Biogeochemical Characterization of Uranium from a Pre-Mining and Post-Mining Uranium in-Situ Recovery Mine.

In-situ recovery (ISR) uranium (U) mine restoration is generally based upon a return of the site to baseline conditions. Little or no scientific information is used to justify utilizing baseline conditions for regulatory compliance and the constituents monitored for compliance have not been evaluated to ensure they are proper indicators of restoration. This study focuses on the novel aspect of baseline characterization of U thereby allowing a complete scientific evaluation of the changes that occur during mining, so that specific recommendations can be made on how best to accomplish restoration of those constituents that most impact human and environmental health.

A thorough understanding of the structures, compositions, occurrence, and stabilities of U species is crucial in order to describe and model natural attenuation processes and to improve in situ reductive approaches of U remediation. Current U remediation approaches focus on reducing uranyl or U(VI) to form the mineral uraninite or UO₂ because of its low water solubility. However, literature studies hint at the formation of reduced, noncrystalline U(IV) species referred to as monomeric or mononuclear U(IV), in systems subject to biological or chemical U(VI) reductions. This study attempts to identify the different U species formed in ore bodies formed before and after mining under different redox and hydrological gradients using traditional wet chemical techniques, synchrotron extended x-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopy and microbial pyrosequencing techniques. Bulk U-EXAFS spectroscopy on the sediment cores indicate that molecular U(IV) might make up more than 25% of the total U found within the ore zone while the remaining fraction constitutes primarily biogenic U(IV) mineral phases. This study is unique since no research, to the best of our knowledge, have been conducted on the potential presence, stability and local coordination chemistry of molecular U(IV) within ore zones or roll-front U deposits till date. This work further highlights the complex interplay between biotic and abiotic factors instrumental in controlling the fate and transport of U in subsurface environments and will provide regulators with sound scientific evidence for optimizing remediation goals.