest

CPS+ Atlas, Alemaña) equipada cunha lámpada de xenon a 550 W m-2 e un filtro

recuberto de cuarzo que impide a emisión a lonxitudes de onda <285 nm. Realizáronse

dous tipos de experimentos: a) en escuridade durante 48 e b) cun ciclo de 12 horas de

luz e 12 horas de escuridade durante 48 h. A temperatura a que se mantiveron as


80

mostras durante o experimento foi de 32±2 ºC na fase luminosa e 25±2 ºC na fase

escura. Tódolos experimentos foron realizados por triplicado.

Para este estudio empregouse unha disolución de metalaxyl (10 mg L-1) e unha

de carbofurano (4 mg L-1) realizadas en auga destilada e filtradas con filtros de nitrato

de celulosa de 0.20 µm de tamaño de poro co fin de eliminar os microorganismos

presentes na disolución. Dez mL destas disolucións foron mesturados con outros 10 mL

de distintas disolucións en viais de 40 mL: a) auga destilada filtrada, b) suspensións

bacterianas extraídas dos solos segundo Bååth (1994) e Bååth et al. (2001), e c)

suspensións con 1 g de solo. Estes viais foron colocados no interior da cámara collendo

mostras de 0.2 mL a intervalos de tempo apropiados e medindo os pesticidas por HPLC-

UV.

Dado que nas condicións destes experimentos o crecemento dos

microorganismos podía estar limitado por algún nutrinte, os experimentos repetíronse

substituíndo as disolucións dos pesticidas con auga destilada filtrada por disolucións

dos pesticidas con un caldo de cultivo filtrado. O caldo de cultivo foi realizado en auga

destilada coa seguinte composición: 1.75 mg L-1 de K2HPO4, 0.50 mg L-1 de KH2PO4,

0.58 mg L-1 de NH4Cl y 0.25 mg L-1 de MgSO4.7H2O.

Nos experimentos con solo, unha vez finalizados os estudios de disipación, a

cantidade de pesticida retido no solo foi extraída con metanol para o metalaxyl e

acetonitrilo para o carbofurano.

Nos experimentos sen solo non se observou disipación de metalaxyl e

carbofurano en escuridade. Na figura nº 22 podemos ver a disipación de ambos

pesticidas en disolucións filtradas en presenza en ausencia de caldo de cultivo durante

os experimentos con 12h luz/12h escuridade. O comportamento dos dous pesticidas foi

moi similar. A disipación de metalaxyl e carbofurano en presenza do caldo de cultivo

foi moito máis rápida, observando que as 10 h de experimento disipouse máis dun 90 %

mentres que nos experimentos sen caldo esta disipación as 10 h era de practicamente a

metade.


81

A

0,0

0,2

0,4

0,6

0,8

1,0

0 10 20 30 40 50 60

Tempo (horas)

Met

alax

yl

(C/C

0)

Sen caldo Con caldo B

0,0

0,2

0,4

0,6

0,8

1,0

0 10 20 30 40 50 60

Tempo (horas)

Car

bofu

rano

(C

/C0)

Sen caldo Con caldo

Figura nº 22: Disipación de metalaxyl (A) e carbofurano (B) en disolucións filtradas en presenza (círculos) e ausencia (triángulos) de caldo de cultivo (12h luz/12h escuridade).

A vida media dos pesticidas baixo as condicións experimentais foi calculada

mediante a seguinte ecuación:

)](1[1 aXbeY −+= ] (4)

Onde Y é a concentración inicial do pesticida (C/C0), X é o logaritmo do tempo

(h), a é o logaritmo da vida media (h) e b é un parametro que indica a velocidade de

disipación do pesticida (h-1)

A vida media baixo condicións de irradiación nas disolucións filtradas sen

caldo de cultivo foi de 13 e 16 h para metalaxyl e carbofurano respectivamente. Con

caldo de cultivo a disipación foi moito maior, obtendo vidas medias de 2.2 h para o

metalaxyl e 1.8 para o carbofurano.

Na figura nº 23 amósanse a disipación dos pesticidas en disolucións con

microorganismos extraídos do solo (S1) en presenza e ausencia de caldo de cultivo

nos experimentos con 12h luz/12h escuridade. O comportamento dos pesticidas nestes

experimentos foi moi similar aos experimentos con disolucións filtradas, unha

disipación importante baixo condicións de irradiación especialmente en presenza de

caldo de cultivo e unha disipación nula en escuridade. Estes resultados indícannos que

non houbo unha degradación bacteriana de forma importante. Isto pode ser debido a que

o tempo de duración do experimento é curto. Asi, Plangklang e Reungsang (2011)

amosan que a vida media do carbofurano degradado por microorganismos é de 127 días

e De Wilde et al. (2010) dan tempos de vida media para o metalaxyl de 131 días.


82

A

0,0

0,2

0,4

0,6

0,8

1,0

0 10 20 30 40 50 60

Tempo (horas)

Met

alax

yl

(C/C

0)

Sen caldo Con caldo B

0,0

0,2

0,4

0,6

0,8

1,0

0 10 20 30 40 50 60

Tempo (horas)

Car

bofu

rano

(C

/C0)

Sen caldo Con caldo

Figura nº 23: Disipación de metalaxyl (A) e carbofurano (B) en disolucións con microorganismos extraídos do solo (S1) en presenza (círculos) e ausencia (triángulos) de caldo de cultivo (12h luz/12h escuridade).

A vida media do metalaxyl nos experimentos con suspensións bacterianas dos

solos foi moi similar a dos experimentos con disolucións filtradas, obtendo tempos de

vida media de 13 h para a suspensión bacteriana do solo 1 e 12 h para a suspensión

bacteriana do solo 2. No caso do carbofurano, os valores da vida media foron máis

baixos que nos experimentos con disolucións filtradas, 6 h para suspensión bacteriana

do solo 1 e 7 horas para a suspensión bacteriana do solo 2. O incremento da disipación

de carbofurano con suspensións bacterianas extraidas do solo pode ser debido a

composición das mesmas, onde algúns elementos poden favorecer a fotodegradación de

carbofurano (Lu et al., 2011; Mahalakshmi et al., 2007).

Nos experimentos con caldo de cultivo, a vida media do metalaxyl foi de 2.2 h e

1.6 h para as suspensións bacterianas dos solos S1 e S2, respectivamente. No caso do

carbofurano, a vida media foi de 1.1 h para a suspensión bacteriana do solo 1 e 1.0 h

para a suspensión bacteriana do solo 2. Do mesmo xeito que nos experimentos con

disolucións filtradas, a presenza do caldo de cultivo incrementou a disipación baixo

condicións de irradiación e non presentou ningún efecto en escuridade. Outros autores

observaron enriquecementos da fotodegradación en compostos orgánicos coa presenza

de ferro, nitratos, fosfatos, acidos húmicos e disolucións de titanio (Lu et al., 2011;

Mahalakshmi et al., 2007; Maheswari et al., 2010; Prabhakaran et al., 2009; Tong et al.,

2011).

Nos experimentos con solo produciuse unha disipación en condicións de

escuridade, pero esta foi máis alta en condicións de irradiación. Na figura nº 24 amosase


83

a disipación dos pesticidas en condicións de irradiación e en condicións de escuridade

para os experimentos con solo.

S1

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Figura nº 24: Disipación de metalaxyl e carbofurano nos experimentos con solo (S1 e S2).

Nos experimentos en condicións de irradiación , a vida media do metalaxyl foi

de 6.9 h e 7.8 h para os solos 1 e 2, respectivamente. No caso do carbofurano, a vida

media foi de 4.2 h para o solo 1 e 5.1 h para o solo 2. Estes valores de vida media son

inferiores aos atopados nos experimentos con disolucións filtradas e con suspensións

bacterianas sen caldo de cultivo, observándose que a disipación de carbofurano e

metalaxyl é máis rápida en presenza de solo cá en ausencia. Este incremento na

disipación pode ser debido a adsorción dos pesticidas no solo ou a degradación por

parte de microorganismos do solo que non estean presentes na suspensión bacteriana.

En condicións de escuridade tamén tivo lugar unha disipación dos pesticidas. Estes

datos obtidos en oscuridad tamén foron axustados a ecuación (4) (R2=0.74-0.85) onde

se obtiveron valores de vida media para o metalaxyl de 104.4 h e 54087.9 h nos solos 1

e 2, respectivamente. No caso do carbofurano, a vida media foi de 153.0 h para o solo 1

e 907.8 h para o solo 2.


84

Nos experimentos con solo en escuridade, a cantidade de metalaxyl disipada

ao final do experimento foi de 31.5±1.7 mg kg-1 para o solo 1 e 8.5±1.3 mg kg-1 para o

solo 2. Estes resultados son moi semellantes aos datos obtidos na extracción do

metalaxyl do solo unha vez finalizados os experimentos de disipación (33.2±2.7 mg kg-1

para o solo 1 e 8.3±1.4 mg kg-1 para o solo 2). No caso do carbofurano sucede o mesmo,

a cantidade disipada ao final dos experimentos en escuridade foi de 11.0±0.5 mg kg-1 no

solo 1 e 4.8±0.1 mg kg-1 no solo 2. A cantidade de carbofurano extraída do solo ao final

do experimento foi de 10.8±1.0 mg kg-1 no solo 1 e 5.2±0.1 mg kg-1 no solo 2. Con todo

isto podemos afirmar que a disipación de metalaxyl en presenza de solo en escuridade é

debida a adsorción dos pesticidas ao solo e non a degradación microbiana.

En condicións de irradiación, a disipación dos pesticidas en presenza de solo ao

final do experimento foi para o metalaxyl de 96.0±0.6 mg kg-1 para o solo 1 e 97.3±2.2

mg kg-1 para o solo 2, no caso do carbofurano a disipación foi de 28.9±1.2 mg kg-1 e

28.3±1.3 mg kg-1 para os solos 1 e 2, respectivamente. Estes valores foron

significativamente máis elevados cós datos obtidos na extracción dos pesticidas ao final

do experimento. A cantidade de metalaxyl extraída baixo condicións de irradiación dos

solos foi de 9.0±0.5 mg kg-1 para o solo 1 e 1.9±0.3 mg kg-1 para o solo 2. Para o

carbofurano a cantidade extraída foi 4.3±0.5 mg kg-1 e 1.7±0.1 mg kg-1 para os solos 1 e

2, respectivamente. Estes resultados amosan que a disipación de metalaxyl e

carbofurano en presenza de solo e baixo condicións de irradiación foi producida por

unha parte por fotodegradación (>85%) e por outra banda por adsorción ao solo

(<15%).

Hai que destacar que as cantidades retidas dos pesticidas no solo nos

experimentos en escuridade foron sempre máis elevadas que nos experimentos en

condicións de irradiación. Isto pode ser atribuido a que nos experimentos en condicións

de irradiación, a cantidade de pesticida presente na disolución é baixa debido a

fotodegradación, e a cantidade adsorbida é altamente dependente da concentración na

disolución.

Nos esperimentos de disipación de metalaxyl e carbofurano en presenza de solo

e caldo de cultivo aconteceu practicamente o mesmo que nos experimentos sen caldo de

cultivo, amosando que a presenza do caldo de cultivo non tivo ningún efecto sobre a

disipación dos pesticidas nos solos.


85


detalle no artigo titulado:

A. Influence of different abiotic and biotic factors on the metalaxyl and

carbofuran dissipation publicado na revista CHEMOSPHERE (DOI:

10.1016/j.chemosphere.2012.10.090) que se presenta como anexo V nesta memoria.

4.4. Contaminación por metalaxyl e nitratos en augas superficiais na

comarca da Limia.

Neste traballo pretendeuse avaliar a posible contaminación causada pola

agricultura na comarca da Limia, xa que o cultivo da pataca foise profesionalizando nos

últimos anos e o mesmo tempo tamén se incrementou o consumo de produtos

fitosanitarios.

Para este estudo realizouse unha mostraxe de augas superficiais (figura nº 25) ao

longo dos canais de drenaxe principais e do río Limia. A zona de estudo consta de 26

km de canais principais e 12 km de río. Recolléronse un total de 22 mostras de auga en

botellas de vidro da seguinte maneira: 16 mostras nos canais de drenaxe principais e 6

mostras no río Limia. A mostraxe realizouse unha vez o mes durante 2 anos (marzo de

2009 ata marzo de 2011). Neste tempo tamén se controlou o caudal nun aforo do canal

de drenaxe principal situado no punto de mostraxe 2.

Unha vez recollidas as mostras realizouse unha extracción sólido-líquido con

disolventes orgánicos (diclorometano:acetona) para determinar a presenza de metalaxyl

por GC-MS. A determinación de nitratos realizouse por cromatografía iónica.


86

N123

456

20 21

22

789101112

1314

1516 17 18 19

50Km

07º 51' 06'' W

42º 05' 08'' N

Main drainage channels

Secondary drainage channels

River

Figura nº 25: Puntos de mostraxe de augas na area de estudo.

En 11 dos 22 puntos de mostraxe de augas atopáronse residuos de metalaxyl e

no 70 % dos puntos de mostraxe nos que se atopou metalaxyl fíxose en máis dunha

ocasión. O punto de mostraxe de augas no que máis veces se atopou metalaxyl foi no

punto 21, onde detectáronse residuos deste pesticida en 10 meses dos 24 que durou a

mostraxe. No punto 22 atopouse metalaxyl nun 33 % das sesións de mostraxe. Nos

puntos 5, 6, 7 e 8 detectouse metalaxyl en 3, 5, 6 e 6 ocasións, respectivamente. A vista

destes resultados podemos distinguir tres zonas dentro da area de estudio:

- Zona 1: que comprendería a parte Norte da área de estudo formada polos

puntos 1- 4 e 20, nos cales non se atoparon residuos de metalaxyl durante os 2 anos que

durou a mostraxe.

- Zona 2: formada polos puntos 5- 8, 21 e 22, situada na parte central da conca,

onde se atoparon as cantidades de metalaxyl máis elevadas (0.288 µg L-1).

- Zona 3: situada ao sur da zona de estudio que comprendería o treito final do

canal de drenaxe principal e a parte do río Limia, e estaría formada polos puntos 9-19,

nos cales atopouse metalaxyl nos puntos 9, 10, 11, 12 e 15 en 3, 1, 1, 2 e 1 ocasión,

respectivamente.

O 22 % das ocasións que se detectaban residuos de metalaxyl excedían o límite

máximo de pesticidas para augas de consumo establecido pola Unión Europea (0.100 µg

L-1; Council Directive 98/83/CE).


87

A presenza de canais de drenaxe secundarios na zona 2 (figura nº 25) e a alta

reversibilidade que presenta a adsorción de metalaxyl ao solo, favorece o transporte do

pesticida dende as parcelas de cultivo ata o canais de drenaxe principais. Na figura nº 26

amósanse os niveis de pesticida detectados na zona 2 durante o período de mostraxe. As

altas concentracións de metalaxyl atopadas nesta zona xorden cando o caudal é baixo e

coincide coas épocas de aplicación do funxicida. A presenza de metalaxyl nos meses

chuviosos non supera os límites de detección (0.02 µg L-1), o que pode ser debido a un

efecto de dilución polo aporte de auga ou, por outra banda, a unha degradación do

composto. De feito a ausencia de metalaxyl noutros puntos de mostraxe onde a

intensidade dos cultivos é similar, como no río Limia, pode ser debido a unha

degradación do composto cando é transportado na disolución do solo cara augas

circundantes. A rápida degradación de metalaxyl foi estudada por diversos autores,

atopando vidas medias no solo de 10 a 40 días (Tomlin, 2003). Traballos levados a cabo

na mesma area de estudo obtiveron valores de vida media no solo de 2 días (López-

Pérez et al., 2006). A presenza de pesticidas en canais de drenaxe tamén foi estudada

por Vryzas et al. (2011) en Grecia, onde atoparon concentracións de pesticidas que

poderían provocar danos aos organismos acuáticos. Hildebrandt et al. (2008) estudaron

tres concas no Norte de España observando que as concentracións de metalaxyl eran

máis altas en augas subterráneas cá en augas superficiais. Bermúdez-Couso et al. (2007)

investigaron a presenza de funxicidas en solos, sedimentos e augas, observando que as

concentracións de metalaxyl eran máis elevadas en solos e sedimentos que nas augas.


88

P 21

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Figura nº 26: Niveis de metalaxyl na zona 2 durante o período de mostraxe.

Na figura nº 27 amósase o caudal e a concentración de nitratos medidos no punto

2 durante o tempo de estudio. Como se pode observar, a concentración de nitratos neste

punto vai relacionada co caudal, incrementándose nos meses nos que tamén se

incrementa o caudal. Sen embargo, estes picos de concentración de nitratos tamén

coinciden cas épocas do ano onde o desenvolvemento vexetal é mínimo, e polo tanto

tamén a incorporación de nitratos as estruturas vexetais.


89

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Figura nº 27: Relación entre a concentración de nitratos e o caudal no punto 2 da mostraxe.

Na figura nº 28 preséntanse outros puntos da área de estudo onde o

comportamento é similar con picos de nitratos nas épocas de maiores chuvias e menor

desenvolvemento vexetal. O contido medio de nitratos en cada punto varía entre 4.99

mg L-1 no punto 2 ata 16.01 mg L-1 no punto 19. O valor máximo de nitratos foi atopado

no punto 12 (60 mg L-1).

O límite máximo de nitratos establecido pola Unión Europea (Council Directive

98/83/CE) para augas de consumo é de 50 mg L-1, este valor foi superado en dúas

ocasións durante o período de estudo, no punto de mostraxe 7 en setembro de 2009

(52.5 mg L-1) e no punto de mostraxe 12 en xaneiro de 2010 (60.1 mg L-1).


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Figura nº 28: Concentración de nitratos nas augas en diferentes puntos de mostraxe e a precipitación mensual.

Tanto as concentracións de nitratos como de metalaxyl amosan unha gran

variación espacial e temporal. Esta variación tamén foi observada por Andrade e Stigter

(2009) os cales suxiren que a variación na concentración de nitratos pode ser debida a

alta fertilización nitroxenada seguida de precipitacións, que provocan unha diminución

na concentración de nitratos por dilución. No presente traballo observamos que a

concentración de nitratos incrementase cando se incrementa o caudal (figura nº 27), o

que atribuímos a que cando as precipitacións son fortes mobilizan os nitratos

incrementando a súa concentración nas augas a pesar do efecto de dilución polo

aumento do caudal.


91


detalle no artigo:

A. Pollution of surface waters by metalaxyl and nitrates from non-point sources

enviado a revista JOURNAL OF ENVIROMENTAL MANAGEMENT en Decembro

de 2012 que se presenta como anexo VI nesta memoria

CONCLUSIÓNS

CONCLUSIÓNS

95

5. CONCLUSIÓNS

Cos resultados obtidos nos traballos realizados así como das interpretacións

feitas a partires deles, concluímos que:

1. A adsorción de carbofurano e metalaxyl polos solos ácidos que estudamos neste

traballo foi, en xeral, baixa. Os datos axustáronse satisfactoriamente a ecuación

de Freundlich, presentando valores de n elevados (valores medios de 0.68 e 0.78

para carbofurano e metalaxyl, respectivamente) o que indica unha baixa

heteroxeneidade dos sitios de adsorción para os dous pesticidas. Os valores de

parámetro KF para o carbofurano variaron entre 0.6 e 8.7 Ln kg-1 µmol(1-n) e

foron correlacionados significativamente co contido en carbono orgánico (r =

0.927, P < 0.01). Para o metalaxyl os valores de KF foron de 0.2-6.3 Ln kg-1

µmol(1-n) correlacionandose significativamente co contido de carbono orgánico (r

= 0.652, P< 0.01), co contido de nitróxeno total (r = 0.668, P < 0.1) e co contido

de arxila (r = 0.561, P< 0.05).

2. Nos experimentos de desorción tipo batch prodúcese, para os dous pesticidas, un

incremento da cantidade desorbida canto maior é a cantidade engadida. Esto

indica que, a medida que nos achegamos á saturación dos sitios de adsorción, o

pesticida vaise unindo a sitios dunha menor especificidade. A desorción de

metalaxyl foi elevada, feito que pode ser debido a baixa hidrofobicidade que

presenta este composto. A porcentaxe de carbofurano liberado está

negativamente relacionado co contido en carbono orgánico do solo, polo que

podemos dicir que esta característica é determinante na irreversibilidade dos

procesos de adsorción de carbofurano no solo.

3. Nos experimentos en cámara de fluxo axitado, a adsoción de carbofiurano e

metalaxyl foi moi rápida, o que indica unha baixa afinidade do pesticida polos

compoñentes do solo. A relación lineal entre a adsorción relativa (q/qmax) e a raíz

do tempo (t1/2) indícanos que a maior parte da adsorción de ambos pesticidas

(>80%) prodúcese mediante reaccións de adsorción rápida. Do mesmo xeito que

nos experimentos batch, a cantidade adsorbida de pesticida viuse afectada polo

contido en carbono orgánico e arxila do solo. A cantidade de carbofurano

desorbida durante os experimentos en cámara de fluxo axitado é independente da

CONCLUSIÓNS

96

cantidade previamente adsorbida, depende do contido de carbono orgánico do

solo. A cantidade desorbida de metalaxyl foi moi elevada, acadando porcentaxes

de desorción do 100%. A velocidade de desorción de ámbolos dous pesticidas foi

máis rápida cá velocidade de adsorción.

4. Os resultados obtidos cos diferentes métodos experimentais (batch, cámara de

fluxo axitado e experimentos en columnas) amósannos comportamentos

similares cando se comparan os dous solos. A capacidade de adsorción de

carbofurano e metalaxyl foi significativamente máis elevada no solo que tiña un

maior contido en carbono orgánico, mentres que a velocidade de adsorción foi

maior no solo que tiña un baixo contido en carbono orgánico. Sen embargo, os

diferentes métodos experimentais proporcionan resultados diferentes cando se

trata do mesmo solo. A adsorción de carbofurano e metalaxyl foi sempre máis

alta nos experimentos de fluxo axitado cá nos outros dous. Os procesos de

desorción foron máis rápidos cós de adsorción nos experimentos de cámara de

fluxo axitado, mentres que nos experimentos en columnas foron semellantes.

5. Os tests de toxicidade amosan que os incrementos na concentración de metalaxyl

na disolución do solo non tiveron efectos negativos na taxa de crecemento

bacteriano. Polo contrario, a presenza de metalaxyl na disolución do solo

estimula o crecemento bacteriano nalgúns casos, onde a comunidade bacteriana

podería usar o metalaxyl como fonte de carbono nos procesos de degradación. A

toxicidade do carbofurano nas comunidades bacterianas do solo foi baixa, aínda

que na disolución do solo cun elevado contido en materia orgánica observouse

unha redución do crecemento bacteriano a partir de 20mg L-1 de carbofurano.

6. A disipación de carbofurano e metalaxyl foi importante nos experimentos baixo

condicións de irradiación, nos cales non se observou degradación bacteriana. A

disipación de ámbolos dous pesticidas viuse favorecida pola presenza do caldo

de cultivo nos experimentos con disolucións filtradas e nos experimentos con

suspensións bacterianas do solo. Nos experimentos con solo houbo disipación de

ámbolos dous pesticidas en condicións de escuridade, debido a adsorción dos

pesticidas ao solo. En condicións de irradiación, a disipación dos pesticidas foi

maior cá nos experimentos en escuridade, pero a súa adsorción ao solo foi

menor. Neste caso, o 85% da disipación de carbofurano e metalaxyl foi atribuida

CONCLUSIÓNS

97

a fotodegradación. A presenza do caldo de cultivo nos experimentos con solo

non presentou ningún efecto na disipación dos pesticidas.

7. O 22 % das ocasións nas que se detectou metalaxyl nas augas superficiais,

superaba o límite establecido pola Unión Europea para augas de consumo (0.100

µg L-1). As concentracións máis elevadas de metalaxyl foron atopadas cando as

precipitacións foron baixas e nunha zona onde o transporte do pesticida dende as

parcelas nas que se aplica está favorecido pola construción dunhas canles de

drenaxe. A concentración de nitratos incrementouse cando aumentaba o caudal,

superando o límite de 50 mg L-1 en dúas ocasións. Ao tratarse dunha zona con

baixa pendente a escorrentía é case insignificante, polo tanto a presenza de

metalaxyl e nitratos nas augas é principalmente consecuencia do seu transporte a

través do solo amosando como resultado unha alta variabilidade temporal e

espacial.

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131

ANEXOS

133

Anexo I. Adsorption and desorption kinetics of

carbofuran in acid soils

A

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Journal of Hazardous Materials 190 (2011) 159–167

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

dsorption and desorption kinetics of carbofuran in acid soils

lipio Bermúdez-Cousoa, David Fernández-Calvinoa,∗, Miriam Pateiro-Mourea,uan Carlos Nóvoa-Munoza, Jesús Simal-Gándarab, Manuel Arias-Estéveza

Soil and Agricultural Science Group, Plant Biology and Soil Science Department, Faculty of Science, University of Vigo, Ourense Campus, E32004 Ourense, SpainNutrition and Bromatology Group, Analytical and Food Chemistry Department, Faculty of Science, University of Vigo, Ourense Campus, E32004 Ourense, Spain

r t i c l e i n f o

rticle history:eceived 5 October 2010eceived in revised form 4 March 2011ccepted 8 March 2011

a b s t r a c t

Carbofuran adsorption and desorption were investigated in batch and stirred flow chamber (SFC) tests.The carbofuran adsorption capacity of the soils was found to be low and strongly dependent on their clayand organic carbon contents. Carbofuran sorption was due mainly (>80%) to fast adsorption processesgoverned by intraparticle diffusion. The adsorption kinetic constant for the pesticide ranged from 0.047

vailable online 15 March 2011

eywords:arbofurandsorptionesorption

to 0.195 min−1 and was highly correlated with constant n in the Freundlich equation (r = 0.965, P < 0.05).Batch tests showed carbofuran desorption to be highly variable and negatively correlated with eCEC andthe clay content. The SFC tests showed that soil organic carbon (C) plays a key role in the irreversibility ofcarbofuran adsorption. Carbofuran desorption increased rapidly at C contents below 4%. The desorptionkinetic constant for the compound (0.086–0.195 min−1) was generally higher than its adsorption kinetic

furan
ineticstirred flow chamber
constant; therefore, carbo

. Introduction

Pesticides are very intensively used against crop diseases inany agricultural areas [1,2]. In some, their use has increasedarkedly during the last few years [3] despite their well-known

azards for human health [4,5]. Carbofuran (2,3-dihydro-2,2-imethyl-7-benzofuranyl-N-methylcarbamate) is a carbamateesticide used against a wide range of insects adversely affecting a

arge number of crops such as potatoes, corn, rice, alfalfa, grapes andther foodstuffs. Carbofuran is directly sprayed onto soil and plantsmmediately after emergence in order to control beetles, nema-odes and rootworms. According to EPA [6], there are considerableisks associated with carbofuran in food and drinking water, riskso pesticide applicators and risks to birds exposed in treated fields.lso, not all products containing carbofuran meet safety standards.his has led EPA to remind growers that carbofuran should not besed on any food crops since it was banned on December 31, 20096].

Carbofuran is moderately persistent in soils [7], where it has aalf-life of 30–117 days [8] depending on the particular agroen-ironmental conditions (e.g. soil organic matter and moisture
ontents, and pH); also, it is highly mobile in soils and easily reachesaters by effect of its high solubility [8]. As a result, carbofuran hashigh potential for contaminating groundwater in aquifers, which
t may enter through leaching and runoff from treated fields. Moni-

∗ Corresponding author. Tel.: +34 988 387070; fax: +34 988 387001.E-mail address: [email protected] (D. Fernández-Calvino).

304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2011.03.021

is more rapidly desorbed than it is adsorbed in soil.© 2011 Elsevier B.V. All rights reserved.

toring pesticide trace levels in waters is important for human healthprotection and environmental control. The European Union has seta maximum allowed concentration of 0.5 �g/L for the combinationof all pesticides, and 0.1 �g/L for individual compounds, in drinkingwater [9].

The fate of pesticides such as carbofuran in soil is influenced bya number of processes [8] specially prominent among which areadsorption and desorption by soil constituents (organic matter andclay, mainly) [10]. Adsorption in soil avoids the presence of pesti-cides in surface and ground waters, but hinders their volatilizationand biodegradation; on the other hand, desorption into the soilsolution facilitates runoff and leaching into water bodies, butmakes the pesticide available to soil microorganisms and facili-tates biodegradation as a result. The risk of carbofuran damages toplants and microorganisms, or its presence at high concentrationsin nearby waters, depends on its concentration in the soil solution,which is in turn a function of the equilibrium governing its adsorp-tion on, and desorption from, soil particles. A better understandingof the underlying mechanisms for this equilibrium should facili-tate the evaluation of the potential adverse effects of carbofuranon agriculture and the environment.

