extracción secuencial-Maskall_1998

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CHEMICAL PARTITIONING OF HEAVY METALS IN SOILS, CLAYS

AND ROCKS AT HISTORICAL LEAD SMELTING SITES

J. E. MASKALL and I. THORNTONEnvironmental Geochemistry Research Group, Centre for Environmental Technology, Royal School

of Mines, Imperial College of Science, Technology and Medicine, London, U.K.

E-mail: [email protected]

(Received 25 June, 1996; accepted in final form 5 October, 1997)

Abstract. The chemical partitioning of lead and zinc is described in contaminated soils and un-

derlying strata at historical lead smelting sites. Sections of soil-rock cores from eight sites of age

200 to c.1900 yr were analysed using a sequential extraction procedure. Of the total amount of

lead and zinc present in soils, only a small proportion is in a readily mobile form. However, this

proportion increases significantly as the pH falls below 5 and for lead reaches 37% in soils at BoleA. A high proportion of lead in soils appears to be associated with the carbonate and specifically

adsorbed phase. It is suggested that this is partly due to the formation of cerussite (PbCO 3) in soils

contaminated with calcareous slag wastes. Lead present in the residual phase in contaminated soils

may be related to the presence of the element in silicate slag particles. Rapid migration of lead to

a depth of 5.6 m in sandstone at Bole A was related to its high solubility in the acidic soils and

rock at this site. Comparable migration at Bole C proceeds by a different mechanism, possibly with

lead in association with Fe-Mn oxides and slag particles. In clay infill in fractured sandstone at

Bole A, anthropogenic lead present at a depth of 4.4 m was extracted predominantly in the fraction

representing Fe-Mn oxides.

Keywords:contamination, lead, migration, partitioning, soil, zinc

1. Introduction

The long term leaching and migration of contaminants from improperly disposed

wastes can result in pollution of ground and surface waters. Data on the mobility

and migration of heavy metals at historically contaminated sites is valuable for the

assessment and management of contemporary pollution problems. The potential

for release of contaminants from waste materials and their movement in soils can

be assessed using a variety of leaching and extraction techniques (Quevauvillier et

al., 1996). In this paper, the chemical partitioning of lead and zinc in contaminated

soils and clays is assessed using a five-step sequential extraction procedure. The

soils and clays were taken from core samples collected from eight historical leadsmelting sites in use between 200 and c.1900 yr ago. The sections of the soil-rock

cores selected for this study comprised slag-contaminated soils and underlying

Present Address Department of Environmental Sciences, University of Plymouth, Drake

Circus, Plymouth, PL4 8AA, UK

Water, Air, and Soil Pollution 108: 391409, 1998.

1998Kluwer Academic Publishers. Printed in the Netherlands.


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392 J. E. MASKALL AND I. THORNTON

clays and rocks into which metals were known to have migrated. The cores have

been analysed previously to establish the extents and rates of vertical migration

of metals in a number of geological parent materials (Maskall et al., 1995, 1996).

In addition, mineralogical analysis of slag wastes has been undertaken at seven of

the sites (Gee et al., 1997). The data for the chemical partitioning of metals are

discussed in relation to the mineralogy of the slag wastes and to the characteristics

of metal migration.

The sequential extraction technique used is based on that of Tessier et al. (1979)

which has been adapted for multi-element analysis by ICP-AES (Li et al., 1995).

Although this method was originally intended for analysis of aquatic sediments,

it has also been successfully applied to the study of soils and dusts (Harrison et

al., 1981; Hickey and Kittrick, 1984; Gibson and Farmer, 1986; Clevenger, 1990).

However, the main limitation of this approach is that the geochemically defined

phases are not perfectly differentiated; there is a certain amount of overlap between

fractions and extraction efficiency can vary with the type of soil under investigation

(Valin and Morse, 1982; Kheboian and Bauer, 1987; Martin et al., 1987). Never-theless, sequential extraction data do provide an indication of the relative bonding

strength of metals in different solid phases and their usefulness can be enhanced

when combined with data from other analytical techniques.

