MECHANISMS AND KINETICS OF ANAEROBIC ABIOTIC AND NITRATE-DEPENDENT BACTERIAL U(IV) OXIDATION

A common strategy used to remove uranium from groundwater is promoting U(VI) reduction and precipitation of U(IV) as uraninite (UO_2(s)) in contaminated aquifers. However, microbially mediated, nitrate-dependent UO_2(s) oxidation under anaerobic conditions may lead to re-mobilization of dissolved U(VI). Anaerobic oxidation of UO_2(s) may proceed by different pathways: abiotic, direct (enzymatic) biotic, or coupled biotic-abiotic. To elucidate the key biotic and abiotic mechanisms underlying UO_2(s) oxidation, flow-through column experiments were carried out with and without addition of Thiobacillus denitrificans. This bacterium contains c-type cytochromes that are thought to be the primary catalysts for anaerobic, nitrate-dependent UO_2(s). Biogenic UO₂(s) was synthesized under anaerobic conditions. Mixtures of biogenic UO_2(s) and quartz, with and without T. denitrificans, were packed into 1-mL or 5-mL polypropylene columns, resulting in an initial porosity of 0.33. Flow was directed upward in the experiments to avoid the presence of N₂ bubbles. The flow rates were controlled with an HPLC pump at flow rates of 0.060 - 0.075 mL/min. Columns were eluted for 100 – 500 pore volumes. Oxidation experiments were carried out in the dark at 25±2ºC in an anaerobic glove box (10%H₂, 90%N₂) in buffered solution (MOPS, pH ~ 7.2). Abiotic oxidation of UO_2(s) by nitrate (1-20 mM) was slow under abiotic conditions. Experiments with nitrite led to higher dissolved U release rates. In the presence of T. dentrificans and nitrate, higher oxidation rates were observed compared with abiotic controls, suggesting that T. dentrificans catalyzed the oxidative dissolution of UO_2(s). Characterization of solids before and after reaction by synchrotron X-ray absorption spectroscopy and scanning transmission X-ray microscopy (STXM) showed that some uranium is retained in the column as U(VI) (sorbed or as a new precipitate). Our results suggest that U(IV) oxidation results from a combination of abiotic nitrite oxidation (from microbiological nitrate reduction) and direct biotic oxidation by T. denitrificans. Reactive transport modeling including equilibrium (aqueous complexation, sorption) and kinetic reactions are used to link molecular-scale mechanisms to macroscopic-scale observations.