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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: j.maskall@plymouth.ac.uk
(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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394 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 395
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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396 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 397
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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398 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 399
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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400 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 401
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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402 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 403
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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404 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 405
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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CHEMICAL PARTITIONING OF HEAVY METALS 407
Figure 8.Bole C: variation of lead concentrations in steps 15 with depth.
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408 J. E. MASKALL AND I. THORNTON
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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CHEMICAL PARTITIONING OF HEAVY METALS 409
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