Thomas Andrew N, Eiche Elisabeth, Göttlicher Jörg, Steininger Ralph, Benning Liane G, Freeman Helen M, Tobler Dominique J, Mangayayam Marco, Dideriksen Knud, Neumann Thomas
Institute of Applied Geosciences, Karlsruhe Institute of Technology, 76137, Karlsruhe, Germany.
Institute of Synchrotron Radiation, Karlsruhe Institute of Technology, 76344, Eggenstein-Leopoldshafen, Germany.
Geochem Trans. 2020 Feb 14;21(1):2. doi: 10.1186/s12932-020-00066-8.
Chromium contamination is a serious environmental issue in areas affected by leather tanning and metal plating, and green rust sulfate has been tested extensively as a potential material for in situ chemical reduction of hexavalent chromium in groundwater. Reported products and mechanisms for the reaction have varied, most likely because of green rust's layered structure, as reduction at outer and interlayer surfaces might produce different reaction products with variable stabilities. Based on studies of Cr(III) oxidation by biogenic Mn (IV) oxides, Cr mobility in oxic soils is controlled by the solubility of the Cr(III)-bearing phase. Therefore, careful engineering of green rust properties, i.e., crystal/particle size, morphology, structure, and electron availability, is essential for its optimization as a remediation reagent. In the present study, pure green rust sulfate and green rust sulfate with Al, Mg and Zn substitutions were synthesized and reacted with identical chromate (CrO) solutions. The reaction products were characterized by X-ray diffraction, pair distribution function analysis, X-ray absorption spectroscopy and transmission electron microscopy and treated with synthetic δ-MnO to assess how easily Cr(III) in the products could be oxidized. It was found that Mg substitution had the most beneficial effect on Cr lability in the product. Less than 2.5% of the Cr(III) present in the reacted Mg-GR was reoxidized by δ-MnO within 14 days, and the particle structure and Cr speciation observed during X-ray scattering and absorption analyses of this product suggested that Cr(VI) was reduced in its interlayer. Reduction in the interlayer lead to the linkage of newly-formed Cr(III) to hydroxyl groups in the adjacent octahedral layers, which resulted in increased structural coherency between these layers, distinctive rim domains, sequestration of Cr(III) in insoluble Fe oxide bonding environments resistant to reoxidation and partial transformation to Cr(III)-substituted feroxyhyte. Based on the results of this study of hexavalent chromium reduction by green rust sulfate and other studies, further improvements can also be made to this remediation technique by reacting chromate with a large excess of green rust sulfate, which provides excess Fe(II) that can catalyze transformation to more crystalline iron oxides, and synthesis of the reactant under alkaline conditions, which has been shown to favor chromium reduction in the interlayer of Fe(II)-bearing phyllosilicates.
在受皮革鞣制和金属电镀影响的地区，铬污染是一个严重的环境问题，而硫酸绿锈已被广泛测试，作为地下水六价铬原位化学还原的潜在材料。报道的该反应产物和机制各不相同，很可能是由于绿锈的层状结构，因为外层和层间表面的还原可能产生具有不同稳定性的不同反应产物。基于对生物源锰（IV）氧化物氧化Cr（III）的研究，含氧土壤中Cr的迁移率受含Cr（III）相溶解度的控制。因此，仔细设计绿锈的性质，即晶体/颗粒尺寸、形态、结构和电子可用性，对于将其优化为修复试剂至关重要。在本研究中，合成了纯硫酸绿锈以及用Al、Mg和Zn替代的硫酸绿锈，并使其与相同的铬酸盐（CrO）溶液反应。通过X射线衍射、对分布函数分析、X射线吸收光谱和透射电子显微镜对反应产物进行表征，并用合成的δ-MnO处理，以评估产物中的Cr（III）被氧化的难易程度。结果发现，Mg替代对产物中Cr的活性有最有利的影响。在14天内，反应后的Mg-GR中存在的Cr（III）被δ-MnO再氧化的比例不到2.5%，并且在对该产物进行X射线散射和吸收分析期间观察到的颗粒结构和Cr形态表明，Cr（VI）在其层间被还原。层间还原导致新形成的Cr（III）与相邻八面体层中的羟基相连，这导致这些层之间的结构相干性增加、独特的边缘域、Cr（III）在抗再氧化的不溶性铁氧化物键合环境中的螯合以及部分转化为Cr（III）取代的铁羟氧化物。基于本研究中硫酸绿锈还原六价铬的结果以及其他研究，通过使铬酸盐与大量过量的硫酸绿锈反应，还可以对这种修复技术进行进一步改进，这会提供过量的Fe（II），可催化转化为更结晶的铁氧化物，以及在碱性条件下合成反应物，这已被证明有利于含Fe（II）层状硅酸盐层间的铬还原。
