Karlsruhe Institute of Technology (KIT), Institute of Applied Geosciences, Adenauerring 20b, 76131 Karlsruhe, Germany.
Dalton Trans. 2018 Aug 14;47(32):11002-11015. doi: 10.1039/c8dt01799a.
Environmental and health hazards associated with the trace element selenium are mainly related to the presence of the highly mobile selenium oxyanions selenite and selenate (oxidation states IV and VI). In this study, we investigated the immobilization of dissolved selenite and selenate during the formation of magnetite in coprecipitation experiments based on the progressive oxidation of an alkaline, anoxic Fe2+ system (pH 9.2). Up to initial selenium concentrations of 10-3 mol L-1 (mass/volume ratio = 3.4 g L-1), distribution coefficient values (log Kd) of 3.7 to 5.1 L kg-1 demonstrate high retention of selenium oxyanions during the mineral formation process. This immobilization is due to the reduction of selenite or selenate, resulting in the precipitation of sparingly soluble selenium compounds. By X-ray diffraction analysis, these selenium compounds were identified as trigonal elemental selenium that formed in all coprecipitation products following magnetite formation. Time-resolved analysis of selenium speciation during magnetite formation and detailed spectroscopic analyses of the solid phases showed that selenium reduction occurred under anoxic conditions during the early phase of the coprecipitation process via interaction with iron(ii) hydroxide and green rust. Both minerals are the initial Fe(ii)-bearing precipitation products and represent the precursor phases of the later formed magnetite. Spectroscopic and electron microscopic analysis showed that this early selenium interaction leads to the formation of a nanoparticulate iron selenide phase [FeSe], which is oxidized and transformed into gray trigonal elemental selenium during the progressive oxidation of the aquatic system. Selenium is retained regardless of whether the oxidation of the unstable iron oxides leads to the formation of pure magnetite or other iron oxide phases, e.g. goethite. This reductive precipitation of selenium induced by interaction with metastable Fe(ii)-containing iron oxide minerals has the potential to influence the mobility of selenium oxyanions in contaminated environments, including the behavior of 79Se in the near-field of nuclear waste repositories.
与微量元素硒有关的环境和健康危害主要与高迁移性硒氧阴离子硒酸盐和亚硒酸盐（氧化态 IV 和 VI）的存在有关。在这项研究中，我们通过基于碱性缺氧 Fe2+ 系统（pH 9.2）的渐进氧化的共沉淀实验，研究了在磁铁矿形成过程中溶解的亚硒酸盐和硒酸盐的固定。在初始硒浓度高达 10-3 mol L-1（质量/体积比=3.4 g L-1）的情况下，分配系数（log Kd）值为 3.7 至 5.1 L kg-1，表明在矿物形成过程中硒氧阴离子的保留率很高。这种固定是由于亚硒酸盐或硒酸盐的还原，导致难溶硒化合物的沉淀。通过 X 射线衍射分析，这些硒化合物被鉴定为在磁铁矿形成后所有共沉淀产物中形成的三方元素硒。在磁铁矿形成过程中对硒形态的时间分辨分析和对固相的详细光谱分析表明，硒还原是在缺氧条件下通过与铁（ii）氢氧化物和绿锈相互作用在共沉淀过程的早期发生的。这两种矿物都是最初含 Fe（ii）的沉淀产物，是后来形成的磁铁矿的前体相。光谱和电子显微镜分析表明，这种早期的硒相互作用导致纳米颗粒铁硒化物相 [FeSe] 的形成，在水系统的渐进氧化过程中，该相被氧化并转化为灰色三方元素硒。无论不稳定的氧化铁的氧化导致纯磁铁矿还是其他氧化铁相（如针铁矿）的形成，硒都被保留下来。与亚稳含 Fe（ii）的氧化铁矿物相互作用引起的硒的这种还原性沉淀有可能影响污染环境中硒氧阴离子的迁移性，包括 79Se 在核废料处置库近场中的行为。
