Uranium Reduction and Structural Incorporation Into Iron Oxides: Competing Sequestration Pathways Under Reducing Conditions.

Uranium retention and sequestration pathways determine the long-term fate of this important contaminant in soils and sediments. The dominant U retention pathway (reduction, incorporation into iron oxides, or adsorption) varies with local biogeochemical conditions. Here we explored the uranium retention mechanisms active during abiotic reduction of U and ferrihydrite by aqueous Fe(II), in the presence of Ca and carbonate ions. Ferrihydrite transformation and U reduction were studied in batch incubations containing Ca (4 mM), carbonate (3.8 mM), ferrihydrite slurry (~180 mg/l), Fe(II) (0.3 mM, 3 mM), and a range of concentrations of uranyl acetate (1 μM-0.5 mM).  Uranium retention pathways were differentiated using extended x-ray absorption fine structure (EXAFS) spectroscopy and synchrotron X-ray powder diffraction. At high U concentrations (0.1-0.5 mM), U(VI) reduction to uraninite was the dominant sequestration pathway. At lower U concentrations (1-10 μM), uraninite precipitation was not observed and incorporation into ferrihydrite transformation products (chiefly goethite) was the dominant sequestration mechanism. Iron(II) concentrations followed the same trend—3 mM Fe(II) promoted U reduction and magnetite formation, while 0.3 mM Fe(II) promoted U incorporation into goethite. Uranium incorporation into goethite may therefore be an important sequestration pathway under environmentally relevant solution conditions in reducing environments.