Department of Soil Science, P.O. Box 84, Lincoln University, Canterbury, New Zealand

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1 Scientific registration n : 120 Symposium n : 25 Presentation : oral Assessment of heavy metal contamination of soils using sequential fractionations Evaluation de la contamination des sols par les métaux lourds à l aide de fractionnements séquentiels McLAREN Ronald Department of Soil Science, P.O. Box 84, Lincoln University, Canterbury, New Zealand Introduction Existing guidelines or regulations for the assessment of heavy metal contamination of soil are based on total soil metal concentrations. However, it is generally recognised that total concentrations do not necessarily provide good information on the potential bioavailability or mobility of metals in soils. Sequential fractionation techniques are being used increasingly to provide more useful assessments of soil heavy metal contamination than is possible with single extractions or total metal concentrations alone. In this paper, three case studies are used to demonstrate the potential of sequential fractionation techniques to assess soils contaminated with heavy metals from different sources. The three case studies are (i) soils contaminated with arsenic (As) from cattle dip (sodium arsenite), (ii) soil contaminated with heavy metals from repeated applications of sewage sludge, and (iii) soil contaminated with copper (Cu), chromium (Cr) and As from timber treatment solution. In all three cases, in addition to the collection of surface samples, the distribution of metals within the soil profile was examined by incremental depth sampling. Soils contaminated with As from cattle dip Soils surrounding cattle dips in Australia are known to be highly contaminated with As and are potentially of concern to the environment and human health. A sequential fractionation scheme, based on the Hedley et al. (1982) scheme for the fractionation of soil P, was developed to assess the chemical nature, and thus the potential bioavailability and mobility of As at the sites (McLaren et al., 1998). Soil As is separated soil into six fractions using the extractants shown in Table 1. By analogy with the P fractionation scheme (Hedley et al., 1982; Tiessen and Moir, 1993), Table 1 also lists the nominal forms of As considered to be present in the six fractions. A study of 11 dip sites revealed considerable surface soil (0-10 cm) contamination with As ( mg As/kg soil). In addition it was shown that considerable movement of As 1

2 down through the soils had taken place with concentrations ranging from mg As/kg soil. Considerable variation was observed in the amounts and proportions of Table 1. Sequential fractionation of soil As Fraction Extractant Nominal As forms extracted 1 Anion exchange membrane Freely exchangeable As M NaHCO 3, ph 8.5 Non-exchangeable but labile As M NaOH As chemisorbed by Fe & Al surfaces 4 Ultrasonic dispersion/0.1 M NaOH As on internal surfaces of aggregates 5 1 M HCl Ca-associated As 6 HCl/HNO 3 digestion Highly recalcitrant As As present in individual fractions between the dip sites examined and between different soil depths. However in spite of this variation, some general trends are apparent. The most labile As fraction (resin-extractable As) accounts for less than 5 % of the total soil As (Table 2). Resin extractable As was highly correlated with water-soluble As (R 2 = 0.84), with watersoluble As concentrations being approximately 15 % of resin-extractable As Table 2. % of total As in individual fractions (n = 26) Fraction Mean Range Resin As NaHCO 3 As NaOH Ultrasonic/NaOH HCl Residue concentrations. Resin-extractable As may have value as an indicator of potential bioavailability and/or mobility of contaminant As in soil. The higher concentrations of As extracted by the resin compared to water can be a distinct advantage as far as analysis of As is concerned. % total As Total As mg kg mm mm mm Soil depth Resin NaHCO 3 NaOH(1) NaOH(2) HCl Residue Figure 1. Distribution of As between fractions at Wagner dip site (McLaren et al. 1998) In most soils, relatively small proportions of total As were also extracted in the second fraction (NaHCO 3 -extractable As). However, the relatively low proportions of resin and NaHCO 3 -extractable As at most sites does not mean that As toxicity or mobility is unlikely to be a problem. The high total As concentrations ensure that the labile forms of As are often well above the levels likely to cause phytotoxicity (Sheppard, 1992). In addition, the relatively high amounts of 2

