Geochemical controls of the microbially mediated redox cycling of uranium and iron
Geochemical controls of the microbially mediated redox cycling of uranium and iron
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DOI：
10.1016/j.gca.2018.05.027
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发表时间：
2018-08-15
影响因子：
5
通讯作者：
Taillefert, Martial
中科院分区：
文献类型：
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作者：
Belli, Keaton M.;Taillefert, Martial
由于氧化态通常是铀在地下环境中迁移的主要决定因素，因此全面了解铀的氧化还原循环对于预测这种污染物的命运至关重要。铁作为铀的还原剂和氧化剂的潜力表明，主要依赖于将铀酰离子（U(VI)）还原为难溶性铀离子（U(IV)）以固定固相铀的修复策略可能会根据铁的生物地球化学转化而增强或阻碍。为了确定铀和铁氧化还原循环的地球化学控制因素，研究人员以U（VI）、水合铁或两者作为终端电子受体，与模型金属还原细菌谢瓦氏菌进行了间歇培养，并改变了溶解无机碳（DIC）、钙和二氧化硅的浓度，以改变水中铀酰的形态和水合铁的二次矿化途径。在几乎所有条件下，与纯铀对照相比，水合铁的存在增加了初始铀从溶液中去除的准一级速率常数，这是由于铁（II）对非生物U（VI）的还原以及铀与水合铁二次矿化产物的固相结合。经过一段初始铀还原期后，铀（IV）被水合铁氧化，导致溶解铀反弹。一旦水合铁被微生物呼吸和二次矿化完全消耗，铀再次通过还原沉淀和与二次矿化产物的结合从溶液中去除。DIC和钙增强了U（IV）固体的氧化溶解，但不影响U(VI)/U（IV）氧化还原对的氧化还原电位，尽管形成了碳酸铀酰水和碳酸铀酰钙。相反，溶解的Fe（II）从溶液中去除以及伴随的Fe(III)/Fe（II）氧化还原对的氧化还原电位的变化是导致铁从U（VI）的还原剂突然转变为U（IV）的氧化剂的原因。在最高DIC浓度和钙存在的情况下，通过菱铁矿沉淀去除溶解的Fe（II）增强了U（IV）的氧化，而二氧化硅的存在限制了铁水化物向磁铁矿的转化和U（IV）的持续氧化。在考虑非生物铀-铁氧化还原循环的热力学有利条件下，建立了一个能再现铀和铁形态孵化时间序列的动力学模型。模拟结果显示，在培养过程中形成的非铀矿U（IV）固体的氧化还原电位比非晶态铀矿低约84 mV，这突出了对非铀矿U（IV）固体进行热力学表征的必要性。本研究的结果表明，在中高DIC浓度下，溶解的Fe（II）浓度是铁在铀氧化还原循环中的主要地球化学控制因素，并强调了在设计特定地点的原位生物修复策略时考虑这一参数的重要性。(C) 2018 Elsevier Ltd.版权所有。
As oxidation state is often a primary determinant of the mobility of uranium in subsurface environments, a comprehensive understanding of the redox cycling of uranium is essential to predict the fate of this contaminant. The potential of iron to serve as both a reductant and an oxidant of uranium suggests that remediation strategies which primarily rely on the reduction of the uranyl ion (U(VI)) to the poorly soluble uranous ion (U(IV)) to immobilize uranium in the solid phase may be either enhanced or hindered depending on the biogeochemical transformations of iron. To identify the geochemical controls of both uranium and iron redox cycling, batch incubations with the model metal-reducing bacterium Shewanella putrefaciens were conducted with either U(VI), ferrihydrite, or both as terminal electron acceptors, and concentrations of dissolved inorganic carbon (DIC), calcium, and silica were varied to alter aqueous uranyl speciation and secondary mineralization pathways of ferrihydrite. The presence of ferrihydrite increased the pseudo-first order rate constant of initial uranium removal from solution in nearly all conditions compared to uranium-only controls due to a combination of abiotic U(VI) reduction by Fe(II) and solid-phase association of uranium with secondary mineralization products of ferrihydrite. Following an initial period of uranium reduction, U(IV) was oxidized by ferrihydrite which led to a rebound in dissolved uranium. Once ferrihydrite was completely consumed by microbial respiration and secondary mineralization, uranium was again removed from solution via reductive precipitation and association with secondary mineralization products. DIC and calcium enhanced the oxidative dissolution of U(IV) solids but did not affect the redox potential of the U(VI)/U(IV) redox couple despite the formation of aqueous uranyl carbonate and calcium-uranyl carbonate species. Instead, removal of dissolved Fe(II) from solution and the accompanied shift in the redox potential of the Fe(III)/Fe(II) redox couple was responsible for the abrupt shift in iron acting as a reductant of U(VI) to an oxidant of U(IV). The removal of dissolved Fe(II) via siderite precipitation at the highest DIC concentration and in the presence of calcium enhanced the oxidation of U(IV), and the presence of silica limited the conversion of ferrihydrite to magnetite and sustained U(IV) oxidation. A kinetic model was developed which could reproduce incubation time series of uranium and iron speciation provided the thermodynamic favorability of abiotic uranium-iron redox cycling was accounted for. The modeling exercise revealed that the non-uraninite U(IV) solids formed in the incubations have a redox potential approximately 84 mV lower than that of amorphous uraninite, highlighting the need for a thermodynamic characterization of non-uraninite U(IV) solids. The results of this study identify dissolved Fe(II) concentration as the primary geochemical control of the role of iron in uranium redox cycling at moderate to high DIC concentrations and emphasize the importance of considering this parameter when designing site-specific, in situ bioremediation strategies. (C) 2018 Elsevier Ltd. All rights reserved.