氧化条件下砷–水铁矿体系结晶转化及砷的再分配机制

Two-line ferrihydrite (Fh) is a poorly crystalline, disordered iron oxyhydroxide ubiquitous occurring in soils, surface and groundwater sediments. It may transform to more crystalline and stable forms of iron oxyhydroxide and inevitable exert some effect on the behavior of the associated arsenic. However, during the crystallization process of Fh which was formed under various media and systems, the arsenic concentration in the liquid phase as well as the redistribution of it in the different transformed and untransformed solid phase in the heterogeneous mixture is still unknown. In this study, the arsenic adsorbed ferrihydrite and the arsenic coprecipitated ferrihydrite produced under different conditions will take as samples to further investigate the intermediate, product, the transformation mechanisms and the fate of arsenic during the Fh crystallization process by sequential chemical extraction combined with solid phase characterization such as XRD, XAS, FTIR, Raman, SEM and TEM. In addition, this project will also investigate on whether or in what extent arsenic can be incorporated into the more crystalline product by using DFT calculations. The objectives of this research are to shed more light on the role and mechanisms of the crystallization process of Fh in the geochemical cycling of arsenic under oxic conditions. The results of this project are of importance to the completeness of arsenic geochemical cycling and the establishment of the method and technology which can be used to control and remediate the arsenic pollution.

2–线水铁矿是土壤及水体沉积物环境中一种常见的结晶程度较差的铁氧化物，会逐渐向结晶程度更高的铁氧化物转化，不可避免的会对所结合的砷产生影响。然而目前对不同体系、不同介质中形成的2–线水铁矿，在其结晶转化过程中砷在固液相之间及固相内部的再分配过程还缺乏完整清晰认识。本项目将主要针对在不同条件下形成的砷–水铁矿吸附/共沉淀样品，利用化学连续提取与XRD、XAS、FTIR、Raman、SEM、TEM等表征手段相结合的方式，深入系统的研究2–线水铁矿结晶转化过程的中间体、产物、转化机理及砷在各相之间的分布情况。同时，借助密度泛函（DFT）计算，研究砷在水铁矿结晶转化过程中能否以及能在多大程度上介入到结晶程度更高的铁氧化物中，揭示氧化环境中2–线水铁矿结晶转化过程在砷的地球化学循环中所起的作用及其机理。项目研究对完善和充实砷的地球化学循环、控制和修复砷污染的方法与技术的建立具有重要意义。

2-线水铁矿广泛存在于土壤以及水体沉积物环境中，是一种常见的结晶度较差的铁的羟基氢氧化物，其与砷在环境中的行为密切相关。在不同pH值以及温度等条件下，水铁矿会逐渐向结晶程度更高的铁氧化物转化，引发与之结合的砷在转化过程中固液两相以及固相内部的再分配问题。因此，本项目系统的研究了在不同条件下形成的砷−水铁矿吸附/共沉淀样品结晶转化过程中砷的再分配过程及其控制机理。研究发现，当固相砷含量相对较低时，砷对水铁矿结晶转化过程影响较小，强碱性条件下（pH 12），无论何种反应介质，液相砷含量在固相结晶转化过程中几乎不变，固相转化为针铁矿；偏碱性条件下（pH 8和10），液相砷含量随着转化的进行而升高，硝酸盐样品的固相转化为赤铁矿，而硫酸盐和氯化物样品的固相则以赤铁矿为主并伴有少量的针铁矿；偏酸性条件下（pH 4），液相砷含量呈现先降低后升高的趋势，硫酸盐和氯化物样品的固相转化产物分别为针铁矿和赤铁矿，而硝酸盐样品的转化产物则以赤铁矿为主并伴有少量的针铁矿；强酸性条件下（pH 2），硫酸盐和硝酸盐样品结晶转化过程中，液相砷含量呈现逐渐降低的趋势，固相转化产物分别为针铁矿和赤铁矿，而氯化物样品结晶转化过程中，液相砷含量呈现先降低再升高的趋势，固相转化产物为赤铁矿。铁砷共沉淀的结晶转化过程同样受到反应介质的影响，当铁砷摩尔比为2时，固相转化产物均为臭葱石，这与反应介质无关，但是砷在固液两相的再分配过程正好相反；当铁砷摩尔比为4时，硫酸盐样品中出现了类臭葱石，而硝酸盐样品中则出现了赤铁矿；而当铁砷摩尔比升高到16时，固相转化产物为钠黄钾铁矾和针铁矿，转化过程中液相砷含量逐渐降低。此外，本项目还对氧化条件下其他典型含砷废渣的长期稳定性以及AMD体系中砷的迁移转化的初步情况进行了研究。项目的成功实施对更为精确的预测环境中砷的迁移转化过程以及防止二次砷污染的发生提供重要依据。