The carbofuran adsorption and desorption capacity of differ-ent types of soils has been the subject of several studies [10–13]which have revealed that the two processes are governed mainly
by the organic matter and clay contents of the soil in additionto its cation exchange capacity. By contrast, the adsorption anddesorption kinetics of carbofuran in soils are less well-known.Adsorption/desorption kinetics can be examined by using varioustechniques [14]. Although many authors have studied carbofuran
dx.doi.org/10.1016/j.jhazmat.2011.03.021

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160 A. Bermúdez-Couso et al. / Journal of Hazar

Table 1Chemical structure and properties of carbofuran.

Common name Carbofuran

Chemical structure

Name 2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-methylcarbamate

CAS n◦ 1563-66-2MWa 221Sb (g L−1) 0.33log Pow

c 1.7Koc

d 209Soil half-lifee 50

Data were obtained from SciFinder® (https://scifinder.cas.org/).a Molecular weight.b Solubility in water at 25 ◦C.c ◦

s[tnmHbwa

ihltdsaogoebWw

2

2

m(sowG(v

after 24 h of contact in all samples.

Octanol/water partition coefficient at 25 C.d Partition coefficient normalized to organic carbon content (mL goc

−1 or L kgoc−1).

e Aerobic soil half-life (Avg, days).

orption kinetics in soils and other materials with batch techniques15–17], batch tests are subject to a number of shortcomings [14]hat are efficiently circumvented by the stirred flow chamber tech-ique [18], which combines the best features of batch and flowethods while avoiding or lessening many of their limitations [19].owever, carbofuran sorption kinetics has seemingly never to dateeen studied with the stirred flow chamber technique. In this work,e used this technique for the first time to investigate carbofuran

dsorption/desorption kinetics.Acid soils are present in large areas of Western Europe where

ntensive agriculture relying on fertilizer and pesticide applicationas been practiced since the second half of the 20th century. This

ed us undertake a study of the carbofuran adsorption and desorp-ion characteristics in acid soils with the aims of (a) providing aetailed description of carbofuran adsorption/desorption on acidoils; (b) relating the sorptive behaviour of carbofuran to soil char-cteristics; (c) examining the adsorption and desorption kineticsf carbofuran in acid soils; and (d) identifying the principal factorsoverning them in acid soils. To this end, we used batch tests inrder to establish the effect of the concentration of carbofuran atquilibrium on its adsorption characteristics and stirred flow cham-er tests to examine the carbofuran adsorption/desorption kinetics.herever possible, the results obtained with the two methodsere compared.

. Materials and methods

.1. Chemicals

Carbofuran (2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-ethylcarbamate) was obtained from Dr. Ehrenstorfer Lab.

Augsburg, Germany) in 99.5% purity. Table 1 shows the chemicaltructure and properties of carbofuran. Ultrapure water wasbtained from a Milli-Ro water purification system. Acetonitrile
as supplied in analytical reagent grade from Merck (Darmstadt,ermany) and all other chemicals were purchased from Panreac
Barcelona, Spain). All aqueous solutions were homogenized byortex agitation.

dous Materials 190 (2011) 159–167

2.2. Soil sampling

Soil sampling was carried out in a former lake area which wasintensively drained for agricultural purposes in the mid-20th cen-tury. Because of its present flat relief, a shallow groundwater tablecan be seen during the rainy season in some areas. For the past 50years, these soils have been intensively managed to grow potatoesand wheat, mainly, under a crop rotation scheme.

A total of 16 soil samples were collected from a soil depth of0–20 cm by using an Edelman probe (Giesbeck, The Netherlands)in sixteen crop stands; care was exercised to span as wide as pos-sible a range of organic matter contents. According to the IUSSWorking Group-WRB 2006 [20], the soils were mostly Phaeozemsand Umbrisols. Phaeozems can be classified as Haplic Phaeozems(Anthric) or Stagnic Phaeozems (Anthric) when they exhibit signsof hydromorphy. When base saturation is less than 50% at any soildepth, soils are classified as Mollic Umbrisols (Anthric) or Stag-nic Umbrisols (Anthric) depending on the presence or absence ofreductive conditions at any time during the year.

After field collection, soil samples were air-dried and sievedthrough a 2 mm mesh, homogenized and stored in polypropylenebottles prior to analysis.

2.3. Soil characteristics

The proportions of sand (2–0.05 mm), silt (0.05–0.002 mm) andclay (<0.002 mm) of the soils were determined by wet sieving forthe size fractions greater than 0.05 mm and with the internationalpipette method for all others [21]. Total carbon content was deter-mined on a ThermoFinnigan 1112 Series NC elemental analyser(Waltham, MA, USA). pH was measured at a soil/water ratio of1:2.5, using a combined glass electrode 10 min after mixing [21].The cation exchange capacity at soil pH (eCEC) was estimated asthe combination of exchangeable base cations extracted by 0.2 MNH4Cl [22] and exchangeable Al extracted by 1 M KCl [23]. Avail-able phosphorus was determined with the Bray II assay [24]. Aloxyhydroxides were quantified by selective extraction with 0.5 MNaOH [25] and Fe oxyhydroxides by dithionite-citrate extraction[26]. The contents in Ca, Mg, Al and Fe were determined by flameatomic absorption spectrometry, and those in Na and K by flameatomic emission spectrometry.

2.4. Carbofuran adsorption and desorption

2.4.1. Batch testsBefore the effect of the concentration of carbofuran on its

adsorption characteristics was studied, four soil samples (no. 2, 5,14 and 15) were used to calculate the time needed to reach theequilibrium. To this end, an amount of soil of 1 g was shaken at dif-ferent times with 10 mL of a 2.3 �M carbofuran solution containing0.005 M CaCl2 as background electrolyte for a variable length oftime (0.5, 1, 4, 8, 16, 24 and 48 h). After incubation, the soil suspen-sions were centrifuged at 2000 rpm and passed through a polyesterfilter of 0.45 �m pore size from Macherey-Nagel (Düren, Germany).Previous tests revealed that these filters adsorb no carbofuran.The resulting supernatant was used to determine the pesticide byHPLC-UV. The amount of soil sorbed carbofuran was calculated bysubtracting that of carbofuran in solution from the amount added.All tests were carried out in triplicate. Fig. 1 shows the amount ofcarbofuran adsorbed as function of time. Equilibrium was alwaysreached after 8–16 h of incubation. Also, adsorption was negligible

Soil adsorption batch tests with carbofuran were performed asfollows: an amount of 1 g of soil was mixed with 10 mL of carbofu-ran aqueous solutions at concentrations ranging from 2 to 18 �M,all containing 0.005 M CaCl2 as background electrolyte. After 24 h of

https://scifinder.cas.org/

A. Bermúdez-Couso et al. / Journal of Hazar

0

1

2

3

4

5

0 10 20 30 40 50

q s(µ

mol

kg-1

)

t (hours)

S 2

0

1

2

3

4

5

0 10 20 30 40 50

q s(µ

mol

kg-1

)

t (hours)

S 5

0

2

4

6

8

10

0 10 20 30 40 50

q s(µ

mol

kg-1

)

S 14

0

1

2

3

4

5

0 10 20 30 40 50

q s(µ

mol

kg-1

)S 15

scsrclwtbt

X

X

w(p(a(Tm

ctiwcbie

2

fusapdt

t (hours) t (hours)

Fig. 1. Carbofuran adsorption kinetics in batch test.

haking (long enough time as previously), the soil suspensions wereentrifuged at 2000 rpm and passed through a filter of 0.45 �m poreize, the resulting supernatant being used to determine carbofu-an by HPLC-UV. The amount of pesticide sorbed by the soil wasalculated by subtracting that present in solution after 24 h of equi-ibrium from the amount initially added. Dissolved organic matter

as determined from the absorbance at 400 nm [27]. Adsorptionests were carried out in triplicate. The limit of quantitation for car-ofuran was 0.04 �mol kg−1. The results were modeled by fittingo the equations of Freundlich (Eq. (1)) and Langmuir (Eq. (2)):

= KFCn (1)

= KLXmC

1 + KLC(2)

here X is the concentration of carbofuran sorbed at equilibrium�mol kg−1); C is the concentration of carbofuran in the aqueoushase equilibrium after 24 h (�M); KF (Ln, kg−1 �mol(1−n)) and ndimensionless) are the Freundlich coefficients; KL (L, �mol−1) isLangmuir constant related to the energy of adsorption and Xm

�mol kg−1) is the maximum adsorption capacity of the sample.he optimum parameter values for these equations were deter-ined by non-linear regression analysis.For the desorption tests, immediately following adsorption of

arbofuran in the soils, the centrifuged residues were weighedo determine the amount of occluded solution and re-suspendedn 10 mL of a 0.005 M CaCl2 solution containing no carbofuran

hich was allowed to equilibrate for 24 h. Then, the samples wereentrifuged at 2000 rpm to remove the supernatant. Occluded car-ofuran was calculated as the difference between the final and

nitial weights, and desorbed carbofuran was then estimated andxpressed in �mol kg−1.

.4.2. Stirred flow chamber (SFC) testsFour soil samples (no. 2, 5, 14 and 15) were used to study carbo-

uran adsorption/desorption in an SFC-based reactor. The reactorsed was one previously employed by the authors [28], albeit with
light modifications. The SFC reactor was made of polypropylenend had an inlet side port at the bottom and a cover with an outletort at the top. Two polytetrafluoroethylene (PTFE) filters 10 mm iniameter and 0.45 �m in pore size were fitted immediately belowhe outlet port and over the inlet port to retain soil samples in the
dous Materials 190 (2011) 159–167 161

chamber. The chamber volume was 1.5 cm3. Both the influent andeffluent were carried through 0.5 mm i.d. PTFE tubing connectedto a Gilson Minipuls 3 peristaltic pump (Gilson S.A.S., Villiers LeBel, France). The temperature was kept at 25 ± 0.1 ◦C by placingthe reactor chamber in a thermostated cabinet. The optimum flowrate (0.2 mL min−1) was monitored throughout and found to oscil-late by <3%. Stirring was provided by a PTFE-coated magnetic bar(3 mm × 1 mm) that was spun at 400 rpm to provide a constantflow. Effluent fractions were collected into 2 mL polypropyleneEppendorf vials by using a Gilson FC 203 G automatic fraction col-lector (Gilson S.A.S., Villiers Le Bel, France).

An amount of about 0.2 g of soil (<2 mm) was placed togetherwith a magnetic stirring bar in the reaction chamber. A 4.5 �Mcarbofuran solution was circulated at the selected flow rate withCaCl2 as background electrolyte (0.005 M). In total, 60 subsamplesof 0.2 mL were collected in different vials (all filled in 1 min each).Immediately after carbofuran sorption, desorption tests were per-formed by circulating the background electrolyte (0.005 M CaCl2).Then, 20 subsamples of 0.2 mL were collected in different vials (allfilled in 1 min each). The subsamples from the adsorption and des-orption runs were used to determine carbofuran by HPLC-UV

2.4.3. Carbofuran determinationLiquid chromatography (LC) analyses were carried out on

a Dionex Corporation liquid chromatograph (Sunnyvale, USA)equipped with a P680 quaternary pump, an ASI-100 autosampler,a TCC-100 thermostated column compartment and a UVD170Udetector. Chromatographic separations were done on a Symme-try C18 column (150 mm long, 4.60 mm i.d., 5 �m particle size)obtained from Waters (Milford, MA, USA) and a C18 guard column(4 mm long, 2 mm i.d., 5 �m particle size) packed with the samematerial. Elution conditions: isocratic elution with 30:70 acetoni-trile/water at room temperature. The flow rate was 1 mL/min−1 andUV detection performed at 278 nm.

2.5. Statistical analysis

Pearson correlation coefficients were determined and a step-wise multiple regression analysis was conducted in order relateadsorption/desorption variables to soil characteristics. Also, apaired two-sample test was used to compare carbofuran desorp-tion at variable added concentrations. The results of these analyseswere considered significant at a probability level of P < 0.05. All sta-tistical processing was done with the Statistical Package for theSocial Sciences (SPSS), version 17.0, for Windows.

3. Results and discussion

3.1. General soil characteristics

The general properties of the studied soils are summarized inTable 2. Most of the samples were loam or sandy loam in texture,and had an organic carbon content in the range 1.4–17%. Also, theywere acid (pH 4.3–5.7) and exhibited a low cation exchange capac-ity (1.92–10.24 cmol(c) kg−1). Their phosphorus contents were highand highly variable (15–703 mg kg−1). Finally, their contents in Aloxyhydroxides ranged from 1.34 to 7.97 mg g−1 and those in Feoxyhydroxides from 1.89 to 9.26 mg g−1.

3.2. Carbofuran adsorption

3.2.1. Batch testsFig. 2 shows the carbofuran adsorption results for the 16 sam-

ples studied in the batch tests. Most of the carbofuran adsorptioncurves were of the L type, with a slope decreasing with increas-

162 A. Bermúdez-Couso et al. / Journal of Hazardous Materials 190 (2011) 159–167

Table 2General soil characteristics.

Sample Sand Silt Clay C pH eCEC P (Bray-II) Aln Fed

% cmol(c) kg−1 mg kg−1 mg g−1 mg g−1

1 48 20 32 4.9 4.3 4.38 282 4.52 2.012 56 22 22 4.3 5.0 4.29 15 3.61 2.183 58 23 19 2.0 5.4 3.17 129 2.75 3.344 77 11 12 1.6 5.4 1.92 119 2.58 2.485 72 15 13 1.4 4.8 2.49 280 1.34 2.266 74 13 13 1.7 5.0 2.35 455 2.50 3.117 67 20 13 1.8 4.9 2.40 278 2.37 3.188 59 25 16 1.6 4.7 3.04 170 2.30 6.219 71 17 12 2.0 4.8 3.26 133 3.18 7.66

10 58 26 16 1.4 4.3 2.84 703 6.05 7.1711 48 30 22 2.7 5.5 6.96 360 2.75 7.8312 58 20 22 2.1 4.9 4.81 387 3.91 6.1313 77 8 15 6.5 5.7 6.18 224 2.50 1.8914 21 35 44 17.1 5.4 10.24 280 7.97 5.2815 42 31 27 4.0 4.9 4.95 331 7.46 9.2616 54 20 26 3.0 5.0 3.35 45 4.73 3.88

0

5

10

15

20

25

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 1

0

5

10

15

20

25

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 2

0

5

10

15

20

25

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 3

0

5

10

15

20

25

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 4

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 5

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 6

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 7

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 8

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 9

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 10

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 11

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 12

0

10

20

30

40

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 13

0

15

30

45

60

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 14

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 15

0

5

10

15

20

0 5 10 15 20

q s(µ

mol

kg-1

)

C (µmol L-1)

S 16

Fig. 2. Carbofuran adsorption equilibria in the 16 soil samples studied. The data shown are the means of triplicate determinations. Coefficients of variation were less than10% in all cases.

A. Bermúdez-Couso et al. / Journal of Hazardous Materials 190 (2011) 159–167 163

Table 3Fitting of the results of the batch tests to the experimental Freundlich equation(mean ± SE).

Sample KF (�mol(1−n) Ln kg−1) n (dimensionless) R2

1 2.9 ± 0.4 0.69 ± 0.06 0.9752 2.5 ± 0.4 0.79 ± 0.07 0.9733 1.7 ± 0.3 0.72 ± 0.07 0.9654 1.9 ± 0.1 0.51 ± 0.03 0.9865 3.0 ± 0.8 0.46 ± 0.12 0.7976 0.9 ± 0.2 0.74 ± 0.08 0.9627 2.9 ± 0.3 0.40 ± 0.04 0.9578 1.4 ± 0.3 0.67 ± 0.09 0.9299 0.8 ± 0.2 0.95 ± 0.08 0.975

10 0.6 ± 0.1 0.92 ± 0.08 0.97511 1.5 ± 0.3 0.74 ± 0.08 0.95712 1.6 ± 0.4 0.70 ± 0.11 0.91413 4.8 ± 1.0 0.74 ± 0.09 0.94114 8.7 ± 1.2 0.71 ± 0.06 0.97015 1.8 ± 0.3 0.67 ± 0.06 0.96516 2.4 ± 0.4 0.54 ± 0.08 0.925

Table 4Significant correlation coefficients between KF in the Freundlich equation and gen-eral soil characteristics.

Soil characteristics r P

Organic carbon (C) 0.927 0.000Sand −0.500 0.049

iaimiactt(Karpswricvopee

K

w

arhiao

0

6

12

18

24

30

36

0 10 20 30 40 50 60

q s(µ

mol

kg-

1 )

t (min)

dt

Clay 0.658 0.006eCEC 0.746 0.001

ng equilibrium concentration of the pesticide. In some cases, thedsorption curve was near-linear and hence of the C type. Judg-ng by its low content (<0.3% in all samples), dissolved organic

atter contributed very little to carbofuran adsorption. The exper-mental data obtained were modeled by fitting to the Langmuirnd Freundlich equations. Fits to the Langmuir equation were dis-arded either because R2 was not statistically significant or becausehe estimated parameters had large associated errors. By contrast,he Freundlich equation provided reasonably good fits in all casesR2 = 0.797–0.975). This equation is an empirical relation whereF can be interpreted as the amount sorbed with C = 1 and n ismeasure of heterogeneity in sorption sites [29]. In this work, n

anged from 0.40 to 0.95 (Table 3) and exceeded 0.5 in most sam-les (mean = 0.68), which is suggestive of low heterogeneity in theorption sites [29]. Constant n exhibited no significant correlationith any of the soil variables measured. On the other hand, KF

anged from 0.6 to 8.7 and was significantly correlated with var-ous soil characteristics (Table 4), but especially with soil organicarbon (r = 0.927, P < 0.05). These results are consistent with pre-iously reported values and confirm the substantial influence ofrganic matter on carbofuran sorption in soil [15,30,31]. A multi-le regression analysis revealed that 96% of the variance in KF wasxplained by the soil characteristics C and Fed through the followingquation:

F = 1.68 + 0.46C − 0.20 Fed

ith R2 = 0.959 and F = 74.1 (P < 0.001).

If an increased value of KF resulted in increased carbofurandsorption, then soil organic matter clearly facilitated carbofu-an adsorption, whereas iron oxides extracted by dithionite-citratead the opposite effect. This suggests that carbofuran adsorption

s influenced by negative charge present in soil colloids. In thesecid soils, organic matter has largely negative charge, whereas ironxides have mainly positive charge.

Fig. 3. Cumulative carbofuran adsorption (qs) as function of time (t) in samples no.2 (triangles), 5 (circles), 14 (diamonds) and 15 (squares).

3.2.2. Stirred flow chamber testsThe results of carbofuran adsorption obtained with the SFC-

based reactor are shown in Fig. 3. The amount of carbofuranretained was always small (7.1–31.9 �mol kg−1). These data arein the same magnitude order than those from the batch tests.This is consistent with the known low capacity of soils to adsorbthis pesticide, which is widely assumed to have a low affinity forsoil components [32,33]. Such a low capacity resulted in very fastadsorption of carbofuran by the soils. Thus, more than 50% wassorbed within 13 min, and 95% of the maximum adsorption reachedwithin 45 min. The carbofuran adsorption kinetics was much fasterin the SFC tests than in the batch tests. Thus, on batch tests at iden-tical starting concentration than in SFC test, 1 h is needed to reach77, 37, 73 and 86% of carbofuran adsorbed at equilibrium in sam-ples no. 2, 5, 14 and 15, respectively, and they need more than 8 hto reach the equlibrium.

Generally, sorption in the SFC-based reactor occurred in twosteps, namely: rapid adsorption during the first few minutes fol-lowed by slow adsorption afterwards [34]. Both types of reactions,fast and slow, could be separated. Fast adsorption reactions are usu-ally diffusion–controlled [35,36]. The relationship among relativeadsorption (qs/qmax) and the square root of time (t1/2) allows one todiscriminate diffusion–controlled processes (i.e., rapid adsorption)from processes controlled by other factors (i.e., slow adsorption).Fig. 4 shows the q/qmax vs. t1/2 curves for the four selected sam-ples. As can be seen, they were largely linear and only bent athigh q/qmax values. The linear portion of the curves corresponds todiffusion–controlled adsorption and their curved portion to slowadsorption reactions. Also, carbofuran adsorption occurred largely(>80%) via fast adsorption reactions.

Krishna and Philip [10] found a highly linear relationshipbetween relative adsorption and the square root of time in carbofu-ran batch sorption tests. However, they only obtained 5 experimen-tal data and were unable to discriminate diffusion–controlled andother processes. This was also the case with our batch tests (Fig. 5).Therefore, SFC tests are advantageous here since they allow sam-ples to be collected at 1 min intervals, which is utterly impossiblewith classical batch techniques. In order to facilitate the analysis ofthe carbofuran retention kinetics, the experimental data were fit-ted to a first-order mathematical model considering one adsorptionsite (Eq. (3)).

dqs = ks(qmax − qs) (3)

where dqs/dt (�mol kg−1 min−1) is the carbofuran adsorption rate,ks a kinetic constant (min−1), qmax (�mol kg−1) the maximum car-bofuran adsorption capacity under the experimental conditions

164 A. Bermúdez-Couso et al. / Journal of Hazardous Materials 190 (2011) 159–167

R² = 0.991

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8

q s/q

max

t1/2

Soil 2

R² = 0.998

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8

q s/q

max

t1/2

Soil 5

R² = 0.998

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8

q s/q

max

t1/2

Soil 14

R² = 0.999

0.0

0.2

0.4

0.6

0.8

1.0

0 2 4 6 8

q s/q

max

t1/2

Soil 15

of t1/2

us

k

Fs

Fig. 4. Fractional cumulative adsorption (qs/qmax) as a function

sed and qs (�mol kg−1) the amount of carbofuran retained by theoil.

Table 5 shows the results of the fitting to this first-orderinetic equation. As can be seen, the maximum adsorption capac-

0.0

0.3

0.6

0.9

1.2

0 2 4 6 8

q s/q

max

t1/2

S 2

0.0

0.3

0.6

0.9

1.2

0 2 4 6 8

q s/q

max

t1/2

S 5

0.0

0.3

0.6

0.9

1.2

0 2 4 6 8

q s/q

max

t1/2

S 14

0.0

0.3

0.6

0.9

1.2

0 2 4 6 8

q s/q

max

t1/2

S 15

ig. 5. Fractional cumulative adsorption (qs/qmax) as a function t1/2 in four of the soilamples used in the batch tests.

in the four soil samples used in the stirred flow chamber tests.

ity (qmax) ranged from 7.3 to 32.8 �mol kg−1; also, it peaked inthe soil with the highest organic carbon and clay contents (sam-ple 14, with 17.1% and 44%, respectively) and was lowest in thesoil with lowest contents in these two components (sample 5,with 1.4% and 13%, respectively). These results were consistentwith those of the batch tests, where Freundlich’s KF was signifi-cantly correlated with soil organic matter and clay (Table 4). Otherauthors previously found the contents in clay and organic matterto play a key role in carbofuran retention by soils [10,30,37,38].However, KF was more markedly influenced by organic matterthan by clay (Table 4). In general, the studied soils had verylow clay contents; therefore, the correlation between clay and KFin the batch tests may have resulted from a close relationshipbetween the clay content and organic carbon in the soils (r = 0.802,P < 0.05).

The kinetic constant ks ranged from 0.047 to 0.195 min−1

(Table 5) and peaked in the soil with the lowest content in organiccarbon. By contrast, the lowest ks value was that for a soil witha medium content of this component (sample 2, with 4.3%). Thisresult shows that the decrease in carbofuran adsorption rate asa function of the amount of pesticide retained is not so clearlydependent on the soil organic carbon content. No relationshipbetween ks and any other soil characteristic was found. On the
other hand, ks was significantly correlated with n (r = 0.965, P < 0.05)in the Freundlich equation. High values of n are indicative ofvery weak forces [39]; therefore, the weaker was the retentionof carbofuran by soil components, the larger was its kinetic con-stant.

A. Bermúdez-Couso et al. / Journal of Hazardous Materials 190 (2011) 159–167 165

Table 5Fitting of the results of the SFC tests to first-order rate equations.

Sample Carbofuran adsorption Carbofuran desorption

qmax (�mol kg−1) ks (min−1) R2 q0 (�mol kg−1) kd (min−1) R2

2 23.0 0.047 0.977 8.0 0.137 0.9875 7.3 0.195 0.950 7.8 0.139 0.952

14 32.8 0.055 0.980 7.1 0.086 0.96715 17.2 0.117 0.977 7.5 0.195 0.983

Table 6Carbofuran desorption (�mol kg−1) as function of the amount previously added and proportion desorbed (in brackets).

Sample Carbofuran added (�mol kg−1)

23 34 45 67 88 134 178

1 nd nd nd nd nd nd nd2 nd nd nd nd nd nd nd3 nd nd 0.4(10) 1.2 (17) 1.2 (15) 1.8 (18) 1.8 (15)4 nd nd 0.8 (20) 2.2 (52) 1.5 (35) 2.2 (41) 3.4 (42)5 nd nd 2.1 (30) 2.6 (31) 4.3 (49) 4.5 (47) 5.0 (49)6 nd nd 0.9 (48) 1.3 (39) 2.1 (38) 3.0 (49) 3.4 (53)7 nd nd 1.6 (30) 2.0 (32) 4.3 (58) 5.4 (71) 6.4 (69)8 nd nd 0.7 (16) 0.8 (17) 0.8 (13) 0.9 (12) 1.2 (12)9 nd nd 1.2 (41) 1.6 (35) 2.2 (39) 2.8 (27) 3.1 (26)

10 nd nd 0.7 (24) 0.8 (28) 0.9 (26) 2.0 (32) 2.4 (31)11 nd nd 0.6 (14) 0.9 (18) 1.0 (18) 1.7 (16) 1.7 (14)12 nd nd 0.9 (18) 1.2 (21) 1.5 (18) 2.1 (23) 2.6 (23)13 nd 0.1 (0) 0.3 (3) 1.3 (8) 1.9 (11) 3.1 (13) 3.0 (8)14 1.1 (13) 1.2 (9) 1.3 (7) 1.6 (7) 1.8 (5) 1.9 (5) 2.9 (6)

n

3

3

EcptbecaelaTsttptad

TPp

A

(Fig. 8); therefore, this variable is a key to the irreversibility of car-bofuran adsorption in soil. As can be seen from Fig. 8, carbofurandesorption increased rapidly at an organic carbon content below4%. One plausible explanation for this result is that clay plays a more

15 nd nd nd16 nd nd 0.5 (13)

d: not detected. Limit of quantitation = 0.04 �mol kg−1.

.3. Carbofuran desorption

.3.1. Batch testsThe results of the desorption batch tests are shown in Table 6.

xpressing the results in �mol kg−1 revealed a marked increase inarbofuran desorption with increase in the amount of carbofuranreviously added to the soil (paired t-test, Table 7). This suggestshat carbofuran is sorbed to less specific sites and with weakeronds when its proportion is close to that of saturation. How-ver, expressing the results as a percentage of previously adsorbedarbofuran exposed no significant differences as a function of themount of pesticide previously added (paired t-test, Table 7). Inter-stingly, no carbofuran desorption was detected (quantificationimit <0.04 �mol kg−1) at low carbofuran doses or in some samplest high carbofuran concentrations (no. 1, 2 and 15; see Table 6).hese results are quite surprising as they relate to none of theoil variables studied. If these three samples are excluded andhe desorption data for the highest carbofuran dose used, thenhe amount of desorbed carbofuran is a potential function of that
reviously adsorbed (Fig. 6). This suggests that carbofuran desorp-ion decreases with increasing sorption capacity of the soil. Above
certain adsorption threshold (about 10 �mol kg−1), desorptionepends on other variables as well. Thus, the percent desorption

able 7aired t-test for desorption batch test expressing the results in �mol kg−1 and as aercentage of previously adsorbed carbofuran.

ACC Carbofuran desorption

(�mol kg−1) (%)

t Sig t Sig45 vs. 67 −4.702 0.001 −1.200 .25367 vs. 88 −2.210 0.047 −0.582 .57188 vs. 134 −6.207 0.000 −1.399 .187134 vs. 178 −3.760 0.003 0.953 .359

CC: added carbofuran concentrations (�mol L−1).

nd nd nd nd0.8 (11) 1.0 (13) 1.4 (16) 1.5 (14)

at the highest carbofuran dose used (an addition of 178 �mol kg−1)was negatively correlated with eCEC (r = −0.732, P = 0.004, n = 13)and the clay content (r = −0.569, P = 0.042, n = 13). On the otherhand, there was no linear relationship between Freundlich’s KFand the amount of carbofuran desorbed at the different concentra-tions used in the tests; consequently, carbofuran is adsorbed anddesorbed in soil via two different mechanisms.

