Uranium in geologic fluids: Estimates of standard partial molal properties, oxidation potentials, and hydrolysis constants at high temperatures and pressures

• Published in: Geochimica et cosmochimica acta Vol. 61; no. 20; pp. 4245 - 4266
• Main Authors: Shock, Everett L., Sassani, David C., Betz, Heidi
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.1997
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(97)00240-8

Theoretical methods are used with the available experimental data to provide estimates of parameters for the revised-HKF equations of state for aqueous uranium species. These parameters are used with standard state thermodynamic data at 25°C and 1 bar to calculate equilibrium constants for redox reactions among the four most common oxidation states of uranium (U(III), U(IV), U(V), and U(VI)), and their hydrolysis reactions at temperatures to 1000°C and pressures to 5 kb. A total of nineteen aqueous uranium species are included. The predicted equilibrium constants are used to construct oxidation potential-pH diagrams at elevated temperatures and pressures and to calculate the solubilities of uraninite as functions of temperature and pH, which are compared to experimental data. Oxidation potential-pH diagrams illustrate the relative stabilities of aqueous uranium species and indicate that U (IV) and U(VI) species predominate in aqueous solution in the U-O-H system. Increasing temperature stabilizes U (VI) and U (III) species relative to U (IV) species, but U (IV) species dominate at oxidation states consistent with mineral-buffer assemblages and near-neutral pH. At low pH, U(VI) is stabilized relative to U (IV) suggesting that uranium transport in hydrothermal systems requires either acidic solutions or potent complexes of U(IV).