2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 336-10
Presentation Time: 3:45 PM

NITRATE MEDIATED URANIUM(IV) OXIDATION IN SHALLOW SUBSURFACE SEDIMENTS


WEBER, Karrie A., School of Biological Sciences and Dept. of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE 68588, HEALY, Olivia M., School of Biological Sciences, University of Nebraska-Lincoln, 232 Manter Hall, Lincoln, NE 68588, SPANBAUER, Trisha L., Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE 68588, NOLAN, Jason, Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588 and SNOW, Daniel, School of Natural Resources, University of Nebraska-Lincoln, Lincoln, NE 68588-0118

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.