Environ Chem Lett (2006) 3: 149–153DOI 10.1007/s10311-005-0022-1

ORIGINAL PAPER

Judit Gal · Andrew S. Hursthouse · Simon J. Cuthbert

Chemical availability of arsenic and antimony in industrial soils

Received: 26 September 2005 / Accepted: 26 September 2005 / Published online: 29 November 2005C© Springer-Verlag 2005

Abstract Total concentrations and extractable fractiona-tions of As and Sb were determined in soil samples fromformer mining sites in Scotland and Italy. Pseudo-totallevels of As and Sb in the sample were between 50–17,428 mg/kg and 10–1,187 mg/kg (Scotland), and 16–691 mg/kg and 1.63–11.44 mg/kg (Italy). Between 0.001–0.63% and <0.001−8.82% of the total soil As and Sb,were extractable using, a single extraction bioavailabilityestimate. Data from an As-specific extraction procedure re-vealed that up to 60% of As was associated to amorphousFe-Al oxyhydroxide phase in all soils. A non-specific-sequential extraction test also showed As to be stronglyassociated with Fe (and Al) oxyhydroxides at both loca-tions. In the case of Sb, in addition to the crystalline Fe-oxide bound Sb the Al-silicate phase also appeared to besignificant. At both sites Sb appears to be chemically moreaccessible than As with consistent availability despite thevaried origin and host soil properties.

Keywords Contaminated soils . Arsenic . Antimony .Availability . Mobility . Sequential extraction . Mining

Introduction

While As has been the focus of environmental concern fordecades, elevated concentrations of Sb around smelters,industrial, mining and mineralized areas have only morerecently become of interest in terms of its detailed envi-ronmental behaviour and impacts (Filella et al. 2002). Stillvery little is known about the environmental mobility andeffects of Sb (Wilson et al. 2004). Currently both elementsrepresent a priority pollutant (Flynn et al. 2003; USEPA2004). Because of the similarities in environmental chem-istry and toxicity between As and Sb (Wilson et al. 2004)

J. Gal · A. S. Hursthouse (�) · S. J. CuthbertSchool of Engineering and Science, University of Paisley,Paisley, PA1 2BE Scotland, UKe-mail: andrew.hursthouse@paisley.ac.ukTel.: +44-141-848-3213Fax: +44-141-848-3204

this study has focused on assessing their mobility in soilsfrom former mining areas.

In this study, three types of extraction procedure havebeen assessed: a single reagent; an element-specific anda non-specific sequential extraction. The methods wereapplied to soil samples collected from two locations in-fluenced by mining activities: the first an abandoned Sbmining and smelting site in SW Scotland (Glendinning)and the second an area in NW Italy (Anzasca Valley,Piemonte), where emissions of As from more recent in-dustrial activities are superimposed on a region where his-torical As contamination arises from Ag/Au mining. Thesites contain intensive mining areas dating from pre-Romantimes (Piemonte) and the eighteenth to nineteenth centuries(Glendinning), with younger soil heaps exposed to weath-ering and percolation of surface waters.

Experimental

Soil sampling

Bulk soil samples (0–10 cm) were collected during fieldcampaigns in April and October 2003 and July 2004. Thesamples were collected downstream along the GlenshanaBurn (Glendinning) and along the Anzasca river plane(Piemonte). A total of 17 (Glendinning – GD) and 14(Piemonte – PI) soil samples were collected at locationsacross the mining areas, and transported to the laboratoryon the same day, and processed immediately.

Determination of total arsenic and antimonyin soil samples

The soil samples were weighed, dried in an oven at30◦C±5, screened through a 2 mm sieve and the <2 mmfraction split for grinding to <150 µm. This portion wasused for ICP-AES analysis. Aliquots of 0.500 g of sam-ple were digested according to the USEPA Method 3051a

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(USEPA 1998) using a CEM MARS X microwave system,to reduce the risk of loss of volatile elements and improveprecision. Following digestion, the soil samples were fil-tered (Whatman 42 ashless, paper filters) and made up to50 ml using UHP water for analysis. Each soil sample wasanalysed in triplicate. All reagents and standards were ofanalytical grade (AnalaR or Spectrosol) and ultra high pu-rity water (18 M�, Elgastat UHP) was used for dilutions.

