The chemical or biological reduction of U(VI) to U(IV) has been studied extensively as a groundwater remediation strategy. The approach relies on the significant decrease in dissolved uranium concentrations when uraninite (UO2) controls uranium solubility. However, recent evidence suggests that uraninite may not be the predominant form of U(IV) in reduced sediments. Laboratory studies have implicated phosphate and bacterial activity as factors contributing to non-uraninite U(IV) speciation. In this study we significantly expand the set of factors that control U(IV) speciation. Using synchrotron x-ray absorption spectroscopy (U LIII-edge XANES and EXAFS) and high-resolution electron microscopy (TEM and SEM) we show that mineral surface sites (TiO2 and Fe3O4) can stabilize adsorbed, non-uraninite U(IV) species for extended periods under reducing conditions. Stabilization occurs at lower surface coverages, which are more typical of natural environments. When the surface coverage of U exceeds a mineral-specific threshold the reduced U(IV) atoms form uraninite nanoparticles. These results suggest that surface sites on the various minerals found in sediments may inhibit the formation of uraninite following U(VI) reduction and indicate that non-uraninite U(IV) speciation can be produced without the direct involvement of biological activity. The significant control of biological and abiotic factors on U(IV) speciation suggests a need to re-examine current geochemical transport models, which typically use only uraninite dissolution and reoxidation reactions to predict the dynamics of U.

Stabilization of adsorbed U(IV) species at high-affinity mineral surface sites under reducing conditions

248th National Meeting (2014)

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