Previous studies on contaminated soils have shown that the partitioning of met-

als depends strongly on their mineralogical and chemical form which in turn is

influenced by the source of contamination. Research by Li (1993) indicated that

variations in the partitioning of lead between old mining and smelting sites in

Derbyshire, U.K. was related to the form of lead present. The proportion of lead

extractable by MgCl2 was higher at the smelting site and this was attributed to the

presence of anglesite (PbSO4) and Pb-oxides in the emission particulates and slag

wastes. In the old mining area however, the relatively low proportion of lead in thesame step was attributed to the presence of lead as cerussite (PbCO 3), galena (PbS)

and pyromorphite (Pb5(PO4)3Cl) and the higher pH of the contaminated soils. A

high proportion of lead at both the smelting and mining sites was extracted in the

step representing the carbonate and specifically adsorbed phase. In comparison,

studies of metal partitioning in urban soils from Lancaster (Gibson and Farmer,

1984) and Glasgow (Gibson and Farmer, 1986) revealed a high proportion of lead

to be present in the reducible fraction and as such was considered to be associated

with Fe-Mn oxides.

Precipitation of metals as carbonates and hydroxycarbonates has been identi-

fied as the dominant control on metal migration in several sedimentary rock types

(Newman and Ross, 1985). Similar results have been reported for metal migration

in glacial deposits (Gibb and Cartwright, 1982) and in a natural clay underlyinga landfill site (Yanful et al., 1988). In all these cases, the presence of carbonate

species leads to elevated pH levels which encourage metal-carbonate precipitation

reactions. Additional controls on metal migration identified in the field include

cation exchange and adsorption (Gibb and Cartwright, 1982; Newman and Ross,


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CHEMICAL PARTITIONING OF HEAVY METALS 393

1985), adsorption by Mn oxides (Kobuty-Amacher et al., 1992) and adsorption to

organic matter (Dorr and Munnich, 1991; Dumontet et al., 1990). In an experi-

mental study of metal movement through soils, Korte et al. (1976) found that soil

texture, surface area, free iron oxide concentration and pH were the most important

factors for predicting movement.

2. Materials and Methods

Sites of former lead smelting activity were identified from historical records and

archaeological studies. Core sampling was undertaken at eight sites; two eighteenth

century cupolas, four medieval boles and two Roman sites, and was concentrated in

areas of slag contamination. Seven of the sites were located in Derbyshire, England

whilst Roman A was located in Clwyd in North Wales. The variations of total metal

concentrations in the soil-rock profiles at these sites have already been published

(Maskall et al., 1995, 1996). The core sections selected for this study comprisedslag-contaminated soils and the underlying clays and rocks into which metals were

known to have migrated. Details of the selected sections of core are given in Table I

in terms of the site, depth, material present and the lead migration rate for the

particular core. The core sections from Cupola A, Bole B, Bole D and Roman A

were chosen in order to investigate the metal partitioning where migration had been

attenuated by clay layers underlying contaminated soils. Conversely, core sections

from Bole A and Bole C were selected as the contaminated soils were directly

underlain by sandstone into which significant migration of lead had occurred.

Soils, clays and rocks were subsampled from the cores, air-dried at 30 C for 72

hr, disaggregated with a pestle and mortar, passed through a 2 mm sieve and milled

to a size of< 180 m. Total metal concentrations were determined by digestingthe milled material with a concentrated nitric/perchloric acid mixture and analysing

by ICP-AES (Thompson and Walsh, 1983). The chemical partitioning of metals

was assessed using a sequential extraction procedure based on that of Tessier et al.

(1979) adapted for ICP-AES by Li et al. (1995). It was carried out progressively

on an initial weight of 1.0 g of milled material using the following extractions:

Step 1: 0.5 m magnesium chloride adjusted to pH 7.0 with 10% ammonia solution.