3 As in labile forms at depth in some soil profiles (e.g. Fig. 1), suggests a significant potential for further leaching. The bulk of the As in soils at the dip sites was present in the NaOH-extractable fractions with a mean 44.7 % in the first extraction and a further 14.2 % after ultrasonic treatment. Relatively small proportions of As were found in the HCl-extractable fraction (mean 6.9 %) and the final residual fraction contained on average 21.1 % As (Table 2). The relative difficulty in extracting this fraction of As, suggests that it should be considered to be extremely tightly bound and thus very unlikely to be bioavailable or mobile. At some sites (e.g. Fig. 1), the significant proportions of As in this fraction at depth suggests that downward movement of mobile forms of As has been followed by transformation into relatively recalcitrant forms. Soils contaminated with metals from sewage sludge Soils were sampled to a depth of 80 cm at four sites which had received sewage sludge at frequent intervals for more than 25 years. Total metal concentrations in the samples were determined using an acid digestion technique. Fractionation of metals was carried out using a simplified scheme based on sequential extraction techniques published by Shuman (1985) and McGrath and Cegarra (1992). The scheme separates metals into just four fractions (Table 3). Table 3 Sequential fractionation of soil metals Fraction Extractant Nominal forms M Ca(NO 3 ) 2 Soluble + exchangeable 2 5 % NaOCl (2 x) Organic-bound 3 Oxalate buffer (ph 3) Oxide-bound plus 0.1M ascorbic acid 4 HNO 3 /HClO 4 digest Residual At all 4 of the sampled sites, there were considerable accumulations of metals in the topsoils (0-10 cm). Concentrations of individual metals ranged from approximately 1-4 mg Cd/kg soil, mg Cr/kg soil, mg Cu/kg soil, mg Ni/kg soil, mg Pb/kg soil and mg Zn/kg soil. There were substantial differences between the metals in their distribution between the four different soil fractions (Fig. 2). Cadmium (37-49 %), Cu (28-64 %) and Ni (41-77%) showed relatively high proportions present in the organic fraction, whereas the highest proportion of Zn was found in the oxide fraction. Chromium (> 90 %) and Pb (74-86 %) were found predominantly in the residual fraction. Only Ni (5-22 %) and Zn (6-21 %) were present in substantial amounts in soluble + exchangeable forms. Both of these metals were observed to have leached downwards through the soil profile to depths of well below the original layer of sludge incorporation (Smith, 1994). 3

4 In addition to the fractionation of metals described above, metal desorption was determined by sequential equilibration of samples in 0.01 M Ca(NO 3 ) 2 (Hogg et al., 1993). All metals, apart from Cr, could be desorbed from the soils using this procedure Exchangeable Organic Oxide Residual % total metal Cd Cr Cu Ni Pb Zn Metal Figure 2. Fractionation of metals in sludged soils (0-10 cm, mean values n = 4) However, the amounts of metal desorbed varied substantially between soils and between the different metals. After 5 successive equilibrations, the cumulative amounts of metal desorbed ranged from a low of µg/kg for Cd to a high of mg/kg for Zn. Desorption of metals from a control soil, which has never received sewage sludge, was barely detectable. Soil contaminated with timber treatment solution (Cu, Cr and As) Copper, Cr and As (CCA) compounds are used extensively in the New Zealand timber preservation industry and soil contamination with these metals at timber treatment sites is not uncommon. Although, in some circumstances, a proportion of the Cr and As in particular can be leached from the soil (Carey et al., 1996), substantial amounts of all three elements generally remain in the soil. The metal fractionation scheme described in Table 3 was used to examine the forms of Cu and Cr remaining in a soil contaminated with CCA, following an extended period of leaching of the soil. Fig. 3 shows the proportional distribution of Cu between the four fractions with depth in the soil profile. Total Cu concentrations ranged from approximately 400 mg/kg in the surface 0-2 cm layer to 10 mg/kg at a depth of cm. In a control soil uncontaminated with CCA, Cu concentrations ranged from 15 mg/kg in the surface to approximately 5 mg/kg at cm. Below 30 cm there was no significant difference in Cu concentration between the control and the CCA treated soil. 4

