Uranium geochemistry on the Amazon shelf: Chemical phase partitioning and cycling across a salinity gradient
- Louisiana Universities Marine Center, Cocodrie, LA (United States)
The size distribution of U was examined in surface waters of the Amazon shelf. Water samples were collected during a low discharge river stage across a broad salinity gradient (0.3-35.4%) and fractionated by planar filtration and tangential-flow ultrafiltration into (1) solution (U{sub s}, <10,000 MW; {approximately}1-10 nm), (2) colloidal (U{sub c}, 10,000 MW-0.4 {mu}m), (3) dissolved (U{sub d} <0.4 {mu}m), and (4) particulate (U{sub p} >0.4 {mu}m) phases. Concentrations of colloidal U comprise up to 92% of the dissolved U fraction at the river mouth and attain highest values ({approximately}0.45 {mu}g/L) in the productive, biogenic region of the Amazon shelf (salinities above {approximately}20%). U{sub d} and U{sub c} distributions are highly nonconservative relative to ideal dilution of river water and seawater, indicating extensive removal at salinities below {approximately}10%. The distribution of U{sub s} also shows some nonconservative behavior, yet removal, if any, is minimal. Saltwater-induced precipitation and aggregation of riverine colloidal material is most likely the dominant mechanism of U removal in the low salinity, terrigenous region of the Amazon shelf. There is evident of a substantial colloidal U input ({approximately}245% of the riverine U{sub c} flux) into surface waters above 5%. Such U{sub c} enrichment most likely is the result of colloidal U-rich porewater diffusion/advection from the seabed and fluid muds or shelf-wide particle-colloid disaggregation. Removal of solution and dissolved phase U via a colloidal intermediate and U{sub c} aggregation in terms of coagulation phase U via a colloidal intermediate and U{sub c} aggregation was examined in terms of coagulation theory. The high reactive nature of all U phases on the Amazon shelf suggests that remobilization and fractionation of U may also occur in other river-influenced coastal environments.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 62005
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 59, Issue 1; Other Information: PBD: Jan 1995
- Country of Publication:
- United States
- Language:
- English
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