title: Evaluating chemical extraction techniques for the determination of uranium oxidation state in reduced aquifer sediments.
abstract: Extraction techniques utilizing high pH and (bi)carbonate concentrations were evaluated for their efficacy in determining the oxidation state of uranium (U) in reduced sediments collected from Rifle, CO. Differences in dissolved concentrations between oxic and anoxic extractions have been proposed as a means to quantify the U(VI) and U(IV) content of sediments. An additional step was added to anoxic extractions using a strong anion exchange resin to separate dissolved U(IV) and U(VI). X-ray spectroscopy showed that U(IV) in the sediments was present as polymerized precipitates similar to uraninite and/or less ordered U(IV), referred to as non-uraninite U(IV) species associated with biomass (NUSAB). Extractions of sediment containing both uraninite and NUSAB displayed higher dissolved uranium concentrations under oxic than anoxic conditions while extractions of sediment dominated by NUSAB resulted in identical dissolved U concentrations. Dissolved U(IV) was rapidly oxidized under anoxic conditions in all experiments. Uraninite reacted minimally under anoxic conditions but thermodynamic calculations show that its propensity to oxidize is sensitive to solution chemistry and sediment mineralogy. A universal method for quantification of U(IV) and U(VI) in sediments has not yet been developed but the chemical extractions, when combined with solid-phase characterization, have a narrow range of applicability for sediments without U(VI).
authors.affiliation: US Geological Survey, Menlo Park, California 94025, USA. dlstoliker@usgs.gov
authors.forename: Deborah L
authors.initials: DL
authors.lastname: Stoliker
authors.forename: Kate M
authors.initials: KM
authors.lastname: Campbell
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Fox
authors.forename: David M
authors.initials: DM
authors.lastname: Singer
authors.forename: Nazila
authors.initials: N
authors.lastname: Kaviani
authors.forename: Minna
authors.initials: M
authors.lastname: Carey
authors.forename: Nicole E
authors.initials: NE
authors.lastname: Peck
authors.forename: John R
authors.initials: JR
authors.lastname: Bargar
authors.forename: Douglas B
authors.initials: DB
authors.lastname: Kent
authors.forename: James A
authors.initials: JA
authors.lastname: Davis
mesh_terms: D000837:Anion Exchange Resins / Q000737:chemistry; D003120:Colorado; D019015:Geologic Sediments / Q000737:chemistry*; D060587:Groundwater; D007220:Industrial Waste / Q000032:analysis; D010084:Oxidation-Reduction; D014501:Uranium / Q000032:analysis*; D017974:Uranium Compounds / Q000032:analysis*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D000837:Anion Exchange Resins; D007220:Industrial Waste; D017974:Uranium Compounds; D014501:Uranium; C047385:uranium oxide