| Abstract:The improper mining and smelting can not only result in soil contamination of heavy metal (Cd, Pb, Zn), but also bring metalloid (As) into soils, which poses a great threat to human beings. Because of the different chemical properties of Cd, Pb and As, a sigle immobilizing agent of heavy metals is difficult to accomplish the immobilization of both Cd^Pb and As. Therefore, the utilization of multi-agents has potential application in the remediation of multi-contaminated soils.In this study, immobilizing agents for stabilization of Cd, Pb As in multi-contaminated soils were screened and further used to search for the optimal combination of immobilizing agents; the technique parameters were optimized to obtain the highest remdiation efficiency; the stabilization of Cd, Pb As in the remdied soils was evaluated. The main results are as follow:(1) The optimal immobilizing agent was obtained by measuring the immobilization effect of available Cd, Pb and As from the clay minerals, phosphate material, industrial by-product and ferric compunds. Ca(H2PO4)2was the optimal immobilizing agent toremedy Cd and Pb contaminated soils. The concentrations of available Cd and Pb in the Ca(H2PO4)2-remedied soils declined by39%and62%, respectively. Moreover, Fe2(SO4)3was chosen as As-immobilizing agent and75%of available As was removed by Fe2(SO4)3addition. Therefore, Ca(H2PO4)2and Fe2(SO4)3were the optimal agents to immobilize Cd, Pb and As in the contaminated soils, respectively.(2) The optimal conditions for Cd and Pb immobilization were as follows:the dosage of Ca(H2PO4)2was ten times of the integral molar quantity of Cd and Pb in soils. Soil water content was60%of maximum field water capacity. The soil particle size was2.44mm. The reaction time was42days. Under the above optimal condition, the removal rates of available Cd and Pb reached up to to51%and99%, respectively.(3) The combination ultlization of Ca(H2PO4)2and Fe2(SO4)3can remedy multi-contaminated soils by Cd Pb and As. The optimal molar ratio of Fe3+/PO43-was2.16:1. Furthermore, separated addition of Ca(H2PO4)2and Fe2(SO4)3had better remediation efficiency than that of coinstantaneous addition. Separated addition of Ca(H2PO4)2and Fe2(SO4)3decreased available Cd, Pb and As by47%、64%and51%, respectively.(4) BCR sequential extraction was used to assess the mobility of the heavy metal. Ca(H2PO4)2addition significantly reduce acid extractable Cd and Pb. Meanwhile Pb mainly existed in reducible and oxidizable forms. Combination ultlization of cand Fe2(SO4)3resulted in the transformation of organic Cd into reducible and residual Cd, as compared to a single Ca(H2PO4)2. However, Ca(H2PO4)2obviously dissolved the residual As into acid extractable and reducible As. Separated addition of Ca(H2PO4)2and Fe2(SO4)3transformed As from acid extractable and reducible fraction into residual fraction.(5) Under simulated acid rain leaching, the leaching amount of Cd and Pb in Ca(H2PO4)2-remedied soils was significant less than that in untreated soil, while the leaching amount of As was higher than that in untreated soil. Compared to a single Ca(H2PO4)2, combination ultlization of cand Fe2(SO4)3reduced the leaching amout of As from10.8mg/L to4.4mg/L.(6)The XRD analysis showed that Cd and Pb in the Ca(H2PO4)2-remedied soils formed insoluble substance. The Ksp values of products of Cd, Pb and As the remedied soil by using Ca(H2PO4)2and Fe2(SO4)3were lower, resulting in more stability of Cd, Pb and As in soils As can adsorbed on manganese oxide and alkaline earth metal or form ternary complexes (Pb3Mn(AsO3)2(AsO2(OH))), which increase the stability of As in the soil. |
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