Adsorption of Uranium(VI) to Manganese Oxides: X‑ray Absorption Spectroscopy and Surface Complexation Modeling

The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U­(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U­(VI) adsorption to synthetic and biogenic MnO2, surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U­(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U­(VI) adsorption to synthetic δ-MnO2 appears to be stronger than to biogenic MnO2, and the differences in adsorption affinity and capacity are not associated with any substantial difference in U­(VI) coordination.