A Study On Heavy Metal Removals From Contaminated Soils By Using Inorganic Extractants

In this paper, paddy soils contaminated with heavy metals from two typical mining areas, Chenzhou Shizhuyuan and Hengyang Shuikoushan in Hunan Province, were collected as the research objects. According to the soil properties and types of heavy metal pollution, extraction experiments were carried out by using representative inorganic extractant, acid (HCl) and salts (Na2S2O3, CaCl2), to determine maximum extraction efficiencies of these extractants for soil heavy metals through single-factor experiments and the orthogonal experiments. Changes in speciation of soil Pb, Zn, Cd and Cu before and after extraction were determined by following the five-step sequential extraction procedures of Tessier. Extraction efficiencies of the extractants for soil heavy metals (Pb, Zn, Cd and Cu) and the related changes in heavy metal speciation were discussed. Finally, effects of soil heavy metal removal through chemical extraction on growth of soybean seedlings were studies. The major experimental results are given as follows:(Ⅰ) Three extractants can effectively remove heavy metals from contaminated soils. The results of single-factor experiments are quite reliable, and are consistent with the results of orthogonal experiments in respect of the extraction conditions and the main extraction factors. However, deviations exist in the secondary factor of the experiments. Based on the results of the two experiments, the most important factor influencing extraction efficiencies of soil heavy metals is the concentration of extractants, followed by the ratio of soil to solution and extraction times. The time and temperature have little effects on heavy metal removal.(Ⅱ) HC1 has the best extraction efficiency among the three extractants for Pb, Zn, Cd, and Cu, and the best extraction conditions for HCl are:concentration-100 mmol·L-1, time-3 h for Chenzhou soil and 4 h for Hengyang soil, temperature-30℃solid-liquid ratio-1:15, more than 3 times consecutive extraction. Under the best extraction conditions, the extraction efficiencies for heavy metals are Pb-51.23%, Zn-20.16%, Cd-82.55%, Cu-44.17% in Chenzhou soil, and Pb-61.02%, Zn-17.52%, Cd-77.34%, Cu-74.58% in Hengyang soil.When Na2S2O3 is used as an extractant, the best extraction conditions are: concentration-100 mmol·L-1, extraction time-3 h, solid-liquid ratio-1:15, temperature-40℃,2-4 times consecutive extraction. Under the best extraction conditions, the extraction efficiencies for heavy metals are Pb-21.11%, Zn-5.11%, Cd-73.15%, Cu-29.12% in Chenzhou soil, and Pb-55.12%, Zn-12.90%, Cd-58.78%, Cu-66.04% in Hengyang soils.CaCl2 has the best extraction efficiencies for Cd from two soils. The best extraction conditions are:concentration-100 mmol-L"1, extraction time-4 h, temperature-40℃for Chenzhou soil and 35℃for Hengyang soil, soil-liquid ratio-1:10 for Chenzhou soil and 1:15 for Hengyang soil, extraction 3 times for Chenzhou soil and 4 times for Hengyang soil. Under the best extraction conditions, the extraction efficiencies for heavy metals are Pb-7.85%, Zn-15.18%, Cd-78.74%, Cu-0.01% for Chenzhou soil, and Pb-39.66%, Zn-18.31%, Cd-65.28%, Cu-20.39% for Hengyang soil.(Ⅲ) The experiments for analysis of speciation of soil heavy metals before and after the extraction show that chemical extraction results in changes in speciation redistribution of soil heavy metals. The contents of exchangeable heavy metals decrease significantly after extraction, and the contents of other forms of heavy metals decrease also. Although the contents of residual metals decrease in different extents, the percentages of residual metals increase in different levels. It shows that chemical extraction can resolve some part of soil residual heavy metals, but the amount is limited.(Ⅳ) The toxicological experiment of soybean seedlings indicates that although the soils are treated with the best extraction conditions, chemical extraction changes soil pH values and causes nutrient elements losing simultaneously when heavy metals removal. The seed germination ratio in the two soils after extraction with CaCl2 is 100%, and those in the two soils treated with HCl and Na2S2O3 are lower. This implies that the soil treated with inorganic extractants needs further improvement and thus will be benefit to plant growth.