Extraction of Arsenic and Heavy Metals from Contaminated Mine Tailings by Soil Washing

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1 Soil and Sediment Contamination, 2: , 211 Copyright Taylor & Francis Group, LLC ISSN: print / online DOI: 1.18/ Extraction of Arsenic and Heavy Metals from Contaminated Mine Tailings by Soil Washing MYOUNG-JIN KIM AND TAESUK KIM Department of Environmental Engineering, College of Engineering, Korea Maritime University, Dongsam-dong, Youngdo-gu, Busan, Korea Introduction In this study, the soil washing technique has been used to treat mine tailings contaminated heavily with arsenic and heavy metals at Jingok mine, which is one of the abandoned mines in Korea. The results showed that phosphoric acid, citric acid, oxalic acid, and sodium metabisulfite were highly effective in extracting arsenic and heavy metals. Among them, oxalic acid was the most effective (especially for Pb, Cu, and Zn), as even a residual fraction of arsenic was partly extracted. The optimum concentration of washing reagent and the ratio between the mine tailings and washing reagent were found to be.5 M and 1:2, respectively. In addition, the extraction kinetics of arsenic and heavy metals was fast, in which the reaction time of 3 minutes was deemed to be a sufficient contact time. From the results, it may be concluded that the low ph of washing solution and the amount of dissolved Fe may be considered as the most important factors in the extraction of arsenic and heavy metals. Keywords Soil washing, arsenic, heavy metals, mine tailings, extraction In general, solidification/stabilization methods are mainly used for the treatment of arsenic and heavy metals in contaminated soils (Mulligan et al., 21). The extractable fraction of pollutants, however, can be leached from the treated soils over a long period of time. Soil washing is one of the ex-situ remediation technologies to extract metalloids such as arsenic and heavy metals from soils and reduce the volume of contaminated soils. The technique has been investigated by many researchers using several types of extractants, such as water, inorganic acids, organic acids, bases, chelating agents, inorganic salts, and other additives (Chang et al., 25; Kantar and Honeyman, 26; Moutsatsou et al., 26; Tao et al., 26; Isoyama and Wada, 27; Min et al., 28; Vaxevanidou et al., 28). Recently, there have been studies to release arsenic using microorganisms, chelating agents, acid, oxalate, and so on (Tao et al., 26; Min et al., 28; Vaxevanidou et al., 28). A combined treatment of iron-reducing microorganism Desulfuromonas palmitatis and ethylenediaminetetraacetic acid (EDTA) was performed for the simultaneous and efficient removal of arsenic and heavy metals (Vaxevanidou et al., 28). Min et al. (28) also Address correspondence to Myoung-Jin Kim, Department of Environmental Engineering, College of Engineering, Korea Maritime University, Dongsam-dong, Youngdo-gu, Busan , Korea. kimmj@hhu.ac.kr 631

2 632 M.-J. Kim and T. Kim reported that both H 3 PO 4 and KH 2 PO 4 were effective to extract arsenic. In addition, arsenic in soil was removed using oxalate or phosphate efficiently (Tao et al., 26). In addition, washing with an acid or a chelating reagent was also found the most effective in removal of heavy metals (Chang et al., 25; Kantar and Honeyman, 26; Moutsatsou et al., 26; Isoyama and Wada, 27). Soil washing with a strong acid, however, may cause not only an increase in the contaminant mobility, but also an adverse change in the soil properties due to the mineral dissolution, leading to a decrease in the soil productivity. Thus, it is necessary to investigate an environmentally friendly and costeffective soil remediation technology. The goal of this study was to develop an environmentally friendly and cost-effective soil washing technique for treating high-concentration, multi-contaminated mine tailings, and reusing the remediated mine tailings. Therefore, we attempted to use an alternate mild reagents to extract contaminants efficiently. In this study, we have investigated a washing process for extracting As, Cd, Cu, Ni, Pb, and Zn from contaminated mine tailings on a laboratory scale. The samples of heavily contaminated mine tailings were treated with potassium phosphate, phosphoric acid, sodium citrate, citric acid, ammonium oxalate, oxalic acid, sodium metabisulfite, and water through a batch process in the range of ph The performance of each washing reagent in extracting the contaminants was evaluated at different concentrations, mixing ratios between the mine tailings and washing solution, and reaction times. Materials and Methods Characteristics of Contaminated Mine Tailings Samples were obtained from mine tailings in an abandoned Jingok mine located in Bongwha, Korea. Various kinds of ores such as Au, Ag, Pb, and Zn had been produced in the mine until about 7 years ago. The mine tailings were collected, using an auger, from the ground surface down to 22 cm, placed in plastic bags, and then stored in a cooler in the field. The samples were dried using a freeze-drier for 3 4 days, sieved through a 2 mm sieve, and homogenized by thorough mixing. Subsequently, the samples were stored in a freezer until further analysis. The size distribution was done with a grain-size analyzer for clay and silt, and a standard sieve for sand and gravel. The conductivity and ph of the mine tailings were determined in suspensions (mine tailings:water = 1:1 by mass) that had been shaken for 1 hour, centrifuged (1, rpm), and then filtered with.45 µm membrane filters. The cation exchange capacity (CEC) was measured using the unbuffered salt extraction method (Grove et al., 1982; Sumner and Miller, 1996). The concentrations of arsenic and heavy metals were determined using an atomic absorption spectrophotometer (AAS, PerkinElmer Analyst 2). The mineralogy of the mine tailings was investigated using X-ray diffraction (XRD, Mac Science Co. M3XHF22). The total concentrations of arsenic and heavy metals were determined by digesting the dry samples using EPA method 61 (Keith, 1998; Kim et al., 22). To measure a point of zero salt effect (point of zero charge, ph PZC ), potentiometric titration analysis was performed on the samples of mine tailings at selected ionic strengths (Zelazny et al., 1996). The sequential extraction for arsenic was conducted before soil washing treatment using the method of Herreweghe (23), which was modified from that of Manful s (1992).

