A Study On Chemical Extraction For Heavy Metals From Contaminated Red Soils By Using Low Molecular Organic Acids

Low molecular organic acids are commonly present in nature and become an important factor in affecting transformation and transfer of soil heavy metals in environment, which influences toxicity and bioavailability of heavy metals to plants.Chemical extraction experiments were conducted to study the effects of six organic acids, including citric acid, oxalic acid, tartaric acid, acetic acid, lactic acid and amino acid, on extraction efficiency of heavy metal from polluted soil at various conditions. The contaminated soils were collected from two red paddy soils from Chenzhou and Hengyang in Hunan Province. The goals of this thesis were making sure the best extraction conditions for rehabilitation of polluted soils. Sometimes the best single condition was not consistent with the best experiment for the result of combination. So the orthogonal experiments were carried on choosing the best combinations. The redistributions of soil Pb, Cu, Cd and Zn were determined by using the sequential extraction procedures of the optimized European Community Bureau of Reference before and after extraction. The effects of extraction of soil heavy metals using citric acid, oxalic acid and tartaric acid on soybean seed germination were studied. The results were summarized as follows:(Ⅰ) It was practical and viable to rehabilitate the soils polluted by heavy metals in Chenzhou and Hengyang through chemical extraction using low molecular organic acids.(Ⅱ) Citric acid, tartaric acid and oxalic acid were effective in heavy metal extraction from the soils. For Chenzhou soil, the suitable extraction conditions for single-factor experiment were:concentration was 100 mmol/L, solid-to-liquid ratio 1:5, temperature 35℃, pH 3, and contact time 24 h. Extraction efficiencies of organic acids removing soil heavy metals increased with concentrations of supporting electrolyte.(Ⅲ) From the orthogonal experiments, the most important factor influencing extraction efficiency of soil heavy metals (Pb, Cu, Cd, and Zn) was concentration of organic acid, followed by type of organic acid, and the third factor was acidity of extractant. The best combination for Pb removal was 100 mmol/L of citric acid with pH 3, and that for Cu, Cd, Zn removal was 100 mmol/L of oxalic acid with pH 2.(Ⅳ) Some kind of relativity existed speciation of soil heavy metals and extraction efficiency of organic acids. The higher the contents of stable heavy metals (residual, sulfuration and organic bound, and iron-manganese oxide bound), the lower the extraction efficiency. For Hengyang soil, three organic acids (tartaric acid, oxalic acid, citric acid) were all effective to remove heavy metals, but the sequence of extraction efficiency was tartaric acid> oxalic acid> citric acid.(Ⅴ) The experiment of soybean seed germination indicated that the germination ratios were related to soil types and organic acids. Distilled water and high concentrations of organic acids resulted in lower germination ratios. The higher the concentrations of organic acids, the lower the germination ratios. For the soils from Chenzhou and Hengyang, the highest germination ratios were 80% and 96%, respectively. Organic acids, such as citric acid, tartaric acid and oxalic acid, were present in soil and could alleviate toxicity of heavy metals to plants.