3.3.2. Stirred flow chamber testsCarbofuran desorption in the SFC tests immediately following

carbofuran adsorption (Fig. 7), ranged from 5.1 to 7.0 �mol kg−1

(16–92% of retained carbofuran). The percent carbofuran desorp-tion was negatively correlated with the soil organic carbon content

y = 80.1x-0.61

R² = 0.616

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70

Adso

rbed

Car

bofu

ran

(µm

ol k

g-1)

Desorption (%)

Fig. 6. Relationship between the amount of carbofuran sorbed and the proportiondesorbed from the most concentrated solution (178 �mol kg−1).

166 A. Bermúdez-Couso et al. / Journal of Hazar

0.0

2.0

4.0

6.0

8.0

0 4 8 12 16 20

q d(µ

mol

kg-

1 )

F(

polcbt(covrmwgcm

wrmef

ai0c

Fo

t (min)

ig. 7. Cumulative carbofuran desorption (qd) as function of time in samples no. 2triangles), 5 (circles), 14 (diamonds) and 15 (squares).

rominent role in carbofuran adsorption in soils containing littlerganic carbon. Wahid and Sethunathan [40] found organic matterevels above 2% to favour the adsorption of pesticides onto organicomponents and lower levels to result in preferential adsorptiony clays and inorganic colloids. Similarly, Weber et al. [41] foundhe mobility of pesticides in soils with low organic matter contents<5%) to be largely governed by the inorganic fractions (particularlylay). Organic matter in our soils, which contained little clay, notnly increased carbofuran adsorption, but also formed more irre-ersible bonds, especially at organic carbon levels above 4%. Theseesults are consistent with those obtained in the batch tests. Sinceost of studied soils had organic carbon contents lower than 5%,e can reasonably assume that carbofuran desorption is mainly

overned by eCEC and the clay content. The data obtained for thearbofuran desorption kinetics were fitted to the first-order kineticodel (Eq. (4)).

dqd

dt= kd(q0 − qd) (4)

here dqd /dt (�mol kg−1 min−1) is the desorption rate of carbofu-an, kd its desorption kinetic constant (min−1), q0 (�mol kg−1) theaximum amount of carbofuran that can be desorbed under the

xperimental conditions used, and qd (�mol kg−1) that desorbedrom the soil.

−1
Parameter q0 ranged from 7.1 to 8.0 �mol kg (Table 5) andccounted for 21 and 100%, respectively, of the soil retention capac-ty (qmax). The desorption kinetic constant, kd, ranged from 0.086 to.195 min−1 and was in general higher than the adsorption kineticonstant (ks). Therefore, carbofuran was more rapidly desorbed
y = 108.56x-0.699

R² = 0.970

0

20

40

60

80

100

0 4 8 12 16 20

Des

orbe

d C

arbo

fura

n (%

)

C (%)

ig. 8. Relationship between the amount of soil organic carbon and the proportionf desorbed carbofuran in the stirred flow chamber tests.

[

[

[

[

[

[

[

[

dous Materials 190 (2011) 159–167

than it is adsorbed by the soil (twice on average, except in soil 5,where adsorption and desorption occurred at a similar rate). Thisis consistent with previous observations of Achik et al. [15].

4. Conclusion

The Freundlich KF values obtained under equilibrium conditionsin the batch tests afford the same conclusion as the qmax values pro-vided by the stirred flow chamber tests: that carbofuran adsorptionis influenced by soil organic carbon and clay. However, as shownin this work, the SFC-based method is clearly superior for studyingthe carbofuran sorption kinetics. In fact, SFC tests allow data to beacquired on a smaller time scale and a considerably greater numberof samples. The SFC tests revealed that carbofuran was adsorbedlargely (>80%) via fast reactions. The irreversibility of carbofuranretention depends strongly on the organic carbon content of thesoil. Thus, the percent carbofuran desorption was relatively low insoils with a proportion of organic carbon higher than 4%. Below thisthreshold, carbofuran desorption increased rapidly up to 100% ofpreviously adsorbed carbofuran. Also, constant n in the Freundlichequation was correlated with the adsorption kinetic constant (ks)as determined from the results of the SFC tests.

Acknowledgement

This work was funded by the INCITE programme of the GalicianCouncil of Innovation and Industry (Ref. 08PXIB383190PR). DavidFernández Calvino was additionally awarded an Ánxeles Alvarinocontract from the Galician Council of Innovation and Industry.

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145

Anexo II. Adsorption and desorption behavior of

metalaxyl in intensively cultivated acid soils

Published: May 26, 2011

r 2011 American Chemical Society 7286 dx.doi.org/10.1021/jf201028q | J. Agric. Food Chem. 2011, 59, 7286–7293

ARTICLE

pubs.acs.org/JAFC

Adsorption and Desorption Behavior of Metalaxyl in IntensivelyCultivated Acid SoilsAlipio Berm�udez-Couso,* David Fern�andez-Calvi~no, Miriam Pateiro-Moure, Beatriz Garrido-Rodríguez,Juan Carlos N�ovoa-Mu~noz, and Manuel Arias Est�evez

�Area de Edafoloxía e Química Agrícola, Departamento de Bioloxía Vexetal e Ciencia do Solo, Universidade de Vigo,Facultade de Ciencias, 32004 Ourense, Spain

bS Supporting Information

ABSTRACT: Metalaxyl adsorption and desorption behavior in acid soils were evaluated via batch and stirred-flow chamberexperiments. On the basis of batch experiments (adsorption curves of the Giles C-type), metalaxyl has a low affinity for acid soils.Also, as derived from batch and stirred-flow chamber tests, its adsorption in acid soils is dictated mainly by their organic matter andclay contents. The high correlation between these two variables makes it rather complicated to resolve their effects. Metalaxyladsorption occurs largely (80�99%) via fast adsorption reactions. On the other hand, the pesticide is desorbed in variableproportions (30�100%). The desorption parameters obtained by fitting the results to a pseudo-first-order reaction were correlatedwith no edaphic variable; however, the q0/qmax ratio, which is a measure of reversibility in the adsorption�desorption process,exhibited significant negative correlation with the organic matter and clay contents.

KEYWORDS: metalaxyl, adsorption/desorption, kinetics, soil

’ INTRODUCTION

Agricultural productivity has increased markedly as a result ofthe development of new, high-yield cultivars, advances in ma-chinery technology, and the widespread use of pesticides ingeneral and herbicides and fungicides in particular.1 Thesepesticides have provided undeniable benefits by effect of theirdramatically reducing the adverse effects of plant diseases andpests on agricultural productivity. However, the excessive use ofsome pesticides has caused serious problems including environ-mental deterioration. In fact, the continual use of these pollutingproducts has raised their concentrations in soils2 to levelsendangering sustainability in some agricultural systems.

Once they reach soil, pesticides undergo a series of mutuallyrelated physical, chemical, and biological processes. Although thesignificance of these processes depends on the chemical nature ofthe pesticide and properties of the soil, degradation and adsorp-tion�desorption processes are usually the most consequential.3

In fact, adsorption�desorption processes govern the mobility ofpesticides and influence their transport, persistence, transforma-tions, and bioaccumulation in soil.4 For example, fairly irrever-sible adsorption of pesticides to soil colloids renders them lessaccessible to soil microorganisms, thereby reducing their persis-tence and restricting their transport in soil. Also, easy desorptionof a pesticide from soil particles facilitates its leaching and drain-age to subsurface or even surface waters.

Metalaxyl [methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate] is a major fungicide of systemic residual action5 andmoderate toxicity.6 This pesticide is widely used to fight andprevent fungal diseases in a variety of crops worldwide. Metalaxylis frequently applied directly to the aerial portion of plants, whichinevitably causes part of the product to fall on the ground duringspraying or to be swept off the plants by rain. The pesticide canalso reach soil from plant debris remaining on soil after harvest or

from plant vestiges detached during treatment.7 As a result,metalaxyl adsorption is favored by a high content of organicmatter;8,9 however, some soils containing large amounts of dis-solved organic matter exhibit moderate to low adsorption ofthe pesticide by effect of its competition with organic matterfor adsorption sites.10,11 Some authors have found metalaxyladsorption to be favored by clay12 and a low pH in the soilsolution containing the pesticide to increase its adsorption in soilparticles.13

Because the adsorption and desorption of contaminants in soilare not instantaneous, accurately studying these processes inmetalaxyl required examining their kinetics for this pesticide.Most studies on metalaxyl adsorption kinetics have relied onbatch8,14 or soil column tests.9 These techniques, however, aresubject to some limitations such as the sluggishness of batch testsor the adverse effects of film diffusion in soil column tests. Theseshortcomings can be circumvented by using the stirred-flowchamber technique,15 which, however, has never previously beenused to examine metalaxyl adsorption kinetics. In the stirred-flowtechnique, the sorbate is continuously circulated through areaction chamber to avoid the potential problems resultingfrom the decrease in concentration in the liquid phase typicallyobserved in batch processes; also, desorption of the pesticide isaccelerated by removing the portion released into the liquidphase. This technique also avoids film diffusion caused bycontinuous agitation in column tests.16

The primary purpose of this work was to examine metalaxyladsorption and desorption in acid soils. To this end, we used both

Received: March 14, 2011Accepted: May 26, 2011Revised: May 25, 2011

7287 dx.doi.org/10.1021/jf201028q |J. Agric. Food Chem. 2011, 59, 7286–7293

Journal of Agricultural and Food Chemistry ARTICLE

batch and stirred-flow chamber tests to (a) obtain a detailedpicture of metalaxyl adsorption/desorption in acid soils; (b)relate metalaxyl adsorption/desorption behavior to soil charac-teristics; (c) elucidate the adsorption/desorption kinetics of thepesticide; and (d) identify the principal factors responsible for itsadsorption and desorption kinetics.

’MATERIALS AND METHODS

Chemicals. Metalaxyl [methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate] was obtained in purity higher than99.5% from Sigma�Aldrich (Steinheim, Germany). Table 1shows its chemical structure and selected properties. Allorganic solvents used for sample preparation were residueanalysis grade and supplied by Panreac (Barcelona, Spain).HPLC grade solvents for HPLC work were also supplied byPanreac. A stock standard solution containing a 1 mg 3mL�1

concentration of metalaxyl in methanol was used to makeworking-strength solutions by appropriate dilution as re-quired. All solutions were stored in amber glass flasks underrefrigeration in order to prevent degradation.Soil Samples. The study area is located in “A Limia” (Galicia,

northwest Spain). In this area, in the last 50 years, the soils wereintensively managed, with potatoes and wheat being the mainproducts cultivated in crop rotation. Sixteen samples obtainedfrom the topmost layer (0�20 cm) of acid soils were collected.Once in the laboratory, the samples were air-dried, sievedthrough a 2 mm sieve, and stored in polyethylene bottles untilanalysis. The particle-size distribution was determined in the <2mm fraction by the internationally recognized pipet method.Mineralogical clay composition was determined by X-ray diffrac-tion with the use of Cu KR radiation with a Philips PM8203diffractometer (Philips, Wavre, Belgium). The soil pH wasmeasured in water (ratio of soil:solution 1:2.5); the total nitrogen(NT) was determined in finely milled samples in an agate mortar(Retsch RM100; Retsch GmbH, Hann, Germany), with a soilanalyzer (Thermo Finnigan 1112 Series NC). The effectivecation-exchange capacity (eCEC) was estimated as the sum ofbase cations (Na, K, Ca, and Mg) displaced with 1 M NH4Ac at

pH 7 and the Al extracted with 1 M KCl. The total soil organiccarbon (CT) was determined by dichromate oxidation. The soilorganic fractions were extracted according to conventionalprocedures based on changes in pH and determined by dichro-mate oxidation.The soils were mainly loam or sandy loam in texture. Clay

mineralogy is similar to those reported from soils developed fromgranitic materials; thus, 1:1 dioctahedral phyllosilicates such askaolinite and halloysite dominated the mineralogical composi-tion of clay fraction, whereas vermiculites (mainly interlayeredaluminum hydroxide) are also frequent. Traces of gibbsite arealso recorded in the soil samples with lowest carbon content.Total organic carbon (CT) ranged from 1.1% to 16.6%. Humicsubstances (fulvic and humic acids) represents on average 62% oftotal organic carbon (range 38�93%). All samples were acidic(pH 4.3�5.7) and possessed a low cation-exchange capacity[1.92�10.24 cmol(c) 3 kg

�1]. Their physicochemical propertiesare summarized in Table 2.Metalaxyl Adsorption/Desorption As Measured in Batch

Tests. Metalaxyl adsorption in the soils was measured via batchexperiments as follows: an amount of 1 g of soil was mixed with10 mL of aqueous solutions containing variable metalaxyl con-centrations from 3 to 100 μmol 3 L

�1 in addition to 0.005 MCaCl2 as background electrolyte. After 24 h of shaking, the soilsuspensions obtained were centrifuged at 700g (15 min) andpassed through a filter of 0.45 μm pore size, the supernatantbeing used to determine the pesticide by HPLC with UVdetection.Experimental data were modeled by fitting to the Freundlich

equation

X ¼ KFCn ð1Þ

and Langmuir equation

X ¼ KLXmC1þ KLC

ð2Þ

where X (qs) is the amount of metalaxyl retained by weight(micromoles per kilogram), C is its concentration in solutionafter 24 h of equilibration (micromoles per liter), KF and n arecoefficients of the Freundlich equation, KL (liters per micro-mole) is a constant dependent on the energy of adsorption,and Xm (micromoles per kilogram) is the maximum adsorptioncapacity of each sample.In the desorption tests, adsorption of metalaxyl by the soil

samples was immediately followed by weighing of the centri-fuged residues to determine the amount of occluded solution.This was followed by resuspension in 10 mL of a 0.005 M CaCl2solution containing no metalaxyl and equilibration for 24 h.Finally, the samples were centrifuged at 700g (15 min) and thesupernatant was used to determine the pesticide by HPLC-UV.Metalaxyl Adsorption/DesorptionAsMeasured in Stirred-

Flow Chamber Tests.Nine soil samples (1, 2, 4, 5, 8, 10, 13, 14,and 15) were used for metalaxyl adsorption/desorption testsin an SFC-based reactor. The 1.5 cm3 propylene stirred-flowreactor used (Figure 1) was a slightly modified version of onepreviously reported by L�opez-Periago et al.16 The reactor had aside inlet at the bottom and an outlet in the lid, the two beingconnected to a Minipuls 3 peristaltic pump from Gilson SAS(Villiers le Bel, France) by 0.5 mm i.d. poly(tetrafluoroethylene)(PTFE) tubing and covered with PTFE filters 10mm in diameterand 0.45 μm in pore size to retain the samples in the reactor.

Table 1. Structure and Chemical Properties of Metalaxyla

aData from Tomlin37 and Kamrin.38. Key: (a) pKa at 25 �C. (b)Molecular weight. (c) Solubility in water at 22 �C. (d) Octanol/waterpartition coefficient at 25 �C. (e) Partition coefficient normalizedto organic carbon content (mL 3 goc

�1 or L 3 kgoc�1. (f) Aerobic soil

half-life (days).

http://pubs.acs.org/action/showImage?doi=10.1021/jf201028q&iName=master.img-000.png&w=193&h=147



Stirring was provided by a PTFE-coated magnetic bar spun at400 rpm, and output solution fractions were collected in 2 mLpolypropylene Eppendorf vials by using a Gilson FC203Gautomatic fraction collector at a rate of 1 vial/min. Tests wereperformed in a cabinet thermostated at 25.0 ( 0.1 �C. The flowrate (0.3 mL 3min

�1) was monitored throughout and found tooscillate by less than (3%.An amount of ca. 0.2 g of soil (<2 mm) was placed together

with a magnetic stirring bar in the reaction chamber and a 7.2 μMmetalaxyl solution was then circulated at the selected flow ratewith 0.005 M CaCl2 as background electrolyte. In total, 60subsamples of 0.2 mL each were collected in different vials thatwere filled for 1 min each. Immediately after metalaxyl wasadsorbed, desorption tests were started by circulating the back-ground electrolyte (0.005 M CaCl2). The pesticide was deter-mined by HPLC-UV in all subsamples from both the adsorptionand desorption runs.Metalaxyl Determination. High-performance liquid chro-

matography (HPLC) analyses were carried out on a liquid

chromatograph from Dionex Corporation (Sunnywale, CA)equipped with a P680 quaternary pump, an ASI-100 autosam-pler, a TCC-100 thermostated column compartment, and aUVD170U detector connected to a PC computer running thesoftware Chromeleon, also from Dionex Corporation. Chroma-tographic separations were done on a Symmetry C18 column(150 mm long � 4.60 mm i.d., 5 μm particle size) from Waters(Milford, MA) and a guard column (50 mm long� 4.60 mm i.d.,5 μm particle size) packed with the same material. The mobilephases were methanol (A) and water (B), and the gradientprogram was as follows: 60% A þ 40% B for 7 min, changeto 95%Aþ 5% B in 3min, hold 5min, change to 60%Aþ 40% Bin 0.1 min, and hold 10 min. The total analysis time was 25 min.The injected volume was set to 50 μL and the flow rate to 0.7 mLmin�1. Metalaxyl was detected at λ = 200 nm.

’RESULTS AND DISCUSSION

Metalaxyl Adsorption. Prior to adsorption tests, two selectedsamples with low (sample 2) and high (sample 14) carboncontent were subjected to a batch kinetic test. As can be seen

Table 2. General Soil Characteristicsa

sand, % silt, % clay, % CT, % NT, % CHA, % CFA, % CHU, % pH eCEC, cmol(c) 3 kg�1

1 48 20 32 5.21 0.48 1.74 0.59 2.88 4.3 4.38

2 56 22 22 3.49 0.26 1.75 0.43 1.31 5.0 4.29

3 58 23 19 1.59 0.15 0.68 0.22 0.69 5.4 3.17

4 77 11 12 1.28 0.01 0.67 0.11 0.50 5.4 1.92

5 72 15 13 1.07 0.13 0.72 0.06 0.29 4.8 2.49

6 74 13 13 2.07 0.12 0.62 0.16 1.29 5.0 2.35

7 67 20 13 1.44 0.13 0.58 0.26 0.60 4.9 2.40

8 59 25 16 1.53 0.13 0.82 0.32 0.39 4.7 3.04

9 71 17 12 1.38 0.17 0.78 0.48 0.12 4.8 3.26

10 58 26 16 1.44 0.13 0.64 0.08 0.72 4.3 2.84

11 48 30 22 2.98 0.23 2.44 0.33 0.20 5.5 6.96

12 58 20 22 2.55 0.27 1.14 0.43 0.97 4.9 4.81

13 77 8 15 5.78 0.48 2.68 0.40 2.70 5.7 6.18

14 21 35 44 16.61 1.18 6.82 0.98 8.81 5.4 10.24

15 42 31 27 3.87 0.31 1.39 0.71 1.77 4.9 4.95

16 54 20 26 3.00 0.28 1.65 0.38 0.97 5.0 3.35aCT andNT, total organic carbon and total nitrogen; CHA, carbon of humic acids; CFA, carbon of fulvic acids; CHU, humin carbon; eCEC, effective cationexchange capacity.

Figure 1. Schematic depiction of the stirred-flow reactor.

Figure 2. Kinetic adsorption curves obtained in batch tests on selectedsamples: (b) sample 2; (2) sample 14.





from Figure 2, equilibrium was reached within 24 h of metalax-yl�soil contact. This led us to use an equilibration time of 24 h inall batch adsorption/desorption runs.Overall, metalaxyl adsorption was low, which is consistent with

its high mobility.17,18 Plotting the amount of pesticide adsorbed(qs, micromoles of metalaxyl per kilogram of soil) against itsequilibrium concentration (C, micromoles per liter) gave near-linear (i.e., C-type, according to Giles et al.19) curves (Figure 3).This suggests that soil particles had little affinity for metalaxyl. Alladsorption curves except those for two samples fitted a linearequation with R2 > 0.900 (Table 3). This reflected constantpartitioning between adsorbing surfaces and the soil solution andallowed the coefficient of distribution, Kd, to be calculated fromthe slopes of the curves. TheKd values thus obtained ranged from0.03 to 3.08 L 3 kg

�1 and averaged 0.91 L 3 kg�1; also, they were

significantly correlated with total soil organic matter (CT andNT), organic matter fractions (CHA, CFA, and CHU) and eCEC(p < 0.01), as well as with sand and clay contents (p < 0.05)(Table S1, Supporting Information).Overall, the adsorption curves fitted the Freundlich equation

better (R2 = 0.865�0.998 versus 0.682�0.995 for the linear fit)than the Langmuir equation, which exhibited nonsignificant R2

values or large errors in the calculated parameters (Table 3).Freundlich fitting parameters are shown in Table 3. Thisequation, which is applicable to soils with nonuniform surfaceproperties,20 is an empirical function whereKF can be interpretedas the amount of sorbate adsorbed at C = 1 and n is a measure ofheterogeneity in adsorption sites. Surface heterogeneity in-creased as n approached 0 and decreases at n approached 1.Constant KF, which ranged from 0.2 to 6.3, was significantly

correlated with soil organic matter, with r = 0.652 for theCT content and r = 0.668 for the NT content. The highestcorrelation coefficient (r = 0.710) was found with CHU fraction.

Figure 3. Metalaxyl adsorption equilibria in the 16 studied soils as determined in the batch tests.

Table 3. Fitting of Adsorption Results to the FreundlichEquation and a Straight Linea

Straight line Freundlich equation

Kd R2 KF n R2

1 0.69( 0.09 0.914 4.7( 1.5 0.60( 0.08 0.963

2 2.24( 0.16 0.975 1.0( 0.5 1.18( 0.13 0.980

3 0.35( 0.05 0.927 4.1( 0.8 0.50( 0.05 0.974

4 0.15( 0.01 0.961 1.2( 0.2 0.57( 0.03 0.992

5 0.26( 0.02 0.981 1.9( 0.2 0.59( 0.03 0.995

6 0.03( 0.01 0.682 3.5( 0.5 0.18( 0.04 0.865

7 0.08( 0.02 0.756 2.1( 0.6 0.37( 0.08 0.895

8 0.38( 0.02 0.987 1.3( 0.4 0.75( 0.08 0.974

9 1.26( 0.08 0.978 0.6( 0.3 1.17( 0.11 0.984

10 1.29( 0.06 0.991 0.2( 0.0 1.42( 0.05 0.998

11 1.18( 0.04 0.995 1.0( 0.3 1.04( 0.07 0.993

12 0.19( 0.01 0.983 4.1( 1.0 0.40( 0.06 0.936

13 1.73( 0.08 0.991 2.8( 0.8 0.90( 0.07 0.988

14 3.08( 0.12 0.994 6.3( 1.8 0.85( 0.07 0.988

15 1.15( 0.05 0.993 1.1( 0.3 1.01( 0.07 0.992

16 0.52( 0.03 0.987 0.7( 0.2 0.92( 0.08 0.987aAmount adsorbed in micromoles per kilogram; equilibrium concentra-tion in micromoles per liter.




The constant was also significantly correlated with eCEC (r =0.498) and clay (r = 0.561) (Table S1, Supporting Information).These results are very similar to those for Kd, which suggests thatmetalaxyl adsorption at low concentrations (KF) occurs viasimilar mechanisms to those prevailing at low to medium con-centrations of the pesticide (Kd). On the other hand, n variedover a wide range (0.18�1.42) and was not significantly corre-lated with the edaphic variables.Figure 4 shows the results of the metalaxyl adsorption tests in

the stirred-flow chamber. The amount of metalaxyl retainedranged from 2.5 to 56.5 μmol kg�1. The curves of Figure 4 werefitted to a mathematical model considering a single adsorptionsite and based on the pseudo-first-order equation of Aharoni andSparks:21 dqs/dt = ks(qmax � qs), where dqs/dt (micromoles perkilogram per minute) is the metalaxyl adsorption rate, ks (perminute) is a kinetic constant, qmax (micromoles per kilogram) isthe maximum metalaxyl adsorption capacity of the soil underexperimental conditions, and qs (micromoles per kilogram) is theamount of pesticide retained by the soil. This model is among themost widely used on account of its simplicity and the fact thatmany reaction kinetics are similarly described by variousmethods.22 The experimental data fitted the pseudo-first-orderequation very closely, with R2 > 0.93 in all cases (Table 4). Themaximum adsorption capacity of the soil, qmax, ranged from 2.6 to60.4 μmol 3 kg

�1; its greatest value was for sample 14, which hadthe highest organic carbon content (16.6%), and the smallest wasfor sample 4, which had one of the lowest CT contents and thelowest clay (12%). qmax was significantly correlated with soiltexture (sand and clay), organic matter (CT and NT), organicmatter fractions (CHA, CFA, and CHU), and eCEC (Table S2,Supporting Information), which were the same variables influen-cing Kd and KF in the batch tests. This confirms the importance

of these variables to the adsorption of metalaxyl by soil. Theresults are consistent with those of other authors who foundmetalaxyl adsorption in soil to be especially marked in soilscontaining abundant organic matter and clay.8,9,23 The highcorrelation between qmax and eCEC may have resulted fromthese variables being strongly dependent on clay and organicmatter; in fact, eCEC was positively correlated with the contentsin both organic CT (r = 0.877, p < 0.05) and clay (r = 0.825,p < 0.05).The high organic matter content of the soil may have adversely

affected its ability to adsorb the pesticide since high contentsof organic matter in solution can reduce the soil retention capa-city through competition of dissolved organic matter for ad-sorption sites11 or the formation of easily leached complexeswith metalaxyl in solution.24�27 However, these mechanisms areunlikely in acid soils, where organic matter is very scarcelydissolved.28 In fact, absorbancemeasurements at 400 nm showedall 16 samples to contain very little organic matter (less than0.3% of total OM) in the soil solution.Resolving the effect of organic matter and clay is very

difficult23 since the two are usually closely associated in soilaggregates. In fact, the organic matter and clay contents wereclosely related in our samples (n = 16, r = 0.834, p < 0.05). As arule, soil organic matter is highly influential on pesticide reten-tion (particularly with low-water-soluble pesticides such aspenconazole). By contrast, organic matter has little effect onhigh-water-soluble pesticides such as metalaxyl.29 Marín-Benitoet al.23 found metalaxyl adsorption to be slightly increased byorganic amendments and little or no adsorption to occur underthe very low hydrophobicity observed in the presence of thispesticide. As a rule, humic acids, which constitute the majorfraction of soil organic matter, have a higher metalaxyl adsorptioncapacity than most soil minerals (montmorillonite excluded).Thus, Andrades et al.8 found the following sequence of adsorp-tion capacity for this pesticide: montmorillonite > humic acids≈illite > vermiculite . kaolinite. Because the clay fraction of thesoils was dominated by low-activity clay materials such ashalloysite and kaolinite, differences in adsorption capacity be-tween samples were probably a result of differences in organicmatter content. This hypothesis is supported by the previousfinding of Pose-Juan et al.30 that metalaxyl adsorption on purekaolinite was 3 times lower than it was on sandy loam soils withan organic C content between 3% and 4%. Also, adsorption hasbeen found to be dictated by inorganic compounds in soils with alow organic matter content and by organic matter in those with ahigh organic matter content18 such as those studied here. Other

Figure 4. Cumulative metalaxyl adsorption (qs) as function of time in the nine soil samples used in the stirred-flow chamber tests.

