Effect And Mechanism Of Immobilization Of Soil Contaminated By Lead And Cadmium Using Clay Mineral Combined With Phosphorous-containing Materials

In recent years,the problem of heavy metal pollution of soil in our country is becoming more and more serious.And the real soil contaminated with metal is often a system of multi-metal combined pollution.It has more practical significance to choose the suitable immobilizing agents for different heavy metal contaminants.In this paper,lead and cadmium were selected as the target contaminant.The optimal immobilizing agents were screened from different kinds of phosphorous-containing materials and clay minerals.On this basis,the clay minerals were under acid and thermal treatments to improve its adsorption capacities for lead and cadmium.The phosphorous-containing materials and the clay minerals were compounded to evaluate the remdiation efficiency of lead and cadmium and the stability of the soil after immobilization were studied,and the mechanism was also analyzed.The main results are as follows:(1)The optimal immobilizing agents were obtained by measuring the immobilization effect of TCLP(Toxicity characteristic leaching procedure)Pb and Cd.The results showed that the concentrations of TCLP Pb were significantly decreased by phosphorous-containing materials treatments,and the remdiation efficiency of Cd was better than Pb by clay minerals treatments.Calcium dihydrogen phosphate and sepiolite were the optimal agents to immobilize Pb and Cd contaminated soil,respectively.(2)The effect of specific surface area of sepiolite and its adsorption capacities for Pb and Cd under different acid and thermal treatments were studied.The results showed that the specific surface area of sepiolite and the remdiation efficiency of Pb and Cd were significantly increased by hydrochloric acid treatment.The optimal conditions for acid treatments were as follows: the concentration of HCl was 6mol/L;the HCl treatment time was 24 h.Under the above optimal condition,the specific surface area of sepiolite were reached the maximum 302.55m2·g-1 and the remdiation efficiency of Pb and Cd were increased by 5.57% and 5.69%,respectively.(3)The remdiation efficiency of Pb and Cd and environmental stability wereevaluated by chemical extraction experiment and column leaching expriment.The results show that the optimal dosage of calcium dihydrogen phosphate was four times of the integral molar quantity of Pb and Cd in soils.And the optimal dosage of modified sepiolite accounting for 5% of the total soil quality.Under the above optimal dosage,the remdiation efficiency of Pb and Cd were reached up to 64.63% and 56.10%,respectively.The treatment could reduce the biological activity and migration of Pb and Cd.Under simulated acid rain leaching,Pb,Cd and P could stabilize in the original soil layer and reduce the possibility of migrating to deep soil and groundwater.(4)The immobilization mechanism was explored by XRD and Visual MINTEQ model.The results show that the main mechanism of P induced Pb immobilization was through pyromorphite like compouds formation after P addition.And sepiolite reduces the ionic activity of Cd by surface adsorption and ion exchange.