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Redistribution of Pb, Zn and Cu Fractions in Tailing Soils Treated with Different Extractants1
Pedosphere 16(3): 312-318, 2006 ISSN 1002-0160/CN 32-1315/P @ 2006 Soil Science Society of China Published by Elsevier Limited and Science Press
Redistribution of Pb, Zn and Cu Fractions in Tailing Soils Treated with Different E x t r a c t a n t s * ' LIU Yun-Guo', WANG Xian-Hail, ZENG Guang-Ming', LI Xin', ZHOU C h ~ n - H u a ' ) ,~FAN Tingl, LI Yong-Li' and YUAN Xin-Zhongl 'Department of Environmental Science and Engineering, Hunan University, Changsha 410082 (China). E-mail: [email protected] Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 11 0016 (China) Graduate School of Chinese Academy of Sciences, Beijing 100039 (China) (Received October 28, 2005; revised February 26, 2006)
ABSTRACT The efficiency of EDTA, HN03 and CaC12 as extractants t o remove Pb, Zn and Cu from tailing soils without varying soil pH was investigated with distributions of Pb, Zn and Cu being determined before and after extraction using the sequential extraction procedure of the optimized European Community Bureau of Reference (BCR). Results indicated that EDTA and HN03 were both effective extracting agents.The extractability of extractants for Pb and Zn was in the order EDTA > HN03 > CaC12, while for Cu it was HNO3 > EDTA > CaClz. After EDTA extraction, the proportion of Pb, Zn and Cu in the four fractions varied greatly, which was related t o the strong extraction and complexation ability. Before and after extraction with HNO3 and CaC12, the percentages of Pb, Zn and Cu in the reducible, oxidizable and residual fractions changed little compared to the acid-extractable fraction. The lability of metal in the soil and the kinds of extractants were the factors controlling the effects of metal extraction. Key Words:
BCR, extraction, fraction, heavy metals, redistribution
INTRODUCTION Heavy metals are considered serious inorganic pollutants because of their toxic effects on biota, having a high enrichment factor and slow removal rate (Alloway and Ayres, 1997). Heavy metals in soil exist in different chemical forms or types of binding. In environmental studies, it is now widely accepted that measuring only the total metal concentration cannot assess the impact of heavy metals on soil (Bufflap and Allen, 1995). Consequently, identification of the geochemical phases to which the metals are bound and the knowledge of the strength of bonds are necessary to evaluate the availability and the capacity of mobilization of heavy metals in soils (Fangueiro et al., 2002). The cleanup of tailing soils has been one of the most difficult tasks for environmental engineering. A number of techniques have been developed that aim to remove heavy metals from tailing soil including exsitu washing with physical-chemical methods (Anderson, 1993) and in-situ phytoextraction (McGrath, 1998). In the ex-situ washing methods, chelating agents or acids are used to enhance heavy metal removals. These chelating agents remove heavy metals with less impact on soil properties than other decontamination systems (Sun et al., 2001). Many studies have shown that EDTA is effective in removing Pb, Zn and Cu from contaminated soils, although extraction efficiency depends on many factors such as the lability of heavy metals in soil, the strength of EDTA, electrolytes, pH and the soil matrix (Blaylock et al., 1997; Brown and Elliott, 1992; Elliott and Shastri, 1999; Papassiopi et al., 1999). Also, after soil washing, EDTA may be recovered and reused through reactions of metal-EDTA complexes with NazS (Hong et al., 1999). Meanwhile, strong acids such as HN03 or HC1 have been used as washing solutions. In some studies *'Project supported by the National High Technology Research and Development Program (863 Program) of China (No. 2001AA644020) and the Natural Science Foundation of HLnan Province (No. 04JJ3013).
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other weaker organic acids such as tartaric, fumaric or pyruvic acids have also been proposed (Wasay et al., 1998). Among these acids, HN03 is the more efficient extracting agent (Neale et al., 1997). These chelating agents and inorganic or organic acids show a significant potential to extract heavy metals from the soils. However, these applications are associated with a number of disordering physical, chemical and biological properties to the soils. Neutral salts, such as CaClz and NaN03, are less aggressive solutions, which are often called ‘soft’ or ‘mild’ extractants and are based on un-buffered salt solutions. These salts mainly dissolve the cation exchangeable fraction although in some cases the complexing ability of the anion (e.g. C1-) can play a certain role (Pueyo et al., 2004; Rauret, 1998). Additionally, a number of sequential extraction procedures exist, such as the Tessier scheme (Tessier et aL, 1979), the Geological Survey of Canada scheme (Hall et al., 1996a,b) and the original European Community Bureau of Reference (BCR) scheme (Ure et al., 1993). The European Community has utilized the optimized BCR approach, a newly developed, standardized sequential extraction procedure (Rauret et al., 1999; Pueyo et al., 2001). A comparison of extractable Pb, Zn and Cu percentages for certified reference soils using the four schemes above indicated significant differences between them with the optimized BCR approach removing significantly more Pb, Zn and Cu from the reducible fraction. Concomitant decreases in the relative importance of the oxidizable and residual fractions also occurred with the application of the optimized BCR approach. The increased removal of metals in the reducible fraction stemmed primarily from the increased hydroxylamine hydrochloride concentration in the optimized BCR scheme (Sutherland and Tack, 2003). Using the optimized BCR approach the present study was carried out t o 1) investigate the efficiency of EDTA (used because of its strong chelating ability for different heavy metals), HN03, and CaC12 extraction in metal-contaminated soils without varying soil pH; and 2) evaluate changes of Pb, Zn and Cu in different forms before and after extraction. MATERIALS AND METHODS Soil sampling, description and analytical measurements
Surface soil samples (0-20 cm) were collected from the tailing areas of Yongzhou Pb/Zn Mine in Hunan Province where soils were contaminated with Pb, Zn and Cu in varying degrees due to mining activities. The soil samples were air-dried, sieved to > 2 mm, homogenized, and stored for lab analysis. The average pH value of the soil was 4.8, and the percentage of organic matter was 6.4%. The percentage of sand, silt and clay was 24.1%, 69.4% and 6.5%, respectively. According to the texture analysis, the soil was classified as silty sand. All the extractions and digestions were performed in triplicate. First, the soil samples we5e digested with HN03-HF-HC103-H202, as used by Medved et al. (1998) with all the reagents used being of analytical grade. Total concentrations of Pb, Zn and Cu in the soil were determined using a flame atomic absorption spectrometer (FAAS). Soil pH was determined using a glass electrode and 1:1 soil to water ratio, while soil particle-size analysis was carried out using a JL-1155 laser particle analyzer. Organic matter was measured with standard methods (Environment Monitoring of China, 1992). Extraction procedures
All glassware was acid-washed prior to use with 4 mol L-’ HN03. Then five grams of soil was placed in 250-mL high-density polyethylene (HPED) bottles. To meet a recommended solid-to-liquid ratio of 0.05 as determined from previous studies (Neale et al., 1997), 100 mL of extracting agents of 0.05 mol L-l EDTA solution, 0.05 mol L-’ HN03 solution, and 0.05 mol L-l CaC12 solution were added to separate 250 mL bottles containing the soils and shaken. Next, the solution pH was adjusted to 4.8 to maintain the original soil acidity. The mixture was then placed in an end-over-end shaker operating at 60 r min-’ for 24 h at room temperature to ensure that chemical equilibrium was reached. Equilibration time of 24 h was selected as other authors in similar studies have used this time limit (Fangueiro et
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al., 2002). Next, the soil solutions were extracted by decantation and passed through a 0.45-pm filter. Following this the soil residues were rinsed with distilled water to remove the residual extractant, and the samples dried. In the soil solution after EDTA, HN03 and CaC12 washing, as well as in the sequential extraction procedure, concentrations of Pb, Zn and Cu were measured in all the acquired liquid solutions.