Table 4. Fitting of Kinetic Adsorption Data for Metalaxyl

sample qmax, μmol 3 kg�1 ks, min

�1 R2

1 26.4( 0.7 0.152 ( 0.012 0.961

2 40.4( 0.6 0.112( 0.004 0.960

4 2.6( 0.3 0.609( 0.217 0.941

5 5.9( 0.3 0.365( 0.045 0.953

8 8.0( 0.2 0.319( 0.024 0.980

10 33.6( 0.3 0.154( 0.004 0.988

13 34.7( 0.4 0.152( 0.005 0.979

14 60.4( 1.0 0.057( 0.002 0.940

15 47.1( 1.4 0.114( 0.007 0.938




studies involving the addition of various surfactants to clayshowed the presence of a cationic surfactant to strongly increasethe sorption capacity of the soil31 and metalaxyl adsorption to berelated to organic C in organoclays.32

The kinetic constant, ks, ranged from 0.057 to 0.609 min�1

(Table 4) and was greatest for sample 4 and smallest for sample14. This suggests that the decrease in metalaxyl adsorption rate isa function of the soil adsorption capacity for the pesticide. In fact,qmax and ks were significantly correlated (r = 0.878, p < 0.05) andthe metalaxyl adsorption rate was inversely proportional to thesoil adsorption capacity. On the other hand, ks was significantlyand negatively correlated only with eCEC and CFA (Table S2,Supporting Information). The relatively high values of ks (0.226min�1 on average, Table 4) reflected in very fast adsorption ofmetalaxyl. Thus, more than 50% of the amount of pesticideadsorbed throughout the experiment (5 min on average) wasadsorbed within 12 min; also, more than 95% of the maximumadsorption (15 min on average) was reached within 38 min.Fast adsorption processes are usually diffusion-controlled.33 Thelinear relationship observed between the metalaxyl relativeadsorption (q/qmax) and the square root of time (t1/2) is in facttypical of diffusion-controlled processes. By way of example,Figure 5 shows two q/qmax versus t

1/2 plots, based on which80�99% of all metalaxyl was adsorbed via fast reactions.Metalaxyl Desorption. The results of the batch metalaxyl

desorption tests are shown in Table 5. The amount of metal-axyl adsorbed ranged from not detected (<0.05 μmol kg�1) to74.9 μmol 3 kg

�1 and was dependent on the particular soil andamount of metalaxyl added. Thus, the amount of pesticidedesorbed increased with increasing amount added, which sug-gests that metalaxyl tended to bind increasingly, and via increas-ingly weaker bonds, to nonspecific adsorption sites as saturationwas approached. Also, metalaxyl adsorption at low rates fellbelow the limit of quantitation (0.05 μmol 3 kg

�1) in nine of the16 samples (Table 5). The amount of metalaxyl desorbed, inmicromoles per kilogram, was significantly correlated with thatpreviously adsorbed (r = 0.951, n = 99). Expressed as a per-centage of metalaxyl previously adsorbed, desorption averaged20% ( 12% and was unrelated to the soil characteristics.Figure 6 shows the metalaxyl desorption results obtained in

the stirred-flow chamber tests as the amounts of metalaxylremaining in the soil following adsorption of the pesticide. Ascan be seen, metalaxyl desorption was very fast. Thus, after 3min,

82% (sample 14) to only 1% (sample 5) remained in the soil, thenine samples averaging at 44%. These proportions are higherthan those obtained in the batch tests, possibly because desorp-tion in the stirred-flow chamber was accelerated by the contin-uous removal of desorbed metalaxyl, whereas desorption of thepesticide in the batch tests was limited by the amount present insolution.The metalaxyl desorption results fitted the first-order kinetic

equation dqd/dt = kd(q0 � qd), where dqd/dt (micromolesper kilogram per minute) is the metalaxyl desorption rate, kd(per minute) is the kinetic desorption constant, q0 (micromolesper kilogram) is the amount of pesticide that can be desorbedunder the experimental conditions, and qd (micromoles perkilogram) is the amount that desorbed in practice. The estimatedq0 values ranged from 2.6 to 37.9 μmol 3 kg

�1 (Table 6), whichamounted to 30�100% (78% on average) of themaximummeta-laxyl retention capacity, qmax.These desorption results are consistent with those of Monkiedje

and Spiteller,34 who concluded that soil�metalaxyl interactionswere weak and the pesticide adsorption in the soil was highlyreversible as a result. The high desorption of metalaxyl was aconsequence of its low hydrophobicity. Thus, Marín-Benitoet al.26 found this highly water-soluble pesticide to be morereadily leached and biodegradable than low-water-soluble pesti-cides such as penconazole.One other especially interesting variable for adsorption/

desorption studies is the ratio of the amount of pesticide poten-tially desorbed to the soil adsorption capacity, q0/qmax. Thisvariable is a measure of irreversibility in soil adsorption processes.As a rule, metalaxyl adsorption in soil is highly reversible.32 Theq0/qmax ratio varied from 0.3 to 1.0 and exhibited significantpositive correlation with the sand content and negative correla-tion with the clay, silt, and CT, CFA, CHU, and NT contents, inaddition to the effective cation-exchange capacity (Table S3,Supporting Information). In previous work, metalaxyl was found

Figure 5. Plot of fractional cumulative adsorption (qs/qmax) vs t1/2 in

two of the soil samples as determined in stirred-flow chamber tests.

Table 5. Metalaxyl Concentration Desorbed in Batch Testsand Percent Desorptiona Relative to the Initially AddedConcentration

initial concn, μmol 3 L�1

18 36 54 72 89

1 6.3 (21.1) 7.9 (18.9) 11.3 (24.2) 19.4 (32.0) 21.0 (30.4)

2 16.3 (66.8) 16.5 (34.7) 24.4 (26.4) 43.1 (29.7) 48.5 (27.7)

3 2.5 (11.6) 3.8 (13.5) 8.4 (25.2) 10.1 (29.4)

4 2.5 (21.6) 3.1 (22.8) 2.8 (19.7)

5 0.9 (6.8) 1.1 (5.5) 1.3 (5.8) 1.4 (5.6)

6 0.4 (5.1) 0.4 (5.3) 0.5 (7.4) 0.6 (8.2)

7 0.9 (11.0) 1.2 (12.1) 2.1 (20.9) 2.3 (23.1)

8 0.4 (2.3) 0.8 (3.7) 1.1 (3.7) 1.7 (5.0)

9 6.0 (32.9) 11.1 (34.2) 13.7 (21.9) 13.5 (18.4) 19.2 (16.6)

10 6.0 (26.5) 7.3 (15.3) 9.5 (13.5) 10.3 (12.3)

11 2.5 (17.1) 5.4 (17.6) 9.1 (16.7) 10.9 (14.9) 14.8 (18.7)

12 0.2 (1.3) 0.6 (3.5) 3.6 (19.8) 5.1 (23.3) 11.4 (43.1)

13 7.9 (27.0) 19.5 (32.4) 28.2 (33.1) 39.3 (37.7) 53.8 (38.2)

14 21.4 (38.3) 33.9 (32.4) 37.8 (28.1) 47.3 (25.2) 74.9 (29.5)

15 7.1 (20.8) 11.2 (22.0) 17.2 (25.2) 21.3 (26.3)

16 4.1 (23.7) 5.7 (22.9) 7.6 (20.3) 8.8 (21.9)a Percent desorption is shown in parentheses.




to be almost completely desorbed from soils with a low contentof organic matter,35 and raising its content by addition of asurfactant caused strong retention of the pesticide. Similarly,column tests conducted byMarín-Benito et al.26 revealed that theaddition of insoluble organic matter to soil decreased the extentand rate of metalaxyl leaching. Also, some authors have foundpeat and organic waste amendments to decrease metalaxylmobility in soils.11,24,36

The kinetic desorption constant, kd, ranged from 0.132 to0.956 min�1 and clearly exceeded its adsorption counterpart, ks(paired t = 2.5, p < 0.05). Therefore, metalaxyl was more rapidlyreleased than it was adsorbed, twice as fast on average. Also, thekinetic desorption constant was not significantly correlated withany edaphic variable (Table S3, Supporting Information). On theother hand, Rodríguez-Cruz et al.35 foundmetalaxyl to be rapidlyleached from soils with a low organic matter content (<2%), andthe addition of a cationic surfactant substantially increased thecontent to make leaching of the pesticide slower.

’ASSOCIATED CONTENT

bS Supporting Information. Three tables showing detailsof correlation coefficients and signification of metalaxyladsorption/desorption parameters with general soil charac-teristics. This material is available free of charge via theInternet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel þ34 988 387070; fax þ34 988 387001; e-mail [email protected].

’ACKNOWLEDGMENT

This work was funded by INCITE Programme 08PXIB38-3190PR of the Galician Council of Innovation and Industry.D.F.-C. additionally acknowledges award of an �Anxeles Alvari~nocontract by the Council.

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Figure 6. Cumulative metalaxyl remaining in the soil (qs) as a function of time during desorption in nine soil samples as measured in the stirred-flowchamber tests.

Table 6. Fitting of Kinetic Desorption Results for Metalaxyl

q0, μmol 3 kg�1 kd, min

�1 R2

1 19.0( 1.4 0.196( 0.028 0.940

2 23.9( 0.6 0.132( 0.005 0.994

4 2.6( 0.1 0.661( 0.091 0.973

5 5.3( 0.0 0.956( 0.006 0.998

8 7.4( 0.1 0.782( 0.030 0.997

10 33.3( 1.1 0.190( 0.011 0.986

13 37.9( 2.9 0.153( 0.020 0.940

14 18.4( 0.6 0.248( 0.022 0.947

15 23.8( 1.3 0.307 ( 0.035 0.975




(14) De Wilde, T.; Mertens, J.; Spanoghe, P.; Ryckeboer, J.; Jaeken,P.; Springael, D. Sorption kinetics and its effects on retention andleaching. Chemosphere 2008, 72, 509–516.(15) Heyse, E.; Dai, D.; Rao, P. S. C.; Delfino, J. J. Development of a

continuously stirred flow cell for investigating sorption mass transfer.J. Contam. Hydrol. 1997, 25, 337–355.(16) L�opez-Periago, J. E.; Arias-Est�evez, M.; N�ovoa-Mu~noz, J. C.;

Fern�andez-Calvi~no, D.; Soto, B.; P�erez-Novo, C.; Simal-G�andara, J.Copper retention kinetics in acid soils. Soil Sci. Soc. Am. J. 2008, 72,63–72.(17) Berm�udez-Couso, A.; Arias-Est�evez, M.; N�ovoa-Mu~noz, J. C.;

L�opez-Periago, E.; Soto Gonz�alez, B.; Simal-G�andara, J. Seasonaldistribution of fungicides in soils and sediments of a small river basinpartially devoted to vineyards. Water Res. 2007, 41, 4515–4525.(18) Kom�arek, M.; �Cadkov�a, E.; Chrastn�y, V.; Bordas, F.; Bollinger,

J. C. Contamination of vineyard soils with fungicides: A review of envi-ronmental and toxicological aspects. Environ. Int. 2010, 36, 138–151.(19) Giles, C. H.; Smith, D.; Huitson, A. A general treatment and

classification of the solute adsorption isotherm. I. Theoretical. J. ColloidInterface Sci. 1974, 47, 755–765.(20) Stumm, W.; Morgan, J. J. Aquatic Chemistry; John Wiley: New

York, 1981.(21) Aharoni, C.; Sparks, D. L. Kinetics of soil chemical reactions—a

theoretical treatment. In Rates of Soil Chemical Processes; Sparks, D. L.,Suarez, D. L., Eds.; Soil Science Society of America: Madison, WI, 1991;pp 1�18.(22) Sparks, D. L.Kinetics of Soil Chemical Processes; Academic Press:

New York, 1989.(23) Marín-Benito, J. M.; S�anchez-Martín, M. J.; Andrades, M. S.;

P�erez-Clavijo, M.; Rodríguez-Cruz, M. S. Effect of spent mushroomsubstrate amendment of vineyard soils on the behaviour of fungicides: 1.Adsorption�desorption of penconazole and metalaxyl by soils andsubsoils. J. Agric. Food Chem. 2009, 57, 9634–9642.(24) Andrades, M. S.; Rodríguez-Cruz, M. S.; S�anchez-Martín, M. J.;

S�anchez-Camazano, M. Effect of the addition of wine distillery wastes tovineyard soils on the adsorption and mobility of fungicides. J. Agric. FoodChem. 2004, 52, 3022–3029.(25) Li, K.; Xing, B.; Torello, W. Effects of organic fertilizer derived

dissolved organic matter on pesticides sorption and leaching. Environ.Pollut. 2005, 134, 187–194.(26) Marín-Benito, J. M.; Rodríguez-Cruz, M. S.; Andrades, M. S.;

S�anchez-Martín, M. J. Effect of spent mushroom substrate amendmentof vineyar soils on the behaviour of fungicides: 2. Mobility of pencona-zole and metalaxyl in undisturbed soil cores. J. Agric. Food Chem. 2009,57, 9634–9642.(27) Barriuso, E.; Andrades, M. S.; Benoit, P.; Houot, S. Pesticide

desorption from soils facilitated by dissolved organic matter comingfrom composts: experimental data and modelling approach. Biogeochem-istry (in press; DOI 10.1007/s10533-010-9481-y).(28) Fern�andez-Calvi~no, D.; Pateiro-Moure, M.; L�opez-Periago, E.;

Arias-Est�evez, M.; N�ovoa-Mu~noz, J. C. Copper distribution and acid�base mobilization in vineyard soils and sediments from Galicia (NWSpain). Eur. J. Soil Sci. 2008, 59, 315–326.(29) Kozak, J. Soil organic matter as a factor influencing the fate of

organic chemicals in the environment. In Humic substances in terrestrialecosystems; Piccolo, A., Ed.; Elsevier: Amsterdam, 1996; pp 625�664.(30) Pose-Juan, E.; Rial-Otero, R.; Paradelo, M.; Simal-G�andara, J.;

Arias, M.; L�opez-Periago, J. E. Behaviour of metalaxyl as copperoxychloride-metalaxyl commercial formulations vs. technical grade-metalaxyl in vineyards-devoted soils. J. Hazard. Mater. 2010, 174,181–187.(31) S�anchez-Martín, M. J.; Rodríguez-Cruz, M. S.; Andrades, M. S.;

S�anchez-Camazano, M. Efficiency of different clay minerals modifiedwith a cationic surfactant in the adsorption of pesticides: Influence ofclay type and pesticide hydrophobicity. Appl. Clay Sci. 2006, 31,216–228.(32) Rodríguez-Cruz, M. S.; Andrades, M. S.; S�anchez-Martín, M. J.

Significance of the long-chain organic cation structure in the sorption of

the penconazole and metalaxyl fungicides by organo clays. J. Hazard.Mater. 2008, 160, 200–207.

(33) Aharoni, C.; Sparks, D. L.; Levinson, S.; Ravina, I. Kinetics ofsoil chemical reactions: relationships between empirical equations anddiffusion models. Soil Sci. Soc. Am. J. 1991, 55, 1307–1312.

(34) Monkiedje, A.; Spiteller, M. Sorptive behaviour of the pheny-lamide fungicides, mefenoxan and metalaxyl, and their acid metabolitein typical Cameroonian and German soils. Chemosphere 2002, 49,659–668.

(35) Rodríguez-Cruz, M. S.; S�anchez-Martín, M. J.; Andrades, M. S.;S�anchez-Camazano, M. Retention of pesticides in soil columns modifiedin situ and ex situ with a cationic surfactant. Sci. Total Environ. 2007,378, 104–108.

(36) Petrovic, A. M.; Barrett, W. C.; Kovach, I. M. L.; Reid, C. M.;Lisk, D. J. The influence of a peat amendment and turf density ondownward migration of metalaxyl fungicide in creeping bentgrass sandlysimeters. Chemosphere 1996, 33, 2335–2340.

(37) Tomlin, C. D. S., Ed.The Pesticide Manuals World Compendium,11th ed.; British Crop Protection Council: Surrey, U.K., 1997.

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155

Anexo III. Comparison of batch, stirred flow chamber,

and column experiments to study adsorption,

desorption and transport of carbofuran within two

acidic soils

Chemosphere 88 (2012) 106–112

Contents lists available at SciVerse ScienceDirect

Chemosphere

journal homepage: www.elsevier .com/locate /chemosphere

Comparison of batch, stirred flow chamber, and column experiments to studyadsorption, desorption and transport of carbofuran within two acidic soils

Alipio Bermúdez-Couso, David Fernández-Calviño, Isabel Rodríguez-Salgado, Juan Carlos Nóvoa-Muñoz,Manuel Arias-Estévez ⇑Área de Edafoloxía e Química Agrícola, Departamento de Bioloxía Vexetal e Ciencia do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense, Spain

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 January 2012Received in revised form 24 February 2012Accepted 25 February 2012Available online 23 March 2012

Keywords:CarbofuranAdsorptionDesorptionKineticsStirred flow chamber

0045-6535/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.chemosphere.2012.02.078

⇑ Corresponding author. Tel.: +34 988368899; fax:E-mail address: [email protected] (M. Arias-Esté

Different methods (batch, column and stirred flow chamber experiments) used for adsorption and desorp-tion of carbofuran studies were compared. All tested methods showed that the carbofuran adsorption washigher in the soil with the higher organic matter content, whereas the opposite behaviour was observed forthe percentage of carbofuran desorbed. However, different methods have revealed some discrepancies incarbofuran adsorption/desorption kinetics. Although batch method showed interesting data on equilib-rium experiments, such as a low heterogeneity for the carbofuran adsorption sites independent of soilorganic matter content, it had some disadvantages for carbofuran adsorption/desorption kinetic studies.The disadvantages were related with the excessive limitations of this method on kinetics, i.e., no differencecould be detected between different soils. However, with column and stirred flow chamber methods thecarbofuran adsorption/desorption kinetics of different soils could be compared. Moreover, the absolute val-ues of carbofuran adsorption/desorption and its rate were higher in the stirred flow chamber than in thebatch and column experiments. Using stirred flow chamber experiments the carbofuran desorption wassignificantly faster than its adsorption, whereas carbofuran using column experiments they were similar.These discrepancies should be considered when the results obtained only with one method is discussed.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction biological processes, with adsorption–desorption processes being

In recent decades, agricultural practices have led to a marked in-crease in the quantity and quality of crops due to the use of newhigh-yield cultivars, advances in agricultural machinery and theuse of pesticides in general, particularly herbicides and fungicides(FAO, 2011). The use of pesticides has been the most significantchange in agriculture, which has drastically reduced the adverse ef-fects of diseases in plants and crops, leading to an increase in theuse of these compounds in only a few years (Dalvie et al., 2009), de-spite it being well known that these compounds are deleterious tothe environment and human health (McKinlay et al., 2008; Everettand Matheson, 2010). In fact, the continued use of these productshas increased their concentration in soil (Hildebrandt et al., 2008)to levels that could threaten the sustainability of some agriculturalsystems.

Carbofuran (2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-methylcarbamate), a pesticide widely used in potato production,is an insecticide used against a wide range of insects. However,according to the EPA (EPA, 2009), the use of carbofuran carries con-siderable risks for the environment and human health. Once in thesoil, pesticides are subjected to various physical, chemical and

ll rights reserved.

+34 988387001.vez).

the most important (Linn et al., 1993) because pesticide degrada-tion, transport, transformation, persistence and bioaccumulationin soils are influenced by adsorption–desorption processes (DeJonge et al., 1996). These processes can be a double-edgedsword—pesticide adsorption into soil colloids prevents the accumu-lation of pesticides in surface and groundwater, while also prevent-ing their volatilisation and biodegradation (Krishna and Philip,2011). However, desorption of pesticides from soils facilitates theirrunoff and leaching into surface and groundwater, which makes thepesticide available to soil microorganisms and facilitates itsbiodegradation.

Because of the risk of harm from pesticides to plants andmicroorganisms or because of the presence of pesticides in highconcentrations in water (dependent upon concentrations in soilsolution—which is dependent on the balance between adsorptionand desorption of pesticides with soil particles), a better under-standing of the mechanisms underlying this balance should facili-tate the assessment of the potentially adverse effects of pesticidesin agriculture and the environment.

The adsorption/desorption of carbofuran in soil has been thesubject of several studies (Yazgan et al., 2005; Arias-Estévez et al.,2006; Liyanage et al., 2006; Rama Krishna and Philip, 2008), whichhave shown that the most important soil characteristics for adsorp-tion/desorption of carbofuran are the contents of organic matter

http://dx.doi.org/10.1016/j.chemosphere.2012.02.078




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A. Bermúdez-Couso et al. / Chemosphere 88 (2012) 106–112 107

and clay. Additionally, it has been observed that the adsorption ofcarbofuran does not vary with modification of soil pH, between 2and 8 (Gupta et al., 2006).

Because adsorption and desorption of pesticides in soil are notinstantaneous processes, accurately studying them in carbofuranrequires an examination of their kinetics. However, studies on car-bofuran adsorption/desorption kinetics are scarce, and were doneusing different methods. The existing work on this topic has beenperformed using the batch (Achik et al., 1991b; De wilde et al.,2008; Kazemi et al., 2009), soil column (Achik et al., 1991a) or stirredflow chamber techniques (Bermúdez-Couso et al., 2011b). However,there are currently no studies comparing these three techniques, asthey are applied to carbofuran adsorption–desorption kinetics. Thecomparison of results obtained with different method in the samesoil is need to know if obtained results are really representative ofsoil processes or if they are influences by the method employed.

The main objective of this study was comparing the carbofuranadsorption/desorption results obtained from the different methods(batch, stirred flow chamber and soil column techniques).

2. Materials and methods

2.1. Chemicals

Carbofuran used in this research [2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-methylcarbamate] was obtained from Dr. Ehren-storfer Lab. (Augsburg, Germany) with a 99.5% purity. Table S1shows the chemical structure and properties of carbofuran. All or-ganic solvents used for sample preparation were of residue analy-sis grade and supplied by Panreac (Barcelona, Spain). HPLC gradesolvents were also supplied by Panreac.

Table 1General soil characteristic.

Property Sample

1 2

Sand (%) 28.6 42.8Silt (%) 30.1 24.5Clay (%) 41.2 32.6Texture Clay Clay-loamC (%) 22.7 2.7N (%) 1.51 0.27pHH2O 4.54 4.44pHKCl 4.09 3.71ECEC 8.63 8.15PBray II (mg g�1) 0.122 0.235Fed (mg g�1) 4.53 1.30Aln (mg g�1) 5.43 1.50

2.2. Soil samples

The samples used were collected within the ‘‘A Limia’’ district(Galicia, NW Iberian Peninsula). This area is devoted to the inten-sive farming of potatoes and wheat in rotation with other crops.Two samples were collected from the topmost layer (0–20 cm),with very different organic carbon contents. Once in the laboratory,the soils were air-dried, sieved through 2 mm mesh and stored inpolyethylene bottles until analysis. The particle size distributionwas determined to within <2 mm fractions by the internationallyrecognised pipette method (Guitián and Carballas, 1976). Soil pHwas measured in water and KCl (ratio of soil:solution 1:2.5) (Gui-tián and Carballas, 1976). Total carbon (CT) and total nitrogen (NT)were determined using a soil analyser (Thermo Finnigan 1112 Ser-ies NC) with samples that were finely milled in an agate mortar(Retsch RM100; Retsch GmbH, Hann, Germany). The effective cat-ion-exchange capacity (eCEC) was estimated as the sum of the basecations (Na, K, Ca and Mg), displaced with 0.2 M NH4Cl (Sumnerand Miller, 1996), and Al extracted using 1 M KCl (Bertsch andBloom, 1996). Available phosphorus was determined using theBray II assay (Bray and Kurtz, 1945). Aluminium oxyhydroxideswere quantified following selective extraction with 0.5 M NaOH(Borggaard, 1985), and iron oxyhydroxides were extracted usingdithionite–citrate (Holgrem, 1967). The concentrations of Ca, Mg,Al and Fe were determined using flame atomic absorption spec-trometry, and the concentrations of Na and K were determinedusing flame atomic emission spectrometry. The physicochemicalcharacteristics of the studied soils are shown in Table 1.

2.3. Carbofuran adsorption and desorption

2.3.1. Batch testA total of 1 g of soil was shaken at different times for variable

lengths of time (0.5, 1, 4, 8, 16, 24 and 48 h) with 10 mL of a

4.5 lM carbofuran solution containing 0.005 M CaCl2 as a back-ground electrolyte. After incubation, the soil suspensions werecentrifuged at 2000 rpm and passed through a polyester filter, withpores sizes of 0.45 lm, (Macherey-Nagel, Düren, Germany). Previ-ous tests revealed that these filters do not adsorb carbofuran. Theresulting supernatant was used to determine the pesticide concen-tration using HPLC–UV. The amount of soil that had adsorbed carbo-furan was calculated by subtracting the amount of carbofuran insolution from the amount added. All tests were performed intriplicate.

In addition, equilibrium tests were conducted using 1 g of driedsieved soil (soil particles were <2 mm in size). The sample wasmixed with 10 mL of aqueous solution, containing variable carbofu-ran concentrations (2–18 lM), with 0.005 M CaCl2 as a backgroundelectrolyte. After 24 h of shaking, the soil suspensions were centri-fuged at 2000 rpm for 15 min and filtered using a 0.45 lm filter. Thecarbofuran concentrations in the supernatant were determinedusing HPLC with a UV detector. The amount of pesticide adsorbedwas calculated as the difference between the amount added andthe amount measured in the solution after being stirred for 24 h.Desorption experiments were performed immediately after adsorp-tion, and the samples were weighed to quantify the amount ofoccluded solution. In total, 10 mL of 0.005 M CaCl2 was added toeach tube. After 24 h of shaking, the samples were centrifuged at2000 rpm, and the supernatant was filtered through 0.45 lm pores.Filtered samples were used to determine the carbofuran concentra-tion using HPLC–UV.

2.3.2. Stirred flow chamber (SFC) testThe reactor used was similar to that used by (López-Periago

et al., 2008), with minor modifications (Fig. S1): a micro-reactor,made of polypropylene (1.5 cm3) with two polytetrafluoroethylene(PTFE) filters 10 mm in diameter with 0.45 lm pores, was fittedinto the outlet port and over the inlet port to ensure that the soilinside the chamber was used. To feed the micro-reactor with thecorresponding solution in each case, the input was connected toa peristaltic pump (Gilson Minipuls 3), and the flow rate was fixedat 0.2 mL min�1. The outlet port was connected with a fraction col-lector (Gilson FC 203 B), in which effluent fractions were collectedinto 2 mL polypropylene Eppendorf vials. During the adsorptionand desorption experiments, the reactor was kept inside a cabinat a temperature of 25 ± 0.1 �C. Stirring was provided by a PTFE-coated magnetic bar (3 mm � 1 mm) that spun at 400 rpm.

In total, 0.2 g of soil (<2 mm) was placed inside the micro-reac-tor with a magnetic stirring bar; then, the solution was treatedwith the pesticide (4.5 lM). This solution contained 0.005 M CaCl2

as a background electrolyte. In total, 60 subsamples of 0.2 mL werecollected in different vials (each filled for 1 min). Once the sampleswere collected, the carbofuran solution was replaced with another

108 A. Bermúdez-Couso et al. / Chemosphere 88 (2012) 106–112

solution of 0.005 M CaCl2, for the desorption experiment. The samenumber of samples was collected within the same time frame. Car-bofuran measurement was performed using HPLC–UV analysiswith all samples.

2.3.3. Column testThe columns used were glass tubes, 8 cm in length and 1.5 cm

in diameter. Soil samples (<2 mm) were added incrementally withdepths of 0.5 cm, followed by gentle tapping of the column to con-solidate the soil and remove any air bubbles (Chotpantarat et al.,2011). The columns’ characteristics are shown in Table S2.

The input was connected to a peristaltic pump (Gilson Minipuls3), which pumped a corresponding solution in each case, with aflow rate of 0.3 mL min�1. The output was connected to a fractioncollector (Gilson FC 203 B). The column experiments were con-ducted at 25 ± 0.1 �C. The soil columns were initially saturatedfrom the bottom with 0.005 M CaCl2 over 24 h. After the saturationprocedure, solutions of 0.005 M CaCl2 with 10 mg L�1 of bromide(Br�) were injected from the bottom at a rate of 0.3 mL min�1.Forty 3 mL samples were collected in glass vials. Then a solutionof 0.005 M CaCl2 was injected in the column, and another 40 sam-ples were collected. The samples were measured in a segmentedflow analyser (Bran Luebbe Auto Analyzer 3). The adsorptionexperiments were performed with 4.5 lM concentrations of carbo-furan, with 0.005 M CaCl2 as a background electrolyte. Forty 3 mLsamples were collected in glass vials, and each sample was filledin 10 min. After adsorption, the pesticide solutions were replacedin 0.005 M CaCl2, and the procedure was repeated similar to thatfor adsorption. The determination of carbofuran was performedusing HPLC–UV analysis with all samples.

2.4. Carbofuran determination

The determination of carbofuran was performed using a highperformance liquid chromatograph (HPLC) with a UV–Visibledetector (Dionex Corporation, Sunnyvale, USA), further equippedwith a P680 quaternary pump, an ASI-100 autosampler, a TCC-10thermostated column compartment and a UVD170U detector.Chromatographic separations were performed with a SymmetryC18 column (4.6 � 150 mm, 5 lm), obtained from Waters (Milford,MA, USA), and a C18 guard column (4.6 � 50 mm, 5 lm) packedwith the same material. The pore size of columns was 100 Å andthe type of packing was end-caped. Elution conditions were iso-cratic, with a ratio of 30:70 acetonitrile:water at room tempera-ture. The flow rate was 1 mL min�1, with injected volumes of150 lL. UV detection was performed at 278 nm with a durationof 15 min for each chromatogram.

2.5. Data analysis

2.5.1. Kinetic dataThe data from carbofuran adsorption and/or desorption kinetics

were fitted to the pseudo-first order model:

dqx

dt¼ kxðqe � qxÞ ð1Þ

The above mentioned equation can be expressed in its inte-grated form:

Lnðqe � qxÞ ¼ LnðqxÞ � kxt ð2Þ

where dqxdt (lmol kg�1 min�1) is the carbofuran adsorption or desorp-

tion rate, kx is the adsorption or desorption rate constant (min�1), qe

(lmol kg�1) is the amount of carbofuran adsorbed or desorbed atequilibrium, qx (lmol kg�1) is the amount of carbofuran adsorbedor desorbed by the soil, and t is the time in minutes. In the equation,

x is the s of our adsorption experiments and the d in the desorptionexperiments.