3 Soil Washing to Treat Contaminated Mine Tailings 633 Six different fractions, including easily soluble, NH 4 F-extractable, NaOH-extractable, reducible, acid soluble, and residual arsenic, were extracted sequentially, starting with 1M NH 4 Cl and ending with a hot mixture of HCl/HNO 3 /HF. Replicate results for all experiments were obtained by repeating the same process on two different days. Soil Washing In order to develop the optimum conditions for soil washing, the experiments were performed based on the following three aspects: 1) washing reagents and their concentrations; 2) mixing ratios between mine tailings and washing reagent; and 3) reaction time. Washing reagents and their concentrations. Contaminated mine tailings were treated using eight washing reagents: potassium phosphate, phosphoric acid, sodium citrate, citric acid, ammonium oxalate, oxalic acid, sodium metabisulfite, and water. Phosphoric acid, citric acid, and oxalic acid were used not only as an extractant of either contaminants or soil minerals, but also as a ligand to build a complex. Salts, such as potassium phosphate, sodium citrate, and ammonium oxalate, were chosen to promote the dissolution of arsenic and heavy metals through the formation of complexes between metal and anion group (Logue et al., 24). In addition, sodium metabisulfite was used as a reductive reagent. In order to determine the optimum washing reagents and their concentrations for extracting arsenic and heavy metals from contaminated mine tailings, soil washing experiments were conducted at various concentrations. The concentrations of each washing reagent, except ammonium oxalate, were set to.25 M,.5 M, and 1. M. Due to the low solubility of ammonium oxalate in water, concentrations were adjusted to.9 M,.18 M, and.35 M. Also, the ph of each washing reagent was measured before the treatment. Two grams of mine tailings and 1 ml of each of the washing reagents were added to a 25 ml Erlemmyer flask, which was then sealed. The suspended mixture was shaken in an incubated shaker for 4 hours at 18 rpm at a constant temperature (25 C ±.3). After centrifuging the mixture at 4 rpm and filtering it, the ph was measured. HNO 3 was added to the filtered solution to obtain a.2% overall concentration of HNO 3. Finally, the concentrations of arsenic and heavy metals in the solution were measured using AAS. This process was repeated for each of the seven washing reagents. Mixing ratios between mine tailings and washing reagent. Four washing reagents showing high extraction efficiency of arsenic and heavy metals in the above experiments were used in this experiment. They included phosphoric acid, citric acid, oxalic acid, and sodium metabisulfite. The concentration of each washing reagent was fixed at.5m, which was determined as the optimum concentration in the previous experiments. The mixing ratio of mine tailings to washing reagent was varied, that is, 1:1, 1:15, 1:2, 1:25, and 1:5 (g/ml) using 2g of mine tailings. Experimental conditions, except for the mixing ratios, and procedures were the same as those in the previous experiments. Reaction time. These experiments were conducted for various reaction time periods:.5, 1., 1.5, 2., 2.5, 3., 3.5, and 4. hours. The ratio of mine tailings to washing reagent was 1:5. The types and concentrations of washing reagents were the same as those in the previous experiments.

