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Influence of Nitrate on Biogeochemical Uranium Redox Cycling and Mobility in Fluvial Sediments
Uranium (U) is a common contaminant in alluvial aquifer systems all over the world. U in the environment exists in two redox states: reduced U(IV) and oxidized U(VI). Oxidized U(VI) species tend to be more soluble and therefore more groundwater mobile than reduced U(IV) species. Thus, redox reactions have a strong control over U mobility into groundwater. A 2015 metadata analysis revealed a statistically significant correlation between groundwater uranium and nitrate, a strong oxidizing agent, in two major US aquifers. Subsequent column experiments using reduced U(IV)-bearing natural sediments confirmed that additions of nitrate to alluvial aquifer sediments will stimulate U(VI) mobilization. However, nitrate dependent Fe(II) oxidation also occurred concurrently and a series of batch U(VI) adsorption experiments demonstrated that more U(VI) was adsorbed onto sediments after oxidation than before oxidation with an increase in the amount of oxidized Fe in the sediments. Thus, an increase in the amount of bicarbonate is required for the net mobilization of U(VI) into groundwater in addition to the input of nitrate. The net input of oxidants, like nitrate, is assumed to always result in net oxidation of reduced chemical species, like Fe(II), sulfide, and U(IV). This paradigm was challenged by results from a field experiment conducted at DOE field challenge site at Rifle Mountain, CO where of a low mass of oxygenated water resulted in a decrease in groundwater U and an increase in dissolved organic carbon. This was tested using batch bioreactors containing a bicarbonate buffered medium and reduced oxbow lake sediments (Riverton, WY). Following the addition of a low level of nitrate, U(VI) decreased concurrently with increases in Fe(II) and sulfide, indicating the establishment of reducing conditions. Nitrate was reduced to nitrite and DOC and cell abundance increased through the duration of the experiment. These results demonstrate that net reducing conditions may persist even with the input of an oxidant.
Geology|Hydrologic sciences|Sedimentary Geology|Physical chemistry
Westrop, Jeffrey P, "Influence of Nitrate on Biogeochemical Uranium Redox Cycling and Mobility in Fluvial Sediments" (2021). ETD collection for University of Nebraska - Lincoln. AAI28419816.