冻融-化学淋洗法协同修复重金属Cd和Pb污染黏性土

针对质地黏重、低渗透性黏性土的淋洗效率低下,该文提出冻融协同化学淋洗的修复方案,并以某冶炼厂受Cd、Pb污染场地黏性土为研究对象,选用乙二胺四乙酸二钠(ethylene diaminetetraacetic acid disodium salt,EDTA)为淋洗剂,进行了冻融-淋洗土柱的实证试验。结果表明,土体的反复冻融(冻胀-吸水、融沉-排水)破坏土体颗粒原有结构,有助于淋洗液与污染物充分接触,淋洗效果明显,经7次冻融后,Cd、Pb去除率分别达到77.24%、37.78%。采用改进的BCR(European Communities Bureau of Reference)连续提取法分析了土柱中Cd、Pb的赋存特征,经7次冻融后,土壤中弱酸提取态、可还原态、残渣态结合的Cd质量分数较淋洗前分别降低了41.46%、63.02%、26.33%,而土壤中可还原态和残渣态结合的Pb质量分数分别降低了32.32%、67.36%。冻融协同化学淋洗修复技术的淋洗剂用量远小于传统淋洗法,为今后利用寒区冻融交替现象,大规模对季冻区重金属污染土壤的异位修复提供了新的思路。

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.