4 634 M.-J. Kim and T. Kim Results and Discussion The coefficients of variation (CV = 1% standard deviation/average) in most results were below 1%. Characteristics of Contaminated Mine Tailings Important physical and chemical properties of the mine tailings are shown in Table 1. The particle size of most mine tailings was smaller than 2 mm while only 5.2% of mine tailings were larger than the reference size. The amount of sand (53 µm < f < 2 mm) was the most (79.1%). The mine tailings were mineralogically complex. According to the XRD result, the major minerals in the mine tailings were quartz, chlorite, kaolinite, jarosite, muscovite, and pyrite. The ph in the solution of the mine tailings was The measurement of ph PZC of the mine tailings showed the value range: The value range was very close to that of kaolinite ( ) (Zelazny et al., 1996). In the viewpoint of electrostatic interaction, since the surface of mine tailings is positively charged at ph < ph PZC, the anionic species, Table 1 Physical and chemical properties of mine tailings Property Unit Value Soil size Clay (<2 µm) % 1.1 Silt (<53 µm) % 14.6 Sand (<2 mm) % 79.1 Gravel (>2 mm) % 5.2 Major minerals a Qz, Ch, Ka, Ja, Mu, Py ph b 4.91 Conductivity b µs/cm 1499 LOI c % 1.59 CEC cmol/kg 2.63 phpzc Metal content d As mg/kg soil 6835 ± 122 Cd mg/kg soil 17.5 ±.1 Cu mg/kg soil 145 ± 1 Ni mg/kg soil 22 ± 5 Pb mg/kg soil 285 ± 75 Zn mg/kg soil 325 ± 5 Fe mg/kg soil ± 225 Mn mg/kg soil ± 138 a: Qz: quartz, Ch: chlorite, Ka: kaolinite, Ja: jarosite, Mu: muscovite Py: pyrite b: measured in supernatant of soil suspension (soil:solution = 1:1 by mass) c: Loss On Ignition d: total concentration by acid digestion

5 Soil Washing to Treat Contaminated Mine Tailings Fraction (%) Step Figure 1. Sequential fractions of arsenic in mine tailings: (1) easily soluble, (2) NH 4 F-extractable, (3) NaOH-extractable, (4) reducible, (5) acid soluble, (6) residual. arsenate, would have strong interaction with the surface of mine tailings and thus high adsorption. As shown in Table 1, the concentrations of arsenic (6835 ± 122 mg/kg), lead (285 ± 75 mg/kg), and zinc (325 ± 5 mg/kg) at the study site were extremely high. In addition, the contents of Fe and Mn were relatively high in the mine tailings (3.2 wt.% and 1.5 wt.%, respectively). In these high Fe content systems, arsenic could form inner sphere complexes with iron-containing minerals (Fendorf et al., 1997). The chemical bonding and speciation of arsenic before soil washing were examined using the method of Herreweghe (23), which consists of 6 steps. As shown in Figure 1, the residual fraction ratio was the highest (52%), whereas those of the easily soluble fraction and NH 4 F-extractable fraction were.21% and 1.11%, respectively. This result means that there is a necessity of using chemical reagents in extracting arsenic from mine tailings. In the next section, how much the treatment developed in this study is effective in extracting the acid-soluble and even residual forms of arsenic is discussed. Soil Washing Experiment Washing reagents and their concentrations. The phs of eight washing solutions are shown in Table 2. They range from.71 to 8.93; however, the ph variation at the three concentrations for each washing solution was not noticeable. The phs in solutions of three acids (phosphoric acid, citric acid, and oxalic acid) were lower than 2 at all the concentrations