2.5.2. Equilibrium dataThe data from carbofuran adsorption at equilibrium provided

by the batch experiments were fitted to the Freundlich (3), Lang-muir (4) and Straight Line (5) equations:

qs ¼ KFCn ð3Þ

qs ¼qmaxKLC1þ KLC

ð4Þ

qs ¼ aþ KdC ð5Þ

where qs is the amount of carbofuran retained by weight(lmol kg�1), C is the carbofuran concentration in solution after24 h of equilibration (lmol L�1), KF and n are coefficients of the Fre-undlich equation, KL (L lmol�1) is a constant dependent on the en-ergy of adsorption, qmax (lmol kg�1) is the maximum adsorptioncapacity, and Kd is the coefficient of distribution (L lmol�1). KF

(Ln kg�1 lmol(1�n)) can be interpreted as the amount of sorbate ad-sorbed at C = 1, and n (dimensionless) is a measure of heterogeneityin adsorption sites. Surface heterogeneity increased as n ap-proached 0 and decreased as n approached 1. The Kd parameter re-flects the pesticide partitioning between adsorbing surfaces and thesoil solution.

3. Results

3.1. Carbofuran adsorption

3.1.1. Batch testsThe results of carbofuran adsorption kinetics via batch experi-

ments are shown in Fig. 1, which shows that equilibrium wasachieved at 24 h of experimentation for the two studied soils.However, at 16 h, the amount of carbofuran adsorbed was veryclose to the carbofuran adsorbed at equilibrium for both soils,(89% and 96% in Soil 1 and Soil 2, respectively). At equilibrium,the amount of carbofuran adsorbed was 19.8 ± 0.0 and4.9 ± 0.0 lmol kg�1 for Soils 1 and 2. The adsorption kinetic datawere well fitted to the pseudo-first order equation (Eq. (1)), withR2 values higher than 0.98 for both soils. The carbofuran adsorptionrate constant was similar for both soils at 0.0022 ± 0.0001 min�1

and 0.0025 ± 0.0002 min�1 for Soil 1 and Soil 2, respectively.The results of carbofuran adsorption at equilibrium are shown

in Fig. 2. The adsorption curves were linear. The results were fittedto Freundlich, Langmuir and straight line equations. Fitting to theLangmuir equation was eventually discarded due to the high errorassociated with the fitted parameters. However, the experimentalresults were well fitted to the Freundlich and straight line equa-tions (R2 range: 0.975–0.990). According to the R2 values, the re-sults were equally satisfactory for both the Freundlich andstraight line equations. Values of Kd were 8.9 ± 0.2 and1.0 ± 0.0 L kg�1 for Soil 1 and Soil 2, respectively. The values forKF were 9.2 ± 0.7 and 1.1 ± 0.1 Ln kg�1 lmol(1�n) for Soil 1 and Soil2, respectively, whereas the values of n were high for both soils(0.99 ± 0.04 and 0.95 ± 0.06 for Soil 1 and Soil 2, respectively).

3.1.2. Stirred flow chamber testFig. 3 shows the amount of carbofuran adsorbed as a function of

time in the studied soils. The adsorption of carbofuran at the end ofthe experiment was higher for Soil 1 (71.6 lmol kg�1) than in Soil 2(12.9 lmol kg�1). The experimental data were fitted to the pseudo-first order equation (R2 0.844 and 0.960 in Soil 1 and Soil 2, respec-tively). The values of qe were 107.1 ± 5.2 and 13.1 ± 0.1 lmol kg�1

for Soil 1 and Soil 2, respectively, whereas the carbofuran adsorp-

µmol

kg-1

)

µmol

kg-1

)

Fig. 1. Carbofuran adsorption kinetics using batch experiments. Soil 1 (open symbols) and Soil 2 (full symbols). In all cases, coefficients of variation were less than 10%.

µmol

kg-1

)

µ

µmol

kg-1

)

µ

Fig. 2. Adsorption equilibrium of Carbofuran in the two studied soils as determined in the batch test. Soil 1 (open symbols) and Soil 2 (full symbols). In all cases, coefficients ofvariation were less than 10%.

µmol

kg-1

)

µmol

kg-1

)

Fig. 3. Cumulative adsorption and desorption of carbofuran as a function of time in the two studied soils, as determined in the stirred flow chamber experiments. Soil 1 (opensymbols) and Soil 2 (full symbols). In all cases, coefficients of variation were less than 10%.


tion rate constant was 0.018 ± 0.001 and 0.129 ± 0.003 min�1 forSoil 1 and Soil 2, respectively.

3.1.3. Columns testThe Breakthrough curves (BTCs) of the tracer and the carbofu-

ran in both studied soils are shown in Fig. 4. The tracer behaviourwas similar in Soil 1 and Soil 2, achieving maximum relative con-centration at 2.5 pore volumes. However, when the carbofuranwas passed through the columns, the results varied significantlyfor Soils 1 and 2. In Soil 2, saturation was achieved at 7 pore vol-umes, whereas in Soil 1, saturation was not achieved during theexperiment time (13VP). The carbofuran outflow concentration

constituted 70% of inflow concentration at the end of the experi-ment (Fig. 4).

The amounts of carbofuran adsorbed during the column experi-ments are shown in Fig. 5. At the end of the experiment (at400 min), the amount of carbofuran adsorbed in Soil 1 was36.8 lmol kg�1, whereas in Soil 2, the amount adsorbed was4.4 lmol kg�1. In Soil 1, equilibrium was not achieved during theexperiment, whereas in Soil 2, equilibrium was achieved at 180 min(7 pore volumes). Values from pseudo-first order equation fitting(R2 values of 0.878 and 0.979 in Soil 1 and Soil 2, respectively) showthat the qe values were 58.7 ± 2.4 and 4.4 ± 0.0 lmol kg�1 for Soil 1and Soil 2, respectively; whereas the carbofuran adsorption rate

Fig. 4. Experimental breakthrough curves of bromide (triangles) and carbofuran(circles). Soil 1 (open symbols) and Soil 2 (full symbols). In all cases, coefficients ofvariation were less than 10%.

Table 2Concentrations of desorbed carbofuran (lmol kg�1) in batch equilibrium tests andpercent desorption relative to the initially added concentration (in parentheses).

Carbofuran added (lmol kg�1)

23 45 68 90 136 181

S1 2.6(29.0)

3.8(23.7)

4.1(18.3)

6.2(14.5)

10.7 (18.7) 13.8 (17.1)

S2 nd nd 1.0(23.1)

1.2(13.4)

1.7 (15.6) 2.9 (19.6)

nd: Below detection limit (0.4 lmol kg�1) In all cases, coefficients of variation wereless than 10%.


constant was 0.003 ± 0.000 and 0.024 ± 0.001 min�1 for Soil 1 and Soil2, respectively.

3.2. Carbofuran desorption

3.2.1. Batch testAfter carbofuran adsorption, desorption in the kinetic batch

experiments was negligible. The results for carbofuran desorptionfollowing carbofuran adsorption in the equilibrium test are shownin Table 2. In Soil 1, the amount of carbofuran desorbed rangedfrom 2.6 to 13.8 lmol kg�1. These quantities increased as theamount of carbofuran previously added increased. However, thepercentage of carbofuran desorbed (14.5–29.0%) did not correlatewith the amount of carbofuran previously added to the soil. Thesame trends were observed for Soil 2, in which the percentage ofcarbofuran desorbed was always lower than 25% and ranged fromundetectable to 2.9 lmol kg�1.

3.2.2. Stirred flow chamber testThe amount of carbofuran remaining in the soil during the car-

bofuran desorption experiments after its adsorption in the stirredflow chamber are shown in Fig. 3. The desorption of carbofuranwas very fast in both soils, with undetectable values reached at18 and 8 min for Soil 1 and Soil 2, respectively. Fitting values tothe pseudo-first order kinetic model (R2 0.934 and 0.856 in Soil 1and Soil 2, respectively) show that the qe values were 32.9 ± 1.0and 11.6 ± 0.0 lmol kg�1 for Soil 1 and Soil 2, respectively. Thesevalues account for 46% and 90% of the carbofuran adsorbed previ-

µmol

kg-1

)

Fig. 5. Cumulative adsorption and desorption of carbofuran as a function of time, as deteIn all cases, coefficients of variation were less than 10%.

ously. The values of the carbofuran desorption rate constant(0.106 ± 0.007 and 0.262 ± 0.025 min�1 for Soil 1 and Soil 2, respec-tively) showed that carbofuran desorption was significantly fasterthan its adsorption.

3.2.3. Column testThe Breakthrough curves (BTCs) for carbofuran desorption

(Fig. 4) show that undetectable carbofuran values were achievedat 9 and 5 pore volumes in Soil 1 and Soil 2, respectively. The car-bofuran amounts remaining in soil in the column desorptionexperiments are shown in Fig. 5. During the experiment, the carbo-furan released from Soil 1 was 14.4 lmol kg�1, whereas in Soil 2,the amount released was 3.8 lmol kg�1. Fitting the experimentaldata to the pseudo-first order equation (R2 0.874 and 0.970 in Soil1 and Soil 2, respectively) provides qe values of 16.1 ± 0.4 and3.8 ± 0.0 lmol kg�1 for Soil 1 and Soil 2, respectively. These valuesrepresent 44% and 86% of carbofuran adsorbed previously. The car-bofuran desorption rate constants were 0.007 ± 0.000 and0.029 ± 0.001 min�1 for Soil 1 and Soil 2, respectively. These valueswere very similar to the ks values.

4. Discussion

Carbofuran adsorption tests at equilibrium (via batch experi-ments) present Giles type-C curves. This curve implies conditionsin which the number of sites remains constant throughout thewhole range of solute concentrations (though not necessarily ofequal energy); generally, type-C curves are associated with lowaffinity of the solute for the substrate (Giles et al., 1974). In previ-ous work (Singh and Srivastava, 2009; Bermúdez-Couso et al.,2011b), Giles type-L and type-S curves were also found for carbo-furan adsorption. However, in other studies, the n parameter fromthe Freundlich equation showed low heterogeneity in the sorptionsites for carbofuran (Yazgan et al., 2005; Rama Krishna and Philip,2008; Bermúdez-Couso et al., 2011b). This finding is in agreementwith the results of our present work (n P 0.95) and also shows that

µmol

kg-1

)

rmined in the columns experiments. Soil 1 (open symbols) and Soil 2 (full symbols).


this low heterogeneity occurred in soil with high organic matter(Soil 1) and low organic matter (Soil 2). Although carbofuran wasnot highly adsorbed by the soils, the KF values and the distributioncoefficient (Kd) were significantly higher in the soil with high or-ganic matter (Soil 1) than in the soil with low organic matter (Soil2). This result is consistent with previous work, which suggests theimportant influence of organic matter content in the soil on its car-bofuran sorption (Singh et al., 1994; Yazgan et al., 2005; Farahaniet al., 2008; Bermúdez-Couso et al., 2011b). As was shown previ-ously, batch experiments are adequate to carbofuran adsorptionat equilibrium. Batch experiments are generally low cost and areeasy to use (Amacher, 1991), therefore this technique is recom-mended for carbofuran adsorption tests at equilibrium.

Carbofuran adsorption kinetics was studied for an initial con-centration of 4.5 lmol L�1, utilising different methods, and the re-sults varied with the method and the soil employed. The values ofqe (carbofuran adsorbed at equilibrium) were consistently higherin Soil 1 than in Soil 2, regardless of the method employed. Theseresults are in agreement with the results obtained in the batchequilibrium test and with the literature discussed previously. Inboth soils, the highest qe values were achieved with the stirredflow chamber experiments. In Soil 1, the qe values were higherusing the column method than with the batch method, whereasin Soil 2, qe values were similar for both methods, although slightlyhigher with the batch method. The differences may be attributableto the different limitations of each method (Amacher, 1991). In thebatch method, the carbofuran concentration in the solution dimin-ished with time and equilibrium was reached at a carbofuran con-centration lower than the initial concentration. This problem isavoided in column and stirred flow chamber experiments, in whichequilibrium is achieved at inflow concentration. Film diffusion isan important limitation in the column experiments, where carbo-furan reach some of the adsorption sites. This shortcoming isavoided in the batch and stirred flow chamber methods.

The carbofuran adsorption rate constants showed the greatestdifferences among the different methods. The lowest ks values wereobtained with the batch method, intermediate values of ks were ob-tained with the column method, and the highest values of ks wereobtained with the stirred flow chamber method. These results sug-gest that the most important limiting factor for the carbofuranadsorption rate is probably equilibrium concentration, followedby film diffusion, and then less importantly, intraparticle diffusion.In this regard, previous work shows us that a substantial portion ofthe carbofuran adsorbed in soil, using stirred flow chamber exper-iments, is kinetically limited by intraparticle diffusion (Bermúdez-Couso et al., 2011b). With regards to the differences between soilswith high and low organic matter content, the batch method re-vealed that the carbofuran adsorption rate constant is similar forboth soils (0.0022 min�1 and 0.0025 min�1, respectively). Thus, inthe batch kinetic experiments, the limitations of the method areextensive, and no difference could be detected between soils withhigh differences in their carbofuran adsorption capacities. There-fore, batch experiments do not seem well suited to studies on theadsorption of carbofuran kinetics. With column and stirred flowchamber methods, differences in ks values were detected. In bothcases, the ks values were higher in the soil with low organic matterthan in the soil with high organic matter content; this means ks maybe inversely proportional to soil adsorption capacity. This behav-iour was previously reported for metalaxyl (Bermúdez-Cousoet al., 2011a), but more work is needed to verify it in carbofuranadsorption. Both, column and stirred flow chamber techniquesseem adequate to evaluate carbofuran adsorption kinetics.

Similar to other pesticides, desorption of carbofuran has beenless studied than its adsorption, and hence, the study of carbofuranliberation from soil is important for understanding carbofuranmovement in the soil. The results on carbofuran desorption from

equilibrium experiments show that the amount of desorbed carbo-furan increased with an increase in the amount of previously addedcarbofuran. Bermúdez-Couso et al. (2011b) have suggested that car-bofuran is adsorbed to less specific sites, forming weaker bonds,when the saturation of sites increases. Similar behaviour was ob-served for metalaxyl (Bermúdez-Couso et al., 2011a). The carbofu-ran desorption kinetic test via batch experimentation could not becompleted, as the concentrations desorbed were lower than the lim-its of detection. However, stirred flow chamber and column testsprovided useful results. The relationship between qe from adsorp-tion and qe from desorption was similar in both methods but differ-ent depending on the soil: 0.3 and 0.9 for Soil 1 and Soil 2,respectively. This means that in soil with high organic content, theirreversibility of carbofuran adsorption is higher than in soil withlow organic matter content. According to our kd values, carbofurandesorption was faster in the stirred flow chamber experiments thanin the column experiments. For both methods, kd values were lowerin Soil 1 than in Soil 2, showing that carbofuran desorption was fas-ter in the soil with lower organic matter than in the soil with higherorganic matter content. Another interesting result is that in the stir-red flow chamber experiments, the desorption rate constant wasmuch higher than the adsorption rate constant; i.e., the desorptionprocesses were faster than the adsorption processes. However, inthe column experiments, ks and kd values have the same magnitudeof order, i.e., the adsorption and desorption of carbofuran rates arequite similar, probably due to the high limitation of film diffusion,with regards to other kinetic limitations. These results showed thatbatch technique is not adequate for carbofuran desorption kinetics,whereas column and stirred flow chamber techniques can be em-ployed. However, there are some differences between both meth-ods; to study intraparticle diffusion rate limitations stirred flowthe stirred flow chamber method should be used, whereas columntechnique is adequate to study the effect of film diffusions on carbo-furan adsorption/desorption rates.

5. Conclusions

When different soils were compared the three tested methodsshowed that the carbofuran adsorption capacity was significantlyhighest (and less reversible) in the soil with the highest organicmatter content. However, batch experiments failed on adsorp-tion/desorption kinetics evaluation. Stirred flow chamber and col-umns experiments was adequate to evaluate carbofuranadsorption/desorption kinetics, showing that the carbofuranadsorption and desorption rates were higher in the soil with thelower organic matter content. However, different experimentalmethods provided different results whenever only one soil wasbeing considered. The carbofuran adsorption/desorption and itsrate were consistently higher in the stirred flow chamber experi-ments than when using the other methods. Also, desorption pro-cesses were faster than adsorption in the stirred flow chamberexperiments, while in the column experiments, adsorption anddesorption processes had similar rates.

Acknowledgments

D. Fernández-Calviño and A. Bermudez-Couso are funded byÁngeles-Alvariño contract from the Galician Government and thepredoctoral programme from the University of Vigo, respectively.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.chemosphere.2012.02.078.



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Hildebrandt, A., Guillamón, M., Lacorte, S., Tauler, R., Barceló, D., 2008. Impact ofpesticides used in agriculture and vineyards to surface and groundwater quality(North Spain). Water Res. 42, 3315–3326.

Holgrem, C.G.S., 1967. A rapid citrate–dithionite extractable iron procedure. Soil Sci.Soc. Am. Proc. 31, 210–211.

Kazemi, H.V., Anderson, S.H., Goyne, K.W., Gantzer, C.J., 2009. Aldicarb andcarbofuran transport in a Hapludalf influenced by differential antecedent soilwater content and irrigation delay. Chemosphere 74, 265–273.

Krishna, K.R., Philip, L., 2011. Bioremediation of single and mixture of pesticide-contaminated soils by mixed pesticide-enriched cultures. Appl. Biochem.Biotechnol. 164, 1257–1277.

Linn, D.M., Carski, T.H., Brusseau, M.L., Chang, F.H., 1993. Sorption and Degradationof Pesticides and Organic Chemicals in Soil. Soil Science Society of America,Madison, WI.

Liyanage, J.A., Watawala, R.C., Aravinna, A.G.P., Smith, L., Kookana, R.S., 2006.Sorption of carbofuran and diuron pesticides in 43 tropical soils of Sri Lanka. J.Agric. Food Chem. 54, 1784–1791.

López-Periago, J.E., Arias-Estévez, M., Nóvoa-Muñoz, J.C., Fernández-Calviño, D.,Soto, B., Pérez-Novo, C., Simal-Gándara, J., 2008. Copper retention kinetics inacid soils. Soil Sci. Soc. Am. J. 72, 63–72.

McKinlay, R., Plant, J.A., Bell, J.N.B., Voulvoulis, N., 2008. Endocrine disruptingpesticides: implications for risk assessment. Environ. Int. 34, 168–183.

Rama Krishna, K., Philip, L., 2008. Adsorption and desorption characteristics oflindane, carbofuran and methyl parathion on various Indian soils. J. Hazard.Mater. 160, 559–567.

Singh, R.P., Srivastava, G., 2009. Adsorption and movement of carbofuran in fourdifferent soils varying in physical and chemical properties. Adsorpt. Sci.Technol. 27, 193–203.

Singh, R.P., Kumari, K., Singh, D., 1994. Influence of different factors on theadsorption of carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl-N-methylcarbamate) on soils. Ecotoxicol. Environ. Saf. 29, 70–79.

Sumner, M.E., Miller, W.P., 1996. Cation exchange capacity and exchangecoefficients. In: Sparks, D.L. (Ed.), Methods of Soil Analysis. Part 3. ChemicalMethods. SSSA Book Series: 5. SSSA and ASA, Madison, WI, pp. 1201–1229.

Yazgan, M.S., Wilkins, R.M., Sykas, C., Hoque, E., 2005. Comparison of two methodsfor estimation of soil sorption for imidacloprid and carbofuran. Chemosphere60, 1325–1331.

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Anexo IV. Metalaxyl dynamics in acid soils: evaluation

using different methods

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Metalaxyl dynamics in acid soils: evaluation using different

methods

Alipio Bermúdez-Couso, David Fernández-Calviño, Juan Carlos Nóvoa-Muñoz,

Manuel Arias Estévez

Área de Edafoloxía e Química Agrícola, Departamento de Bioloxía Vexetal e Ciencias do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense. CITI (Centro de Investigación, Transferencia e Innovación) - Universidade de Vigo, Tecnopole, San Cibrao das Viñas,32900. Ourense, Spain

ABSTRACT

BACKGROUND: The importance of adsorption-desorption processes on the metalaxyl concentration in the soil solution and other processes necessitates a thorough understanding of these mechanisms in the soil to prevent the potential adverse effects of metalaxyl in the soil and to improve pesticide management in agriculture. The aim of this work was to evaluate the adsorption-desorption kinetics and transport of metalaxyl in two acid soils with different organic matter concentrations using three different methods: batch, stirred flow chamber and soil column experiments. RESULTS: The metalaxyl adsorption capacity in one soil is different depending on the method employed in the following sequence: stirred flow chamber > column > batch for the same initial metalaxyl concentration. CONCLUSION: The differences among the three tested methods can be attributed to the limitations of each methods, in batch experiments, the suspension was continuously stirred, however, the amount of metalaxyl in the solution decreased in the time and equilibrium was reached at a metalaxyl concentration lower than the initial concentration. The metalaxyl desorption could be limited by the amount present in solution. In the column experiments, the soil remains without stirring, and therefore film diffusion could be an important limitation, and the metalaxyl could not reach certain adsorption sites. These shortcomings are eliminated in the stirred flow chamber methods in which the soil suspension is continuously stirred, and equilibrium is reached at the inflow metalaxyl concentration

KEYWORDS: metalaxyl, adsorption-desorption, batch, stirred flow chamber, column, soil

1 INTRODUCTION

Metalaxyl (methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate) is a systemic pesticide with moderate toxicity1,2 that is widely used to combat and prevent fungal diseases. The intensive use of this pesticide in intensive agricultures could lead to high accumulations in the soil that could present considerable risks to the environment and human health3. In the soil, metalaxyl is subjected to various physical, chemical and biological processes, such as chemical and biological degradation, leaching, runoff, uptake by plants, transformation, persistence and bioaccumulation.4 However, the most important processes are adsorption and desorption of the pesticide in the soil because the other processes described previously are influenced by pesticide adsorption and desorption.5 The adsorption and desorption of pesticides could have different effects on pesticide persistence in the soil. Adsorption

avoids the movement of pesticides into the water system but also prevents the volatilization and biodegradation of pesticides.6 Desorption of pesticides from the soil facilitates runoff and leaching but makes the pesticide available to soil microorganisms, facilitating its biodegradation. Moreover, desorption facilitates volatilization.

The importance of adsorption-desorption processes on the metalaxyl concentration in the soil solution and other processes necessitates a thorough understanding of these mechanisms in the soil to prevent the potential adverse effects of metalaxyl in the soil and to improve pesticide management in agriculture. The adsorption of metalaxyl in soils was studied previously. Most of these studies were performed at equilibrium using a batch technique,7-12 leaching experiments using soil columns13 or a combination of both techniques.14,15 However, metalaxyl adsorption-desorption kinetics studies are scarce.16 Kinetic

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studies are important because the adsorption and desorption of pesticides in the soil are not instantaneous processes; therefore, studies on the rates of adsorption and desorption of metalaxyl in the soil are needed to improve knowledge of the pesticide’s behavior in the soil.

The aim of this work was to evaluate the adsorption-desorption kinetics and transport of metalaxyl in two acid soils with different organic matter concentrations using three different methods: batch, stirred flow chamber and soil column experiments.

Table 1. Chemical structure and properties of metalaxyl

Common name

Metalaxyl

Chemical structure

Name methyl N-methoxyacetyl-N-(2,6-

dimethylphenyl)-DL-alaninate CAS nº 57837-19-1 MWa 279 Sb (g L-1) 8.4 Log Pow

c 1.8 Kocd 30-300 Soil half-life f

10-40

Data were obtained from SciFinder (http://scifinder.cas.org/) a Molecular weight, b Solubility in water at 25ºC, c Octanol/water partition coefficient at 25ºC, d Partition coefficient normalized to organic carbon content (mL goc-1 or L klgoc-1), f Aerobic soil half-life (days).

2 MATERIALS AND METHODS

2.1 Chemicals

Metalaxyl [methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate] was obtained in purity greater than 99.5% from Sigma–Aldrich (Steinheim, Germany). Table 1 shows the chemical structure and selected properties of metalaxyl. All of the organic solvents used for sample preparation were residue analysis grade and supplied by Panreac (Barcelona, Spain). The HPLC grade solvents for HPLC measurements were also supplied by Panreac.

2.2 Soil Samples

Two soil samples were collected in the region of “A Limia” (Galicia, NW Spain) from the topmost soil layer (0-20 cm) in plots devoted to intensive farming. In the laboratory, the samples were air-dried, sieved (< 2 mm) and stored in polyethylene jars until analysis. The soil samples were determined using the same methods employed in previous work16 in which the methods used for the general characterization are described in detail. The general characteristics of the soils are shown in Table 2.

Table 2. General soil characteristics

Property Sample S1 S2 Sand (%) 29 43 Silt (%) 30 24 Clay (%) 41 33 Texture Clay Clay-Loam C (%) 22.7 2.7 N (%) 1.51 0.27 pH H2O 4.5 4.4 eCEC (cmol(c) kg-1 8.63 8.15

2.3 Metalaxyl adsorption and desorption using batch experiments

Two different experiments were performed using the batch technique: metalaxyl adsorption kinetics and metalaxyl adsorption at equilibrium.

The kinetic experiments were conducted as follows: 1 g of soil was shaken with 10 mL of a 7.2 µM metalaxyl and 0.005 M CaCl2 solution for different lengths of time (0.5, 1, 4, 8, 16, 24 and 48 hours). After incubation, the soil suspensions were centrifuged at 2000 rpm and passed through a polyester filter with pores sizes of 0.45 µm (Macherey-Nagel, Düren, Germany). Previous tests revealed that these filters do not adsorb metalaxyl. The resulting supernatant was used to determine the pesticide concentration using HPLC-UV. The amount of metalaxyl adsorbed on the soil was calculated by subtracting the amount of metalaxyl in solution from the amount of metalaxyl that was previously added. All tests were performed in triplicate.

Equilibrium experiments were conducted as follows: a 1 g amount of soil was mixed with 10 mL of aqueous solutions containing variable metalaxyl concentrations (9 to 90 µM) and 0.005 M CaCl2 as a background electrolyte. After 24 h of shaking, the soil suspensions were centrifuged at 2000 rpm for 15 min and passed through a filter with a 0.45-µm pore size. The supernatant was used to determine the metalaxyl concentrations by HPLC using a UV detector.

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The amount of pesticide adsorbed was calculated as the difference between the amount added and the amount measured in the solution. The results were fitted to Freundlich (1), Langmuir (2) and Straight Line (3) equations.

nFCKX = (1)

CK

CXKX

L

mL

+=

1 (2)

CKX D= (3)

where X is the concentration of metalaxyl adsorbed in the soil (µmol kg-1); C is the concentration of metalaxyl in the aqueous phase after equilibrium (µM); KF (Ln kg-1 µmol(1-n)) and n (dimensionless) are coefficients of the Freundlich equation; KL (L µmol-1) is a constant dependent on the energy of adsorption; Xm (µmol kg-1) is the maximum adsorption capacity of each sample; and KD (L µmol-1) is the partitioning coefficient.

In both the kinetic and equilibrium experiments, desorption experiments were performed immediately after adsorption, weighing the samples to quantify the amount of occluded solution. To each tube, 10 mL of 0.005 M CaCl2 was added. After 24 h of shaking after the equilibrium adsorption experiments and variable times after the kinetic experiments, the samples were centrifuged at 2000 rpm, and the supernatant was filtered through a 0.45-µm filter and was used to determine the metalaxyl concentrations using HPLC-UV.

2.4 Metalaxyl adsorption and desorption using stirred flow chamber experiments

The reactor used was similar to that used by López-Periago et al.17 with minor modifications (Figure 1). The micro-reactor used in this study was made of polypropylene (1.5 cm3) with two polytetrafluoroethylene (PTFE) filters, measuring 10 mm in diameter and 0.45 µm in pore size, fitted in the outlet port and over the inlet port to retain the soil inside the chamber. The input was connected to a peristaltic pump, Gilson Minipuls 3. The flow rate was fixed at 0.2 mL min-1, and the outlet port was connected to a fraction collector, Gilson FC 203 B, in which the effluent fractions were collected into 2 mL polypropylene Eppendorf vials. The reactor was located inside a cabin at a temperature of 25 ± 0.1 ° C during the adsorption and desorption experiments. Stirring was provided by a PTFE-coated magnetic bar (3 mm x 1 mm) that spun at 400 rpm. The adsorption experiments were performed as follows: a 0.2 g amount of soil was placed inside the micro-reactor, and a solution with 7.2 µM of metalaxyl and 0.005 M CaCl2 as a background electrolyte was subsequently passed

Figure 1. Schematic depiction of the stirred flow chamber.

through the reactor. In total, 60 0.2-mL subsamples were collected in different vials (all of which were filled in 1 min). After the adsorption experiments, the metalaxyl solution was replaced by another solution with only 0.005 M CaCl2 to perform the desorption experiments. The same number of samples (in the same time period) as used in the adsorption experiments were collected. The determination of metalaxyl was performed by HPLC-UV in all samples.