Analysis Before the extraction procedure, the distribution of four fractions: acid-extractable, reducible, oxidizable and residual fractions, for Pb, Zn and Cu in soils was determined with the optimized BCR procedure. Then the sequential extraction procedure using the optimized BCR sequential extraction procedure was carried out to ascertain redistributions of Pb, Zn and Cu after extraction. Metal concentrations and extraction efficiencies €or Pb, Zn, and Cu were compared using total concentration; concentration extracted by extractants EDTA, HN03 and CaC12; and extractable percentage. Then the redistributions of Pb, Zn and Cu, after using the extractants, were evaluated. RESULTS AND DISCUSSION Extraction efficiencies with EDTA, HNO3 and CaC12 Table I showed that the soil was heavily contaminated with high concentrations of total Pb, Zn and Cu. Thus, because of low pH and organic matter level in this silty sand, it was difficult to support plant growth and not easy to use phytoremediation. TABLE I Total concentrations of Pb, Zn, and Cu, the concentrations extracted by extractants EDTA, HN03 and CaC12 and the extractable percentages Metal
Pb Zn cu
ct,ta)
mg kg-' 6 872 2 982 547
EDTA
HN03
Cextb)
%extC)
Cext
mg kg-' 2 129 875 237
%
mg kg1033 858 283
31.0 29.3 43.3
CaClz
'
%ext
Cext
%ext
%
mg kg-' 961 585 158
%
15.0 28.8 51.7
14.0 19.6 28.9
")Ct,t means the total concentration of metals; b)Cext means the concentration of metals extracted by extractant; means the extractable percentage of metals.
")%,,t
The sum of the four fractions (Table 11) was reasonably similar to the total concentration obtained after digestion of the original samples with HN03-HF-HC103-Hz02 (Table I). Table I also compares the removal efficiencies of the extracted metals for the three extractants. The results clearly indicated that both EDTA and HN03 were more effective in extracting Pb, Zn and Cu than CaCl2. Percentage extractabilities of metals for HN03 and CaC12 followed the sequence Cu > Zn > Pb. It was likely that the lability of metals in soil played an important role. Lead was more tightly bound in the soil than Zn and Cu, and therefore, was possibly more difficult to extract. The extractability of the extractants for Pb and Zn was in the order of EDTA > HN03 > CaC12, while for Cu it was in the order of HN03 > EDTA > CaC12. Also, the concentrations of Pb and Zn extracted with the EDTA procedure were always higher than those with HN03 or CaCl2. Because EDTA could release the metals that had formed a complex or had been adsorbed by organic substances, it could enhance the solubility and mobility of metals in the soil (Petruzzelli, 1989). The chelating agent EDTA could form not only soluble complexes with metals, but it could also influence the distribution of metals from low water soluble fractions to a high soluble fraction. However, the Cu extractability with the HN03 procedure was clearly better than the other solutions. The metals extracted were virtually
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TABLE I1 Sequential-extracted fractions of Pb, Zn and Cu before and after extraction with EDTA, H N 0 3 and CaC12 Metal
Concentration")
Acid-extractable
Reducible
Oxidizable
Residual
Sum
335 12 658 690
2397 797 1167 1194
mg kg-I 197 94 147 237
3955 3639 3796 3705
6884 4542 5768 5826
488 42 91 210
241 158 172 198
1034 950 980 956
1186 935 952 933
2949 2085 2195 2297
169 44 32 86
78 46 34 75
190 156 115 139
117 86 88 83
554 332 269 383
~
a)Cb,f means the fraction concentration of metals before extraction; CEDTA,C H N Oand ~ Ccaclz are the fraction concentrations of metals after extraction with EDTA, HNO3 and CaCl2, respectively.
all 1:1 metal-EDTA complexes. However, because the formation constants were similar for Pb-EDTA (19.0), Zn-EDTA (17.5) and Cu-EDTA (19.7), they alone could not explain this phenomenon. Nevertheless, the difference in the chemical forms and the degree of crystallization of the metal could have caused the 1:l metal-EDTA complexes (Sun et al., 2001). Also, calcium chloride could effectively remove the metals in the CEC fraction, and due to the strong cation exchange ability and the complexation with chloride, CaC12 could be applied to extract the metals from soils (Lebourg et al., 1998). The extraction of exchangeable fractions could reduce the hazard of the soil, which was particularly true for Pb. Redistribution of Pb after extraction Before extraction, Pb mostly accumulated in the residual fraction (57.4% of the total amount) and in the reducible fraction (34.8%)of the soil (Fig. 1). The acid-extractable fraction and oxidizable fraction of P b represented 4.9% and 2.9% of the total, respectively. However, the redistribution of P b in the soil changed dramatically with EDTA, HN03 and CaC12 (Fig. 1). The most noticeable change was the percentage of the reducible fraction that decreased from 34.8% t o 17.6%, 20.2% and 20.5% in soils treated with EDTA, HN03 and CaCl2, respectively. The solubility of Fe and Mn when utilizing the three extractants mainly caused the decreases in the reducible fractions. 100
a, 0)
m c
80
c
a, 0
t
Eg
n m
v
60 40
4-
Acid
H Reducible El Oxidizable
0 Residual
0
20 4-
X
W
Fig. 1 Percentage fraction of Pb, Zn and Cu before and after extraction. Pbef means the percentage fraction in un~ PCaClz are the fraction percentages in soils treated with EDTA, HNO3 and CaClz, treated soil and PEDTA,P H N Oand respectively.
