HR: 1340h
AN: H23D-1630    [Abstracts]
TI: Experimental Study of U(VI) Release Kinetics from Aquifer Sediments from a Former Uranium Mill Tailings Site (Rifle, Colorado, USA)
AU: * Hyun, S
EM: sphyun@umich.edu
AF: University of Michigan, 1351 Beal Avenue, Ann Arbor, MI 48109, United States
AU: Campbell, K M
EM: kcampbell@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Hayes, K F
EM: ford@umich.edu
AF: University of Michigan, 1351 Beal Avenue, Ann Arbor, MI 48109, United States
AU: Davis, J A
EM: jadavis@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Uranium(VI) release kinetics from aquifer sediments from a former uranium mill tailings site in Rifle, Colorado was studied to understand uranium distribution within the sediments. The sediments were sampled at depths of 3.5-3.8 m in December 2004. The samples were air-dried, sieved, and the <2 mm fraction was collected and used in this study. Total uranium content in the sediments, determined by gamma-radiometry, was 4.1 μg/g sediment. The labile fraction of U(VI) in the sediments was determined using carbonate/bicarbonate extractions, which should cause complete desorption of U(VI) in the absence of mass transfer limitations. Carbonate/bicarbonate extraction of the sediments showed very slow release kinetics, with only 12 % of the labile U(VI) in the sediments being released during the first 96 hours of extraction. This is much less than found in a previous study at a different mill tailings site (Naturita, Colorado), in which more than 80 % of labile U(VI) was released during the same period of extraction. Up to two months of carbonate/bicarbonate extraction released 1 μg U(VI) per gram of Rifle sediment, which is 25 % of the total U in the sediment. Extraction with an artificial groundwater prepared to simulate the field groundwater chemistry showed 0.26 μg U/g sediment was released during the initial 94 hours of extraction, with a gradual increase of released U(VI) with time, while other major and minor elements (except Si) rapidly reached steady-state concentrations during the first few hours of reaction. Two hypotheses are under consideration to explain the slow U(VI) release kinetics: 1) colloidal clay fraction particles cementing larger grains of the sediments are creating nanoscale interparticle pores that act as a diffusion barrier to U(VI) desorption, and 2) a U(IV) solid phase exists whose oxidation and dissolution control the U(VI) release rate. To test the hypotheses, oxidation and extraction of the sediments have been conducted using oxidants such as hydrogen peroxide. The results of this study are expected to contribute to better understanding of U(VI) sorption behavior on the sediments and will contribute to the development of a comprehensive bioremediation strategy at the Rifle uranium mill tailings site.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0412 Biogeochemical kinetics and reaction modeling (0414, 0793, 1615, 4805, 4912)
DE: 0418 Bioremediation
DE: 0461 Metals
DE: 0471 Oxidation/reduction reactions (4851)
SC: Hydrology [H]
MN: 2007 Fall Meeting