氧化还原电位对土壤中重金属环境行为的影响研究进展

E_h（redox potential，氧化还原电位）代表土壤氧化性和还原性的相对程度，是以电位反映土壤所处氧化还原状态的指标.由于重金属易与对E_h敏感的组分发生吸附、络合、沉淀等化学反应，因此E_h的变化会直接/间接影响重金属形态从而决定其毒性和活性.E_h变化对重金属活性往往存在双向影响:①淹水环境使E_h降低，引发反硝化反应、铁锰氧化物和硫酸盐还原，使pH向中性靠拢，从而间接导致酸性土壤中游离态重金属沉淀或碱性土壤中吸附态重金属释放.②厌氧条件在使E_h发生变化的同时，会引起铁锰氧化物、有机质、硫化物的形态或含量发生改变，如铁锰氧化物还原、大分子有机质分解，分解产物与重金属离子结合增加了可溶性重金属的浓度，但厌氧条件也可以使重金属离子和硫化物产生沉淀，且还原Cr、Hg等元素，降低毒性；在氧化条件下，硫化物氧化形成硫酸盐促进了重金属释放；但铁锰离子沉淀及其氧化物形态变化形成的具有强吸附力的无定形铁锰氧化物可重新吸附游离态金属离子.这些复杂的化学变化对定量化研究E_h对重金属的影响造成困难，从而降低了E_h波动环境（水稻土、周期性覆水潮滩沉积物等）中重金属污染风险评价的准确性，也限制了通过改变E_h来原位修复重金属污染土壤的方法实施.因此，需进一步对E_h诱导下土壤颗粒界面上发生的与重金属行为相关的反应进行深入研究，并开发检测分析技术，如无损伤的物理探测方法或同位素稀释技术等，建立不同土壤中E_h与重金属形态、活性的定量关系，以揭示E_h对重金属形态的影响机制. 

Influence of Redox Potential on Heavy Metal Behavior in Soils: a Review

Redox potential (E_h) is an index used to assess the oxidation and reduction conditions in the environment. Since heavy metals (HMs) easily react with E_h-sensitive substances by adsorption, complexation and precipitation etc., E_h fluctuation has a significant impact on the toxicity and lability of HMs. Researches have showed that the changes of E_h have a bidirectional effect on heavy metal lability:firstly, E_h declines in flooded environments, triggering denitrification and reduction of iron and manganese oxides and sulfides, and pH tends to be neutral, which indirectly leads to the precipitation of exchangeable HMs in acidic soils or the release of adsorbed HMs in alkaline soils; secondly, alteration in the form and content of iron and manganese oxides, organic matter and sulfides can result from the change of E_h. In anoxic environments, HM solubility maybe enhanced by the reduction of iron and manganese oxides and the decomposition of organic matter into dissolved organic matter which can complex HM ions. In contrast, the HM ions may also precipitate out as metal sulfides and be reduced to less toxic species such as Cr, Hg etc.; In oxic conditions, HMs in metal sulfides can be oxidized into sulfates, but they can also be adsorbed onto amorphous Fe/Mn oxides, formed by Fe, Mn precipitation or alteration in oxides, as free metal ions. These complex chemical reactions make it more difficult to quantify the impact of E_h on HM behavior, thus reducing the accuracy of the risk assessment of HMs in an E_h fluctuation environment (paddy soil, cyclical tidal flat sediments, etc.). This also limits the implementation of in situ remediation procedures by changing E_h for HM-contaminated soil. Therefore, it is necessary to carry out further research on HM-related reactions induced by E_h change on soil surfaces and develop analytical techniques, such as non-destructive physical detection methods or isotope dilution techniques, to establish quantitative relationships between E_h and HMs speciation and lability in different soils, so that the mechanism of the effect of E_h on heavy metal speciation can be revealed.