6 636 M.-J. Kim and T. Kim Table 2 phs of initial washing solutions and after the soil washing for 4 hours ph Concentration Washing reagents (M) initial after 4h Potassium phosphate Phosphoric acid Sodium citrate Citric acid Ammonium oxalate Oxalic acid Sodium metabisulfite , Water used. Among them, the ph in solution of oxalic acid was the lowest ( ). The phs in solutions of the salts such as potassium phosphate, sodium citrate, and ammonium oxalate were in the range of Finally, the phs of water and the solution of sodium metabisulfite were and , respectively. Figure 2 shows the efficiencies of soil washing at three different concentrations using the washing reagents. The efficiencies are expressed in terms of the percentages of extracted arsenic and heavy metals as compared with the original values. The results of soil washing experiments indicate that phosphoric acid, citric acid, oxalic acid, and sodium metabisulfite are highly effective in removing arsenic and heavy metals. A detailed explanation of their efficiencies is as follows: Phosphoric acid was effective to extract As ( 65% of total concentration), Cd ( 1%), and Zn ( 6%). Citric acid was effective for Cd ( 8%) and Pb ( 51%). Oxalic acid worked well for As ( 72%), Cd ( 8%), Cu ( 89%), Pb ( 65%), and Zn ( 76%). Sodium metabisulfite, on the other hand, was effective for the removal of Cd ( 1%) and Ni ( 68%). Fe was also extracted effectively using phosphoric acid ( 41%) and oxalic acid ( 54%). It was, however, very low (<1%) using other reagents. In addition, 96.7% and 99.8% of the Mn was extracted using oxalic acid and sodium

7 Soil Washing to Treat Contaminated Mine Tailings M.5M 1M As extracted (%) 6 4 Cd extracted (%) (a) (b) (c) (d) (e) (f) (g).25m.5m 1M 2 (a) (b) (c) (d) (e) (f) (g) Figure 2. Efficiencies of soil washing at three different concentrations for each washing reagent: (a) potassium phosphate, (b) phosphoric acid, (c) sodium citrate, (d) citric acid, (e) ammonium oxalate, (f) oxalic acid, (g) sodium metabisulfite. The concentrations of all the washing reagents except ammonium oxalate (.9M,.18M, and.35m) were adjusted to.25m,.5m, and 1M. (Continued)

8 638 M.-J. Kim and T. Kim M.5M 1M Cu extracted (%) 6 4 Ni extracted (%) (a) (b) (c) (d) (e) (f) (g).25m.5m 1M 2 (a) (b) (c) (d) (e) (f) (g) Figure 2. (Continued)

9 Soil Washing to Treat Contaminated Mine Tailings M.5M 1M Pb extracted (%) 6 4 Zn extracted (%) (a) (b) (c) (d) (e) (f) (g).25m.5m 1M 2 (a) (b) (c) (d) (e) (f) (g) Figure 2. (Continued)

10 64 M.-J. Kim and T. Kim metabisulfite, respectively. Among the washing reagents examined in this study, oxalic acid was found to be the most effective in extracting arsenic. For example, 72% of arsenic was extracted using oxalic acid even though over 5% of total arsenic was in the form of residual fraction. The extraction efficiency increased with increasing concentrations of washing reagents, especially under slightly acidic conditions. For example, approximately 333 mg/kg of arsenic was extracted in 4 hours with.25m oxalic acid, but 4643 mg/kg was extracted with.5m, showing that.5m oxalic acid could extract arsenic from mine tailings about 1.5 times more than.25m oxalic acid. Based on the results, the optimum washing reagent was thus found to be.5m oxalic acid. Several mechanisms contribute to the extraction of arsenic and heavy metals from mine tailings using an acid solution and its salt: (1) desorption of arsenic and metal cations via ion exchange; (2) dissolution of compounds containing arsenic and heavy metals; and (3) dissolution of soil mineral components (e.g. Fe-Mn oxides) that may contain the contaminants (Kuo et al., 26; Dermont et al., 28). The enhanced extraction of arsenic and heavy metals in the presence of citrate or oxalate can be explained through the following processes. They include the extraction of secondary coatings (e.g., Fe and Mn), which results in the liberation of arsenic and metals into solution, and the formation of stable and soluble complexes between heavy metals and anions (citrate and oxalate). In addition, acids and their salts could promote the dissolution of Fe- and Mn-oxides through the formation of surface complexes via ligand exchange between the carboxylic and phosphate Metal extracted (%) As Cu Zn Fe extracted (%) Figure 3. Relation of extracted iron with extracted arsenic and heavy metals.

