Formation of a U(VI)-Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides.
Formation of a U(VI)-Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides.
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DOI：
10.1021/acs.est.9b03180
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发表时间：
2019-12
影响因子：
11.4
通讯作者：
L. Townsend;S. Shaw;Naomi E. R. Ofili;N. Kaltsoyannis;A. Walton;J. Mosselmans;Thomas S. Neill;J. Lloyd;S. Heath;Rosemary Hibberd;K. Morris
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文献类型：
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作者：
L. Townsend;S. Shaw;Naomi E. R. Ofili;N. Kaltsoyannis;A. Walton;J. Mosselmans;Thomas S. Neill;J. Lloyd;S. Heath;Rosemary Hibberd;K. Morris
在放射性废物处理和受污染土地情况下，铀都是一种风险驱动的放射性核素。在这些环境中，一系列生物地球化学过程可以发生，包括硫酸盐还原，这可以诱导铁（氧）氧化物矿物相的硫化。在硫化过程中，已知不稳定的U（VI）还原为相对不流动的U(IV)；然而，在这些生物地球化学反应过程中，铀形态变化的详细机制却知之甚少。在这里，我们在pH 7和pH 9.5下对吸附在水合铁上的U（VI）进行了高度控制的硫化实验，并利用地球化学分析、x射线吸收光谱（XAS）和计算模型对该系统进行了研究。XAS数据分析表明，在硫化反应过程中形成了一种新的瞬态U(VI)-过硫化物配合物作为中间物质，同时铀向溶液中短暂释放。扩展x射线吸收精细结构（EXAFS）模型表明，过硫配体在铀酰部分的赤道平面上配位，计算模型支持该物种的形成。铀的最终形态是纳米粒铀（IV）铀矿，在pH 7下2天和pH 9.5下1年这一阶段都很明显。我们在环境相关条件下鉴定出一种新的、不稳定的U(VI)-过硫化物物种，可能对放射性废物处理和污染土地情景中硫化物环境中U的迁移性有影响。
Uranium is a risk-driving radionuclide in both radioactive waste disposal and contaminated land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. Here, we performed highly controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling. Analysis of the XAS data indicated the formation of a novel, transient U(VI)-persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.