Presentation on theme: "Marine Geochemistry of Uranium J. Kirk Cochran School of Marine & Atmospheric Sciences Stony Brook University (SUNY) Stony Brook, NY 11794-5000."— Presentation transcript:
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 g/L, 2.45 dpm 238 U/L at 35 per mil salinity 234 U/ 238 U activity ratio = 1.144 ± 0.002 (due to preferential chemical weathering of 234 U from continents) Present as U 6+, speciated as UO 2 (CO 3 ) 3 4- Long residence time in seawater (~450,000 y)
U Speciation In solution, U 6+ as UO 2 (CO 3 ) 3 4- (Langmuir, 1978) In special environments (anoxic basins, reducing marine sediments), reduction can occur, leading to rapid removal of U 4+ from solution But, even in such environments, U in solution is U 6+
Where in the oceans is U 6+ reduced? An anoxic basin U 4+ U 6+ and Total U U in the Cariaco Basin water column (Anderson, 1987) U in anoxic basin sediment pore water (Barnes and Cochran, 1990)
U in nearshore sediments Separation of oxidation states by ion exchange chromatography Samples of sediment pore water (anoxic) have U in solution as U 6+ But total U concentrations in pore waters are less than predicted from salinity, indicating removal Cochran et al. (1986)
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)
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)
Large Scale Removal of U from Aqueous Solution (Seawater?) Zero valent iron (ZVI, Fe 0 ) Used to remediate groundwater contaminants, for example Fe is oxidized as contaminant is reduced For U: Fe 0 + UO 2 2+ → Fe 2+ + UO 2 ; G° = -164 kJ mol-1 Luna-Velasco et al. (2010) IN OUT
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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