Fate of tetravalent uranium under reducing conditions

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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.