Marine Geochemistry of Uranium

J. Kirk CochranSchool of Marine & Atmospheric Sciences

Stony Brook University (SUNY)Stony Brook, NY 11794-5000

U in Seawater

• Concentration- ~3.3 g/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)

U vs Salinity

Pates & Muir (2007)

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+

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)

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)

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, Fe0)

•Used to remediate groundwater contaminants, for example

•Fe is oxidized as contaminant is reduced

•For U: Fe0 + UO22+ → Fe2+ + UO2; G° = -164 kJ mol-1

Luna-Velasco et al. (2010)

IN

OUT

FlowU6+ U4+

U6+ U4+

U6+ U4+

Luna-Velasco et al. (2010)

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.