Stock solutions of commercially produced (Plasma PureStandard Solutions, Leeman Labs Incorporation), multi –element standard solutions (500 mg/l) in 10% nitric acid,were used to prepare appropriate elemental calibration stan-dards for the ICP-AES (Perkin Elmer Optima 3000). Allsample concentrations were reported as mg/kg dry weight.

Analytical quality was checked by analysis of Metals inSoil Certified Reference Material (Resource TechnologyCorporation No. CRM020-050) with very good recoveryfor arsenic and antimony (97 and 85%, respectively). Inaddition spikes and blank checks were run routinely.

Extraction of arsenic and antimony

1. Single-stage extraction: A single-step extraction proce-dure using 1 M NH4NO3 solution to determine readilysoluble and plant available metal contents (DIN197301997; Hammel et al. 2000) was used. The mobile frac-tion was extracted by shaking 10 g of soil with 25 ml1 M NH4NO3 for 2 h followed by filtration (DIN197301997).

2. Element-specific (As) sequential extraction: A five-step sequential extraction procedure was applied to1.5 g of soil (Wenzel et al. 2001) for As analysis.The fractions were characterised as non-specificallysorbed (0.05 M (NH4)2SO4), specifically sorbed (0.05 MNH4H2PO4), amorphous and crystalline hydrous ox-ides of Fe and Al (0.2 M NH4-oxalate buffer), well-crystallised hydrous oxides of Fe and Al (0.2 M NH4-oxalate buffer+ascorbic acid) and residual fractions(aqua-regia digest).

3. Non-specific sequential extraction: The CISED(Chemometric Identification of Substrates and Ele-ment Distribution) method (Cave et al. 2004) usesthe relative solubility of the solid material at differentrates as the concentration of aqua regia is increased.The chemical composition of the solid responsible foreach extracted phase is derived from a chemometricdata processing method and XRD/SEM data on sam-ple composition. The more easily extractable phasessuch as carbonates are dissolved in the lower acid con-centration extracts while the less soluble components,such as iron oxide are dissolved in the higher strengthacid extracts. This method only provides information onthe nitric/hydrochloric acid soluble components in thesoil material and captures all components released inconventional extraction schemes. Each extracted solu-tion was analysed for 22 elements, by ICP-AES, usingcertified solution standards (as above). The extraction

and subsequent data processing were carried out at theBritish Geological Survey (Keyworth, UK).

Statistical treatment of data

Statistical assessment including regression analysis andsignificance tests of correlation coefficients, were carriedout using SYSTAT Version 8. The data-processing ofthe non-specific sequential extraction (CISED) methodwas undertaken at the British Geological Survey usinga MatLab (The Mathworks, Natick, MA, USA) basedmodeling protocol.

Results and discussion

Soil characteristics

The total concentrations of As in the soils were between50–17,428 mg/kg (811 mg/kg, median) (GD) and 16–691 mg/kg (96 mg/kg, median) (PI) (normal distribution).While the Sb levels were lower, (log-normal distribution),ranging between 10–1,187 mg/kg (69 mg/kg, median) (GD)and between 1.63 and 11 mg/kg (8.12 mg/kg, median) (PI).The concentrations of As and Sb in the soils collected fromboth sites, were well above the background levels and gen-erally exceeded the International Guidelines (VROM andBodemsanering 1983).

Soils at both sites were acidic to slightly acidic, pH(CaCl2) varied between 2.62–6.81 (GD) and 3.55–4.83 (PI)while Eh (CaCl2) ranged from −41 to 224 mV (GD) and 89to 159 mV (PI). At low pH values As(V) tends to exist whileat higher pH values the corresponding arsenic(III) speciescan be present. As the pH increases, As is desorbed into thesoil solution. In a sulphur-free environment, As is predictedto be very mobile under almost all conditions (Vink 1996).Antimony, especially under oxic conditions, is very soluble(Krupka and Serne 2002) while more reducing conditionslower Sb solubility. It is therefore expected that soil param-eters like pH/Eh will influence elemental toxicity due tochanging availability (solubility or mobility).