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Considerable decreases in the oxidizable fraction of Pb after extraction with EDTA (Table 11) were consistent with the solubility of P b bound in organic matter (Petruzzelli, 1989; Li and Shuman, 1996). After extraction with HN03 and CaClz, the concentration of P b in the acid-extractable fraction increased. The residual HN03 and CaClZ trapped in the soil and the fact that these extractants could form stable water-soluble complexes, which were weakly adsorbed on soil particles (Yu and Klarup, 1994) maybe the reason for this result. The proportion of the residual fraction in the EDTA treated soil increased to nearly 80.0%, whereas in H N 0 3 and CaClz treated soils it was 65.0%, which indicated that much Pb was probably occluded in minerals like quartz, feldspars and so on (Fuentes et al., 2004). The high extractable efficiency of P b by EDTA (31.0%, Table I) suggested that EDTA had the potential to remediate P b from contaminated soil. The smaller extraction efficiency for P b with HN03 and CaClz (Table I) was probably related to the low amount of Pb in the acid-extractable or other extractable forms. The P b extraction efficiency of H N 0 3 was higher than that of CaClz (Table I), which could be attributed to HN03 being able to dissolve more P b bound in carbonates.
Redistribution of Z n after extraction Zinc in the untreated soil was mostly concentrated in the residual fraction (40.2%) and the oxidizable fraction (35.1%) (Fig. 1). This could have been due to Zn (or Cu) providing a greater enrichment in the oxidizable fraction than P b (Zhang and Ke, 2004), and also having a high percentage in the acid-extractable fraction (16.6%) (Fig. 1). The percentage of Zn in the reducible fraction and residual fraction before extraction remained similar to the results after the extraction with the three extractants. The extractable efficiencies of these three extractants in the oxidizable fraction were obvious with the percentage of the oxidizable fraction increasing from 35.0% to about 50.0%. The most notable differences, however, took place with the acid-extractable fraction. The percentage of acid-extractable fraction after extraction with EDTA, HN03 and CaC12 decreased from 16.6% to 2.0%, 4.1% and 9.1%, respectively. Another noteworthy change was that the concentration of Zn in the residual fraction decreased fractionally (Table 11), after extraction with the three extractants, which implied that about 20.0% of the residual Zn was extracted as a consequence of the partial disaggregation of the silicate matrix (Barona et d.,2001).The extractable efficiency of Zn with HN03 was higher than that of Pb (Table I). Perhaps more Zn was bound to carbonates and because of the low soil pH, HN03 could solubilize them effectively. The CaClz solution solubilized some Zn from the contaminated soil, possibly due to a large amount of Ca2+ in the CaC12 solution that could displace Zn from adsorption sites on the soil solids. The C1- could also form ZnCl+, which had a low affinity for the reacting surfaces in the soil (Barrow and Ellis, 1986). However, the Ca2+ and C1- could only solubilize the exchangeable or outer-sphere associated Zn from the soil surface.
Redistribution of Cu after extraction In untreated soil, Cu was widely distributed in the four fractions, with the sum of the acid, reducible and oxidizable fractions being 69.5% (Fig. 1). In the acid-extractable fraction, the percentage reached 30.5%, which was considerably higher than that of P b and Zn (Fig. 1). After extraction with EDTA and HN03, the measured percentages of Cu in the acid-fraction decreased from 30.5% to 13.3% and 11.9%, respectively (Fig. I), and the concentration of the other three fractions remained relatively unchanged (Table 11). This indicated that much Cu was associated with sulfides and probably occluded in the minerals. The extractable efficiency of CaC12 was fair and this salt only dissolved Cu by cation exchange and the complexing ability of the anion (e.g. Cl-). Because of the high percentage in the acid-extractable fraction (Fig. l),a high extractable efficiency could be reached. Thus, the extractable efficiencies with EDTA, HN03 and CaC12 were 43.3%, 51.7% and 28.9% respectively (Table I), and for Cu, HN03 had the best effect. The high extractable efficiency with H N 0 3 was possibly due to two reasons: 1)large amounts of metals associated with the clay mineral fraction could be exchangeable with protons (Hf) replacing them, and 2) a large amount of Cu was
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bound to carbonates. In regards to the second point, the pK,, value for Cu-carbonate was 9.7. Therefore, Cu had a strong affinity to carbonates forming carbonate precipitates or co-precipitates, and the strong acid, HN03 could dissolve the carbonate more easily than EDTA or CaC12. The large portion of Cu in the acid-extraction fraction suggested that cation exchange and inorganic ligands such as CaC12 could effectively release Cu from the soil, which reinforced the results of this study. CONCLUSIONS The solution of EDTA extracted more Pb and Zn than HN03 and CaClz due to its strong ability to form stable complexes with a variety of metals and to release the metals forming a complex or being adsorbed by organic substances. The HN03 solution extracted more Cu than EDTA and CaClz, probably due to the dissolved metals bound in the carbonates and Cu re-absorption to soil components. Extractabilities of metals for HN03 and CaCl2 followed the sequence: Cu > Zn > Pb. However, drawbacks associated with the extraction application included extraction of soil macronutrients important for plant growth and increasing soil acidity. In addition, the increased metal mobility could result in groundwater contamination. Further research such as the optimum plant-extractantmetal combination and the most suitable field application method is necessary to fully address these issues. REFERENCES Alloway, B. J. and Ayres, D. C. 1997. Chemical Principles of Environmental Pollution. Chapman & Hall, UK. Anderson, W. C. (ed.). 