B079-0001
Influx of nitrate into reduced organic-rich sediments stimulates U(VI) reduction

Monday, 14 December 2020
Poster
Jeffrey Westrop1, Pooja Yadav2, Karrie A Weber3, John Bargar4, Vincent Noel4, Arjen van Veelen5, Xiaoqin Wu6 and Romy Chakraborty7, (1)University of Nebraska Lincoln, Lincoln, NE, United States, (2)University of Nebraska-Lincoln, Biological Sciences, Lincoln, NE, United States, (3)University of Nebraska-Lincoln, School of Biological Sciences, Lincoln, NE, United States, (4)SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource, Menlo Park, CA, United States, (5)University of Southampton, Southampton, United Kingdom, (6)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (7)Lawrence Berkeley Nat'l Lab, Berkeley, CA, United States
Abstract:
The existing paradigm describes oxidation of reduced chemical species, such as Fe(II) and U(IV), following an influx of oxidants, dissolved oxygen (DO) or nitrate. Prior field research results challenged this paradigm whereby low concentrations of an oxidant (DO) injected into a biostimulated reduced region of an alluvial aquifer stimulated a decrease in aqueous U concentrations (Rifle, CO; (Pan et al., 2018)). Here we experimentally test the impact of a highly soluble oxidant, nitrate, on redox state using organic-rich, reduced-U bearing oxbow lake sediments (collected from Riverton, WY; SLAC SFA). Triplicate batch reactors of reduced sediments were amended with anoxic bicarbonate buffered medium with and without the addition of nitrate at high or low concentrations (<14 mg/L-N>) following preincubation with added uranyl chloride (final concentration 5 ppm). Following the addition of the nitrate, aqueous U concentrations increased in reactors amended with “high” nitrate. However, in “low” nitrate reactors aqueous U decreased. No significant decrease was observed in unamended controls. XANES analysis of sediments collected from batch reactors (10 hours of incubation) revealed an increase in solid-phase U(IV) relative to the unamended controls (85% and 40% U(IV) respectively) indicating a rapid reduction of U(VI) to U(IV) in response to nitrate. An increase in aqueous Fe(II) concentrations further supported the onset of reducing conditions. While a decrease in sulfide was observed, PHREEQC modeling of geochemical data indicated Fe-sulfide precipitation. Reduction activity occurred concurrent with an increase in dissolved organic carbon (DOC) in nitrate-amended treatments. This increase in DOC was observed with an increase in cell abundance. Microbial activity was confirmed to catalyze this process in batch reactors amended with an antibiotic, chloramphenicol. The addition of chloramphenicol significantly suppressed nitrate reduction as well as the loss of aqueous U, supporting the role of microbial activity catalyzing reduction with the addition of low nitrate concentrations into organic-rich sediments. Together these results demonstrate that in organic rich sediments an influx of an oxidant can lead to the increase in DOC supporting microbial activity and reducing conditions.
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