2.5 Metalaxyl adsorption and desorption using column experiments

The columns used were glass tubes 8 cm in length and 1.5 cm in diameter. The soil (<2 mm) was added in increments of 0.5 cm depth followed by gentle tapping of the column to consolidate the soil and remove any air bubbles.18 The input was connected to a peristaltic pump, namely, the Gilson Minipuls 3. The output was connected to a fraction collector, the Gilson FC 203 B. The column experiments were conducted at 25 ± 0.1 ºC, and the flow rate was 0.3 mL min-1. The experiments were conducted as follows: the soil columns were initially saturated from the bottom with 0.005 M CaCl2 during 24 h. After the saturation procedure, different experiments were performed on the two studied soils: a) a solution of 0.005 M CaCl2 containing 10 mg L-1 of bromide (Br-) was injected from the bottom. Next, 40 samples of 3 mL each were collected in glass vials. A solution of 0.005 M CaCl2 was subsequently injected, and another 40 samples were collected. The bromide concentration in each sample was measured in a segmented flow analyzer, Bran Luebbe Autoanalizar 3; b) a solution with 7.2 µM metalaxyl with 0.005 M CaCl2 as a background electrolyte was injected from the bottom. Next, 40 samples of 3 mL each, filled in 10 min, were collected in glass

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vials. After metalaxyl adsorption, the metalaxyl solution was replaced by 0.005 M CaCl2, and the procedure was repeated, as in the case of adsorption. The determination of metalaxyl was performed by HPLC-UV in all samples.

2.6 Metalaxyl determination

The determination of metalaxyl was performed using a high performance liquid chromatograph (HPLC) with a UV-Visible detector (Dionex Corporation, Sunnyvale, EE.UU.), equipped with a P680 quaternary pump, an ASI-100 autosampler, a TCC-10 thermostated column compartment and a UVD170U detector. Chromatographic separation was done on a Symmetry C18 column (4.6 x 150 mm, 5 µm) obtained from Waters (Milford, MA, USA) and a C18 guard column (4.6 x 50 mm, 5 µm) packet with the same material. The elution conditions for metalaxyl were as follows: the mobile phases were methanol (A) and water (B), and the gradient program was 60% A + 40% B for 7 min, changing to 95% A + 5% B for 3 min, holding for 5 min, changing to 60% A + 40% B for 0.1 min and holding for 10 min. The total analysis time was 25 min. The injected volume was 50 µL, and the flow rate was 0.7 ml min-1. Metalaxyl was detected to λ = 200 nm.

3 RESULTS AND DISCUSSION

3.1 Metalaxyl adsorption using batch experiments

Figure 2 shows the amount of metalaxyl adsorbed as a function of the incubation time. In both soils, equilibrium was reached at 24 hours of incubation, although after 16 hours, the amount of metalaxyl adsorbed was 95 % and 98 % of the metalaxyl adsorbed at equilibrium in Soil 1 and Soil 2, respectively. Once at equilibrium, the amount of metalaxyl adsorbed was 32.8 and 6.4 µmol kg-1 for Soils 1 and 2, respectively. The metalaxyl adsorption kinetic data were fitted to the pseudo-first order equation (4).

dqs/dt = ks (qmax – qs) , (4)

where dqs/dt (µmol kg-1 min-1) is the metalaxyl adsorption rate, ks (min-1) is the metalaxyl adsorption rate constant, qmax (µmol kg-1) is the amount of metalaxyl adsorbed at equilibrium and qs (µmol kg-1) is the amount of metalaxyl adsorbed in the soil. The above mentioned equation (4) can be expressed in its integrated form (5), where t is the time.

Ln (qs) = Ln (qmax – qs) + ks t (5)

The data from the batch kinetic experiments were well fitted to the pseudo-first order equation with an R2 of 0.92 and 0.99 for Soil 1

and Soil 2, respectively. The metalaxyl adsorption rate constants (ks) were 0.0037 min-1

0

10

20

30

40

0 1000 2000 3000

qS

(µm

ol k

g-1

)

t (min)

Figure 2. Metalaxyl adsorption as a function of time in the batch experiments. Soil 1 (circles); Soil 2 (triangles).

for Soil 1 and 0.0045 min-1 for Soil 2. Figure 3 show the metalaxyl adsorption curves at equilibrium in the two studied samples. The metalaxyl adsorption curves were linear for both soils, i.e., Giles type C curves.19 This type of curve implies a constant partition between the amounts of metalaxyl adsorbed in the soil and the metalaxyl concentration in solution, i.e., a constant coefficient of distribution (KD). This coefficient was calculated fitting the adsorption results obtained at equilibrium to a Straight Line equation (3). The data were well fitted to the lineal equation (Table 3), with R2 values higher than 0.95 for the two studied soils. The KD values were 5.94 L kg-1 in Soil 1 and 0.58 L kg-1 in Soil 2. This difference could be attributed to differences in the organic matter concentration between the two studied soils. In fact, other authors showed that soil organic matter plays a key role in metalaxyl adsorption.11,14 The KD value for Soil 2 was in the range previously reported.9,11,14-16 However, Soil 1 had a KD value higher than all previously reported soils with one exception, specifically, a soil with 0.5% organic carbon content, 68% in clay and 32% in silt and KD 9.17 L kg-1.15

The adsorption data obtained at equilibrium was also fitted to the Freundlich (1) and Langmuir (2) equations. Fitting to the Langmuir equation was discarded because of the high level of error associated with the fitted parameters. However, fitting the metalaxyl adsorption data to the Freundlich equation was satisfactory with R2 values higher than 0.95 for the two studied soils (Table 3). The fitted values of the Freundlich constant (KF) were 12.15 Ln kg-1 µmol(1-n) for Soil 1 and 0.88 Ln kg-1 µmol(1-n) for Soil 2, whereas the n parameter (dimensionless) was 0.82 for Soil 1 and 0.90 for Soil 2. The Freundlich equation is an empirical function,

171

where KF can be interpreted as the amount of sorbate adsorbed at C = 1, i.e., a measure of metalaxyl adsorption at low concentrations, whereas the n parameter is a measure of the adsorption site’s heterogeneity. This heterogeneity increased as n approached 0 and decreased as n approached 1.20

The n parameter values of the present work were in the range previously reported,7,14-16 showing a notably low heterogeneity in the metalaxyl adsorption sites. The KF values followed the same pattern as the KD values with higher values in Soil 1 than in Soil 2.

3.2 Metalaxyl desorption using batch experiments

The metalaxyl desorption after its adsorption in the kinetic batch experiments was negligible (<0.5 µmol kg-1). However, after the metalaxyl adsorption experiments at equilibrium, the

results of metalaxyl desorption were detectable above the 90 µmol kg-1 that was added (Table 4). In the two studied soils, the amount of metalaxyl desorbed increased as the amount of metalaxyl previously added increased, ranging from 4.0 to 121.1 µmol kg-1 in Soil 1 and from 1.3 to 19.4 µmol kg-1 in Soil 2. In general, the percentage of metalaxyl desorbed also increased as the amount of metalaxyl previously added increased, ranging between 9.2 and 37.8 % in Soil 1 and between 21.7 and 42.6% in Soil 2. This behavior suggests that the strength of the metalaxyl’s bond to the soil diminishes as the amount of metalaxyl adsorbed in the soil increased.16 Results also show a high irreversibility of adsorption processes because more than 50% of the previously adsorbed metalaxyl remained in the soil after the desorption experiments in all cases.

0

100

200

300

400

0 20 40 60 80 100

qS

(µm

ol k

g-1

)

C (µmol L -1)

S 1

0

100

200

300

400

0 20 40 60 80 100

qS

(µm

ol k

g-1

)

C (µmol L -1)

S 2

Figure 3. Adsorption of metalaxyl in the two studied soils, as determined in the batch experiments performed at equilibrium.

Table 4. Metalaxyl desorbed (µmol kg-1) after batch experiments at equilibrium and percentage of metalaxyl desorbed (in brackets) with respect to the concentration of metalaxyl added

Metalaxyl added (µmol kg-1)

90 179 358 537 716 895

S1 4.0(9.2) 8.1(10.0) 26.5(18.6) 43.2(20.9) 65.4(25.4) 121.1(37.8)

S2 1.3(21.7) 2.0(15.7) 6.9(32.1) 11.1(40.3) 15.4(42.6) 19.4(39.4)

The percentage of metalaxyl desorbed was higher in Soil 2 than in Soil 1, suggesting that the irreversibility of metalaxyl adsorption was increased as the soil organic matter content increased, as was suggested previously by Marin-Benito et al.11

3.3 Metalaxyl adsorption using stirred flow chamber experiments

Figure 4 shows the amount of metalaxyl adsorbed in the soil as a function of time in the stirred flow chamber experiments. During the experiment, 49.4 µmol kg-1 of metalaxyl was

adsorbed in Soil 1, whereas in Soil 2, the amount of metalaxyl adsorbed was 10.1 µmol kg-1. The data shown in Figure 4 were well fitted to the pseudo-first order equation (4), with R2 values of 0.87 and 0.98 being observed for Soil 1 and Soil 2, respectively. The amount of metalaxyl adsorbed at equilibrium (qmax) was 54.7 µmol kg-1 for Soil 1 and 10.1 µmol kg-1 for Soil 2. These values were 68 and 58% higher than those found in the batch kinetic experiments for Soil 1 and Soil 2, respectively. Moreover, the metalaxyl adsorption was significantly faster than in the batch

172

experiments. In Soil 1, equilibrium was not achieved during the experiment (60 min), but at 30 min, the metalaxyl adsorbed was 62% of the metalaxyl that can be adsorbed at equilibrium, and at 60 min, this percentage increased to 90%. In Soil 2, equilibrium was reached at 17 min into the experiment. This faster metalaxyl adsorption than in batch experiments was reflected in higher ks values, 0.032 min-1 in Soil 1 and 0.275 min-1 in Soil 2, i.e., in Soil 1, ks was 8.6 times higher than in the batch experiments, and in Soil 2, ks was 61.1 times higher. These ks results also showed that the adsorption of metalaxyl was faster in Soil 2 than in Soil 1, suggesting that the metalaxyl adsorption rate is inversely proportional to the adsorption capacity. The differences between the results obtained using the batch and stirred flow chamber methods may be attributable to the different limitations of each method.21 In the batch method, the concentration of metalaxyl in the solution diminishes with time until equilibrium, which is stated at a lower concentration than the initial concentration. This problem is avoided in the stirred flow chamber method in which the equilibrium is achieved at

flow concentration. The lower equilibrium concentration in the batch method could be a notably high limitation in metalaxyl adsorption, reducing the amount of metalaxyl adsorbed and its adsorption rate. In fact, the ks parameter was similar in the two soils that were used, despite the high differences in metalaxyl adsorption capacity, suggesting that the metalaxyl adsorption rate in the batch experiment was limited by the method independently of the soil used. In the stirred flow chamber method, the metalaxyl adsorption rate was controlled mainly by intraparticle diffusion,16 providing different results with different soils.

3.4 Metalaxyl desorption using stirred flow chamber experiments. Figure 5 shows the amount of metalaxyl desorbed from the soil as a function of time in the stirred flow chamber experiments. The metalaxyl desorption was rapid; at 5 minutes, the amount of metalaxyl desorbed represented 50% of the metalaxyl that was desorbed during the experiment, whereas in Soil 2, this percentage increased to 80 %.

0

15

30

45

60

0 20 40 60

qS

(µm

ol k

g-1

)

t (min)

S 1

0

15

30

45

60

0 20 40 60

qS

(µm

ol k

g-1

)

t (min)

S 2

Figure 4. Cumulative metalaxyl adsortion (qs) as a function of time (t) in the two studied soils, as determined in the stirried flow chamber experiments.

0

10

20

30

40

0 5 10 15 20

qd

(µm

ol k

g-1

)

t (min)

S 1

0

10

20

30

40

0 5 10 15 20

qd

(µm

ol k

g-1

)

t (min)

S 2

Figure 5. Cumulative metalaxyl desorption (qd) as a function of time (t) in the two soil samples, as determined in the stirred flow chamber experiments.

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The data from Figure 5 were fitted to a pseudo-first order model (6).

dqd/dt = kd (q0 – qd) (6)

where dqd/dt (µmol kg-1 min-1) is the metalaxyl desorption rate, kd (min-1) is the metalaxyl desorption rate constant, q0 (µmol kg-1) is the amount of metalaxyl that could be desorbed under experimental conditions, and qd (µmol kg-

1) is the amount of metalaxyl desorbed from the soil.

The metalaxyl desorption kinetics were well fitted to the pseudo-first order model (6) with R2 values of 0.93 for Soil 1 and 0.90 for Soil 2. The amount of metalaxyl that could be desorbed under experimental conditions (q0) was 43.25 and 9.21 µmol kg-1 for Soil 1 and Soil 2, respectively. These values represented 88 and 91% of the metalaxyl previously adsorbed. Monkiedje and Spiteller9 suggest that the soil-metalaxyl interactions are notably weak, making the metalaxyl adsorption in

Table 5. Summary of the column parameters. C0: initial concentration, L: column length, D: column diameter, ρρρρ: bulk density determined experimentally, θθθθ: porosity, q: flow rate, v: average pore-water velocity, R: retardation factor.

º C0

(µM)

L

(cm)

D

(cm)

ρρρρ

(g cm-3)

θθθθ

(cm3 cm-3)

q

(cm3 min-1)

v

(cm min-1)

R

S1 7.2 8 1.5 0.64 0.63 0.30 0.27 7.8

S2 7.2 8 1.5 1.06 0.56 0.30 0.31 2.1

the soil highly reversible. This weak bond could be attributed to the low hydrophobicity of metalaxyl.11 These percentages of metalaxyl desorption were significantly higher than those found in the batch equilibrium test. In a previous work, Bermudez-Couso et al.16 suggested that these differences could be attributed to the continuous removal of desorbed metalaxyl in the stirred flow chamber experiments, whereas in the batch experiments, the metalaxyl desorption could be limited by the amount present in solution. Higher percentages of desorption using stirred flow chamber experiments than using batch experiments were also found for other organic contaminants22 and for heavy metals.23 The metalaxyl desorption rate constant (kd) was 0.118 min-1 for Soil 1 and 0.337 min-1 for Soil 2. These values were higher than the ks values, indicating that metalaxyl desorption is a faster process than metalaxyl adsorption. Moreover, the metalaxyl desorption from Soil 2 was faster than from Soil 1.

3.5 Column test

3.5.1 Tracer and metalaxyl transport in soil columns

Figure 6 shows the breakthrough curves (BTCs) for bromide and metalaxyl in the two studied soils. In all cases, a strong symmetry between adsorption and desorption was found,

suggesting similar limitations for adsorption and desorption processes. These results were different from those found by Marin-Benito et al.13 in soil columns under unsaturated conditions in which no symmetry was found between adsorption and desorption. This discrepancy suggests that the breakthrough curves could be influenced by the saturation conditions.

In both soils, the BTCs for the tracer (Br-) were similar, reaching the inflow concentration at approximately 2.5 pore volumes (PV). This finding suggests that the differences in the leaching of metalaxyl between soils were not related to water flow, and therefore the differences in metalaxyl adsorption could be studied by this method. The BTCs for metalaxyl were significantly different in Soil 1 and in Soil 2. In Soil 2, the inflow concentration was reached at 7 PV, whereas in Soil 2, only 85% of the inflow solution was reached at the end of the experiment (13.5 PV).

Another difference between the BTCs of Soil 1 and Soil 2 was the slope of the curves (Figure 6), which was significantly higher in Soil 2 than in the Soil 1, suggesting faster adsorption and desorption processes in Soil 2 than in Soil 1. This result agrees with the results from the stirred flow chamber, suggesting that differences in the

metalaxyl adsorption capacity were more important than differences in the column characteristics. Table 5 shows the column characteristics and the retardation factor (R),

which was calculated according to the next equation (7):24

R = 1 + (ρ KD/θ) (7)

174

where ρ is the bulk density, KD is the coefficient of distribution (cm3 g-1) and θ is the porosity (cm3 cm-3). When there is no adsorption, KD = 0, and R = 1.

The values of ρ and θ (Table 5) suggest that R should be higher in Soil 2 than in Soil 1. However, R was 7.8 for Soil 1 and 2.1 for Soil 2, indicating that the KD parameter was more important than ρ and θ, i.e., the differences in the metalaxyl adsorption capacity were more important than the differences in the column characteristics. The R values of both soils are in the same order of magnitude as those found by Marin-Benito et al.13 under unsaturated conditions.

3.5.2 Metalaxyl adsorption using column experiments

Figure 7 shows the amount of metalaxyl adsorbed as a function of time.

The amount of metalaxyl adsorbed at the end of the experiment was 43.9 µmol kg-1 in Soil 1 and 7.3 µmol kg-1 in Soil 2. These results agreed with the batch and stirred flow chamber experiments. Therefore, the three methods tested in this study provided the same qualitative result, i.e., the soil with higher organic matter and clay content has a higher capacity to adsorb metalaxyl. This result agrees with previous work,11,14,16 and therefore the three methods are adequate to compare the metalaxyl adsorption capacity from different soils.

The results from Figure 7 were well fitted to the pseudo-first order equation (4) with R2 values of 0.94 and 0.96 for Soil 1 and Soil 2, respectively. The qmax values were 49.0 µmol kg-1 for Soil 1 and 7.3 µmol kg-1 for Soil 2.

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Figure 6. Experimental breakthrough curves for bromide (circles) and metalaxyl (triangles) in the column experiments.

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Figure 7. Cumulative metalaxyl adsorption (qs) as a function of time (t) in the two studied soils, as determined in the column experiments.

175

S1

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Figure 8. Cumulative metalaxyl desorption (qd) as a function of time (t) in the two soil samples, as determined in the column experiments.

These values were higher than those found with the batch method (32.8 and 6.4 µmol kg-1) but lower than those found with the stirred flow chamber (54.7 and 10.1 µmol kg-1), i.e., the metalaxyl adsorption capacity in one soil is different depending on the method employed in the following sequence: stirred flow chamber > column > batch for the same initial metalaxyl concentration. The metalaxyl adsorption rate constants (ks) obtained using the column experiments were 0.006 min-1 for Soil 1 and 0.031 min-1 for Soil 2. As with the qmax, the ks values were higher in the stirred flow chamber experiments (0.032 and 0.275 min-1 in Soil 1 and Soil 2, respectively) than in the column experiments, and the results for both experiments were higher than in the batch experiments (0.0037 min-1 in Soil 1 and 0.0045 min-1 in Soil 2). The differences in both qmax and ks among the three methods tested could be attributed to the different limitations of each method.21 In the batch methods, the soil suspension with metalaxyl was continuously stirred; however, the metalaxyl concentration in the solution diminished with time, and equilibrium was reached at a metalaxyl concentration lower than the initial concentration. In the column experiments, the

soil remains without stirring, and therefore film diffusion could be an important limitation, and the metalaxyl could not reach certain adsorption sites. These shortcomings are eliminated in the stirred flow chamber methods in which the soil suspension is continuously stirred, and equilibrium is reached at the inflow metalaxyl concentration. In the stirred flow chamber, the metalaxyl is adsorbed mainly via fast reactions,16 and it is well-known that fast reactions are limited by intraparticle diffusion.25 These results show that the equilibrium concentration had more importance than film diffusion on the metalaxyl adsorption capacities and kinetics. Moreover, intraparticle diffusion is a limitation with less importance than the limitations in the other two processes.

3.5.3 Metalaxyl desorption using column experiments

The results for the metalaxyl desorption kinetics using the column technique are shown in Figure 8. These results were well fitted to the pseudo-first order model (6) with R2 values of 0.94 in Soil 1 and 0.98 in Soil 2. The values of q0 were 34.4 and 6.7 µmol kg-1 for Soil 1 and Soil 2, respectively, representing 78 and 92% of the metalaxyl previously adsorbed in the

e soil. These results are notably similar to those found with the stirred flow chamber and higher than those found with the batch experiments. This means that the continuous removal of desorbed metalaxyl is important to an adequate determination of metalaxyl adsorption reversibility. With respect to the metalaxyl desorption rate constant (kd), using column experiments, the rate was 0.007 min-1 in Soil 1 and 0.026 min-1 in Soil 2. These values were close to the ks values found in metalaxyl adsorption using the column test; the adsorption and desorption rates are similar. This discrepancy with respect to the stirred flow chamber experiments was probably due to the

high limitation of film diffusion with respect to the other kinetic limitations in the column experiments.

4 CONCLUSION

The three methods tested in this study show that the soil with higher organic matter and clay content has a higher capacity to adsorb metalaxyl. The metalaxyl adsorption capacity in one soil is different depending on the method employed in the following sequence: stirred flow chamber > column > batch for the same initial metalaxyl concentration. The percentage of metalaxyl desorbed was higher in Soil 2 than in Soil 1 in the three methods tested, suggesting

176

that the irreversibility of metalaxyl adsorption was increased as the soil organic matter content increased. The differences among the three tested methods can be attributed to the limitations of each methods, in batch experiments, the suspension was continuously stirred, however, the amount of metalaxyl in the solution decreased in the time and equilibrium was reached at a metalaxyl concentration lower than the initial concentration. The metalaxyl desorption could be limited by the amount present in solution. In the column experiments, the soil remains without stirring, and therefore film diffusion could be an important limitation, and the metalaxyl could not reach certain adsorption sites. These shortcomings are eliminated in the stirred flow chamber methods in which the soil suspension is continuously stirred, and equilibrium is reached at the inflow metalaxyl concentration.

ACKNOWLEDGMENT

This work was funded by the INCITE program of the Galician Council of Innovation and Industry (Ref. 08PXIB383190PR). A. Bermudez-Couso is funded by the predoctoral program of the University of Vigo.

REFERENCES

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2 Kamrin, M.A. Pesticide profiles. Toxicity, environmental impact, and fate, CRC Press: Boca Raton: FL, (1997).

3 EPA 40 CFR Part. 180 of May 15, 2009, 74 (93), Rules and Regulations 23045-23095. (2009).

4 Arias-Estévez M, López-Periago E, Martínez-Carballo E, Simal-Gándara J, Mejuto JC and García-Río L, The mobility and degradation of pesticides in soils and the pollution of groundwater resources. Agric Ecosyst Environ 123:247-260 (2008).

5 De Jonge RJ, Breure AM and Van Andel JG, Reversibility of adsorption of aromatic compounds onto powdered activated carbon (PAC). Water Res 30:883-892 (1996).

6 Krishna KR and Philip L, Bioremediation of single and mixture of pesticide-contaminated soils by mixed pesticide-enriched cultures. Appl Biochem Biotechnol 164:1257-1277 (2011).

7 Sharma D and Awasthi MD, Adsorption and movement of metalaxyl in soils under unsaturated flow conditions. Plant Soil 195:293-298 (1997).

8 Sukul P and Spiteller M, Influence of biotic and abiotic factors on dissipating metalaxyl in soil. Chemosphere 45:941-947 (2001).

9 Monkiedje A and Spiteller M, Sorptive behavior of the phenylamide fungicides, mefenoxam and metalaxyl, and their acid metabolite in typical Cameroonian and German soils. Chemosphere 49:659-668 (2002).

10 Arias M, Paradelo M, López E and Simal-Gándara J, Influence of pH and soil copper on adsorption of metalaxyl and penconazole by the surface layer of vineyard soils. J Agric Food Chem 54:8155-8162 (2006).

11 Marín-Benito JM, Sánchez-Martín MJ, Soledad-Andrades M, Pérez-Clavlto M and Rodríguez-Cruz MS, Effect of spent mushroom substrate amendment of vineyard soils on the behavior of fungicides: 1. Adsorption-desorption of penconazole and metalaxyl by soils and subsoils. J Agric Food Chem 57:9634-9642 (2009).

12 Baglieri A, Gennari M, Arena M and Abbate C, The adsorption and degradation of chlorpyriphos-methyl, pendimethalin and metalaxyl in solid urban waste compost. J Environ Sci Health Part B Pestic Food Contamin Agric Wastes 46:454-460 (2011).

13 Marín-Benito JM, Rodríguez-Cruz MS, Andrades MS and Sánchez-Martín MJ, Effect of spent mushroom substrate amendment of vineyard soils on the behavior of fungicides: 2. Mobility of penconazole and metalaxyl in undisturbed soil cores. J Agric Food Chem 57:9643-9650 (2009).

14 Andrades MS, Sánchez-Martín MJ and Sánchez-Camazano M, Significance of soil properties in the adsorption and mobility of the fungicide metalaxyl in vineyard soils. J Agric Food Chem 49:2363-2369 (2001).

15 Fernandes MC, Cox L, Hermosín MC and Cornejo J, Adsorption-desorption of metalaxyl as affecting dissipation and leaching in soils: Role of mineral and organic components. Pest Manag Sci 59:45-552 (2003).

16 Bermúdez-Couso A, Fernández-Calviño D, Pateiro-Moure M, Garrido-Rodríguez B, Nóvoa-Muñoz JC and Estévez MA, Adsorption and desorption behavior of metalaxyl in intensively cultivated acid soils. J Agric Food Chem 59:7286-7293 (2011).

17 López-Periago JE, Arias-Estévez M, Nóvoa-Muñoz JC, Fernández-Calviño D, Soto B, Pérez-Novo C and Simal-Gándara J, Copper retention kinetics in acid soils. Soil Sci Soc Am J 72:63-72 (2008).

18 Chotpantarat S, Ong SK, Sutthirat C and Osathaphan K, Competitive sorption and transport of Pb2+, Ni2+, Mn2+, and Zn2+ in lateritic soil columns. J Hazard Mater 190:391-396 (2011).

19 Giles CH, Smith D and Huitson A, A general treatment and classification of the solute adsorption isotherm. I. Theoretical. J Colloid Interface Sci 47:755-765(1974).

20 Vidal M, Santos MJ, Abrão T, Rodríguez J and Rigol A, Modeling competitive metal sorption in a mineral soil. Geoderma 149:189-198 (2009).

21 Amacher MC, Methods of obtaining and analyzing kinetic data, In Rates of Soil Chemical Processes, Sparks, D.L. and Suarez, D.L., Eds.; SSSA Special Publication No. 27: Madison, WI, pp. 19-59 (1991).

22 Bermúdez-Couso A, Fernández-Calviño D, Pateiro-Moure M, Nóvoa-Muñoz JC, Simal-Gándara J and Arias-Estévez M, Adsorption and desorption kinetics of carbofuran in acid soils. J Hazard Mater 190:159-167 (2011).

23 Fernández-Calviño D, Pérez-Novo C, Bermúdez-Couso A, López-Periago E and Arias-Estévez M, Batch and stirred flow reactor experiments on Zn sorption in acid soils. Cu competition. Geoderma 159:417-424 (2010).

24 Toride N, Leij FJ and van Genuchten MT, The CXTFIT Code for Estimating Transport Parameters from Laboratory or Field Tracer Experiments, Version 2.1. Research Report No. 137: U.S. Salinity Laboratory, USDA, ARS, Riverside, Ca, (1999).

25 Aharoni C and Sparks DL, Kinetics of soil chemical reactions – A theoretical treatment. In Rates of Soil Chemical Processes, Sparks, D.L. and Suarez, D.L., Eds.; SSSA: Madison, WI, pp. 1-18 (1991).

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Anexo V. Influence of different abiotic and biotic

factors on the metalaxyl and carbofuran dissipation

Chemosphere xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Chemosphere

journal homepage: www.elsevier .com/locate /chemosphere

Influence of different abiotic and biotic factors on the metalaxyl andcarbofuran dissipation

Alipio Bermúdez-Couso, Juan Carlos Nóvoa-Muñoz, Manuel Arias-Estévez, David Fernández-Calviño ⇑Área de Edafoloxía e Química Agrícola, Departamento de Bioloxía Vexetal e Ciencia do Solo, Universidade de Vigo, Facultade de Ciencias, 32004 Ourense, SpainCITI (Centro de Investigación, Transferencia e Innovación), University of Vigo, Tecnopole, San Cibrao das Viñas, Ourense, Spain

h i g h l i g h t s

" Metalaxyl and carbofuran dissipation are unaffected by soil microbes in short time scales." Metalaxyl and carbofuran dissipation depends on photodegradation and adsorption." Photodegradation has a stronger effect than adsorption.

a r t i c l e i n f o

Article history:Received 25 July 2012Received in revised form 19 October 2012Accepted 29 October 2012Available online xxxx

Keywords:SoilCarbofuranMetalaxylDissipationAdsorptionPhotodegradation

0045-6535/$ - see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.chemosphere.2012.10.090

⇑ Corresponding author. Address: Área de EdafDepartamento de Bioloxía Vexetal e Ciencia do SFacultade de Ciencias, 32004 Ourense, Spain. Tel.988387001.