1993. Soil Washing/Soil Flushing. Amer. Acad, Environmental Engineers, Annaplois, MD. pp. 42-56. Barona, A., Aranguiz. I. and Elias, A. 2001. Metal associations in soils before and after EDTA extractive decontamination: Implications for the effectiveness of further clean-up procedures. Enwiron. Polht. 113: 79-85. Barrow, N. J. and Ellis, A. S. 1986. Testing a mechanistic model. V. The points of zero salt effect for phosphate retention, for zinc retention and for acid/alkali titration of a soil. J . Soil Sci. 37: 303-310. Blaylock, M. J., Salt, D. E., Dushenkov, S., Zakharova, O., Gussman, C., Kapulnik, Y., Ensley, B. D. and Raskin, I. 1997. Enhanced accumulation of P b in Indian mustard by soil-applied chelating agents. Enwiron. Sci. Technol. 31: 860-865. Brown, G. A. and Elliott, H. A. 1992. Influence of electrolytes on EDTA extraction of P b from polluted soil. Water, Air and Soil Pollution. 62: 157-165. Bufflap, S . E. and Allen, H. E., 1995. Sediment pore water collection methods for trace metal analysis: A review. Water Res. 29: 165-177. Elliott, H. A. and Shastri, N. L. 1999. Extractive decontamination of metal-polluted soils using oxalate. Water, Air and Soil Pollution. 110: 335-346. Environment Monitoring of China. 1992. Neoteric Analytical Methods of Elements in Soils (in Chinese). Science and Environment Literature Publisher, Beijing. 184pp. Fangueiro, D., Bermond, A., Santos, E., Carapuca, H., and Duarte, A . 2002. Heavy metal mobility assessment in sediments based on a kinetic approach of the EDTA extraction: Search for optimal experimental conditions. Analytic. Chimzc. Acta. 459: 245-256. Fuentes, A,, Llorens, M., Saez, J., Soler, A., Aguilar, M. I., Ortuno, J. F. and Meseguer, V. F. 2004. Simple and sequential extractions of heavy metals from different sewage sludges. Chemosphere. 54: 1039-1 047. Hall, G . E. M., Gauthier, G., Pelchat, J. C., Pelchat, P. and Vaive, J. E. 1996a. Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements. J . Anal. Atom. Spectrom. 11: 787-796. Hall, G . E. M., Vaive, J. E., Beer, R. and Hoashi, M. 1996b. Selective leaches revisited, with emphasis on the amorphous Fe oxyhydroxide phase extraction. J . Geochem. Explor. 56: 59-78. Hong, P. K. A,, Li, C., Banerji, S. K. and Regmi, T . 1999. Extraction, recovery, and biostability of EDTA for remediation of heavy metal-contaminated soil. Sod and Sediment Contaminatzon. 8: 81-103. Lebourg, A., Sterckeman, T., Ciesielski, H. and Proix, N. 1998. Trace metal speciation in three unbuffered salt solutions used to assess their bioavailability in soil. J . Enwiron Qual. 27: 584-590. Li, Z. and Shuman, L. M. 1996. Redistribution of forms of zinc, cadmium and nickel in soils treated with EDTA. The Science of the Total Environment. 191: 95-107. McGrath, S. P, 1998. Phytoextraction for soil remediation. I n Brooks, R. R. (ed.) Plants That Hyperaccumulate Heavy Metals. CAB International, Wallingford, UK. pp. 261-287.
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Medved, J., Stresko, V., Kubova, J. and Polakovicova, J. 1998. Efficiency of decomposition procedures for the determination of some elements in soils by atomic spectroscopic methods. Fesenius Journal of Analytical Chemistry. 360: 219-224. Neale, C. N.,Bricka, R. M. and Chao, A. C. 1997. Evaluating acids and chelating agents for removing heavy metals from contaminated soils. Environmental Progress. 16: 274-280. Papassiopi, N., Tambouris, S. and Kontopoulos, A. 1999. Removal of heavy metals from calcareous contaminated soils by EDTA leaching. Water, A i r and Soil Pollution. 109: 1-15. Petruzzelli, G. 1989. Recycling wastes in agriculture: Heavy metal bioavailability. Agric. Ecosyst. Environ. 27: 493-503. Pueyo, M., L6pea-SBnchez, J. F. and Rauret, G. 2004. Assessment of CaC12, NaN03 and NH4N03 extraction procedures for the study of Cd, Cu, Pb and Zn extractability in contaminated soils. Analytica Chimica Acta. 504: 217-226. Pueyo, M., Rauret, G., Luck, D., Yli-Halla, M., Quevauviller, P. and L6pez-SBnchez, J. F. 2001. Certification of the extractable contents of Cd, Cr, Cu, Ni, P b and Zn in a freshwater sediment following a collaboratively tested and optimized three-step sequential extraction procedure. J . Environ. Monit. 3: 243-250. Rauret, G. 1998. Extraction procedures for the determination of heavy metals in contaminated soil and sediment. Talanta. 46: 449-455. Rauret, G., L6pez-SBnchez, J. F., Sahuquillo, A. Rubio R., Davidson C., Ure A. and Quevauviller Ph. 1999. Improvement of the BCR three-step sequential extraction procedure prior to the certification of new sediment and soil reference materials. J . Environ. Monitor. 1: 57-61. Sun, B., Zhao, F. J., Lombi, E. and McGrath, S. P. 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113: 111-120. Sutherland, R. A. and Tack, F. M. G. 2003. Fractionation of Cu, P b and Zn in certified reference soils SRM 2710 and SRM 2711 using the optimized BCR sequential extraction procedure. Advances in Environmental Research. 8: 37-50. Tessier, A,, Campbell, P. G. C. and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51: 844-851. Ure, A. M., Quevauviller, Ph., Muntau, H. and Griepink, B. 1993. Speciation of heavy metals in soils and sediments: An account of the improvement and harmonization of extraction techniques undertaken under the auspices of the BCR of the Commission of the European Community. Int. J . Environ. Anal. Chem. 51: 135-151. Wasay, S. A., Barrington, S. F. and Tokunaga, S. 1998. Remediation of soils polluted by heavy metals using salts of organic acids and chelating agents. Environmental Technology. 19: 369-380. Yu, J. and Klarup, D. 1994. Extraction kinetics of copper, zinc, iron and manganese from contaminated sediment using disodium ethylenediaminetetraacetate. Water, A i r and Soil Pollution. 75: 205-225. Zhang, M.K. and Ke, Z. X. 2004. Copper and zinc enrichment in different size fractions of organic matter from polluted soils. Pedosphere. 14(1): 27-36.