11 Soil Washing to Treat Contaminated Mine Tailings 641 groups, and surface hydroxyl groups (Logue et al., 24; Kantar and Honeyman, 26). Abumaizar and Smith (1999) also reported that the extraction of arsenic and heavy metals from contaminated soil is a process highly controlled by the dissolution of the metalmineral bond, followed by the dispersion of the pollutant metal in the washing solution as an emulsion, complex, or suspension. In this study, it was not possible to obtain peaks for these complexes with the FT-IR because the concentrations were very low. High concentrations of not only arsenic and heavy metals, but also Fe and Mn, were found in wastewater treated with acids. The reason can be explained as follows. While acids (phosphoric acid, citric acid, and oxalic acid) are able to dissolve metallic components of mine tailings, such as Fe and Mn, which usually adsorb arsenic and heavy metals, the contaminants could also be concomitantly dissolved into solution (Alam et al., 21; Tokunaga and Hakuta, 22). As shown in Figure 3, more arsenic and heavy metals (Cu and Zn) were released with increasing iron concentration in wastewater. As shown in Figure 4, the dissolution of arsenic and heavy metals, except Cd and Ni, increased with decreasing ph and reached a minimal value in the high ph range (8 9) (Alam et al., 21). For example, the extraction percentage was up to 89.6% at ph lower than 2, whereas it was 31.2% or lower in the ph range of 8 9. The equilibrium ph did not change significantly from the initial ph due to much stronger buffer function of anions such as phosphate, citrate and oxalate than that of the mine tailings (Alam et al., 21). A control experiment with water at the ph 5.6 resulted in only less than 7.4% extraction, Metal extracted (%) As Cu Pb Zn ph Figure 4. Percentage of extracted arsenic and heavy metals according to ph.

12 642 M.-J. Kim and T. Kim Figure 5. Percentage of extracted arsenic and heavy metals according to ratio of washing reagent to soil. (Continued)

13 Soil Washing to Treat Contaminated Mine Tailings 643 Figure 5. (Continued)

14 644 M.-J. Kim and T. Kim Figure 5. (Continued)

15 Soil Washing to Treat Contaminated Mine Tailings 645 (a) 1 8 Metal extracted (%) 6 4 As Cd Cu Ni Pb Zn Time (min) (b) 1 8 Metal extracted (%) 6 4 As Cd Cu Ni Pb Zn Time (min) Figure 6. Percentage of extracted arsenic and heavy metals according to the reaction time: (a) phosphoric acid, (b) citric acid, (c) oxalic acid, (d) sodium metabisulfite. (Continued)

16 646 M.-J. Kim and T. Kim (c) 1 8 Metal extracted (%) 6 4 As Cd Cu Ni Pb Zn (d) Metal extracted (%) Time (min) As Cd Cu Ni Pb Zn Time (min) Figure 6. (Continued)

17 Soil Washing to Treat Contaminated Mine Tailings 647 indicating that the much higher extraction of arsenic and heavy metals is mainly due to the function of salt anions such as phosphate, citrate, and oxalate. Reducing reagents such as Na 2 S 2 O 5 can maximize the solubility of the metal by reducing it to a lower oxidation state or by weakening the bond between the metal and mine tailings (Abumaizar and Smith, 1999). Mixing ratios between mine tailings and washing reagent. Four washing reagents (phosphoric acid, citric acid, oxalic acid, and sodium metabisulfite), which showed relatively high efficiencies in extracting arsenic and heavy metals, were used in this experiment. Figure 5 shows the percentage of extraction for arsenic and heavy metals (Cd, Pb, Cu, and Zn) according to the ratio between the mine tailings and washing reagents. In general, the percentage of extracted arsenic and heavy metals increased by increasing the ratio up to 1:2, and then reached a steady state. However, Cd and Ni did not follow the trend, showing that their extraction increased continuously up to 1:5 ratio. It is necessary to make the ratio as low as possible in order to minimize the volume of wastewater produced and capacity of equipment needed after soil washing. The ratio of 2 g:4 ml (1:2) was found to be optimal for environmentally friendly treatment. Reaction time. Figure 6 shows the percentage of extracted arsenic and heavy metals (Cd, Pb, Cu, and Zn) according to the reaction time in the range of 3 minutes to 4 hours. The percentage increased very slowly with the reaction time. In most of these experiments, arsenic and heavy metals were extracted efficiently in the initial stage of the treatment. Of the total amounts of arsenic and heavy metals extracted after washing for 4 hours, 68 81% was extracted within the first 3 minutes. Based on the results shown in Figure 6, a washing period of 3 minutes was considered appropriate because no considerable amounts of arsenic and heavy metals were extracted after this time. Conclusion In this research, mine tailings severely contaminated with arsenic and heavy metals have been efficiently remediated using a soil washing technique. Based on the batch experiments in this study, it is concluded that an optimized soil washing technique with phosphoric acid, citric acid, oxalic acid, or sodium metabisulfite can be an efficient, environmentally friendly, and time-saving method to treat contaminated mine tailings containing high amounts of arsenic and heavy metals. Further, this method does not cause serious environmental problem related to the neutralization of wastewater produced after soil washing because only weak acid and organic acid were used as washing reagents. The remediated mine tailings could be subsequently disposed of in a landfill, or treated using the technique of solidification/stabilization for remaining heavy metals. References Abumaizar, R. J., and Smith, E. H Heavy metal contaminants removal by soil washing. J. Hazard. Mater. B7, Alam, M. G. M., Tokunaga, S., and Maekawa, T. 21. Extraction of arsenic in a synthetic arseniccontaminated soil using phosphate. Chemosphere 43, Chang, S. H., Wang, K. S., and Kuo, C. Y. 25. Remediation of metal-contaminated soil by an integrated soil washing-electrolysis process. Soil & Sediment Contamination 14,

