Fate of tetravalent uranium under reducing conditions

刺激微生物将可溶六价铀[U(VI)]还原为稀溶四价铀[U(IV)]已被开发为一种原位固定污染含水层中铀的策略。这种策略的成功取决于作为微生物还原产物形成的U（IV）相的低溶解度。最近的研究表明，这些还原产物不仅包括稳定的结晶矿物相，如铀矿，而且还与生物质，如非结晶U（IV）物种有关。虽然这些生物质相关的铀（IV）的热力学性质、动员机制和速率尚不清楚，但实验证据似乎表明它们更具流动性，并可能影响铀生物修复的效果。此外，在湿地中鉴定出主要由有机质和铁组成的含U（IV）胶体，表明有机配体在调动U（IV）中的重要性。最后，以前的研究表明，生物配体可以加速结晶铀矿的溶解，我们认为它们也可能促进非结晶铀（IV）的动员。在此背景下，提出的工作将解决生物配体和还原性腐殖质动员非结晶U（IV）的动力学和机制，以及它们与铀矿的比较。我们还将探讨配体对非结晶U（IV）向结晶U（IV）的潜在转化的影响，因为这一过程预计会影响U（IV）的稳定性。为了在复杂的自然系统中建立识别这些过程的工具，我们研究了铀同位素分馏作为铀还原机制的适用性，通过与有机配体络合诱导非晶体铀（IV）的动员，以及非晶体铀（IV）向晶体铀（IV）的潜在转化。我们建议，同位素分馏的定量也可以用于阐明这些过程的分子机制，在现场和微观研究。最后，我们将开发定量反应输运模型，包括动力学过程和同位素分馏，这些模型将通过柱实验进行测试，以近似现场规模铀还原和再动员的复杂性。该模型不仅为预测还原条件下铀的输运提供了有用的工具，而且还有助于解释沿流动路径的同位素分馏。这项研究的结果将为铀从地下污染源（如矿山尾矿的浸出或土壤中未爆炸的贫铀弹药）中迁移提供新的见解。此外，我们期望它们提供可用于调整补救战略的设计以及评估其可持续性的宝贵信息。

The stimulation of microbial reduction of the soluble hexavalent U [U(VI)] to sparingly soluble tetravalent U [U(IV)] has been exploited as an in-situ strategy for the immobilization of uranium in contaminated aquifers. The success of this strategy rests on the low solubility of U(IV) phases that are formed as the product of microbial reduction. Recent research efforts have shown that these reduction products do not only consist of stable crystalline mineral phases such as uraninite, but are also associated with biomass such as non-crystalline U(IV) species. While the thermodynamic properties and the mechanisms and rates of mobilization of such biomass-associated U(IV) are unknown, experimental evidence seems to indicate that they are more mobile and may impact the efficacy of uranium bioremediation. Additionally, the identification of U(IV)-bearing colloids composed primarily of organic matter and iron in a wetland, suggests the importance of organic ligands in mobilizing U(IV). Finally, it was previously shown that biogenic ligands can accelerate the dissolution of crystalline uraninite and we suggest that they may also promote the mobilization of non-crystalline U(IV). In this context, the proposed work will address the kinetics and mechanisms of the mobilization of non-crystalline U(IV) by biogenic ligands and reduced humic substances and how they compare to those of uraninite. We will also probe the impact of ligands on the potential transformation of non-crystalline U(IV) to crystalline U(IV) species since this process is expected to impact the stability of U(IV). In order to establish tools to identify these processes in complex natural systems, we investigate the suitability of uranium isotope fractionation as a proxy for the mechanisms of U reduction, of mobilization of non-crystalline U(IV) induced e.g. by complexation with organic ligands, as well as of potential transformation of non-crystalline to crystalline U(IV). We propose that the quantification of isotope fractionation may also be used for the elucidation of the molecular mechanisms of these processes in field and microcosm studies. Finally, we will develop quantitative reactive transport models including kinetic processes and isotope fractionation that will be tested against column experiments that approximate the complexity of field scale uranium reduction and re-mobilization. This model will not only provide a useful tool for predicting uranium transport under reducing conditions, but also aide the interpretation of isotope fractionation along the flow path.The results of this research will deliver new insights into the mobility of U from underground sources of U contamination such as leaching from mine tailings or from unexploded depleted uranium ammunition in soil. We furthermore expect them, to provide valuable information that may be used to adjust the design of remediation strategies as well as to evaluate their sustainability.