淋洗修复后残留土壤中重金属的再释放及环境风险

淋洗修复后残留土壤中的重金属会因外界环境的改变可能会发生再释放过程而使环境风险增加，淹水是农田土壤常见的管理模式之一，然而淹水介导的土壤环境条件的改变对淋洗修复后残留农田土壤中重金属的再释放和环境风险的变化影响尚不清楚.因此，本研究开展了持续淹水培养（180 d）对经柠檬酸（CA）、EDTA、FeCl₃和HCl淋洗修复后残留土壤中Cd、Pb和Zn的有效性、形态分配和环境风险的影响研究.结果表明随着培养时间的延长，经CA、EDTA和HCl淋洗后土壤中有效态Cd含量呈先增加后减少的趋势，有效态Pb含量增加，有效态Zn含量减少.经FeCl₃淋洗后土壤中有效态Cd、Pb和Zn含量随培养时间延长而增加.形态分析结果表明，淹水会使土壤E_h降低而致使铁锰氧化物的溶解，因此导致可还原态Pb含量向弱酸提取态Pb转移是有效态Pb含量增加的原因；土壤弱酸提取态Cd含量变化与土壤有效态Cd含量变化基本一致；淹水对淋洗修复后Zn形态的影响较小.基于改进的潜在生态风险指数法评价结果表明淹水使CA、EDTA和HCl淋洗后土壤Cd、Zn环境风险和重金属环境总风险减少，使Pb的环境风险增加.淹水后FeCl₃淋洗后土壤Cd、Pb和Zn环境风险及重金属环境总风险增加.由于FeCl₃淋洗后的土壤重金属污染状况较低且环境总风险（低风险）明显小于其它3种淋洗剂淋洗的土壤，因此，FeCl₃可成为Cd-Pb-Zn复合污染土壤的淋洗剂选择，但是需关注其淋洗修复后残留土壤中重金属的再释放并采取调控措施.本研究结果可为农田重金属污染土壤合适淋洗剂的选择和淋洗法修复重金属污染农田土壤的效果评估提供更为合理的指导思路.

Release and environmental risk of residual heavy metals in leached-remediated soils

The release of residual heavy metals in soils having been remediated by leaching might occur due to the changes of the external environment, and could increase environmental risk. Flooding is one of the common management modes for farmland soil, but its effect on the release of residual heavy metals is unclear. Therefore, a 180 days of flooding incubation and the effects on the availability, redistribution and environmental risk of residual Cd, Pb and Zn in citric acid (CA), EDTA, FeCl₃ and HCl leached-remediated soils were conducted in present study. The results showed that soil available Cd increased at first and then decreased, available Pb increased and available Zn decreased with the prolonging of incubation time in CA, EDTA and HCl leached-remediated soils. However, soil available Cd, Pb and Zn in the FeCl₃ leached-remediated soil increased. The results of BCR sequential extraction demonstrated that flooding could decrease soil Eh and hence the dissolution of iron and manganese oxides, which caused the transformation of Pb from easily reducible fraction to weak acid extractable fraction, thus soil available Pb increased in leached-remediated soils. The change of soil Cd weak acid extractable fraction was similar to that of soil available Cd, and the effect of flooding on Zn redistribution was relatively low in in leached-remediated soils. The evaluation results based on modified potential ecological risk index presented that environmental risk of Cd, Zn and the total environmental risk decreased for CA, EDTA and HCl leached-remediated soils under flooding incubation, however environmental risk of Pb increased. Flooding incubation increased the environmental risk of Cd, Pb and Zn and the total environmental risk for FeCl₃ leached-remediated soil. Finally, since that soil heavy metal contamination was relatively low and the total environmental risk (low risk) was lower than that of the other three leaching reagents in FeCl₃ leached-remediated soil, FeCl₃ could be the alternative as a leaching reagent for the Cd-Pb-|Zn contaminated soil. But the release of heavy metals in the residual soil in FeCl₃ leached-remediated soil should be paid more attentions, and measures should be taken to regulate and control the environmental risk. The results of this study can provide a more reasonable guideline for the selection of suitable leaching agents and evaluation of remediation effect by leaching for heavy metal contaminated farmland soils.