Step 2: 1 m sodium acetate adjusted to pH 5.0 with acetic acid.

Step 3: 0.04 m hydroxylamine hydrochloride in 25% acetic acid.

Step 4: 30% hydrogen peroxide in 0.02 m nitric acid

Step 5: 60% perchloric acid, 70% nitric acid and 35% hydrofluoric acid.

The method is intended to distinguish five fractions representing the following

phases; exchangeable (step 1), carbonate and specifically adsorbed (step 2), iron

and manganese oxide (step 3), organic and sulphide (step 4) and residual (step

5). However, the amount of metal extracted in each step does not necessarily cor-


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TABLE I

Samples selected for metal partitioning study

Site Core(s) Material Depth (m) n Pb migration

rate (cm y

1)

Cupola A 1 Soil 00.5 4 0.11

Clay 0.51.6 6

Cupola B 1+3 Soil 00.8 9 1.48

Bole A 2 Soil 00.4 2 0.79

Sandstone 0.44.3 12

Bole B 1 Soil 00.5 4 0.31

Clay 0.51.2 3

Bole C 1+3 Soil 00.5 5 0.720.77

Clay 0.50.6 1

Sandstone 0.63.2 4

Bole D 2 Soil 01.0 5 1.44

Clay 1.01.5 2

Sandstone 1.51.6 1

Roman A 1 Soil 1.72.3 3 0.07

Clay 2.34.8 2

Roman B 3 Soil 00.2 1 0.54

Clay 0.20.7 3

Limestone 0.71.3 1

respond to that present in each geochemically defined phase in the test material.The geochemical phases at each extraction step are largely operationally defined

and indicate relative rather than absolute chemical speciation. The main interpre-

tations are based on the solubility of metals but are supplemented with additional

mineralogical analyses where available. In this paper, the fractions are referred to

in terms of the extraction step and the geochemical phase considered to be the

predominant source of metal is added in parentheses e.g. step 1 (exchangeable).

3. Results and Discussion

3.1. ACCURACY OF THE SEQUENTIAL EXTRACTION PROCEDURE

The overall accuracy of the sequential extraction procedure was assessed by analy-

sis of reference materials from the National Institute of Standards and Technology

(NIST). The sums of the concentrations from the five steps were compared with the

certified values for the total concentrations and the results, expressed in terms of


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TABLE II

Recovery of metals from sequential extraction procedure (%)

Sample Element Mean Range SD n

NIST SRM 2710 Pb 91 8894 2 4

Zn 92 8793 3 4

NIST SRM 2711 Pb 93 9295 1 3

Zn 89 8790 1 3

Contaminated Soils Pb 97 84112 6 33

Zn 100 82121 10 33

Underlying Clays Pb 114 92217 29 17

Zn 105 74168 25 17

Underlying Rocks Pb 105 84139 13 18

Zn 107 88141 12 18

% recovery, are shown in Table II. The recovery rates are generally high (> 80%)

and the mean rates lie within or very close to the target range of 90 110%. Mean

recovery rates of metals in soils, clays and rocks were determined by comparing

the sum of the analyses for the five steps with the total metal concentration deter-

mined by digestion with nitric and perchloric acids. The results also fall generally

within the target range (Table II). However, recovery rates for metals in underlying

clays reached elevated levels in a limited number of samples where the total metal

concentrations were very low.

3.2. GENERAL TRENDS IN METAL PARTITIONING

The concentrations of metals in the five fractions in contaminated soils and un-

derlying clays show that the amounts of metals that are extracted at each stage

can vary widely (Table III). The proportions of lead and zinc extracted in step 1

(exchangeable) are generally low and on average range between 26% of the total

metal. In comparison with results gained by Li (1993), the lead data for this study

are more similar to those of the mining area than the smelting area (Table IV).