Redistribution of Pb, Zn and Cu Fractions in Tailing Soils Treated with Different E x t r a c t a n t s * ' LIU Yun-Guo', WANG Xian-Hail, ZENG Guang-Ming', LI Xin', ZHOU C h ~ n - H u a ' ) ,~FAN Tingl, LI Yong-Li' and YUAN Xin-Zhongl 'Department of Environmental Science and Engineering, Hunan University, Changsha 410082 (China). E-mail: [email protected] Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 11 0016 (China) Graduate School of Chinese Academy of Sciences, Beijing 100039 (China) (Received October 28, 2005; revised February 26, 2006)
ABSTRACT The efficiency of EDTA, HN03 and CaC12 as extractants t o remove Pb, Zn and Cu from tailing soils without varying soil pH was investigated with distributions of Pb, Zn and Cu being determined before and after extraction using the sequential extraction procedure of the optimized European Community Bureau of Reference (BCR). Results indicated that EDTA and HN03 were both effective extracting agents.The extractability of extractants for Pb and Zn was in the order EDTA > HN03 > CaC12, while for Cu it was HNO3 > EDTA > CaClz. After EDTA extraction, the proportion of Pb, Zn and Cu in the four fractions varied greatly, which was related t o the strong extraction and complexation ability. Before and after extraction with HNO3 and CaC12, the percentages of Pb, Zn and Cu in the reducible, oxidizable and residual fractions changed little compared to the acid-extractable fraction. The lability of metal in the soil and the kinds of extractants were the factors controlling the effects of metal extraction. Key Words:
BCR, extraction, fraction, heavy metals, redistribution
INTRODUCTION Heavy metals are considered serious inorganic pollutants because of their toxic effects on biota, having a high enrichment factor and slow removal rate (Alloway and Ayres, 1997). Heavy metals in soil exist in different chemical forms or types of binding. In environmental studies, it is now widely accepted that measuring only the total metal concentration cannot assess the impact of heavy metals on soil (Bufflap and Allen, 1995). Consequently, identification of the geochemical phases to which the metals are bound and the knowledge of the strength of bonds are necessary to evaluate the availability and the capacity of mobilization of heavy metals in soils (Fangueiro et al., 2002). The cleanup of tailing soils has been one of the most difficult tasks for environmental engineering. A number of techniques have been developed that aim to remove heavy metals from tailing soil including exsitu washing with physical-chemical methods (Anderson, 1993) and in-situ phytoextraction (McGrath, 1998). In the ex-situ washing methods, chelating agents or acids are used to enhance heavy metal removals. These chelating agents remove heavy metals with less impact on soil properties than other decontamination systems (Sun et al., 2001). Many studies have shown that EDTA is effective in removing Pb, Zn and Cu from contaminated soils, although extraction efficiency depends on many factors such as the lability of heavy metals in soil, the strength of EDTA, electrolytes, pH and the soil matrix (Blaylock et al., 1997; Brown and Elliott, 1992; Elliott and Shastri, 1999; Papassiopi et al., 1999). Also, after soil washing, EDTA may be recovered and reused through reactions of metal-EDTA complexes with NazS (Hong et al., 1999). Meanwhile, strong acids such as HN03 or HC1 have been used as washing solutions. In some studies *'Project supported by the National High Technology Research and Development Program (863 Program) of China (No. 2001AA644020) and the Natural Science Foundation of HLnan Province (No. 04JJ3013).
REDISTRIBUTION OF TAILING METAL FRACTIONS
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other weaker organic acids such as tartaric, fumaric or pyruvic acids have also been proposed (Wasay et al., 1998). Among these acids, HN03 is the more efficient extracting agent (Neale et al., 1997). These chelating agents and inorganic or organic acids show a significant potential to extract heavy metals from the soils. However, these applications are associated with a number of disordering physical, chemical and biological properties to the soils. Neutral salts, such as CaClz and NaN03, are less aggressive solutions, which are often called ‘soft’ or ‘mild’ extractants and are based on un-buffered salt solutions. These salts mainly dissolve the cation exchangeable fraction although in some cases the complexing ability of the anion (e.g. C1-) can play a certain role (Pueyo et al., 2004; Rauret, 1998). Additionally, a number of sequential extraction procedures exist, such as the Tessier scheme (Tessier et aL, 1979), the Geological Survey of Canada scheme (Hall et al., 1996a,b) and the original European Community Bureau of Reference (BCR) scheme (Ure et al., 1993). The European Community has utilized the optimized BCR approach, a newly developed, standardized sequential extraction procedure (Rauret et al., 1999; Pueyo et al., 2001). A comparison of extractable Pb, Zn and Cu percentages for certified reference soils using the four schemes above indicated significant differences between them with the optimized BCR approach removing significantly more Pb, Zn and Cu from the reducible fraction. Concomitant decreases in the relative importance of the oxidizable and residual fractions also occurred with the application of the optimized BCR approach. The increased removal of metals in the reducible fraction stemmed primarily from the increased hydroxylamine hydrochloride concentration in the optimized BCR scheme (Sutherland and Tack, 2003). Using the optimized BCR approach the present study was carried out t o 1) investigate the efficiency of EDTA (used because of its strong chelating ability for different heavy metals), HN03, and CaC12 extraction in metal-contaminated soils without varying soil pH; and 2) evaluate changes of Pb, Zn and Cu in different forms before and after extraction. MATERIALS AND METHODS Soil sampling, description and analytical measurements
Surface soil samples (0-20 cm) were collected from the tailing areas of Yongzhou Pb/Zn Mine in Hunan Province where soils were contaminated with Pb, Zn and Cu in varying degrees due to mining activities. The soil samples were air-dried, sieved to > 2 mm, homogenized, and stored for lab analysis. The average pH value of the soil was 4.8, and the percentage of organic matter was 6.4%. The percentage of sand, silt and clay was 24.1%, 69.4% and 6.5%, respectively. According to the texture analysis, the soil was classified as silty sand. All the extractions and digestions were performed in triplicate. First, the soil samples we5e digested with HN03-HF-HC103-H202, as used by Medved et al. (1998) with all the reagents used being of analytical grade. Total concentrations of Pb, Zn and Cu in the soil were determined using a flame atomic absorption spectrometer (FAAS). Soil pH was determined using a glass electrode and 1:1 soil to water ratio, while soil particle-size analysis was carried out using a JL-1155 laser particle analyzer. Organic matter was measured with standard methods (Environment Monitoring of China, 1992). Extraction procedures
All glassware was acid-washed prior to use with 4 mol L-’ HN03. Then five grams of soil was placed in 250-mL high-density polyethylene (HPED) bottles. To meet a recommended solid-to-liquid ratio of 0.05 as determined from previous studies (Neale et al., 1997), 100 mL of extracting agents of 0.05 mol L-l EDTA solution, 0.05 mol L-’ HN03 solution, and 0.05 mol L-l CaC12 solution were added to separate 250 mL bottles containing the soils and shaken. Next, the solution pH was adjusted to 4.8 to maintain the original soil acidity. The mixture was then placed in an end-over-end shaker operating at 60 r min-’ for 24 h at room temperature to ensure that chemical equilibrium was reached. Equilibration time of 24 h was selected as other authors in similar studies have used this time limit (Fangueiro et
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al., 2002). Next, the soil solutions were extracted by decantation and passed through a 0.45-pm filter. Following this the soil residues were rinsed with distilled water to remove the residual extractant, and the samples dried. In the soil solution after EDTA, HN03 and CaC12 washing, as well as in the sequential extraction procedure, concentrations of Pb, Zn and Cu were measured in all the acquired liquid solutions.
