Fate of tetravalent uranium under reducing conditions

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. In this project, we 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 noncrystalline 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 and will thus provide valuable information that may be used to adjust the design of remediation strategies.