This is because the soils in this study were invariably collected from the most

contaminated part of the smelting sites and contained therefore a high proportion

of calcareous slag wastes which tend to elevate the soil pH (Maskall et al., 1995,

1996). Indeed for both soils and clays, the proportions of metals extracted in step 1

(exchangeable) are significantly and inversely related to pH (Figure 1) and similar

relationships have been reported by Iyengar et al. (1981) and Li (1993). The one

site which featured a high proportion (37%) of lead in soils in step 1 (exchangeable)

was Bole A which had the lowest pH of all the sites with an average of 3.9.


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TABLE III

Concentrations of metals in different fractions in slag contaminated soils (n=33) and

underlying clays (n=17) (g g1)

Slag Contaminated Soils Underlying Clays

Extraction Meana Meanb Range Meana Meanb Range

Step 1 Pb 1052 5 103960 12 2 0.368

Zn 44 6 0.6399 83 4 0.1409

Step 2 Pb 28219 45 3689500 257 31 41780

Zn 238 13 0.6977 70 6 0.9449

Step 3 Pb 11060 22 3830600 215 30 21520

Zn 752 31 25630 356 24 71770

Step 4 Pb 3551 15 217400 58 18 2353

Zn 292 10 14650 447 13 14650Step 5 Pb 7796 13 1341000 91 19 5366

Zn 554 40 22560 237 53 51300

a Expressed as a concentration (g g1).b Expressed as a proportion of the total concentration (%).

TABLE IV

Mean proportions (%) of metals extracted in each fraction for a range of soils

Extraction Cupola Aa Mining Areaa Glasgow Soilb Lancaster Soilc

(n=10) (n=11) (n=397) (n=4)

Step 1 Pb 21 5 2 1

Zn 8 0.5 3 3

Step 2 Pb 29 27 11 26

Zn 12 8 7 31

Step 3 Pb 21 37 51 44

Zn 27 37 17 34

Step 4 Pb 25 28 19 12

Zn 11 5.5 29 9

Step 5 Pb 4 3 17 17

Zn 42 49 43 23a Li (1993).b Gibson and Farmer (1986).c Gibson and Farmer (1984).


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Figure 1.All sites: variation of the proportion of lead extracted in step 1 with soil pH.

Higher proportions of lead are extracted in the second step (carbonate and

specifically adsorbed) which on average accounts for 45 and 31% of the total

concentration in soils and clays respectively. The value for soils is particularly high

and exceeds the corresponding figures from other studies of urban soils and soils

contaminated by smelting and mining activities (Table IV). This may be due to therelatively high pH levels in soils at the study sites which have been elevated due to

the release of calcium and carbonate compounds from the slag wastes (Maskall et

al., 1995, 1996). The proportion of lead extracted in step 2 is significantly related to

pH (r= 0.73) and increases from approximately 20 to 80% over the pH range 4.0

7.2. Previous work has indicated that the importance of this fraction as a lead sink

rises with increases in the pH and calcite contents of soils and dusts (Harrison et al.,

1981; Gibson and Farmer, 1986). Soils from the study sites are highly contaminated

and their lead content generally exceeds the theoretical maximum which can be

adsorbed by the CEC. This suggests that some of the lead is present occluded in

slag particles, specifically adsorbed to soil constituents or precipitated. As the lead

level in soils increases, the percentage of lead extractable in step 2 also increases ( r

= 0.68). This provides further evidence that lead may be present in the specifically

adsorbed form or precipitated as carbonates in contaminated soils.

Step 3 is operationally defined as the fraction bound to Fe-Mn oxides. However,

it has been shown that in some carbonaceous soils, the second extraction step may

not be effective in removing all the carbonate minerals into solution (Jouanneau


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et al., 1983). Metals extracted in step 3 therefore may contain a proportion of

the carbonate forms in addition to those bound to Fe-Mn oxides, particularly in

slag contaminated soils which have been shown to have higher carbonate content

(Maskall et al., 1995). Nevertheless, step 3 (Fe-Mn oxides) accounts for 22% of

the lead in soils which is low compared to soils from mining and urban areas

(Table IV) and probably reflects the dominant role of step 2 for this element in

this study. A higher proportion of lead is extracted from the clays (30%). Step 3

