Marine Geochemistry of Uranium

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.