E-mail address: [email protected] (D. Fernández-C

Please cite this article in press as: Bermúdez-CoChemosphere (2012), http://dx.doi.org/10.1016

a b s t r a c t

Metalaxyl and carbofuran dissipation was studied in response to different factors (soil bacterial commu-nities, light irradiation, presence of an inorganic culture medium and presence of soil) and combinationsof these factors in short-term experiments (48 h). The soil microbial communities have no effect on met-alaxyl or carbofuran dissipation in the time scale employed. Light irradiation and soil promote metalaxyland carbofuran dissipation by photodegradation and adsorption, respectively. However, photodegrada-tion has a stronger effect on metalaxyl and carbofuran dissipation than the adsorption of the pesticidesin the soil. The addition of the culture medium have no direct effect on pesticide dissipation, degradationby microbial communities or adsorption but its presence greatly increased photodegradation.

� 2012 Published by Elsevier Ltd.

1. Introduction

The increase of agricultural production during the last centurywas associated with the use of pesticides to fight against plant dis-eases. However, only a small amount of an applied pesticide is incontact with the organisms responsible for diseases (Pimentel,1995), whereas a large amount of the pesticide reaches the soil,causing potential risks to the environment and to human health(EPA, 2009).

Pesticides in the soil can be affected by physical, chemical andbiological processes such as adsorption on soil colloids, uptakeby plant roots, transport via soluble and particulate runoff, infiltra-tion, volatilisation, and chemical and biological degradation (Arias-Estévez et al., 2008). The dissipation of pesticides from soil solution

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uso, A., et al. Influence of differ/j.chemosphere.2012.10.090

has a key role in their potential to damage plants and soil macroand microorganisms and to contaminate nearby water sources.The dissipation depends on the pesticide structure and chemicalcharacteristics, the physicochemical soil characteristics, the soilbiology and various environmental factors (Arias-Estévez et al.,2008).

Carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methyl-carbamate) is a carbamate insecticide that is highly usedworldwide. Carbofuran has a high solubility in water (700 mg L�1)(Worthing, 1991), thus it can reach the groundwater via lixiviationand could cause problems relating to human health. Metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyl) alanine methyl ester] is asystemic fungicide used in many countries to fight against plantdiseases caused by fungi such as Pythium spp. and Phytopthoraspp. Its solubility in water (8400 mg L�1) is higher than that of car-bofuran; therefore, metalaxyl can also more easily reach thegroundwater.

Different metalaxyl and carbofuran dissipation mechanismswere studied. Metalaxyl and carbofuran sorption was studied bymany authors, showing that their dissipation from soil solution

ent abiotic and biotic factors on the metalaxyl and carbofuran dissipation.





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2 A. Bermúdez-Couso et al. / Chemosphere xxx (2012) xxx–xxx

depends mainly on clay and organic matter content (Andradeset al., 2001; Rama Krishna and Philip, 2008; Bermúdez-Cousoet al., 2011a, 2011b). Other processes, such as photodegradationand microbial degradation, were also studied (Trabue et al.,2001; Massoud et al., 2008; Baker et al., 2010; Lu et al., 2011;Plangklang and Reungsang, 2012). These processes could have anantagonistic effect. For example, pesticide adsorption on soil col-loids removes them from the soil solution, thereby preventing deg-radation (Krishna and Philip, 2011). The effects of the differentprocesses should be studied to improve the overall dissipation ofpesticides from the soil. However, studies comparing all possibledissipation mechanisms and their interactions in short periods oftime have not been performed.

The main objective of this study was to evaluate the effects ofsorption, photodegradation and microbial degradation and theirinteractions on metalaxyl and carbofuran dissipation in short peri-ods of time. To this end, experiments have been carried out inwater solutions in the absence and presence of soils and in thepresence of light or darkness.

2. Materials and methods

2.1. Chemicals

Carbofuran (2,3-dihydro-2-2-dimethyl-7-benzofuranyl-N-meth-ylcarbamate) with 99.5% purity was obtained from Dr. EhrenstorferLab. (Augsburg, Germany). Metalaxyl [methyl N-methoxyacetyl-N-(2,6-dimethyl)-DL-alaninate] with a purity >99.5% was obtained fromSigma–Aldrich (Steinheim, Germany). The chemical structures andselected properties of carbofuran and metalaxyl are shown in TableS1. All organic solvents used for sample preparation were residueanalysis grade and were supplied by Panreac (Barcelona, Spain). HPLCgrade solvents for HPLC work were also supplied by Panreac.

2.2. Soil samples

The soil samples used were collected in the ‘‘A Limia’’ district(Galicia, NW Iberian Peninsula). This area is devoted to intensivefarming of potato and wheat in rotation with other crops. Twosamples were collected from the topmost layer (0–20 cm) andwere air dried, sieved through a 2 mm mesh and stored in polyeth-ylene bottles until analysis. The soil characteristics were previouslydescribed by Bermúdez-Couso et al. (2012). The texture of the soilswas clay for Soil 1 and clay loam for Soil 2; both samples were acid(pH 4.5 for Soil 1, pH 4.4 for Soil 2). Soil organic carbon was 22.7%and 2.7% for Soil 1 and for Soil 2, respectively.

2.3. Pesticide toxicity to soil bacteria

The pesticide toxicity to the bacterial community was measuredusing the leucine (Leu) incorporation technique (Kirchman et al.,1985) to determine the bacterial growth. The bacterial communitywas extracted from the soil with the homogenisation and centrifu-gation technique described by Bååth (1994) and Bååth et al. (2001).The soils (2 g, fresh weight) were mixed at maximum intensitywith 20 ml distilled water for 3 min using a multivortex shaker.The mixtures were then centrifuged at 1000 g for 10 min to createa bacterial suspension in the supernatant. Aliquots of 1.35 ml wereadded to microcentrifugation vials. Then, 0.15 ml of different con-centrations of either metalaxyl or carbofuran in water was addedto each vial. Seven different concentrations of each pesticide wereused for each soil type. A control with distilled water substitutedfor pesticide solution was included for each soil. Next, 2 lL[3H]Leu (37 MBq ml�1 and 5.74 TBq mmol�1; Amersham) wasadded along with unlabeled Leu to each tube, resulting in

Please cite this article in press as: Bermúdez-Couso, A., et al. Influence of differChemosphere (2012), http://dx.doi.org/10.1016/j.chemosphere.2012.10.090

275 nM Leu in the bacterial suspensions. After 2 h of incubation,bacterial growth was stopped with 75 lL of 100% trichloroaceticacid. Washing and subsequent measurement of radioactivity wereperformed as described by Bååth et al. (2001) using an scintillatorTriCarb 2810TR. The results for the different pesticide concentra-tions were standardised with respect to the bacterial growth inthe presence of distilled water.

2.4. Pesticide dissipation experiments

The dissipation experiments were performed with metalaxyl(10 mg L�1) and carbofuran (4 mg L�1) solutions at different solu-tion conditions to separate the different pesticide dissipationmechanisms: photodegradation, microbial degradation, chemicaldegradation and adsorption. The pesticide solutions were madewith distilled water and then passed through 0.20 lm pore sizecellulose nitrate filters to remove the microorganisms from thesolutions. A total of 10 mL of each of these solutions was thenmixed with 10 mL of one of three different solutions in the40 mL EPA vials: (a) distilled filtered water, (b) a soil bacterial sus-pension extracted according to the methods of Bååth (1994) andBååth et al. (2001), and (c) suspension with 1 g of soil. Then, thevials were placed in the simulation light cabin equipped with a xe-non lamp with an output of 550 W m�2 and quartz filters with acut-off at 285 nm (Atlas Suntest CPS + Atlas, Germany). Two typesof experiments were performed for all treatments: (a) in darknessfor 48 h and (b) with a cycle of 12 h light and 12 h darkness over48 h. The temperature during the experiments was kept at32 ± 2 �C in the light phase and 25 ± 2 �C during the darknessphase. All experiments were performed in triplicate. Subsamplesof 0.2 mL were taken at appropriate time intervals, and the metal-axyl or carbofuran concentrations were measured.

Because it had been previously found that the microorganisms’growth could be limited by the absence of any nutrient, the experi-ments were reproduced but changing the distilled filtered watersolution containing the pesticides by a filtered culture medium con-taining the same pesticide concentrations. The culture medium wasmade with filtered distilled water and 1.75 mg L�1 K2HPO4, 0.50 mgL�1 KH2PO4, 0.58 mg L�1 NH4Cl and 0.25 mg L�1 MgSO4�7H2O.

In the experiments with the soil, once the dissipation tests werefinished, the pesticides retained in the soil were extracted. The pro-cedure employed was the same for both carbofuran and metalaxyl,except that methanol was used as the solvent for metalaxyl andacetonitrile was the solvent for carbofuran. The procedure was asfollows: the vials were opened and incubated at 40 �C to evaporateall the liquid. Once dry, 10 mL of solvent was added to the vials,which were then shaken for 30 min. The vials were placed in asonicator for 5 min and centrifuged at 2000 rpm for 10 min. Aftercentrifugation, the supernatant was transferred to a 60 mL EPAvial. This procedure was repeated three times. The combined ex-tracts (10 + 10 + 10 mL) were concentrated using a nitrogen evap-orator (Caliper LS, Turbovap LV, Caliper, USA). The dried residuewas dissolved in 1 mL of distilled water, transferred to a 2 mL vialand then analysed by HPLC-UV. The pesticide extracted by thismethod is considered the adsorbed pesticide. The concentrationof pesticides in soil studied before dissipation experiments werelower than the detection limit (0.1 lmol kg�1 and 0.04 lmol kg�1

for metalaxyl and carbofuran, respectively).

2.5. Carbofuran and metalaxyl determination

The determination of pesticides was performed using high per-formance liquid chromatography (HPLC) with a UV–Visible detec-tor from Dionex Corporation (Sunnyvale, EE.UU.), equipped with aP680 quaternary pump, an ASI-100 autosampler, a TCC-10thermostatic column compartment and a UVD170U detector.



A. Bermúdez-Couso et al. / Chemosphere xxx (2012) xxx–xxx 3

Chromatographic separation was performed on a Symmetry C18column (4.6 � 150 mm, 5 lm) obtained from Waters (Milford,MA, USA) and a C18 guard column (4.6 � 50 mm, 5 lm) packetwith the same material. Test conditions for Carbofuran were as fol-lows: isocratic elution with 30:70 acetonitrile/water at room tem-perature, flow rate 1 mL min�1, injected volume of 150 lL, UVdetection performed at k = 278 nm and a 15 min duration for thechromatogram. For metalaxyl, the mobile phases were methanol(A) and water (B), and the gradient program was as follows: 60%A + 40% B for 7 min, change to 95% A + 5% B for 3 min, hold for5 min, change to 60% A + 40% B for 0.1 min, and hold for 10 min.The total analysis time was 25 min, the injected volume was50 lL and the flow rate was 0.7 mL min�1. Metalaxyl was detectedat k = 200 nm.

3. Results and discussions

3.1. Pesticide toxicity to soil bacteria

In both soils, toxicity tests showed that increases in the metal-axyl concentration in the soil solution had no negative effects onthe soil bacterial community growth rates (Fig. 1a). On the con-trary, the metalaxyl presence in the soil solution stimulated bac-terial growth in some cases; in these cases, the bacterialcommunity could use metalaxyl as a carbon source in degradationprocesses (Baker et al., 2010). These results showed that metal-axyl concentrations below 200 mg L�1 had no toxicity effects onbacterial communities in either soil solution. Carbofuran toxicitytests (Fig. 1b) showed that the Soil 2 bacterial communities werenot inhibited by carbofuran concentrations below 200 mg L�1.However, the Soil 1 bacterial communities were inhibited by car-bofuran concentrations over 20 mg L�1. These results suggest thatthe Soil 1 bacterial communities were more sensitive to carbofu-ran concentrations than the Soil 2 bacterial communities. This dis-crepancy may be explained by the different organic mattercontents of the soils. Soil 1 had significantly higher organic mattercontent than Soil 2, thus the carbofuran adsorption capacity wassignificantly higher in Soil 1 (10.8 ± 0.1 mg kg�1) than in Soil 2(5.2 ± 0.1 mg kg�1). The development of tolerance to a toxicantis based on the fact that the organisms in a toxic environment sur-vive only if they are tolerant to the toxicant. The most tolerantorganisms will outgrow the more sensitive organisms, resultingin an increased average community tolerance (Blanck et al.,1988). Due to the Soil 2 low carbofuran adsorption capacity, itsbacterial communities were more exposed to carbofuran thanthose found in Soil 1, resulting in the Soil 1 bacterial communitiesbeing less tolerant to carbofuran than the bacterial communitiesfrom Soil 2.

0.0

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Fig. 1. Relative bacterial growth as a function of the pest


3.2. Pesticide dissipation

3.2.1. Experiments with filtered distilled waterThe dissipation experiments performed in filtered distilled

water showed no dissipation of either metalaxyl or carbofuran inthe darkness (Fig. 2). However, during the 12 h light/12 h dark test,the dissipation of both pesticides occurred quickly and at high lev-els (Fig. 2). After 10 h of irradiation, the percentages of the pesti-cides dissipated were 47 ± 4% for metalaxyl and 43 ± 5% forcarbofuran. After 48 h (24 h of irradiation), the percentages dissi-pated were 90 ± 3 and 87 ± 2% for metalaxyl and carbofuran,respectively. The half-lives of the pesticides under the experimentalconditions were calculated using a logistic model: Y = 1/[1 + eb(X�a)](Doelman and Haanstra, 1989), where Y is the measured level ofpesticides relative to the initial concentration (C/C0), X is the loga-rithm of the time (h), a is the logarithm of the half life (h), and bis a slope parameter indicating the pesticide dissipation rate. Allof the irradiation experimental data fit well with the model withR2 values higher than 0.95 for both the metalaxyl and carbofuranexperiments. The half-life of the pesticides with irradiation and dis-tilled filtered water were 13 and 16 h for metalaxyl and carbofuran,respectively. This half-life period is less than those reported byother authors. Thus, Tomlin (2003) reported a range of half-lifefor metalaxyl between 10 and 40 d, although the presence of lightcan stimulate the degradation of metalaxyl and decreasing itshalf-live (Komárek et al., 2010). Carbofuran and metalaxyl photo-degradation in a few hours was also observed in previous studies,except with important differences in the observed rates of degrada-tion (Sukul, 1992; Lu et al., 2011). The differences could be attrib-uted to the different light intensities employed for differentstudies because the degradation of pesticides increases as lightintensity increases (Mahalakshmi et al., 2007).

3.2.2. Experiments with bacterial suspensionsThe dissipation of metalaxyl and carbofuran in the presence of

bacterial suspensions extracted from the two studied soils wassimilar to that observed in the experiments with distilled filteredwater, i.e., increased dissipation with irradiation and an absenceof dissipation under conditions of darkness (Fig. 2). This result im-plies that there was little bacterial degradation. The lack of an ef-fect by microorganisms could be attributed to the short durationof the experiments. In this respect, Plangklang and Reungsang(2011) showed that the half-life of carbofuran degradation by soilmicroorganisms was 127 d, and De Wilde et al. (2010) found a half-life greater than 131 d for metalaxyl. However, our experimentswere performed in 2 d. Baker et al. (2010) observed that physico-chemical characteristics such as pH were more important for met-alaxyl degradation than the microbial composition. For metalaxyl,the half-life obtained under irradiation was similar to that for

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icide concentration. Soil 1 (triangles), Soil 2 (circles).



(a)

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(a2)

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(b2)

Fig. 2. Metalaxyl and carbofuran dissipation in filtered distilled water (a and b) and in the presence of a soil bacterial suspension from Soil 1 (a1 and b1) and Soil 2 (a2 andb2).


distilled filtered water with both soil bacterial suspensions (13 hfor the Soil 1 bacterial suspension and 12 h for the Soil 2 bacterialsuspension). However, for carbofuran, the half-life was signifi-cantly reduced in response to the irradiation with respect to theexperiments performed with distilled filtered water. The carbofu-ran half-lives were 6 and 7 h in the presence of the bacterial sus-pensions from Soil 1 and Soil 2, respectively. The increase incarbofuran degradation could be attributed to the chemical com-position of the bacterial suspension (Table S2) because some ele-ments such as nitrates can enhance organic compoundsphotodegradation (Nélieu et al., 2009; Gatidou and Iatrou, 2011).

3.2.3. Experiments with soilIn the experiments with soil, the dissipation of both metalaxyl

and carbofuran was produced in darkness and with radiation, butthe dissipation was higher with radiation than in darkness for bothpesticides (Fig. 3). In the irradiation experiments, the half-lives formetalaxyl were 6.9 and 7.8 h for Soil 1 and Soil 2, respectively,whereas for carbofuran, the half-lives were 4.2 and 5.1 h for Soil 1and Soil 2, respectively. These values were clearly lower than thosefound with filtered distilled water and with the bacterialsuspensions, showing that the dissipation of metalaxyl and


carbofuran is faster in presence of soil compared to the dissipationrates that occur in the absence of soil. The higher dissipation of pes-ticides in the presence of soil can be attributed to the adsorption ofpesticides in the soil or to the presence of microorganisms in thesoil that were not present in the bacterial suspension. The half-livesfor the darkness experiments were significantly higher than thosefor the irradiation experiments for both soils and both pesticides(104.4 and 54087.9 h for metalaxyl in Soil 1 and Soil 2, respectively;153.0 and 907.8 h for carbofuran in Soil 1 and Soil 2, respectively).Moreover, the results that fit well with the logistic model presentedthe worst R2 results, with values between 0.74 and 0.85. In theexperiments performed with soil, the metalaxyl and carbofuran dis-sipation could be attributed to pesticide degradation or to adsorp-tion on soil particles. In the absence of irradiation, the amount ofmetalaxyl dissipated was 31.5 ± 1.7 mg kg�1 with Soil 1 and8.5 ± 1.3 mg kg�1 with Soil 2 (Table S3). These amounts were simi-lar to those extracted from the soil once the experiments werefinished (33.2 ± 2.7 and 8.3 ± 1.4 mg kg�1 from Soil 1 and Soil 2,respectively) (Table S3). The pattern for carbofuran was similar.The carbofuran dissipation in darkness was 11.0 ± 0.5 and4.8 ± 0.1 mg kg�1 with Soil 1 and Soil 2, respectively (Table S3).The amount of carbofuran extracted from soils after the dissipation



(a) (b)

(c) (d)

Fig. 3. Metalaxyl and carbofuran dissipation in the presence of soils. Soil 1 (a and c), Soil 2 (b and d).


experiments was 10.8 ± 1.0 1 mg kg�1 in Soil 1 and5.2 ± 0.1 mg kg�1 in the Soil 2 (Table S3). In darkness both metal-axyl and carbofuran dissipation was dependent on the characteris-tics of the soil, especially organic carbon: Soil 1 with 22.7% oforganic carbon showed higher values of dissipation; Soil 2 with2.7% of organic carbon showed less values of dissipation. Therefore,the metalaxyl and carbofuran dissipation in the presence of soil indarkness was due to adsorption on soil particles and not to micro-bial degradation. For metalaxyl, the pesticide dissipation underirradiation in the presence of soil after 48 h was 96.0 ± 0.6 mg kg�1

for Soil 1 and 97.3 ± 2.2 mg kg�1 for Soil 2, whereas for carbofuran,the dissipation was 28.9 ± 1.2 for Soil 1 and 28.3 ± 1.3 mg kg�1 forSoil 2 (Table S3). These concentrations were significantly higherthan the pesticide concentrations extracted from the soils afterthe dissipation experiments were finished. The amount of metal-axyl extracted was 9.0 ± 0.5 mg kg�1 in Soil 1 and 1.9 ± 0.3 mg kg�1

in Soil 2. For carbofuran, the amount extracted was 4.3 ± 0.5 and1.7 ± 0.1 mg kg�1 in Soil 1 and Soil 2, respectively (Table S3). Theseresults showed that under irradiation, the metalaxyl and carbofu-ran dissipation was produced mainly by photodegradation (>85%),with a small degree of the dissipation occurring via adsorption onthe soil particles. It is remarkable that the amount of pesticide ad-sorbed in the soil was always higher in the experiments in darknessthan in the experiments under irradiation for both pesticides andboth soils. This behaviour could be attributed to the lower pesticideconcentration in the solution under irradiation due to the photo-degradation; in the adsorption experiments, the amount adsorbedin the soil was highly dependent on the pesticide concentration inthe soil solution. These results show that when the degradation ofpesticides occurs, the amount of the pesticides that are adsorbedin the soil decreases significantly.

3.2.4. Experiments with filtered distilled water and culture mediumThe results from the pesticide dissipation experiments per-

formed with filtered culture medium are shown in Figs. 4 and 5.


In the presence of filtered distilled water, the dissipation was neg-ligible in darkness, but under irradiation, both the metalaxyl andcarbofuran dissipation was high (>90% after 10 h of irradiation).The half-life was 2.2 h for metalaxyl and 1.8 h for carbofuran.The pesticide dissipation with irradiation in the presence of theculture medium was significantly higher than with distilled water,but in the darkness, there was no effect on pesticide dissipation.These results indicate that the culture medium has no effect onpesticide dissipation by itself, but it greatly enhanced photodegra-dation. The enhancement of the photodegradation observed formetalaxyl, carbofuran and other organic compounds was also ob-served for iron, nitrates, humic acid and titanium solutions (Maha-lakshmi et al., 2007; Lu et al., 2011; Tong et al., 2011). For someantibiotics (Prabhakaran et al., 2009) and fungicides (Maheswariet al., 2010), a significant increase in photolysis was observed inthe presence of phosphates, the main element in our culturemedium.

3.2.5. Experiments with bacterial suspensions and culture mediumWhen the experiments with filtered culture medium and bacte-

rial suspensions were performed in darkness, the metalaxyl andcarbofuran dissipation was negligible for the bacterial suspensionsfrom Soil 1 and from Soil 2. These results confirm that the pesticidedegradation by bacterial activity was not important on the timescale employed in our experiment. Under irradiation, degradationwas important for both metalaxyl and carbofuran, with metalaxylhalf-life values of 2.2 and 1.6 h for Soil 1 and Soil 2 suspensions,respectively, and carbofuran half-life values of 1.1 and 1.0 h for Soil1 and Soil 2 suspensions, respectively. These values were similar tothose found with filtered culture medium and filtered distilledwater, showing that in the presence of culture medium, the effectof the bacterial suspension chemical composition on pesticide dis-sipation was masked by the chemical composition of the culturemedium.



(a) (b)

(a1)

(a2)

(b1)

(b2)

Fig. 4. Metalaxyl and carbofuran dissipation in the presence of culture medium and filtered distilled water (a and b) and Metalaxyl and carbofuran dissipation in the presenceof culture medium and a soil bacterial suspension from Soil 1 (a1 and b1) and Soil 2 (a2 and b2).


3.2.6. Experiments with soils and culture mediumThe metalaxyl and carbofuran dissipation in the presence of soil

and culture medium was similar to the dissipation of these pesti-cides in the presence of soil and filtered distilled water both in dark-ness and with irradiation. With irradiation, the metalaxyl half-lifewas 9.4 and 12.0 h for Soil 1 and Soil 2, respectively, whereas forcarbofuran, the half-life was 4.2 and 6.5 h for Soil 1 and Soil 2,respectively. Similar to the results for soil and filtered distilledwater, the dissipation values in darkness were not well fitted tothe logistic model (R2 < 0.88), and in the case of Soil 2, the datacould not be fitted to the model at all. The half-lives in darknessfor Soil 1 and both pesticides were exceeded 200 h. At the end ofthe experiments performed in darkness, the amount of pesticidedissipated in Soil 1 was 32.5 ± 0.8 and 10.4 ± 1.7 mg kg�1 for metal-axyl and carbofuran, respectively, whereas in Soil 2, the dissipationwas 9.8 ± 1.3 mg kg�1 for metalaxyl and 6.1 ± 0.1 mg kg�1 for car-bofuran (Table S3). These values were similar to those obtainedwith the extraction performed after the dissipation experiments.


The amount of extracted metalaxyl was 30.9 ± 1.2 and9.4 ± 1.0 mg kg�1 for Soil 1 and Soil 2, respectively, while theamount of extracted carbofuran was 11.1 ± 1.3 mg kg�1 for Soil 1and 5.5 ± 0.1 mg kg�1 for Soil 2 (Table S3). As in the experimentsperformed with soil and filtered distilled water, the metalaxyland carbofuran dissipation with irradiation was significantly higherthan the dissipation of these pesticides in darkness. The amount ofmetalaxyl dissipated at the end of the experiments was 95.3 ± 0.8and 96.7 ± 1.5 mg kg�1 for Soil 1 and Soil 2, respectively, whilethe amount of carbofuran dissipated was 28.6 ± 1.0 mg kg�1 for Soil1 and 28.7 ± 0.5 mg kg�1 for Soil 2 (Table S3). These values werehigher than those obtained in the extractions performed after theexperiments. The amount of metalaxyl extracted was8.2 ± 0.4 mg kg�1 and 1.8 ± 0.1 mg kg�1 from Soil 1 and Soil 2,respectively, while the amount of carbofuran extracted was4.3 ± 0.5 and 1.2 ± 0.3 mg kg�1 from Soil 1 and Soil 2, respectively(Table S3). Both the amount of pesticide dissipation and the amountof pesticide adsorbed in the soil were quite similar to those found



(a) (b)

(c) (d)

Fig. 5. Metalaxyl and carbofuran dissipation in the presence of culture medium and soils. Soil 1 (a and c), Soil 2 (b and d).


with soil and filtered distilled water, showing that the culture med-ium had no effect on pesticide dissipation in the presence of soil.

4. Conclusions

Soil bacterial communities of studied soils seem to have a hightolerance to metalaxyl and carbofuran under tested experimentalconditions. The toxicity of these pesticides, estimated as the inhibi-tion of bacterial growth, was not observed at any concentrationstudied (0–200 mg L�1) in the case of metalaxyl, although for oneof the soils a reduction in bacterial growth was detected for carbo-furan concentration above 20 mg L�1.

It was found that light irradiation exerts a crucial role in the dis-sipation of metalaxyl and carbofuran in short-term experiments.However the results also suggested that soil composition, throughchemicals released to bacterial suspensions, could raise slightly thedissipation of carbofuran.

Soil adsorption was the main responsible of metalaxyl and car-bofuran dissipation when the experiments were carried out in thepresence of soil and under darkness, whereas the effect of soilmicroorganisms in the degradation of the pesticides was negligi-ble. The presence of light (during the irradiation experiments)was crucial to explain the dissipation of pesticides, contributingto degrade above 85% of the pesticide in all cases whereas soiladsorption of the pesticides was reduced.

Nutrient supply through a culture medium revealed no effect onpesticide degradation by microorganisms. On the contrary, thecombination of culture medium and irradiation resulted in an in-crease of the pesticide dissipation in those experiments which fil-tered distilled water or bacterial suspensions were used. Theaddition of a culture medium was not a factor of variation of thedissipation of pesticides in presence or absence of soil.


Appendix A. Supplementary material

Supplementary data associated with this article can be found, in theonline version, at http://dx.doi.org/10.1016/j.chemosphere.2012.10.090.

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187

Anexo VI. Pollution of surface waters by metalaxyl and

nitrates from non-point sources

189

Pollution of surface waters by metalaxyl and nitrates from non-point sources

Alipio Bermúdez-Cousoa, David Fernández-Calviñoa, Manuel Ali Álvarez-Enjob,

Jesús Simal-Gándarac, Juan Carlos Nóvoa-Muñoza, Manuel Arias-Estéveza

a) Soil and Agricultural Science Group, Plant Biology and Soil Science Department, Science Faculty, University of Vigo, Ourense Campus, 32004 Ourense, Spain b) Hydraulic Engineering Group, School of Civil Engineering, Canals and Ports, University of A Coruña, Elviña Campus, 15701 A Coruña, Spain c) Nutrition and Bromatology Group, Analytical and Food Chemistry Department, Science Faculty, University of Vigo, Ourense Campus, 32004 Ourense, Spain

ABSTRACT

The mobility of contaminants in soil is highly dependent upon the characteristics of the contaminant chemical and the properties of the soil. As a model study area, we studied the district of “A Limia” (Galicia, NW Spain)—a cropland devoted to growing potatoes, where the soil had been managed intensively over the last 50 years. The soil was characterised by low slopes with the water table located very close to the soil surface. Our aim was to study the influence of high and intensive crop production on the water bodies and non-point source contamination, with a particular focus on metalaxyl and nitrates. The highest concentrations of metalaxyl occurred when rainfalls were low and in a study area where drainage was favoured by the construction of numerous drainage canals. Analysis of variance indicated that the interaction between sampling site and season can explain the variability in metalaxyl concentrations. The spatial and temporal distribution of the nitrates also showed a high variability, with the interaction between seasons and sampling area being the most significant factor in explaining the levels found.