represents the second most significant sink for zinc after the step 5 (residual) and

accounts for 31% of the element in soils and 24% in the clays. This fraction has also

been found to hold large amounts of zinc in soils contaminated by copper smelting

emissions (Kuo et al., 1983; Hickey and Kittrick, 1984). In soils, the percentage of

zinc extracted in step 3 increases significantly with pH (r= 0.69) which probably

reflects the enhanced scavenging of metals by Fe-Mn oxides at higher pH levels.

Step 4 is operationally defined as the organic and sulphide fraction but it has

been shown that the primary sulphide minerals, including PbS, can not be totally

dissolved by this step (Forstner, 1985). Although the term organic/sulphide is usedin the text, it should be regarded as the organic fraction with partial dissolution of

the primary sulphide phase (Kim and Fergusson, 1991). However, sulphide min-

erals were not identified as a dominant lead bearing phase in slags at the study

sites by Gee et al. (1997). Step 4 (organic and sulphide) accounts for relatively

small proportions of metals in soils and clays (Table III). In contaminated soils, the

percentage of metal extracted in this fraction increase with the CEC for both lead

(r= 0.79) and zinc (r= 0.48).

Step 5 (residual) accounts for a relatively high proportion of zinc in soils (40%)

which reflects results gained by Li (1993) for both smelting and mining areas in

Derbyshire. The particularly high proportion of zinc present in clays (53%) in this

step may represent residual zinc from within clay minerals as found by Iyengaret al(1981). In comparison, the proportions of lead extracted in soils (13%) and clays

(19%) are low. The percentage of lead extracted in step 5 increases significantly

with the total lead concentration (r= 0.59). This may reflect the presence of lead

occluded in slag particles in highly contaminated soils and this is discussed further

in Section 3.3.

3.3. METAL PARTITIONING IN RELATION TO MINERALOGY

Mineralogical analysis of slag wastes and contaminated soils from seven of the

study sites was undertaken by Gee et al. (1997) using a combination of Scanning

Electron Microscopy and X-Ray Diffraction. In large slag fragments (> 2 mm),

lead was found to occur in several forms including lead oxide (PbO), pyromor-

phite (Pb5(PO4)3Cl), cerrusite (PbCO3), hydrocerrusite (Pb3(CO3)2(OH)2), galena

(PbS), anglesite (PbSO4) and leadhillite (Pb4SO4(CO3)2(OH)2). In addition, lead

was identified as a component of a number of silicate phases, some of which were

glassy in nature. In contaminated soils, silicate slag particles (< 2 mm) were still


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recognisable but were more weathered than the larger fragments. In these particles,

the lead phases had generally weathered to cerussite or hydrocerrusite but galena

occasionally occurred, usually trapped within silicate material. It was suggested

that this precipitation of lead mainly as cerrusite is related to the release of calcium

and carbonate compounds from the slag wastes and the consequent elevation of

pH in the contaminated soils (Gee et al., 1997). In addition, significant amounts

of glassy lead silicate material were found in soils, particularly at a sites with

relatively high pH.

The high proportion of lead extracted in step 2 (carbonate and specifically ad-

sorbed) at the study sites is further evidence of the widespread presence of cerrusite

in soils indicated by Gee et al. (1997). Furthermore, the observation that the per-

centage of lead extracted in step 2 increases with the total lead content of the soils

suggests that the formation of cerrusite is favoured in the most highly contaminated

soils. These soils also tend to have the highest pH levels and would favour therefore

the formation of cerrusite on thermodynamic grounds as the mineral is usually

stable at a pH above 6.0 (Brookins, 1988). Lead in a glassy silicate form in soilswould be expected to be extracted in step 5 (residual). In this step, we again observe

that the percentage of lead extracted increases with the total lead concentration of

the soils. This may indicate the presence of lead bearing silicates in the most highly

contaminated (slag rich) soils but further work would be required to confirm this.

