稻田土壤中亚铁氧化驱动的自由基氧化砷的化学机制

The toxicity and stability of Arsenic (As) in paddy soil is largely determined by its valence state and speciation, thus the transformation of As(III), which is more toxic and transferable, to less toxic As(V) species is the most critical step to reduce the bioavailability of As in the contaminated soil. It has been reported that under the fluctuating redox conditions in paddy soil, the redox cycling of iron is closely coupled to the transformation and migration of As, but the predominant mechanism governing such coupled processes is still not known, not to mention the cascade generation of reactive oxygen species from continuous Fe(II) oxidation at the anoxic-oxic boundaries in paddy soil and their potential impact on As oxidation. This project will focus on the transformation of arsenic species mediated by Fe(II) oxidation and associated generation of reactive oxygen species in paddy soil. By recasting the spatial and temporal characteristics of paddy soil systems in the laboratory, we aim to investigate the impact of continuous oxidation of Fe(II) and associated generation of various reactive oxygen species on the transformation and mobilization of As. Further, detailed kinetic model consisting of proposed mechanistically-based reaction sets with intrinsic rate constant for each elementary reaction will be developed to describe the results obtained and the predominant mechanisms operative in this system. The kinetic model established in this study should improve our conceptual and quantitative understanding of the contribution of Fe(II) oxidation to As migration and transformation in paddy soil and thus contributing to a holistic assessment of the mechanism of iron redox cycling induced As behavior in the soil.

稻田土壤中砷的形态决定了其迁移性和毒性，而如何将高毒性、易迁移的三价砷转化为低毒性的五价砷一直是解决土壤砷污染问题的关键过程。虽已有研究表明在稻田土壤氧化还原交换条件下，铁的氧化还原过程和砷的迁移转化有着至关重要的耦合关系，但是目前这一耦合过程的机理尚不明确，尤其是稻田淹水厌氧环境下产生的大量亚铁离子被氧化时生成的大量氧化性极强的活性氧自由基对砷转化的影响还关注甚少，因此本项目主要关注稻田土壤中铁氧化过程驱动的自由基生成过程，以及氧化砷的效应与机制。通过实验室构建土壤-亚铁-三价砷模式体系，研究表层土壤和水-土界面中亚铁氧化、自由基产生过程和砷的转化迁移过程的反应途径和反应机理；建立反应动力学模型，解析控制该过程的关键因素，并定量评估水稻土铁氧化过程中不同影响因素对有砷固化过程的促进或抑制作用，深入揭示稻田土壤中有氧环境下铁循环耦合砷行为的反应机制。

稻田土壤中砷的形态决定了其迁移性和毒性，如何将高毒性、易迁移的三价砷转化为低毒性的五价砷一直是解决土壤砷污染问题的关键过程。稻田土壤中周期性的氧化还原条件，以及丰富的铁含量导致土壤剖面的不同层次中持续发生亚铁的氧化反应并产生大量的活氧自由基。这些氧化性极强的物种密切影响着土壤中铁及其他金属的形态转化及生物有效性。初步研究表明在稻田土壤氧化还原交换条件下，铁的氧化还原过程和砷的迁移转化有着至关重要的耦合关系。因此稻田淹水厌氧环境下产生的大量亚铁离子被氧化时生成的大量氧化性极强的活性氧自由基对砷转也产生重要的影响。本项目主要关注了稻田土壤中铁氧化过程驱动的自由基生成过程，以及氧化砷的效应与机制。主要通过实验室构建土壤-亚铁-三价砷模式体系，研究表层土壤和水-土界面中亚铁氧化、自由基产生过程和砷的转化迁移过程的反应途径和反应机理；依据实验数据建立反应动力学模型，解析控制该过程的关键因素，并定量评估水稻土铁氧化过程中不同影响因素对有砷固化过程的促进或抑制作用，深入揭示稻田土壤中有氧环境下铁循环耦合砷行为的反应机制。