Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, CO

Abstract

Reductive bioremediation is currently being explored as a possible strategy for uranium-contaminated aquifers such as the Old Rifle site (Colorado). The stability of U(IV) phases under oxidizing conditions is key to the performance of this procedure. An in situ method was developed to study oxidative dissolution of biogenic uraninite (UO2), a desirable U(VI) bioreduction product, in the Old Rifle, CO, aquifer under different variable oxygen conditions. Overall uranium loss rates were 50–100 times slower than laboratory rates. After accounting for molecular diffusion through the sample holders, a reactive transport model using laboratory dissolution rates was able to predict overall uranium loss. The presence of biomass further retarded diffusion and oxidation rates. These results confirm the importance of diffusion in controlling in-aquifer U(IV) oxidation rates. Upon retrieval, uraninite was found to be free of U(VI), indicating dissolution occurred via oxidation and removal of surface atoms. Interaction of groundwater solutes such as Ca2+ or silicate with uraninite surfaces also may retard in-aquifer U loss rates. These results indicate that the prolonged stability of U(IV) species in aquifers is strongly influenced by permeability, the presence of bacterial cells and cell exudates, and groundwater geochemistry.

Long-Term in Situ Oxidation of Biogenic Uraninite in an Alluvial Aquifer: Impact of Dissolved Oxygen and Calcium

“…Electron microscopy measurements showed the resulting cleaned nanoparticles had particle sizes of 1.5 to 2 nm. 31 X-ray diffraction measurements cannot be performed on gel pucks because of the large X-ray background scattered from the gels. Also, the EXAFS data quality from gel pucks is poor in comparison to an ungelled sample.…”

“…In both cases, uraninite was present in loose agglomerations. 31 Biogenic Uraninite-Doped Gel Pucks for Mass Balance. Polyacrylamide gels acted as an inert matrix that did not dissolve or chemically modify uraninite reactivity.…”

“…Permeable sample cells (2 mL volume with cellulose ester membranes) were described in detail previously. 31 Suspensions of uraninite (∼50 to 300 mg of uraninite per sample cell) or polyacrylamide gel pucks (∼10−20 mg uraninite per sample cell, for mass balance), all in ultrapure water (initial resistivity >18.2 MΩ-cm), were installed into the cells in the field in a N 2 -filled anaerobic chamber. Uraninite samples were not preequilibrated with Rifle GW.…”

Biogeochemical Controls on the Product of Microbial U(VI) Reduction

“…Cells were harvested by centrifugation at 8000g for 10 min and washed in simple buffered medium (referred to as BP medium), composed of 30 mM NaHCO 3 and 20 mM 1,4-piperazinediethanesulfonic acid (PIPES buffer) adjusted to pH 6.8. Washed cells were suspended to an optical density (OD 600 ) of 1.0 in BP medium amended with solutes as described in Table 1, or in filter-sterilized groundwater from a U-contaminated site at Old Rifle, Colorado 16 (RGW) ( Table 1). 2.2.…”

“…Duplicate reduction batches with the appropriate solute composition were amended with 20 mM lactate as an electron donor and varying concentrations of U(VI) acetate (100 to 600 μM) as a source of U(VI) under anoxic conditions. The relatively high concentrations of uranium (in comparison to field concentration ranging from 0.4 to 1.8 μM 16 ) was necessary in order to allow sufficient U(IV) accumulation for Scanning Transmission X-ray Microscopy (STXM) characterization. Our model systems were amended with 30 mM bicarbonate, and a circumneutral pH was set in order to maintain the added dissolved U(VI) in the aqueous phase.…”