Subsurface sediments are a heterogeneous mixture containing buried soil horizons and organic matter. These organic-rich deposits are recognized to drive metal/radionuclide reduction and trap reduced chemical species (e.g, (Fe(II)) including contaminants such as uranium (U(IV)) in both anthropogenically contaminated as well as natural systems. Upon the influx of an oxidant, such as dissolved oxygen (DO) and/or nitrate, the trapped reduced minerals may be subject to oxidation. The change in redox state can alter metal/radionuclide behavior and, in the case of uranium, contribute to the generation of soluble and mobile U(VI) species from reduced U(IV). Prior research has demonstrated that microbial catalysis can facilitate U(IV) oxidation and U mobility.  To test the role of microbially catalyzed mobilization of naturally occurring U by nitrate, sediments and groundwater were collected from an alluvial aquifer with high groundwater uranium concentrations in a region exhibiting fluctuating redox conditions.  Upflow meso-scale column reactors were packed with reduced sediment contain naturally occurring uranium and sterile sand (50% mass/mass) with bicarbonate buffered (pH 7.1) artificial groundwater serving as the influent. Bicarbonate initially released adsorped U from the sediment matrix.  Following bicarbonate-mediated U release, nitrate was added to duplicate columns to stimulate U mobilization mediated by the influx of an oxidant.  The addition of high nitrate concentrations (above MCL) in the influent stimulated U mobilization to concentrations that exceeded the MCL relative to controls in which nitrate was omitted.  Oxidation of U was coupled to the observed reduction of nitrate to nitrite.  No significant uranium was measured in the effluent in treatments amended with sodium azide, demonstrating inhibition of nitrate respiration and uranium oxidation. Thus, the influx of oxidants such as nitrate into reduced sediments bearing naturally occurring solid-phase U(IV) stimulates microbial activity resulting in mobilization of sedimentary U.   <br/>

Microbially-mediated metal/radionuclide oxidation coupled to nitrate reduction in an alluvial aquifer

255th National Meeting (2018)

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