Analysis Before the extraction procedure, the distribution of four fractions: acid-extractable, reducible, oxidizable and residual fractions, for Pb, Zn and Cu in soils was determined with the optimized BCR procedure. Then the sequential extraction procedure using the optimized BCR sequential extraction procedure was carried out to ascertain redistributions of Pb, Zn and Cu after extraction. Metal concentrations and extraction efficiencies €or Pb, Zn, and Cu were compared using total concentration; concentration extracted by extractants EDTA, HN03 and CaC12; and extractable percentage. Then the redistributions of Pb, Zn and Cu, after using the extractants, were evaluated. RESULTS AND DISCUSSION Extraction efficiencies with EDTA, HNO3 and CaC12 Table I showed that the soil was heavily contaminated with high concentrations of total Pb, Zn and Cu. Thus, because of low pH and organic matter level in this silty sand, it was difficult to support plant growth and not easy to use phytoremediation. TABLE I Total concentrations of Pb, Zn, and Cu, the concentrations extracted by extractants EDTA, HN03 and CaC12 and the extractable percentages Metal
Pb Zn cu
ct,ta)
mg kg-' 6 872 2 982 547
EDTA
HN03
Cextb)
%extC)
Cext
mg kg-' 2 129 875 237
%
mg kg1033 858 283
31.0 29.3 43.3
CaClz
'
%ext
Cext
%ext
%
mg kg-' 961 585 158
%
15.0 28.8 51.7
14.0 19.6 28.9
")Ct,t means the total concentration of metals; b)Cext means the concentration of metals extracted by extractant; means the extractable percentage of metals.
")%,,t
The sum of the four fractions (Table 11) was reasonably similar to the total concentration obtained after digestion of the original samples with HN03-HF-HC103-Hz02 (Table I). Table I also compares the removal efficiencies of the extracted metals for the three extractants. The results clearly indicated that both EDTA and HN03 were more effective in extracting Pb, Zn and Cu than CaCl2. Percentage extractabilities of metals for HN03 and CaC12 followed the sequence Cu > Zn > Pb. It was likely that the lability of metals in soil played an important role. Lead was more tightly bound in the soil than Zn and Cu, and therefore, was possibly more difficult to extract. The extractability of the extractants for Pb and Zn was in the order of EDTA > HN03 > CaC12, while for Cu it was in the order of HN03 > EDTA > CaC12. Also, the concentrations of Pb and Zn extracted with the EDTA procedure were always higher than those with HN03 or CaCl2. Because EDTA could release the metals that had formed a complex or had been adsorbed by organic substances, it could enhance the solubility and mobility of metals in the soil (Petruzzelli, 1989). The chelating agent EDTA could form not only soluble complexes with metals, but it could also influence the distribution of metals from low water soluble fractions to a high soluble fraction. However, the Cu extractability with the HN03 procedure was clearly better than the other solutions. The metals extracted were virtually
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TABLE I1 Sequential-extracted fractions of Pb, Zn and Cu before and after extraction with EDTA, H N 0 3 and CaC12 Metal
Concentration")
Acid-extractable
Reducible
Oxidizable
Residual
Sum
335 12 658 690
2397 797 1167 1194
mg kg-I 197 94 147 237
3955 3639 3796 3705
6884 4542 5768 5826
488 42 91 210
241 158 172 198
1034 950 980 956
1186 935 952 933
2949 2085 2195 2297
169 44 32 86
78 46 34 75
190 156 115 139
117 86 88 83
554 332 269 383
~
a)Cb,f means the fraction concentration of metals before extraction; CEDTA,C H N Oand ~ Ccaclz are the fraction concentrations of metals after extraction with EDTA, HNO3 and CaCl2, respectively.
all 1:1 metal-EDTA complexes. However, because the formation constants were similar for Pb-EDTA (19.0), Zn-EDTA (17.5) and Cu-EDTA (19.7), they alone could not explain this phenomenon. Nevertheless, the difference in the chemical forms and the degree of crystallization of the metal could have caused the 1:l metal-EDTA complexes (Sun et al., 2001). Also, calcium chloride could effectively remove the metals in the CEC fraction, and due to the strong cation exchange ability and the complexation with chloride, CaC12 could be applied to extract the metals from soils (Lebourg et al., 1998). The extraction of exchangeable fractions could reduce the hazard of the soil, which was particularly true for Pb. Redistribution of Pb after extraction Before extraction, Pb mostly accumulated in the residual fraction (57.4% of the total amount) and in the reducible fraction (34.8%)of the soil (Fig. 1). The acid-extractable fraction and oxidizable fraction of P b represented 4.9% and 2.9% of the total, respectively. However, the redistribution of P b in the soil changed dramatically with EDTA, HN03 and CaC12 (Fig. 1). The most noticeable change was the percentage of the reducible fraction that decreased from 34.8% t o 17.6%, 20.2% and 20.5% in soils treated with EDTA, HN03 and CaCl2, respectively. The solubility of Fe and Mn when utilizing the three extractants mainly caused the decreases in the reducible fractions. 100
a, 0)
m c
80
c
a, 0
t
Eg
n m
v
60 40
4-
Acid
H Reducible El Oxidizable
0 Residual
0
20 4-
X
W
Fig. 1 Percentage fraction of Pb, Zn and Cu before and after extraction. Pbef means the percentage fraction in un~ PCaClz are the fraction percentages in soils treated with EDTA, HNO3 and CaClz, treated soil and PEDTA,P H N Oand respectively.
