| As one of an important part of the natural environment, soil is the basic material for human survival and society. While, with the rapid development of the mining and smelting industries in recent years, waste liquids, gases and solids containing heavy metals produced in lead and zinc smelting process which would resulting in soil ecosystem destruction and soil quality degradation. Soil environment quality is not only related to the ecosystem security but also crucial to the safe of agricultural production. Therefore, the remediation of heavy metal contaminated soil is very imminent. Among the many heavy metal contaminated soil remediation technologies, soil washing or leaching had been proved to be an efficient method which could be used in the contaminated site remediation due to most of the pollutants could be removed from soil during the washing process. However, the different chemical chelates leaching efficiency on soil washing and the utilizable value of soil after leaching were still need to be explored.In this study, batch leaching experiments were conducted to examine the effects of different chemical chelates on Cd, Cu, Zn and Pb extraction from different contaminated soils and the utilizable value of soils after leaching. The redistributions of heavy metals in soils were determined using the BCR sequential extraction procedure before and after treatment. The utilizable value of leached soil was also assessed by the ryegrass pot experiment. The main results obatained in this study could be described as below.(1) For the four kinds of chelates, EDTA, EDDS, EGTA and NTA, the removal rates of Cd, Cu, Zn, Pb were different. Cd was the easiest to be leached with the leaching efficiency in the sequence of EDTA > NTA > EDDS > EGTA. At the optimum EDTA leaching conditions, 0.1mol/L EDTA solution with p H 4 could remove 76.4%-91% Cd, 38.1%-42.5% Cu, 19.2%-57% Zn, and 45.4%-52.0% Pb, respectively.(2) The organic acids on the soil Cd、Cu、Zn and Pb extraction study showed that heavy metal removal rate increased with organic acid concentration increase. The heavy metals removal rates could reach the highest points with 0.1 mol/L citric acid and tartaric acid were used respectively; the removal rates of Cd、Cu、Zn and Pb by citric acid were slightly higher than that of tartaric acid. But from the perspective of the removal rate of 4 kinds of heavy metal, 0.1 mol/L citric acid and tartaric acid were the best eluants, and citric acid is more suitable for repairing seriously polluted area, tartaric acid more suitable for repairing light polluted area.(3) Compared with the above four kinds of organic acid, HCl has better heavy metals removal rate. When HCl concentration is 0.1 mol/L, the maximum removal rate of heavy metals were Cd(88.3%) > Zn(60%) > Pb(44.1%) > Cu(23.7%). Acid was often used in leaching heavy metals pollution in soil, but the soil structure and nutrient composition would be destroyed during the washing. Hence, the HCl should not be recommended in the soil washing.(4) The redistributions of heavy metals were investigated by using the soil BCR sequential extraction procedure before and after treatment. The results showed that EDTA、NTA、tartaric acid and citric acid leaching could effectively decrease the content of heavy metals in acid soluble and reducible, also reduce the biological effectiveness of heavy metals and environmental risks.(5) After washing, heavy metals uptakes in ryegrass were significantly reduced when compared with non leached soil. Moreover, soil respiration rate of EDTA and NTA leached soils after planting perennial ryegrass were not changed remarkable, soil enzyme activity after EDTA leaching was higher than that of NTA. After tartaric acid leaching, the soil respiration and soil enzyme activity increased.(6) Considering the leaching agents cost, efficiency, soil potential utilization value and the other related factors, it can be conclude that EDTA and citric acid were suitable for heavy metals extraction from contaminated soil and the environmental risks were faint. |
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