18 648 M.-J. Kim and T. Kim Dermont, G., Bergeron, M., Mercier, M., and Richer-Lafleche, M. 28. Soil washing for metal removal: A review of physical/chemical technologies and field applications. J. Hazard. Mater. 152, Fendorf, S., Eick, M. J., Grossl, P., and Sparks, D. L Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ. Sci. Technol. 31, Grove, J. H., Fowler, C. S., and Sumner, M. E Determination of the charge character of selected acid soils. Soil Sci. Soc. Am. J. 46, Herreweghe, S. V., Swennen, R., Vandecasteele, C., and Cappuyns, V. 23. Solid phase speciation of arsenic by sequential extraction in standard reference materials and industrially contaminated soil samples. Environ. Pollut. 122, Isoyama, M. and Wada, S. I. 27. Remediation of Pb-contaminated soils by washing with hydrochloric acid and subsequent immobilization with calcite and allophonic soil. J. Hazard. Mater. 143, Kantar, C., and Honeyman, B. D. 26. Citric acid enhanced remediation of soils contaminated with uranium by soil flushing and soil washing. J. Environ. Eng.-ASCE 132, Keith, L. H Compilation of EPAs Sampling and Analysis Methods, 2 nd.ed., Lewis Publishers, Boca Raton, FL, pp Kim, M. J., Ahn, K. H., and Jung, Y. 22. Distribution of inorganic arsenic species in mine tailings of abandoned mines from Korea. Chemosphere 49, Kuo, S., Lai, M. S., and Lin, C. W. 26. Influence of solution acidity and CaCl 2 concentration on the removal of heavy metals from metal-contaminated rice soils. Environ. Pollut. 144, Logue, B. A., Smith, R. W., and Westall, J. C. 24. Role of surface alteration in determining the mobility of U(VI) in the presence of citrate: Implication for extraction of U(VI) from soils. Environ. Sci. Technol. 38, Manful, G Occurrence and Ecochemical Behavior of Arsenic in a Goldsmelter Impacted Area in Ghana, PhD dissertation, University of Ghent, Ghent, Belgium, Centrum voor milieusaneringen aan de RUG. Min, Z., Bohan, L., Ming, L., Yong, Z., Qingru, Z., and Bin, O. 28. Arsenic removal from contaminated soil using phosphoric acid and phosphate. J. Environ. Sci. 2, Moutsatsou, A., Gregou, M., Matsas, D., and Protonotarios, V. 26. Washing as a remediation technology applicable in soils heavily polluted by mining-metallurgical activities. Chemosphere 63, Mulligan, C. N., Yong, R. N., and Gibbs, B. F. 21. Remediation technologies for metal-contaminated soils and groundwater: An evaluation. Eng. Geo. 6, Sumner, M. E., and Miller, W. P Cation exchange capacity and exchange coefficients, In: Methods of Soil Analysis. Part 3. Chemical Methods, Soil Science Society of America, Inc., Madison, Wisconsin, pp Tao, Y., Zhang, S., Jian, W., Yuan, C., and Shan, X. 26. Effects of oxalate and phosphate on the release of arsenic from contaminated soils and arsenic accumulation in wheat. Chemosphere 65, Tokunaga, S., and Hakuta, T. 22. Acid washing and stabilization of an artificial arseniccontaminated soil. Chemosphere 46, Vaxevanidou, K., Papassiopi, N., and Paspaliaris, I. 28. Removal of heavy metals and arsenic from contaminated soils using bioremediation and chelant extraction techniques. Chemosphere 7, Zelazny, L. W., He, L., and Vanwormhoudt, A Charge analysis of soils and anion exchange, In: Methods of Soil Analysis. Part 3. Chemical Methods, Soil Science Society of America, Inc., Madison, Wisconsin, pp