KEYWORDS: metalaxyl; nitrates; surface waters; pollution; non-point sources.

1. Introduction The increase in pesticide application on land devoted to agriculture during the last several decades revealed the potential hazard of pesticides to human health and the environment. The possible contamination of surface waters depends on the agricultural land, the soil properties, the characteristics of the water bodies (depth and flow rate), the characteristics of the land close to the water bodies (soil use, slope, and distance from water bodies) and the climatic conditions (temperature, rainfall, moisture and wind) (Kreuger, 1998; Kreuger and Törnqvist, 1998; Capel et al., 2001; Neumann et al., 2002; Tesfamichael and Kaluarachchi, 2006; Ramos et al., 2000). The mobility of the pesticide in the soil is dependent on physical, chemical and biological processes, including adsorption-desorption, volatilisation, chemical and physical degradation, absorption by plants, runoff and lixiviation (Vryzas et al., 2007; Arias-Estévez et al., 2008). All of these processes are highly dependent on the chemical characteristics of the pesticide and the properties of the soil (Linn et al., 1993). On land devoted to a potato crop, the use of inorganic (NPK) and organic fertilisers is very high. Due to the high water needs of this crop, the pollutants present in the soils devoted to potatoes could be transported to the water bodies by drainage, runoff or spray drift (Bach et al., 2001). In areas with very low slopes, the pollutants movement in the soil is mainly due to

the drainage thought the soil. In this sense, areas with a high presence of intensive agriculture represent an important source of non point pollution, in a lot of cases due to a large concentration of nitrates in the water bodies (Núñez-Delgado et al. 1995; Burkart and Feher, 1996). Also, in areas with high relative moisture in the air and with the water table close to the soil surface the use of high amounts of fungicides is needed to avoid crop yield losses. The district of “A Limia” (Galicia, Northwest Spain) is a cropland where the soil has been intensively managed over the last 50 years. The cropland devoted to potatoes is found on low slopes, with the water table located very close to the soil surface. Therefore, this area has a significant risk of environmental pollution that could cause problems to human health. The water bodies can be polluted by high nitrate concentrations via organic and inorganic fertilisation (the binding capacity of the nitrates to the soil colloids is very low). However, the pesticides used for the protection of crop yields can also reach water bodies. With respect to human health, The European Union Council Directive 98/83/CE on water for human use establishes a maximum limit of 50 mg L-1 for NO3-, of 0.1 µg L-1 for individual pesticides and 0.5 µg L-1 for the sum of all pesticides. In a previous study performed on experimental plots located in the same area, metalaxyl was the only pesticide detected in potatoes and water used for irrigation (López-Pérez et al., 2006)

190

because of its high solubility in water and, hence, its high mobility. The high concentration of nitrates in water bodies is typical in agricultural areas due to the high mobility of nitrates in the soil (Núñez-Delgado et al., 1997; Núñez-Delgado et al., 2002; López Periago et al., 2002). Sustainable management of water resources requires improved information about the relevance of pesticide input. However, river monitoring programs for pesticide contamination are restricted to a limited number of catchments. The aim of this work was to study the influence of highly intensive crop production on the non-point source contamination of the water bodies, with a particular focus on metalaxyl and nitrates. 2. Experimental 2.1. Study area The study area is located in a region called A Limia (Ourense, NW Spain; Figure 1). Geologically, the region is dominated by an old tertiary depression (630 m above sea level) which was intermittently filled with quaternary sediments reaching a thickness up to 200 m (IGME, 1974). The sediments that filled the depression were eroded from its borders where several types of granite, schist and gneiss were the most abundant rocks. With time, the progressive infilling caused the depression to evolve towards a semi-endorreic system crossed in an E-SW orientation by the River Limia, the main outstream of the area that flows into the Atlantic Ocean. The River Limia shows the highest annual flow peaks in January, February, May, August and September (Gómez Nieto, 1996). As result of the interaction between geology, geomorphology and climatology factors through the years, the study area has become a very flat surface (slope <3%) and the origin of one of Spain’s largest natural ponds. This wetland, called Lagoa de Antela, has an extension of 4200 ha and a water depth between 60 and 300 cm. Today, the climatology of the study area is dominated by an Atlantic influence, although some features of the Mediterranean climate such as continentality and severe droughts during summer are also present. Mean annual temperature is 11 ºC (5.1 ºC in January and 18.4 ºC in July), with total annual precipitation at 881 mm on average, irregularly distributed through the year (120 mm in January and 20 mm in July and August). The soils of the area developed from sediments, showing surface horizons with coarse texture while a clay layer appears at a variable depth. Soils are generally acid (water pH <6.0), even those dedicated to agriculture, rich in organic matter in those areas where hydromorphic conditions are still present but relatively poor in well drained areas (approximately 30 g kg-1). The most

characteristic soil types are Leptosols, Umbrisols, Camisols, Gleysols and Histosols (FAO, 1998). 2.2. Land Use In the middle of the 20th century, a serious change in the land use took place due to agricultural exploitation of the study area. A large pattern of desiccation began with the construction of an elaborate system of ditches, draining the water of the pond towards a main drainage canal, which, in turn, flows to the River Limia. Consequently, 3243 ha of the former pond was desiccated and dedicated to cultivation of cereals and potato crops, especially potatoes because of the economic importance of this crop. In the study area, potato, grown in rotation with wheat, extends through 3804 ha showing a crop yield of 30 Mg ha-1 (IGE, 2012) that produces 90000 Mg of potatoes annually (Figure 1). Due to its economic significance, soil water availability, fertiliser management and the effects of plant diseases were the major concerns for the potato crop in the study area. The water requirement of the potato crop and the summer droughts led to the construction of locks on the main drainage canal to keep the water level, using the accumulated water to irrigate the crops. Consequently, the water table rose, and extensive areas were flooded during rainy periods. Because the potato crop demands a large amount of nutrients, organic (mainly chicken manure) and inorganic fertilisers (100-200 kg N ha-1; 100-200 kg P2O5 ha-1; 100-150 kg K2O ha-1) were used as pre-sowing fertilisation (López-Mateo, 2007). Depending on climatic conditions, potato sowing takes place between March and May, and the application of fungicides (mainly metalaxyl) begins as the potato plant commences its growing period up to 21 days before the harvest. Metalaxyl is sprayed above the plant, and the number of treatments depends on climatic conditions during the growing season. The content of metalaxyl in commercial fungicides ranges between 5-25%, whereas the treatment dose varies from 0.50 to 0.08% (5-0.8 kg ha-1, respectively). Figure 2 shows the sowing, fertilisation and metalaxyl application to the potato crop.. 2.3. Water Sampling and flow measurement. Water samples were collected from the water courses in HCl pre-washed glass bottles of 2.5 L of volume with the help of a telescopic water sampler. The first mL of water sampled in each sampling point were used for an additional bottle washed and then discarded, leaving the bottle ready for the definitive water sample. The strategy of water sampling was focused in those areas that showed the highest agricultural

191

(a)

N123

456

20 21

22

789101112

1314

1516 17 18 19

50Km

07º 51' 06'' W

42º 05' 08'' N

Main drainage channels

Secondary drainage channels

River

(b)

N

50Km

07º 51' 06'' W

42º 05' 08'' N

Crops

Pastures

Deciduous trees

Pine

Timber species

Heather

Figure 1. Study area and sampling points (a), together with land use of watershed (b).

activity, being extended in a total 38 km of water courses that included the main drainage canal (also its secondary arm) and the river Limia (Figure 1). Twenty-two sampling points were distributed as follows: 16 in the main

drainage canals, and 6 in the river Limia. Water samples were collected monthly during two years, since March 2009 to March 2011. The water flow was in the main drainage canal was monitored through the sampling period after

192

1 2 3 4 5 6 87 9 10 1211

Month

Fertilization Sowing Collection

Metalaxyl

Figure 2. Agronomic treatments throughout the potatoes growing cycle.

measure of the water height in a Parshall flume located at the beginning of the main drainage canal and that showed a 152 cm wide throat section (Figure 3). Water flow was calculated using the equation Q=C x Hn, where Q is the flow rate (m3 s-1), H is the height of the water column measured in the Parshall flume (m) and C and n are constants depend on Parshall size. Thus for a Parshall flume with a 152 cm wide throat section, the values of C and n are 3.73 and 1.59, respectively. The relationship between precipitation and flow is presented in Figure 4.

Rainfall is scarce in summer being August and September the driest months. The months of greatest rainfall were ranging from October to February. The maximum values of flow match the wettest months. The highest flow value was 1.1 m3 s-1 in January 2010. In the summer months there is virtually no water circulation. The response of flow to precipitations is very rapid; no delays occur at the peaks of flow with respect to precipitation (Figure 4). 2.4. Chemical characterization of water samples The concentration of nitrates in water samples was carried out in 0.45µm filtered samples using ion chromatography using a Bran + Luebbe AA3 flow injection analyser (Norderstedt, Germany). For the determination of metalaxyl, 100 mL of a 75:25 dichloromethane:acetone solution was added to 500 mL of water sample in a decanting funnel. The mixture was shaken for 15 min. and

WD

A

CP

R

H

Flow Direction

M B T G

a

K N

E

W = 1.524A = 1.981a = 1.321B = 1.943

D = 2.302C = 1.829

G = 0.914

K = 0.076

T = 0.610

P = 3.080R = 0.610

N = 0.229M = 0.457

Measurent Point

Dimensions (metres)

Top view

Side view

E = 0.914

H = 0.926

Figure 3. Description of Parshall system used for flow measurements. then allowed to settle. Later, the organic phase was collected in a 250 mL volumetric flask. This extraction process was repeated twice adding 50 mL of the organic solvents solution. All the organic phase was evaporated till dryness under a stream of nitrogen in a TurboVap LV equipment (Caliper Life Science, Madrid, Spain) and re-dissolved with 1 mL of acetone and the resulting content was transferred to a 2 mL vials. The concentration of metalaxyl was determined using a Trace CG Thermo Finnigan gas chromatograph (Rodano,

Italy) equipped with a PolarisQ ion trap mass spectrometric detection system. Measuring conditions were as follows: the injection volume was set to 2µL, the flow rate of the mobile phase was 1 mL min-1, the initial temperature was 80 ºC, ramped at 8 ºC min-1 to 200 ºC and hold for 15 min, ramped again at 2 ºC min-1 to 210 ºC and hold for 5 min, finally ramped at 20 ºC min-1 to 270 ºC and hold for 2.5 min. The retention time was 15.9 min and the selection ions (m/z) used for the detection and

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quantification of metalaxyl were 160 (100%), 205.9 (52.2%) and 278.8 (2%). 2.5. Statistical analyses All statistical analyses were performed using Statgraphics Plus v. 5.1 for Windows. The statistical significance of differences between group means was estimated by analysis of variance (ANOVA) and least significant difference tests. Basic and descriptive statistics

together with Pearson correlations were also calculated. 3. Results 3.1. Metalaxyl Table 1 shows the sampling points, the number of collected samples, and metalaxyl detection frequency in the study period. At 11 of the 22 sampling points (50%) where samples were analysed, metalaxyl residues were found. At 70% (8 sampling points), metalaxyl was detected on more than one occasion. The sampling point at which the pesticide appeared most frequently is sampling point 21. For 10 months of the study, metalaxyl appeared 24 times (42%). Sampling point 24 was followed by sampling point 22 where metalaxyl was detected 33% of the sampling times. At sampling points 5, 6, 7 and 8, the metalaxyl was detected 3, 5, 6 and 6 times, respectively (Table 1). At sampling points 9 and 12, metalaxyl was detected only in 3 and 2 months, respectively, of the months when samples were analysed (24).

Table 1. Metalaxyl occurrence in the different sampling points. Sampling point

Number of samples

Number of detections

Frequency of detection (%)

Minimum concentration (µg/L)

Median concentration (µg/L)

Maximum concentration (µg/L)

1 19 0 0 nd nd nd 2 23 0 0 nd nd nd 3 24 0 0 nd nd nd 4 24 0 0 nd nd nd 5 23 3 13 0.059 0.098 0.155 6 24 5 21 0.037 0.123 0.230 7 24 6 25 0.037 0.101 0.288 8 24 6 25 0.024 0.079 0.125 9 24 3 12 0.032 0.057 0.076 10 24 1 4 0.028 nd 0.028 11 24 1 4 0.035 nd 0.035 12 24 2 8 0.031 0.063 0.096 13 24 0 0 nd nd nd 14 24 0 0 nd nd nd 15 23 1 4 0.045 nd 0.045 16 24 0 0 nd nd nd 17 24 0 0 nd nd nd 18 24 0 0 nd nd nd 19 24 0 0 nd nd nd 20 24 0 0 nd nd nd 21 24 10 42 0.022 0.070 0.103 22 24 8 33 0.039 0.098 0.276

nd: not detected (<0.02 µg L-1)

Finally, in sampling points 10 and 11, metalaxyl was detected only once. At the exit of the basin, after the junction of the river with the main canal (point 14), metalaxyl was not detected in any of the 24 months studied. We can distinguish three areas of concentration of metalaxyl. The first area corresponds to the first four sampling points (1-4, 20; Table 1) at the start of the drainage canals (Figure 1) where metalaxyl had not been detected for the two years when samples were analysed. The second area corresponds to the area of secondary canals (sampling points 5, 6, 7, 8, 21 and 22; Figure 1; Table 1) where the highest concentrations of metalaxyl were found (> 0.100 µg L-1), being maximum concentration of 0.288 µg L-1 at

sampling point 7. The third zone corresponds to the sampling points at the river and at the basin outlet after the reunion of the branches of the main drainage canal where metalaxyl had not been detected in any of the samples over the two years of the study (sampling points 9-19), with the exception of only point 15 where metalaxyl was detected on one occasion (Table 1). At point 14, out of the basin, after the junction of the river and the main canal waters, no metalaxyl was detected. Figure 5 shows the distribution of metalaxyl over time in the aforesaid second zone (sampling points 21, 22, 5-8) where metalaxyl was detected most frequently. This distribution

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Figure 5. Metalaxyl levels and rainfall during the study period. shows a clear dependence on rainfall since higher concentrations were obtained when rainfall was low (low flows). However, along the second year of sampling, metalaxyl does not appear in sampling points 5 and 6; a saw tooth look in its concentration levels plot suggests that its occurrence in water was related to application and degradation cycles. 3.2. Nitrates Nitrate concentrations in the waters are shown in Table 2. Nitrate concentrations have a heterogeneous distribution both in space and in time. Regarding the spatial distribution, the mean value for 24 months at each point ranged from 4.99 mg L-1 for point 2 to 1.16 mg L-1 for point 19 (Table 2). The maximum value (60 mg L-1) was observed at point 12 at the secondary canal. Standard deviations were very high, ranging between 4.25 and 12.73 mg L-1 for points 18 and 12, respectively. This high spatial variation of nitrate concentrations can also be observed by analysing the coefficients of variation that were generally high, ranging from 42 to 110%. In sampling point 2, a flume was located to be able to relate the flow with the concentration of nitrates. The results indicate that the

concentration was clearly dependent on the flow level because nitrate concentration increased when the average flow rate also increased (Figure 5). However, these nitrate concentration peaks also coincided with seasons where plant development was minimal, and therefore the incorporation of nitrate into plant structures occurred. Figures 6 and 7 show other points where the behaviour is similar, with nitrate peaks at times of increased rainfall and of reduced vegetative development. In the points at the head of the drainage canals and at the river area, concentration variations were much lower.

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Table 2. Descriptive statistics of nitrate concentration (mg L-1) in different sampling points.

Sampling point Mean Standard deviation Max Min Median Variation coefficient (%) 1 6.63 6.35 21.69 0.26 4.84 96 2 4.99 5.49 20.71 <0.03 3.21 110 3 5.83 6.55 23.47 <0.03 3.85 112 4 13.58 9.23 38.50 0.36 10.44 68 5 15.79 6.62 30.62 4.00 14.03 42 6 12.92 10.07 34.00 0.49 9.90 78 7 13.55 10.82 52.49 0.39 10.48 80 8 15.48 9.54 40.01 1.06 14.48 62 9 12.98 8.47 32.05 1.00 12.17 65 10 14.78 7.77 35.01 1.83 15.21 53 11 11.89 7.69 33.57 2.45 9.76 65 12 12.50 12.73 60.09 0.46 8.87 102 13 10.49 6.82 26.14 2.68 9.77 65 14 7.12 5.12 20.57 0.71 4.84 72 15 9.97 4.66 19.93 0.94 9.98 47 16 10.69 4.44 21.06 3.27 10.50 42 17 11.65 5.83 29.60 0.44 10.68 50 18 7.49 4.25 15.26 0.73 7.95 57 19 16.01 6.76 39.83 6.63 15.75 42 20 6.04 5.54 18.00 0.23 4.38 92 21 11.79 8.52 27.27 0.59 12.18 72 22 11.37 10.17 32.03 0.60 10.81 89

4. Discussion 4.1.Metalaxyl Monitoring of pesticides in surface water basins in areas of intensive agricultural cultivation is particularly important worldwide, but the heterogeneity of watershed study cases in geomorphology, soil and climate features makes modelling very difficult. This topic has been highlighted in a study on the modelling of the presence of pesticides in surface waters in Germany (Huber et al., 2000). In this work, 22 sampling points in a basin in NW Spain were sampled for 24 consecutive months. The key feature of this basin was that the soils were acid, and their slope was only slight; thus, runoff processes were reduced to a minimum. The results of the analysis of metalaxyl in the surface water indicated that the situation is not of concern from an environmental standpoint. However, for 22% of the times that the compound was detected, the maximum limit set by the European Union for water intended for human consumption (0.100 mg L-1) was exceeded. In the basin with a strong human intervention to construct canals for drainage of what was an ancient lagoon, the metalaxyl concentrations detected in water indicate that continuous monitoring should be performed (Figure 1). The canals favour the transport of pesticide from the farming plots to the surrounding waterways, despite the fact that the soils in the area of the secondary canals have a higher content of organic matter relative to other soils in the watershed study. A study of 16 acid soils of the basin indicated that metalaxyl has a low affinity for soils containing organic matter. This characteristic plays an important role in the reversibility of the adsorption-desorption processes of metalaxyl (Bermúdez-Couso et al., 2011); the soil features

therefore do not seem to be as important for the presence of metalaxyl in surface water relative to the formation of drainage canals. The presence of pesticides in drainage canals has been highlighted in a study in Greece by Vryzas et al. (2011), who found concentrations of several pesticides that could harm aquatic organisms.. Moreover, Hildebrandt et al. (2008) studied three watersheds in northern Spain finding that metalaxyl concentrations are higher in groundwater relative to surface water. Bermúdez-Couso et al (2007) studied the presence of metalaxyl in soils, sediments and surface waters, finding that metalaxyl conentrations are higher in soils and sediments than in surface waters. The highest concentrations of metalaxyl in the basin secondary canals were found when the canal flow was low and coincided with the fungicide application (Figure 2). The concentration of metalaxyl decreased to undetectable levels (<0.02 µg L-1) in the rainiest months, due to the dilution effect of the compounds present in the water or due to its photolytic degradation. The absence of metalaxyl at other sampling points where the crop intensity is similar, as in the case of the points at the main riverbed, could be due to degradation of metalaxyl during transport by the soil solution to surrounding waters. The rapid degradation of metalaxyl has been highlighted by several authors: half-life values of 10-40 days in soil were found by Tomlin (2003). In experimental plots of potatoes in the same study area (López-Pérez et al., 2006), we found that the half-life of metalaxyl in soils was about two days, and that the pesticide was detected in the plants washing water due to its high water solubility. In experiments to study the laboratory degradation of metalaxyl, we also found that its half-life for two soils with different organic matter content under

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Figure 7. Nitrate concentrations at different sampling points (P) and monthly precipitation.

simulated light was between 11.5-15.6 hours (Bermúdez-Couso et al. 2012). Using an analysis of variance study, we found that the concentration of metalaxyl was dependent on year of study (F = 22.93, P = 0.0000), the season (F = 7.36, P = 0.0001) and

the sampling area (F = 23.72, P = 0.0000). Also were significant interactions between variables as year and season of sampling with the sampling area (F = 5.17, P = 0.0000 and F = 5.35, P = 0.0000, respectively). These last two significant relationships are shown in Figure 8,

indicating that metalaxyl concentration is higher in the first year primarily in sampling area 2 (Figure 8A), on the other hand also the concentration is higher metalaxyl in the spring-summer seasons mainly in zone 2 (Figure 8B). This indicates that both the sampling area and the time of year are essential in establishing the quality of surface waters. 4.2. Nitrates Nitrates, like metalaxyl, show great variation both temporal and spatial. This variation has already been outstanding by other authors, so Andrade and Stigter (2009) argue that this

variation was due the high nitrogen fertilization and rainfall causing a decrease in nitrate concentration by dilution. Our data indicate that the nitrate concentration was increased with increasing flow (Figure 6), which we attribute to that rainfall is a strong mobilization of soil nitrates that increases its concentration despite the effect dilution by increased flows. In this sense, different water flows can affect the concentration of nitrates in water: higher water

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Figure 8. Analysis of variance. Relationship between the three sub-areas of study and year of sampling (A) and season (B).Area 1: Samples 1, 2, 3, 4 and 20; Area 2: Samples 5, 6, 7, 8, 21 and 22; Area 3: Samples 9, 10, 11, 12, 13 ,14, 15, 16, 17, 18 and19. Season 1: Spring; Season2: Summer; Season 3: Autumn; Season 4: Winter.

flows favour the transport and dilution processes of nitrates while low water flows favour the accumulation of nitrates; this fact has already been highlighted by Arauzo et al. (2011). As for the temporal variation, using analysis of variance found that nitrate concentration was different depending on the year of sampling (F = 14.915, p = 0.0000) of the season (F = 11.69, P = 0.0000) and the area sampling (F = 1.29, P = 0.0000). The interaction between the sampling area and season was significant in explaining the variation of nitrate concentration (F = 5.72, P = 0.0000).

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Figure 9. Analysis of variance of nitrate concentration depending on variables such as sampling area and season Area 1: Samples 1, 2, 3, 4 and 20; Area 2: Samples 5, 6, 7, 8, 21 and 22; Area 3: Samples 9, 10, 11, 12, 13 ,14, 15, 16, 17, 18 and19. Season 1: Spring; Season 2: Summer; Season 3: Autumn; Season 4: Winter.

Figure 9 shows this relationship and shows that nitrate concentration was higher in autumn-winter, mainly in the sampling area 2. This coincides with the findings of Pauwels et al (2001), which found that the highest concentrations of nitrates in winter and lowest in late summer. In our case we believe this reduction in nitrate concentrations in autumn-winter is because the plant activity decreases dramatically at this time, and therefore, also reduces nitrates absorption by plants. In general, both the flow and the sampling period are shown as essential variables when setting the concentration of nitrates in water. Frequent sampling a single point or a single time during the year does not seem to be sufficient to establish water quality. 5. Conclusions Monitoring of surface water in a watershed devoted to intensive cultivation of potatoes with low slopes showed a high temporal and spatial variability. The low slope in the study area makes runoff negligible and, therefore, the potential presence of contaminants such as metalaxyl and nitrates in water occurs primarily by transport through the soil. For 22% of the times that metalaxyl was detected, the maximum limit set by the European Union for water intended for human consumption (0.100 mg L-1) was exceeded. The highest concentrations of metalaxyl occurred when rainfall was low and in a study area where drainage was favoured by the construction of numerous drainage canals. Analysis of variance indicated that the interaction between sampling site and season can explain the variability in metalaxyl concentrations. Only twice did nitrate concentrations exceed the limit value of 50 mg L-1 set by the European Union as the maximum limit for drinking water. The spatial and temporal distribution of nitrates also showed a high variability, with the interaction between seasons and sampling area being the most significant factor explaining the levels of nitrates found..

ACKNOWLEDGEMENTS This work was funded by the INCITE programme of the Galician Council of Innovation and Industry (Ref. 08PXIB383190PR), together with the EU FEDER programme. A. Bermúdez-Couso is funded by the predoctoral programme from the University of Vigo. The authors acknowledge C.A.C.T.I. of the University of Vigo for some of the analytical measurements carried out in sampled waters. REFERENCES Andrade, A. I. A. S. S. and Stigter, T. Y., 2009. Multi-

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Anexo VII. Información e criterios de calidade das

publicacións

201

INFORMACIÓN E CRITERIOS DE CALIDADE DAS

PUBLICACIÓNS.

Na normativa interna sobre estudos oficiais de postgrado da Universidade de

Vigo sobre as Teses de Doutoramento presentadas por artigos, indicase que para a súa

admisión debe incluírse unha descrición do factor de impacto ou de outros criterios de

calidade das publicacións que forman parte da memoria. Quedando claramente

explícitos os nomes de tódolos autores, a súa orde, a súa filiación, o nome da

publicación, a editorial e o seu ISSN ou ISBN.

A continuación dáse a información requirida de cada un dos artigos, incluíndo o

factor de impacto das revistas onde están publicados e as súas posicións dentro dos

epígrafes nos que se atopan.

Artigo I

Autores: Alipio Bermúdez-Cousoa, David Fernández-Calviñoa, Miriam Pateiro-Mourea,

Juan Carlos Nóvoa-Muñoza, Jesús Simal-Gándarab e Manuel Arias-Estéveza.

Filiación: a Área de Edafoloxía e Química Agrícola. Facultade de Ciencias de Ourense.

Universidade de Vigo.

b Área de Nutrición e Bromatoloxía. Facultade de Ciencias de Ourense.


Título: Adsorption and desorption kinetics of carbofuran in acid soils.

Revista: Journal of Hazardous Materials, 190:159-167 (2011)

Editorial: Elsevier Science BV

ISSN: 0304-3894

Epígrafe: Environmental Sciences

Factor de Impacto: 4.173

Posición relativa da revista no epígrafe (2011): 17/205

202

Artigo II

Autores: Alipio Bermúdez-Couso, David Fernández-Calviño, Miriam Pateiro-Moure,

Beatriz Garrido-Rodríguez, Juan Carlos Nóvoa-Muñoz e Manuel Arias

Estévez.

Filiación: Área de Edafoloxía e Química Agrícola. Facultade de Ciencias de Ourense.


Título: Adsorption and Desorption Behavior of Metalaxyl in Intensively Cultivated

Acid Soils.

Revista: Journal of Agricultural and Food Chemistry, 59: 7286-7293. (2011).

Editorial: AMER CHEMICAL SOC

ISSN: 0021-8561

Epígrafe: Agriculture, multidisciplinary


Posición relativa da revista no epígrafe (2011): 3/57

Artigo III

Autores: Alipio Bermúdez-Couso, David Fernández-Calviño, Isabel Rodríguez-

Salgado, Juan Carlos Nóvoa-Muñoz e Manuel Arias-Estévez



Título: Comparison of batch, stirred flow chamber, and column experiments to study

adsorption, desorption and transport of carbofuran within two acidic soils.

Revista: Chemosphere, 88: 106-112. (2012).

Editorial: PERGAMON-ELSEVIER SCIENCIE LTD

ISSN: 0045-6535

Epígrafe: Environmental Sciencies


Posición relativa da revista no epígrafe (2011):32/205

203

Artigo IV

Autores: Alipio Bermúdez-Couso, David Fernández-Calviño, Juan Carlos Nóvoa-

Muñoz e Manuel Arias Estévez.



Título: Metalaxyl dynamics in acid soils: evaluation using different methods

Revista: Pest Management Sciencie (Enviado)

Artigo V

Autores: Alipio Bermúdez-Couso, Juan Carlos Nóvoa-Muñoz, Manuel Arias-Estévez e

David Fernández-Calviño.



Título: Influence of different abiotic and biotic factors on the metalaxyl and carbofuran

dissipation.

Revista: Chemosphere, DOI:10.1016/j.chemosphere.2012.10.090

Editorial: PERGAMON-ELSEVIER SCIENCIE LTD

ISSN: 0045-6535

Epígrafe: Environmental Sciencies


Posición relativa da revista no epígrafe (2011):32/205

Artigo VI

Autores: Alipio Bermúdez-Cousoa, David Fernández-Calviñoa, Manuel Ali Álvarez-

Enjob, Jesús Simal-Gándarac, Juan Carlos Nóvoa-Muñoza, Manuel Arias-

Estéveza.

Filiación: a Área de Edafoloxía e Química Agrícola. Facultade de Ciencias de Ourense.


204

b Grupo de Enxeñería Hidráulica. Escola Técnica Superior de Enxeñeiros de

Camiños, Canais e Portos -Universidade da Coruña.

c Área de Nutrición e Bromatoloxía. Facultade de Ciencias de Ourense.


Título: Pollution of surface waters by metalaxyl and nitrates from non-point sources.

Revista:Journal of Enviromental Management (Enviado).

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