NITRATE MEDIATED URANIUM(IV) OXIDATION IN SHALLOW SUBSURFACE SEDIMENTS

Nitrate mediated Uranium (U) oxidation has been demonstrated in a variety of sediments, soils, and groundwater. To date, studies evaluating U bio-oxidation and mobilization have primarily focused on anthropogenically contaminated sites. U is commonly deposited as insoluble reduced uranium minerals. These reduced U minerals are subject to reoxidation by available oxidants, such nitrate, in situ. Soluble U from natural sources is a recognized contaminant in public water supplies throughout the US. Here we evaluate the potential of nitrate mediated oxidation of U in shallow subsurface sediments from an alluvial aquifer in Nebraska. Subsurface sediments and groundwater (20-64ft.) containing nitrate (from fertilizer) and natural iron and uranium were collected. The reduction potential revealed a reduced environment and was confirmed by the presence of Fe(II) and U(IV) in sediments. Although sediments were reduced, nitrate persisted in the groundwater. Nitrate concentrations decreased, 38 mg/L to 30 mg/L, with increasing concentrations of Fe(II) and U(IV). Dissolved U, primarily as U(VI), increased with depth, 30.3 μg/L to 302 μg/L. Analysis of sequentially extracted U(VI) and U(IV) revealed that virtually all U in sediments existed as U(IV). The presence of U(IV) is consistent with reduced Fe (Fe(II)) and low reduction potential. The increase in aqueous U concentrations with depth suggests active U cycling. Tetravalent U (U(IV)) phases are stable in reduced environments, however the input of an oxidant such as molecular oxygen or nitrate into these systems would result in oxidation. Thus co-occurrence of nitrate suggests that nitrate could be used by bacteria as a U(IV) oxidant. Most probable number enumeration of nitrate-dependent U(IV) oxidizing microorganisms demonstrated an abundant community ranging from 1.61x10⁴ to 2.74x10⁴ cells g^-1 sediment. Enrichments initiated verified microbial U reduction and U oxidation coupled to nitrate reduction. Not only could nitrate serve as an oxidant, but Fe(III) could also contribute to U mobilization. Nitrate-dependent Fe(II) oxidation is an environmentally ubiquitous process facilitated by a diversity of microorganisms. These processes could also have a confounding effect on the mobility of naturally occurring U and water quality in subsurface environments.