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Considerable decreases in the oxidizable fraction of Pb after extraction with EDTA (Table 11) were consistent with the solubility of P b bound in organic matter (Petruzzelli, 1989; Li and Shuman, 1996). After extraction with HN03 and CaClz, the concentration of P b in the acid-extractable fraction increased. The residual HN03 and CaClZ trapped in the soil and the fact that these extractants could form stable water-soluble complexes, which were weakly adsorbed on soil particles (Yu and Klarup, 1994) maybe the reason for this result. The proportion of the residual fraction in the EDTA treated soil increased to nearly 80.0%, whereas in H N 0 3 and CaClz treated soils it was 65.0%, which indicated that much Pb was probably occluded in minerals like quartz, feldspars and so on (Fuentes et al., 2004). The high extractable efficiency of P b by EDTA (31.0%, Table I) suggested that EDTA had the potential to remediate P b from contaminated soil. The smaller extraction efficiency for P b with HN03 and CaClz (Table I) was probably related to the low amount of Pb in the acid-extractable or other extractable forms. The P b extraction efficiency of H N 0 3 was higher than that of CaClz (Table I), which could be attributed to HN03 being able to dissolve more P b bound in carbonates.
Redistribution of Z n after extraction Zinc in the untreated soil was mostly concentrated in the residual fraction (40.2%) and the oxidizable fraction (35.1%) (Fig. 1). This could have been due to Zn (or Cu) providing a greater enrichment in the oxidizable fraction than P b (Zhang and Ke, 2004), and also having a high percentage in the acid-extractable fraction (16.6%) (Fig. 1). The percentage of Zn in the reducible fraction and residual fraction before extraction remained similar to the results after the extraction with the three extractants. The extractable efficiencies of these three extractants in the oxidizable fraction were obvious with the percentage of the oxidizable fraction increasing from 35.0% to about 50.0%. The most notable differences, however, took place with the acid-extractable fraction. The percentage of acid-extractable fraction after extraction with EDTA, HN03 and CaC12 decreased from 16.6% to 2.0%, 4.1% and 9.1%, respectively. Another noteworthy change was that the concentration of Zn in the residual fraction decreased fractionally (Table 11), after extraction with the three extractants, which implied that about 20.0% of the residual Zn was extracted as a consequence of the partial disaggregation of the silicate matrix (Barona et d.,2001).The extractable efficiency of Zn with HN03 was higher than that of Pb (Table I). Perhaps more Zn was bound to carbonates and because of the low soil pH, HN03 could solubilize them effectively. The CaClz solution solubilized some Zn from the contaminated soil, possibly due to a large amount of Ca2+ in the CaC12 solution that could displace Zn from adsorption sites on the soil solids. The C1- could also form ZnCl+, which had a low affinity for the reacting surfaces in the soil (Barrow and Ellis, 1986). However, the Ca2+ and C1- could only solubilize the exchangeable or outer-sphere associated Zn from the soil surface.
Redistribution of Cu after extraction In untreated soil, Cu was widely distributed in the four fractions, with the sum of the acid, reducible and oxidizable fractions being 69.5% (Fig. 1). In the acid-extractable fraction, the percentage reached 30.5%, which was considerably higher than that of P b and Zn (Fig. 1). After extraction with EDTA and HN03, the measured percentages of Cu in the acid-fraction decreased from 30.5% to 13.3% and 11.9%, respectively (Fig. I), and the concentration of the other three fractions remained relatively unchanged (Table 11). This indicated that much Cu was associated with sulfides and probably occluded in the minerals. The extractable efficiency of CaC12 was fair and this salt only dissolved Cu by cation exchange and the complexing ability of the anion (e.g. Cl-). Because of the high percentage in the acid-extractable fraction (Fig. l),a high extractable efficiency could be reached. Thus, the extractable efficiencies with EDTA, HN03 and CaC12 were 43.3%, 51.7% and 28.9% respectively (Table I), and for Cu, HN03 had the best effect. The high extractable efficiency with H N 0 3 was possibly due to two reasons: 1)large amounts of metals associated with the clay mineral fraction could be exchangeable with protons (Hf) replacing them, and 2) a large amount of Cu was
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bound to carbonates. In regards to the second point, the pK,, value for Cu-carbonate was 9.7. Therefore, Cu had a strong affinity to carbonates forming carbonate precipitates or co-precipitates, and the strong acid, HN03 could dissolve the carbonate more easily than EDTA or CaC12. The large portion of Cu in the acid-extraction fraction suggested that cation exchange and inorganic ligands such as CaC12 could effectively release Cu from the soil, which reinforced the results of this study. CONCLUSIONS The solution of EDTA extracted more Pb and Zn than HN03 and CaClz due to its strong ability to form stable complexes with a variety of metals and to release the metals forming a complex or being adsorbed by organic substances. The HN03 solution extracted more Cu than EDTA and CaClz, probably due to the dissolved metals bound in the carbonates and Cu re-absorption to soil components. Extractabilities of metals for HN03 and CaCl2 followed the sequence: Cu > Zn > Pb. However, drawbacks associated with the extraction application included extraction of soil macronutrients important for plant growth and increasing soil acidity. In addition, the increased metal mobility could result in groundwater contamination. Further research such as the optimum plant-extractantmetal combination and the most suitable field application method is necessary to fully address these issues. REFERENCES Alloway, B. J. and Ayres, D. C. 1997. Chemical Principles of Environmental Pollution. Chapman & Hall, UK. Anderson, W. C. (ed.). 