3.4. METAL PARTITIONING IN RELATION TO METAL MIGRATION

As the proportion of metals that are extracted in step 1 (exchangeable) in the

contaminated soils is generally low, it would be expected that the amounts of

metal available for downwards migration would be limited. Indeed, previous work

has indicated that the amounts of lead and zinc that had migrated and had beenretained by the underlying strata were low compared to the amounts present in the

soils (Maskall et al., 1995, 1996). Furthermore, previous work also indicated that

for sites with similar geology, metal mobility tended to increase at lower soil pH

(Maskall et al., 1995). This is supported by the observation that at lower soil pH

levels the proportion of lead in step 1 (exchangeable) increases. However, the par-

titioning data also reveal that at lower pH levels the proportion of metals extracted

in step 2 (carbonate and specifically adsorbed) decreases. In the contaminated soils

at the study sites therefore, metal mobility increases under conditions of low pH

apparently via the dissolution of metal species held in the major reservoir classed

as carbonate and specifically adsorbed leading to an increase in metals in the

exchangeable fraction.

The underlying clays selected for this study are moderately contaminated and

represent material in which the migration of metals has occurred but has been

limited by the process of attenuation. Taking the clays as a group (excluding one

outlier), the partitioning data show that as the total lead concentration increases, the

percentage of lead extracted increases significantly in step 2 (r= 0.86) and step 3 (r


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Figure 2.Cupola A: variation of lead concentrations in steps 15 with depth.

= 0.64) only. Similar results are found for zinc and suggest that specific adsorption,

precipitation and adsorption to Fe-Mn oxides are important mechanisms for the

attenuation of metals in clays although further work is required to confirm this.

Yanful et al. (1988) report that the mobility of metals in a clay liner under a landfill

site was limited by precipitation as carbonates under conditions of high pH.

The variation of metal partitioning with depth in a soil-clay profile at Cupola

A is shown in Figures 25. The clay has limited the movement of lead to a few

centimetres below the soil-clay interface whilst zinc has penetrated to a depth of

1.6 m. The greater mobility of zinc may be related to the higher proportion of the


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Figure 3.Cupola A: variation of the proportion of lead in steps 1-5 with depth.

element extracted in step 1 (exchangeable) compared to lead (Figures 3 and 5).

High proportions of lead (41%) and zinc (35%) are extracted in step 4 (organic

and sulphide) in the clay. The metals may be adsorbed onto organic matter which

is present at a relatively high concentration (LOI = 29%). This is supported by

the observation that in all the clays the percentage of lead extracted in step 4 is

significantly related to the organic matter content (r= 0.52).

Rapid migration of lead to a depth of several metres was recorded in Crawshaw

Sandstone at Bole A, an area where underlying clay layers were absent (Maskall

et al., 1995). The variation of lead partitioning with depth in Core 2 is presented


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Figure 4.Cupola A: variation of zinc concentrations in steps 15 with depth.

in Figures 6 and 7. The low pH of the soils at this site results in a large proportion

(37%) of lead extracted in step 1 (exchangeable). Lead appears to be migrating pre-

dominantly in the exchangeable form which remains the commonest lead species to

a depth of nearly 4 m, perhaps maintained by the acid nature of the sandstone. This

core from Bole A was studied by Whitehead et al. (1997) using lead isotope tracing

and estimates were made of the proportions of lead in the sandstone originating

from (i) the anthropogenic slag contamination at the surface and (ii) the natural

background. It was found that for lead in contaminated sandstone at a depth of 1.4

2.3 m, 88% was of anthropogenic origin and for lead in contaminated fracture clay


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Figure 5.Cupola A: variation of the proportion of zinc in steps 1-5 with depth.

infill at 4.5 m, 98% was of anthropogenic origin. Table V shows the lead partition-

ing data for two of the contaminated sandstone samples used in the isotope study, a

similar fracture clay infill sample from the same core and an uncontaminated sand

from Cupola A. The data confirm that in the contaminated sandstone, a high pro-

portion of the anthropogenic lead is present in step 1 (exchangeable). In the fracture

clay infill however, a disproportionately high concentration of anthropogenic lead

is extracted in step 3 suggesting that Fe-Mn oxides are important in attenuation of

the element in subsurface clays. Results from Bole A support the suggestion that

rapid and significant metal migration is facilitated by a high metal solubility in

soils which in this case is due to the relatively low soil pH.


