Synergistic remediation of heavy metal Cd and Pb contaminated clay by freeze-thaw and chemical washing
(2.State Key Laboratory of Frozen Soil Engineering, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China 730000)
(3.No. 2 Institute of Geological & Mineral Resources Survey of Henan, Zhengzhou, China 450001)
(4.Henan Yuhan Environmental Governance Co., Ltd., Xinxiang, China 450001)
【Abstract】Contamination of heavy metals in agricultural soil has been a worldwide challenge for the food security and people’s health. Especially, cadmium (Cd) and lead (Pb) contamination in soil is a serious problem in China. Therefore, it is imperative to develop remediation techniques able to remove contaminants in a highly efficient and cost effective way. The traditional washing method is widely used for soil remediation on account of its high efficiency and simple operation. However, the efficiency of traditional washing method is limited by soil permeability, so this method can only remediate a small range of heavy metal-contaminated soil and has poor removal effect for the clayey soil, which cannot be popularized at a large scale. In order to solve the problem of low washing efficiency resulted from the heavy texture and low permeability of clayey soil, we proposed a cooperative remediation by freeze-thaw and chemical washing method. Taking the Cd and Pb contaminated soil in a smelter as the research object, we conducted the empirical tests of freeze-thaw and washing (FTW) for soil columns with 0.1 mol/L ethylenediamine tetraacetic acid disodium salt (EDTA). The results showed that repeated freezing and thawing (frost heave–water absorption, thaw settlement–drainage) of soil destroyed the original cohesive force and soil skeleton structure between the soil particles, so that the soil particles were rearranged, which contributed to the fully contact with the eluent and contaminants. The washing effect was significantly improved by this method. The removal rates of Cd in FTW3 (FTW treatment with three freeze-thaw cycles; adding EDTA at 1st and 3rd cycle), FTW5 (FTW treatment with five freeze-thaw cycles; adding EDTA at 1st and 2nd cycle), and FTW7 (FTW treatment with seven freeze-thaw cycles; 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 forms 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 extractable Cd increased by 20.76%, the average reducible Cd decreased by 41.58%, and the residual Cd increased by 193.45% in FTW3. The weak acid extractable Cd decreased by 0.39%, the average reducible Cd decreased by 45.75%, and the residual Cd increased by 43.73% in FTW5. The weak acid extractable Cd, average reducible Cd, and residual Cd decreased by 41.46%, 63.02%, and 26.33%, respectively, in FTW7. The average reducible Pb increased by 11.23%, the average oxidizable Pb decreased by 63.12%, and the residual Pb increased by 53.97% in FTW3. The average reducible Pb decreased by 0.12%, the average oxidizable Pb decreased by 64.13%, and the residual Pb increased by 30.68% in FTW5. The average reducible Pb, average oxidizable Pb, and residual Pb decreased by 32.32%, 62.05%, and 67.36%, respectively, in FTW7. Moreover, the freeze-thaw and washing method had a lower ratio of liquid to soil, which 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, this study 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.
【Keywords】 heavy metals; pollution; soils; freeze-thaw cycle; chemical washing; contaminated clayey soil; EDTA;
(Translated by LIU T)
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