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Marine Geochemistry of Uranium

Marine Geochemistry of Uranium. J. Kirk Cochran School of Marine & Atmospheric Sciences Stony Brook University (SUNY) Stony Brook, NY 11794-5000. U in Seawater. Concentration- ~3.3 m g/L, 2.45 dpm 238 U/L at 35 per mil salinity

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Marine Geochemistry of Uranium

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  1. Marine Geochemistry of Uranium J. Kirk Cochran School of Marine & Atmospheric Sciences Stony Brook University (SUNY) Stony Brook, NY 11794-5000

  2. U in Seawater • Concentration- ~3.3 mg/L, 2.45 dpm 238U/L at 35 per mil salinity • 234U/238U activity ratio = 1.144 ± 0.002 (due to preferential chemical weathering of 234U from continents) • Present as U6+, speciated as UO2(CO3)34- • Long residence time in seawater (~450,000 y)

  3. U vs Salinity Pates & Muir (2007)

  4. U Speciation • In solution, U6+ as UO2(CO3)34- (Langmuir, 1978) • In special environments (anoxic basins, reducing marine sediments), reduction can occur, leading to rapid removal of U4+ from solution • But, even in such environments, U in solution is U6+

  5. Where in the oceans is U6+ reduced? An anoxic basin U4+ U6+ and Total U U in the Cariaco Basin water column (Anderson, 1987) U in anoxic basin sediment pore water (Barnes and Cochran, 1990)

  6. U in nearshore sediments • Separation of oxidation states by ion exchange chromatography • Samples of sediment pore water (anoxic) have U in solution as U6+ • But total U concentrations in pore waters are less than predicted from salinity, indicating removal Cochran et al. (1986)

  7. U Removal in Nearshore Sediments • Uranium removal rate correlates with sulfate reduction rate in nearshore (estuarine) sediments • U can be reduced by sulfate- reducing bacteria Barnes & Cochran (1993)

  8. The Oceanic U Balance • Dominant input is from rivers • Dominant sink are sediments (suboxic, anoxic) • Removal associated with alteration of oceanic crust (low T weathering of marine basalts, high T alteration associated with hydrothermal circulation) Barnes and Cochran (1990)

  9. Large Scale Removal of U from Aqueous Solution (Seawater?) • Zero valent iron (ZVI, Fe0) • Used to remediate groundwater contaminants, for example • Fe is oxidized as contaminant is reduced • For U: Fe0 + UO22+→ Fe2+ + UO2; DG° = -164 kJ mol-1 IN OUT Luna-Velasco et al. (2010)

  10. U6+ U4+ Flow U6+ U4+ U6+ U4+ Luna-Velasco et al. (2010)

  11. References • Anderson, R.F. (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3, 145-164. • Barnes, C. and J.K. Cochran (1990) Uranium removal in oceanic sediments and the oceanic U balance. Earth and Planetary Science Letters 97, 94-101. • Barnes, C. and J.K. Cochran (1993) Uranium geochemistry in estuarine sediments: Controls on removal and release processes. Geochimica et Cosmochimica Acta 57, 555-569. • Langmuir, D. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochimica et Cosmochimica Acta 42, 547-569. • Cochran, J.K. (1993) The oceanic chemistry of the U- and Th-series nuclides. In: Uranium Series Disequlibirium, (eds. M. Ivanovich & R.S. Harmon), 2d edition, Clarendon Press, Oxford, UK. • Cochran, J.K., A.E. Carey, E.R. Sholkovitz and L.D. Surprenant (1986) The geochemistry of uranium and thorium in coastal marine sediments ands sediment porewaters. Geochimica et Cosmochimica Acta 50, 663-680. • Luna-Velasco, A., R. Sierra-Alvarez, B. Castro and J. A. Field (2010) Removal of nitrate and hexavalent uranium from groundwater by sequential treatment in bioreactors packed with elemental sulfur and zero-valent iron. Biotechnology and Bioengineering, doi: 10.1002/bit.22881.

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