Synergistic remediation of heavy metal Cd and Pb contaminated clay by freeze-thaw and chemical washing

Abstract: Contamination of heavy metals in agricultural soil has been a worldwide challenge for the food security and the health. Especially, cadmium and lead contamination in soil is a serious problem in China. Therefore, it is imperative to develop remediation techniques, which are able to removal contaminants in a high efficient and cost effective way. Now, the traditional washing method is used for soil remediation on account of its' high efficiency and simple operation. However, the efficiency of traditional washing method is limited by permeability of soils, so this method can only repair a small range of heavy metal contaminated soil, which cannot be produced on a large scale, and the removal effect of the clay soil is generally poor. In order to solve the problem of low washing efficiency resulted from heavy texture and low permeability in clay soil, a cooperative remediation by freeze-thaw and chemical washing method was proposed. In this paper, taking Cd and Pb contaminated soil in a smelter as the research object, the empirical tests of freeze-thaw and washing (FTW) soil columns were conducted with 0.1 mol/L EDTA (ethylene diaminetetraacetic acid disodium salt). The results show that repeated freezing and thawing (frost heave-water absorption, thaw settlement-drainage) of soil destroys the original cohesive force and soil skeleton structure between the soil particles, so that the soil particles were rearranged, which was contributed to the fully contact with the eluent and contaminants. After the freeze-thaw and soil washing tests, the washing effect was significantly improved. The removal rates of Cd in FTW3(FTW treatment with 3 freeze-thaw cycle; adding EDTA at 1st and 3rd cycle), FTW5(FTW treatment with 5 freeze-thaw cycle; adding EDTA at 1st and 2nd cycle) and FTW7(FTW treatment with 7 freeze-thaw cycle; adding EDTA at 1st to 3rd cycle) groups were 9.05%, 64.90% and 77.24% respectively, and the removal rates of Pb in FTW3, FTW5 and FTW7 groups were 2.06%, 14.42% and 37.78%, respectively. The morphology of heavy metals at different depths in the soil column after washing were analyzed by the three-stage continuous extraction method (BCR method) proposed by the European Community Bureau of Reference. The weak acid extracted Cd increased by 20.76%, the average Cd of reducible state decreased by 41.58%, and the residual Cd increased by 193.45% in FTW3. The weak acid extracted Cd decreased by 0.39%, the average Cd of reducible state decreased by 45.75%, and the residual Cd increased by 43.73% in FTW5. The results showed that the weak acid extracted Cd decreased by 41.46%, the average Cd of the reducible state decreased by 63.02%, and the residual Cd decreased by 26.33% in FTW7. And the average Pb of reducible state increased by 11.23%, the average Pb of oxidizable state decreased by 63.12%, and the residual Pb increased by 53.97% in FTW3. The average Pb of reducible state decreased by 0.12%, the average Pb of oxidizable state decreased by 64.13%, and the residual Pb increased by 30.68% in FTW5. The average Pb of reducible state decreased by 32.32%, the average Pb of oxidizable state decreased by 62.05%, and the residual Pb decreased by 67.36% in FTW7. Moreover, the freeze-thaw and washing method has a lower ratio of liquid to soil, the ratio of liquid to soil were 0.32, 0.47 and 0.62, respectively, so that the amount of eluent was much smaller than that of the traditional washing method. Besides, through this study, it provides a method for the ex-situ remediation of heavy metal-contaminated soil in seasonally frozen areas by using the phenomenon of freeze-thaw alternation in cold regions in the future.