title: In situ mobility of uranium in the presence of nitrate following sulfate-reducing conditions.
abstract: Reoxidation and mobilization of previously reduced and immobilized uranium by dissolved-phase oxidants poses a significant challenge for remediating uranium-contaminated groundwater. Preferential oxidation of reduced sulfur-bearing species, as opposed to reduced uranium-bearing species, has been demonstrated to limit the mobility of uranium at the laboratory scale yet field-scale investigations are lacking. In this study, the mobility of uranium in the presence of nitrate oxidant was investigated in a shallow groundwater system after establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species. A series of three injections of groundwater (200 L) containing U(VI) (5 μM) and amended with ethanol (40 mM) and sulfate (20 mM) were conducted in ten test wells in order to stimulate microbial-mediated reduction of uranium and the formation of reduced sulfur-bearing species. Simultaneous push-pull tests were then conducted in triplicate well clusters to investigate the mobility of U(VI) under three conditions: 1) high nitrate (120 mM), 2) high nitrate (120 mM) with ethanol (30 mM), and 3) low nitrate (2 mM) with ethanol (30 mM). Dilution-adjusted breakthrough curves of ethanol, nitrate, nitrite, sulfate, and U(VI) suggested that nitrate reduction was predominantly coupled to the oxidation of reduced-sulfur bearing species, as opposed to the reoxidation of U(IV), under all three conditions for the duration of the 36-day tests. The amount of sulfate, but not U(VI), recovered during the push-pull tests was substantially more than injected, relative to bromide tracer, under all three conditions and further suggested that reduced sulfur-bearing species were preferentially oxidized under nitrate-reducing conditions. However, some reoxidation of U(IV) was observed under nitrate-reducing conditions and in the absence of detectable nitrate and/or nitrite. This suggested that reduced sulfur-bearing species may not be fully effective at limiting the mobility of uranium in the presence of dissolved and/or solid-phase oxidants. The results of this field study confirmed those of previous laboratory studies which suggested that reoxidation of uranium under nitrate-reducing conditions can be substantially limited by preferential oxidation of reduced sulfur-bearing species.
authors.affiliation: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States. Electronic address: charlesjparadis@gmail.com.
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Paradis
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States.
authors.forename: Sindhu
authors.initials: S
authors.lastname: Jagadamma
authors.affiliation: Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States.
authors.forename: David B
authors.initials: DB
authors.lastname: Watson
authors.affiliation: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States.
authors.forename: Larry D
authors.initials: LD
authors.lastname: McKay
authors.affiliation: Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, United States; Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States; Department of Microbiology, University of Tennessee, Knoxville, TN 37996, United States; Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, United States; Center for Environmental Biotechnology, University of Tennessee, Knoxville, TN 37996, United States; Institute for a Secure and Sustainable Environment, University of Tennessee, Knoxville, TN 37996, United States.
authors.forename: Terry C
authors.initials: TC
authors.lastname: Hazen
authors.affiliation: School of Civil and Construction Engineering, Oregon State University, Corvallis, OR 97331, United States.
authors.forename: Melora
authors.initials: M
authors.lastname: Park
authors.affiliation: School of Civil and Construction Engineering, Oregon State University, Corvallis, OR 97331, United States.
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Istok
mesh_terms: D052918:Environmental Restoration and Remediation; D000431:Ethanol / Q000737:chemistry; D009566:Nitrates / Q000737:chemistry*; D010084:Oxidation-Reduction; D013431:Sulfates / Q000737:chemistry; D013457:Sulfur Compounds / Q000737:chemistry; D013714:Tennessee; D014501:Uranium / Q000032:analysis* / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry
chemical_list: D009566:Nitrates; D013431:Sulfates; D013457:Sulfur Compounds; D014874:Water Pollutants, Chemical; D000431:Ethanol; D014501:Uranium
keywords: Mobility; Nitrate; Oxidation; Reduction; Sulfate; Uranium