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TABLE V

Partitioning of lead in sandstones and fracture clay infill (g g1)

Site Bole A Bole A Bole A Cupola A

Depth (m) 1.4 2.0 4.4 1.3

Material Contaminated Contaminated Fracture Infill Uncontaminated

Sandstone Sandstone Clay Sand

Step 1 41.4 52.8 30.0 0.3

Step 2 14.4 12.0 84.0 3.6

Step 3 10.6 6.0 222.0 2.4

Step 4 0.4 0.2 9.4 2.4

Step 5 13.5 12.0 30.0 4.5

Total Lead 80.3 83.0 375.4 13.2

Anthropogenic 70.7 73.0 370.4 0

Leada

a Calculated using isotope ratios from Whiteheadet al. (1997).

Considerable movement of lead and zinc have also been recorded in Namurian

Sandstone in Core 3 at Bole C to a depth of over 4 m (Maskall et al., 1996).

However, at this site, migration does not appear to have occurred by the same

mechanism as at Bole A. A very low proportion of lead was found in step 1

(exchangeable); 0.5% in the slag contaminated soil and 0.2% on average in the

sandstone with similar levels for zinc (Figures 8 and 9). This is probably related

to the high pH (6.9) of the contaminated soil which is also fairly rich in organic

matter. 46% of the lead in the contaminated sandstone is extracted in step 3 (Fe-

Mn oxide) and 34% in step 5 (residual). Most of the zinc is extracted in step5 (residual) and the mean concentration (292 g g1) is too high to be entirely

due to the natural background. Metal contamination at this site appears to be due

to the downwards movement into the fractured sandstone of lead-rich iron and

manganese oxides along with particles of slag containing lead and zinc although

further work is required to confirm this.

4. Conclusions

Of the total amounts of lead and zinc in contaminated soils and underlying clays

taken from historical lead smelting sites, only relatively small proportions were

extracted in a readily mobile form. However, these proportions increase with low-

ered soil pH and at Bole A, where mean pH is 3.9, 37% of lead present in soils

was extracted in step 1 (exchangeable). A large proportion of lead in soils (mean

= 45%) was extracted in step 2 (carbonate and specifically adsorbed) and this pro-

portion increases as the soils become more contaminated. This is partly related to


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Figure 6.Bole A: variation of lead concentrations in steps 15 with depth.

the presence of cerrusite (PbCO3) which forms as a weathering product in soils in

the presence of calcium and carbonate compounds leached from the slag wastes.

The proportion of lead extracted in step 5 (residual) also rises with contamination in

soils and it is suggested that this is due to the increased presence of lead occluded in

silicate slag particles. In the contaminated soils, metal mobility is enhanced under

conditions of low pH apparently via the dissolution of metal species present in

step 2 (carbonate and specifically adsorbed) leading to an increase in the fraction

representing exchangeable metal. At Bole A, the high mobility of lead in soils is

linked to the rapid migration of the metal to a depth of 5.6 m. A high proportion of


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Figure 8.Bole C: variation of lead concentrations in steps 15 with depth.


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Figure 9.Bole C: variation of the proportion of lead in steps 1-5 with depth.

Acknowledgements

This work is funded by the International Lead Zinc Research Organisation and The

BOC Foundation for the Environment. The authors are grateful to Dr. Xiangdong

Li and to postgraduate research students Keith Whitehead and Clare Gee for theircontributions.


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