Effect of Ca2+ and Zn2+ on UO2 dissolution rates.
The dissolution of UO(2) in a continuously stirred tank reactor (CSTR) in the presence of Ca(2+) and Zn(2+) was investigated under experimental conditions relevant to contaminated groundwater systems. Complementary experiments were performed to investigate the effect of adsorption and precipitation reactions on UO(2) dissolution. The experiments were performed under anoxic and oxic conditions. Zn(2+) had a much greater inhibitory effect on UO(2) dissolution than did Ca(2+). This inhibition was most substantial under oxic conditions, where the experimental rate of UO(2) dissolution was 7 times lower in the presence of Ca(2+) and 1450 times lower in the presence of Zn(2+) than in water free of divalent cations. EXAFS and solution chemistry analyses of UO(2) solids recovered from a Ca experiment suggest that a Ca-U(VI) phase precipitated. The Zn carbonate hydrozincite [Zn(5)(CO(3))(2)(OH)(6)] or a structurally similar phase precipitated on the UO(2) solids recovered from experiments performed in the presence of Zn. These precipitated Ca and Zn phases can coat the UO(2) surface, inhibiting the oxidative dissolution of UO(2). Interactions with divalent groundwater cations have implications for the longevity of UO(2) and the mobilization of U(VI) from these solids in remediated subsurface environments, waste disposal sites, and natural uranium ores.
Department of Energy, Environmental, and Chemical Engineering, One Brookings Drive, Washington University, St Louis, Missouri 63130, United States. cerratoj@wustl.edu
José M
JM
Cerrato
Charles J
CJ
Barrows
Lisa Y
LY
Blue
Juan S
JS
Lezama-Pacheco
John R
JR
Bargar
Daniel E
DE
Giammar
D000327:Adsorption; D002118:Calcium / Q000737:chemistry*; D004777:Environment; D007700:Kinetics; D008855:Microscopy, Electron, Scanning; D010084:Oxidation-Reduction; D012995:Solubility; D012996:Solutions; D017974:Uranium Compounds / Q000737:chemistry*; D014865:Waste Disposal, Fluid; D014867:Water / Q000737:chemistry; D056928:X-Ray Absorption Spectroscopy; D015032:Zinc / Q000737:chemistry*
D012996:Solutions; D017974:Uranium Compounds; D014867:Water; D015032:Zinc; C047385:uranium oxide; D002118:Calcium