Extractable soil As and Sb by single extraction

Levels of NH4NO3 extracted As and Sb in the samplesranged from 0.01 to 3.10 mg/kg (0.19 mg/kg, median) (As,GD) and from <0.001–4.12 mg/kg (0.51 mg/kg, median)(Sb, GD) and between <0.001–0.13 mg/kg (0.03 mg/kg,median) (As, PI) and <0.001–0.30 mg/kg (0.06 mg/kg,median). These concentration values, indicating the mobilefraction represent <1% (Sb, PI) of the pseudototal Asconcentrations for both sites while for Sb it was between0.01–8.82% (GD) and 0.01–3.43% (PI). However whilst,Sb appears to be proportionally more mobile than As,the results indicate that neither of the elements could betermed strongly bioavailable.

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Fig. 1 Arsenic released by As-specific sequential extraction fromthe (a) Glendinning (n=17) and (b) Piemonte (n=14) soil samples.F1: non-specifically sorbed; F2: specifically sorbed; F3: amorphousand crystalline hydrous oxides of Fe and Al; F4: well-crystallisedhydrous oxides of Fe and Al; F5: residual phases

The NH4NO3 extract is considered as the portion avail-able for plants (DIN19730 1997; Hammel et al. 2000).Other studies have found relatively low values of ex-tractable Sb using this method for example Hammel et al.(2000) reported values of 0.02–0.29 mg/kg (0.06–0.59% ofthe total soil Sb content).

The redox state and pH of soils has a major influenceon As and Sb speciation and solubility (Krupka and Serne2002). A strong and significant relationship was found be-tween soil pH and mobile As content (R2=0.50; p=0.008)with a less significant relationship for Sb (R2=0.26;p=0.075).

Extractable soil As by specific sequential extraction

Application of this method was to describe As distributiononly. Analysis of Sb using this extraction scheme producedvery poor recoveries, highlighting the differences in so-lution chemistry of these elements. The results from thesoil extractions are presented as a box and whisker plotin Fig. 1. In both locations, step 3 dominates, reflecting

the strength of association with Fe/Al-oxyhydroxides inGD samples (70% mean), and PI 60% (between 9 and450 mg/kg). The next most significant fractions were theresidual (18% mean, GD) and crystallised Fe/Al hydrousoxides (∼20% mean, PI). This latter fraction is consid-ered poorly mobilizable but still available (Fedotov et al.2005). The most mobile/available fraction extracted in step1 was found to be equivalent to <0.001–32 mg/kg (GD)and <0.001–2.2 mg/kg (PI) As. No significant differencewas observed between the sum of As extracted during the5-step sequential extraction and the pseudototal As (t-test,95% confidence interval).

The association of As with Fe is in agreement with SEM-EDS observations and agrees with previous reports for con-taminated soils (Wenzel et al. 2001; Taggart et al. 2004).

When the As associated with the single extraction stepand first step from the As-specific method are compared,the single-step extract from both locations was consid-erably lower than the amount in the first stage of theAs-specific sequential extraction. However, a very strongand positive linear relationship was found between them(y=10.121x+0.93, R2=0.84, P<0.0001). No other signifi-cant relationship was found between the NH4NO3 extractedAs and the other fractions from the sequential extraction.The differences are likely to be a reflection of specificchemical attack, but it is important to note the relativecomparability of the two techniques. Absolute values maynot agree but relative trends do. This has implications forthe potential use of a simple extraction test for wider as-sessment of soil-As contamination.

The relationship of the more available As and Sb com-ponents (first step of the As-specific extraction and thesingle extraction) relative to total soil content was assessedby multiple correlation and strongly significant (P<0.001–<0.01) relationships between soil As and the extracted Aswere found in samples with the highest total concentra-tions, despite the differing source terms between elementsand locations. In Piemonte for Sb this relationship was notso strong (<0.05), probably due to the lower solubility andtotal Sb concentrations.

Extractable soil As and Sb by non-specific sequentialextraction

Using the CISED extraction method a range of geochemi-caly distinct components were identified. Both As and Sbshowed differences in their association with change in acidconcentration. However, the association of both metals wasdominated by a number of Fe-rich components which, frompowder XRD, suggested goethite and hematite dissolutionwas taking place.