5 % total soil copper Soil depth (cm) Exchangeable Organic Oxide Residual Figure 3. Fractionation of Cu in CCA contaminated soil In addition to the elevated Cu concentrations in the CCA contaminated soil compared to the control, there is also a major difference in the distribution of Cu between the different soil fractions. Table 4 shows the mean proportional distribution of Cu in the top 10 cm of soil for both the control and CCA-contaminated profiles. The contaminated soil has much higher proportions of Cu in the organic and oxide fractions compared to the control soil, and much less Cu in the residual fraction. The same is true for Cr (Table 4). Table 4. Percentage distribution of Cu and Cr between fractions in topsoil (0-10 cm) of contaminated and uncontaminated soils Sample Exchangeable Organic Oxide Residue Copper Control Contaminated Chromium Control Contaminated The higher proportions of metal in the more labile fractions in contaminated, compared to uncontaminated soils, appears to be a fairly common phenomenon, e.g. Levy et al., 1992; McGrath and Cegarra, In the absence of suitable control soils or knowledge of total metal concentrations in uncontaminated soils, examination of the proportional distribution of metals between fractions could well provide information regarding whether soils have in fact been contaminated with metals. 5

6 Conclusions The use of sequential fractionation techniques to assess metal-contaminated soils is not without problems. Few reagents used in such procedures are completely selective for particular forms of metals, and the amounts of metal extracted can be highly dependent on the conditions of extraction, e.g. ph, temperature, particle size, mixing, time of extraction. For detailed consideration of the issues associated with the development and use of fractionation schemes, the reviews by Beckett (1989) and Kersten and Förstner (1995) should be consulted. However, in comparison with total soil metal concentrations alone, fractionation data can provide a much more useful assessment of metal-contaminated soil. This is clearly demonstrated with reference to the three case studies described above. Irrespective of the actual fractionation scheme used, the most labile (and most easily extracted) soil metal fractions, provide estimates of potential bioavailability and mobility of metals in contaminated soils. Conversely, the metals in those fractions most resistant to extraction can be considered relatively inert and therefore of little environmental concern. In addition, in the absence of other information, the proportional distribution of metals between different soil fractions can be used to distinguish between metal-contaminated and uncontaminated soils. References Beckett, P.H.T The use of extractants in studies on trace metals in soils, sewage sludges, and sludge-treated soils. Advances in Soil Science 9: Carey, P.L., McLaren, R.G., Cameron, K.C. and Sedcole, J.R Leaching of copper, chromium, and arsenic through some free-draining New Zealand soils. Australian Journal of Soil Research 34, Hedley, M.J., Stewart, J.W.B. and Chauhan, B.S Changes in inorganic and organic soil phosphorus fractions induced by cultivation practices and by laboratory incubations. Soil Science Society of America Journal 46: Hogg, D.S., McLaren, R.G. and Swift, R.S Desorption of copper from some New Zealand soils. Soil Science Society of America Journal 57: Kersten, M. and Förstner, U Speciation of trace metals in sediments and combustion waste. In: Chemical Speciation in the Environment (eds. A.M. Ure and C. M. Davidson), Blackie Academic & Professional, London. pp Levy, D.B., Barbarick, K.A., Siemer, E.G. and Sommers, L.E Distribution and partitioning of trace metals in contaminated soils near Leadville, Colorado. Journal of Environmental Quality 21: McGrath, S.P. and Cegarra, J Chemical extractability of heavy metals during and after long-term applications of sewage sludge to soil. Journal of Soil Science 43: McLaren, R.G., Naidu, R., Smith, J. and Tiller, K.G Fractionation and distribution of arsenic in soils contaminated by cattle dip. Journal of Environmental Quality (in press). Sheppard, S.C Summary of phytotoxic levels of soil arsenic. Water, Air and Soil Pollution 64:

7 Shuman, L.M Fractionation method for soil microelements. Soil Science 140: Smith, E.R.G Long-term fate of heavy metals applied in sewage sludge to a sandy soil. BSc (Hons) Thesis, Lincoln University, New Zealand. Tiessen, H. and Moir, J.O Characterization of available P by sequential extraction. pp In M.R. Carter (ed), Soil sampling and methods of analysis. Lewis Publishers, Boca Raton, Florida. Key words: bioavailability, fractionation, heavy metals, soil contamination Mots clés : disponibilité biologique, fractionnement, métaux lourds, contamination des sols 7