1993. Soil Washing/Soil Flushing. Amer. Acad, Environmental Engineers, Annaplois, MD. pp. 42-56. Barona, A., Aranguiz. I. and Elias, A. 2001. Metal associations in soils before and after EDTA extractive decontamination: Implications for the effectiveness of further clean-up procedures. Enwiron. Polht. 113: 79-85. Barrow, N. J. and Ellis, A. S. 1986. Testing a mechanistic model. V. The points of zero salt effect for phosphate retention, for zinc retention and for acid/alkali titration of a soil. J . Soil Sci. 37: 303-310. Blaylock, M. J., Salt, D. E., Dushenkov, S., Zakharova, O., Gussman, C., Kapulnik, Y., Ensley, B. D. and Raskin, I. 1997. Enhanced accumulation of P b in Indian mustard by soil-applied chelating agents. Enwiron. Sci. Technol. 31: 860-865. Brown, G. A. and Elliott, H. A. 1992. Influence of electrolytes on EDTA extraction of P b from polluted soil. Water, Air and Soil Pollution. 62: 157-165. Bufflap, S . E. and Allen, H. E., 1995. Sediment pore water collection methods for trace metal analysis: A review. Water Res. 29: 165-177. Elliott, H. A. and Shastri, N. L. 1999. Extractive decontamination of metal-polluted soils using oxalate. Water, Air and Soil Pollution. 110: 335-346. Environment Monitoring of China. 1992. Neoteric Analytical Methods of Elements in Soils (in Chinese). Science and Environment Literature Publisher, Beijing. 184pp. Fangueiro, D., Bermond, A., Santos, E., Carapuca, H., and Duarte, A . 2002. Heavy metal mobility assessment in sediments based on a kinetic approach of the EDTA extraction: Search for optimal experimental conditions. Analytic. Chimzc. Acta. 459: 245-256. Fuentes, A,, Llorens, M., Saez, J., Soler, A., Aguilar, M. I., Ortuno, J. F. and Meseguer, V. F. 2004. Simple and sequential extractions of heavy metals from different sewage sludges. Chemosphere. 54: 1039-1 047. Hall, G . E. M., Gauthier, G., Pelchat, J. C., Pelchat, P. and Vaive, J. E. 1996a. Application of a sequential extraction scheme to ten geological certified reference materials for the determination of 20 elements. J . Anal. Atom. Spectrom. 11: 787-796. Hall, G . E. M., Vaive, J. E., Beer, R. and Hoashi, M. 1996b. Selective leaches revisited, with emphasis on the amorphous Fe oxyhydroxide phase extraction. J . Geochem. Explor. 56: 59-78. Hong, P. K. A,, Li, C., Banerji, S. K. and Regmi, T . 1999. Extraction, recovery, and biostability of EDTA for remediation of heavy metal-contaminated soil. Sod and Sediment Contaminatzon. 8: 81-103. Lebourg, A., Sterckeman, T., Ciesielski, H. and Proix, N. 1998. Trace metal speciation in three unbuffered salt solutions used to assess their bioavailability in soil. J . Enwiron Qual. 27: 584-590. Li, Z. and Shuman, L. M. 1996. Redistribution of forms of zinc, cadmium and nickel in soils treated with EDTA. The Science of the Total Environment. 191: 95-107. McGrath, S. P, 1998. Phytoextraction for soil remediation. I n Brooks, R. R. (ed.) Plants That Hyperaccumulate Heavy Metals. CAB International, Wallingford, UK. pp. 261-287.
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Medved, J., Stresko, V., Kubova, J. and Polakovicova, J. 1998. Efficiency of decomposition procedures for the determination of some elements in soils by atomic spectroscopic methods. Fesenius Journal of Analytical Chemistry. 360: 219-224. Neale, C. N.,Bricka, R. M. and Chao, A. C. 1997. Evaluating acids and chelating agents for removing heavy metals from contaminated soils. Environmental Progress. 16: 274-280. Papassiopi, N., Tambouris, S. and Kontopoulos, A. 1999. Removal of heavy metals from calcareous contaminated soils by EDTA leaching. Water, A i r and Soil Pollution. 109: 1-15. Petruzzelli, G. 1989. Recycling wastes in agriculture: Heavy metal bioavailability. Agric. Ecosyst. Environ. 27: 493-503. Pueyo, M., L6pea-SBnchez, J. F. and Rauret, G. 2004. Assessment of CaC12, NaN03 and NH4N03 extraction procedures for the study of Cd, Cu, Pb and Zn extractability in contaminated soils. Analytica Chimica Acta. 504: 217-226. Pueyo, M., Rauret, G., Luck, D., Yli-Halla, M., Quevauviller, P. and L6pez-SBnchez, J. F. 2001. Certification of the extractable contents of Cd, Cr, Cu, Ni, P b and Zn in a freshwater sediment following a collaboratively tested and optimized three-step sequential extraction procedure. J . Environ. Monit. 3: 243-250. Rauret, G. 1998. Extraction procedures for the determination of heavy metals in contaminated soil and sediment. Talanta. 46: 449-455. Rauret, G., L6pez-SBnchez, J. F., Sahuquillo, A. Rubio R., Davidson C., Ure A. and Quevauviller Ph. 1999. Improvement of the BCR three-step sequential extraction procedure prior to the certification of new sediment and soil reference materials. J . Environ. Monitor. 1: 57-61. Sun, B., Zhao, F. J., Lombi, E. and McGrath, S. P. 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113: 111-120. Sutherland, R. A. and Tack, F. M. G. 2003. Fractionation of Cu, P b and Zn in certified reference soils SRM 2710 and SRM 2711 using the optimized BCR sequential extraction procedure. Advances in Environmental Research. 8: 37-50. Tessier, A,, Campbell, P. G. C. and Bisson, M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Anal. Chem. 51: 844-851. Ure, A. M., Quevauviller, Ph., Muntau, H. and Griepink, B. 1993. Speciation of heavy metals in soils and sediments: An account of the improvement and harmonization of extraction techniques undertaken under the auspices of the BCR of the Commission of the European Community. Int. J . Environ. Anal. Chem. 51: 135-151. Wasay, S. A., Barrington, S. F. and Tokunaga, S. 1998. Remediation of soils polluted by heavy metals using salts of organic acids and chelating agents. Environmental Technology. 19: 369-380. Yu, J. and Klarup, D. 1994. Extraction kinetics of copper, zinc, iron and manganese from contaminated sediment using disodium ethylenediaminetetraacetate. Water, A i r and Soil Pollution. 75: 205-225. Zhang, M.K. and Ke, Z. X. 2004. Copper and zinc enrichment in different size fractions of organic matter from polluted soils. Pedosphere. 14(1): 27-36.