Figure 2 presents results for both As and Sb distributionin different components in the Gleninning soils from theCISED test. Most of the extractable As (∼70–90%) is inthe Fe-dominated components (amorphous and crystallineFe oxyhydroxide, crystalline Fe-Al oxide).

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Fig. 2 Box and whisker plot showing the range of total extracted As(upper) and Sb (lower) associated with the mineralogical componentsidentified by CISED in the Scottish soils (n=5)

A smaller portion is found in the exchangeable compo-nents (31 mg/kg, maximum value). Two Ca-Mg rich com-ponents were identified, probably derived from calcite ordolomite (presence in samples confirmed by powder XRDanalysis), associated with ∼2% of the extractable As.

For Sb, again the Fe-rich components dominate (Fe-Al oxides/oxyhydroxides, ∼90% mean). However a sig-nificant difference is the contribution from Fe/Fe-Pb sul-phides (7% mean). In the mineralization of the region, Sbis deposited as stibnite (Sb2S3) and the low availability(<0.001–4.12 mg/kg), in the single-step extraction is likelyto be due to this association. It is worth noting that in twosamples collected from the abandoned smelting locationareas almost 50% of the total Sb was associated with theFe/Fe-Pb sulphide extract.

In the Piemonte samples (Fig. 3) a similar distributionis seen for As. Most of the extractable As (∼70–90%mean) was found in the Fe-dominated oxide components,(Fe oxide, amorphous Fe oxyhydroxide). A much smallerAl oxide fraction is the next most significant component

Fig. 3 Box and whisker plot showing the range of total extractedAs (a) and Sb (b) associated with the mineralogical componentsidentified by CISED in the Italian soils (n=2)

followed by the pore water/exchangeable fraction (5.95%mean) For Sb only two components, pore water and Alsilicate are responsible for most of the extracted Sb. In asample from the industrially contaminated area in PieveVergonte (Piemonte) 3 mg/kg Sb was extracted from thepore water/exchangeable component, representing 90% ofthe total extracted amount. While in samples collected froma fluvial terrace mining area most of the extracted antimonyappeared to be in the Al silicate (Ca-Al-Mn-Mg-Si) com-ponent (95% of the total extracted Sb). The Fe-rich compo-nents (Fe oxides and clay like materials) contain only traceamounts of Sb (ca 2%).

Conclusion

Although total As and Sb values were high, the readilyavailable fractions only exceeded international guidelinesin the case of As. According to the results of two sequen-tial extractions the greatest portion of soil arsenic (from

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both sites), was strongly retained by the Fe-oxide boundfractions. The behaviour of Sb is more site-specific. Inthe Scottish soils it is retained by Fe oxide and sulphidephases while in the Italian soils, under more recent indus-trial influence, a significant portion is more available butsuperimposed on a large portion bound in the silicate frac-tion, reflecting primary mineralisation origins. Statisticallysignificant relationships were obtained between soil pHand available As and Sb fractions indicating that pH playsa significant role in regulating solubility /mobility.

The single-step extraction scheme provides an indicationof relative “availability” of the two elements, but for As,underestimates absolute levels when compared to the As-specific extraction. The CISED extraction approach pro-vides considerably more mechanistic information aboutmetal association with soil phases, but with considerablymore information needed to apply it.

This study has provided data that confirm availabil-ity/mobility of As and Sb is related to the degrada-tion/weathering of the host phases and highlighted sig-nificant differences between the two elements at each site.The influence of the mineralogical form on bioavailabilityis well known (Ko et al. 2003) but from this study, we showthat minor portions of soil As and Sb are mobile in the mostbioavailable fractions. Significant portions of each elementare also associated with phases which are sensitive to long-term changes in the physico-chemical characteristics of thesites, which have implications for longer term supply of themobile pool.

Acknowledgements This study was funded by the University ofPaisley. The authors would like to thank David Stirling for help withICP-AES for Aurellio Facchinelli and Claudia Bruno (Universityof Turin, Italy) for their help in sample collection and sequentialextraction. We also acknowledge Mark Cave, Ben Klinck, BarbaraPalumbo-Roe and Jo Wragg, BGS (Keyworth) for their support andhelp with the data processing.

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