水稻土硫酸盐还原耦合铁还原钝化镉的化学和微生物机理

Cadmium (Cd) is the most widespread heavy-metal soil contaminant in China, and iron and sulfate reductions are important soil processes in paddy field ecosystems. The interactions between iron reduction, sulfate reduction and Cd immobilization can set a model for studying the coupling of surface biogeochemical processes. With the hypothesis that Cd immobilization is influenced by iron and sulfur transformation driven by microbial iron and sulfate reductions, this project is aimed to investigate: (i) the effects on Cd immobilization of key iron reduction processes, i.e. reductive dissolution of Fe(III) oxides, Fe(II)-catalyzed recrystallization of iron oxides, and formation of secondary Fe(II) minerals, via incubation of an iron-reducing bacterium; (ii) the chemical mechanisms whereby Cd immobilization is coupled with iron and sulfate reductions, by incubation of an iron- and sulfate-reducing bacterium and determination of the status of Cd, Fe, S, and O during incubation; and (iii) the microbial and chemical mechanisms whereby Cd immobilization is coupled with iron and sulfate reductions, by incubation of microbial enrichments from paddy soils and analyses of the relationship between change in Cd status during iron and sulfate reductions and in structure of microbial community. This project could contribute to a better understanding of the influence of biogeochemical processes on environmental behavior of Cd, which will help in the remediation of Cd-contaminated soils.

镉是我国最严重的土壤重金属污染物；铁还原和硫酸盐还原是水稻土中重要的元素循环过程。铁还原、硫酸盐还原、镉形态转化之间的内在联系可作为地表多过程耦合的模式。其科学假设是：微生物驱动的铁和硫酸盐还原过程控制水稻土铁硫的矿物转化，从而影响镉的形态转化。主要研究内容包括：(1) 铁还原菌培养体系中，阐明氧化铁还原溶解、亚铁催化的氧化铁晶相转变、次生氧化铁矿物的生成耦合镉形态转化的化学机制； (2) 兼具铁还原功能的硫酸盐还原菌培养体系中，解析镉与铁、硫、氧元素之间的结合状态，阐明铁还原—硫酸盐还原—镉形态转化耦合过程的化学机制；(3) 水稻土富集微生物培养体系中，探讨镉的形态转化与铁和硫酸盐还原过程、微生物群落组成变化之间的关系，阐明铁还原—硫酸盐还原—镉形态转化耦合过程的微生物机制与化学机制。旨在加深对元素循环过程与镉行为之间关系的认识，为寻找实用有效的稻田镉污染控制技术提供理论依据。

镉是我国检出率最高的土壤重金属污染物；铁还原和硫酸盐还原是水稻土中重要的元素循环过程。为了阐明铁还原、硫酸盐还原、镉形态转化之间的内在联系，本项目主要研究了纯菌培养体系中铁还原及土壤培养体系中铁、硫还原对镉行为的影响，并阐释了其微生物和化学机制。纯菌培养体系中，镉能削弱纤铁矿还原并阻止纤铁矿向磁铁的转变；而微生物铁还原可以固定具有实际意义的低浓度的镉，其机制是铁氧化物和微生物细胞的吸附作用。进一步的土壤培养实验表明，向土壤中添加有机质能促进土壤本身所含的铁氧化物的还原，从而有效地固定具有实际意义的低浓度的镉；单独添加铁氧化物或硫酸盐效果有限或无效，因为不能消除电子供体的限制。水稻土微生物群落调查证实了，铁还原菌及硫酸盐还原菌的相对多度并不随铁或硫含量增加而增加。以上工作可为深入认识元素循环过程与镉行为之间关系做出贡献，并可为寻找实用有效的稻田镉污染控制技术提供参考信息：对于一般镉污染水稻土，淹水加秸秆还田可能是一项有潜力的修复措施。