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

Everett Shock, David C. Sassani, Heidi Betz

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Abstract

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).

Original languageEnglish (US)
Pages (from-to)4245-4266
Number of pages22
JournalGeochimica et Cosmochimica Acta
Volume61
Issue number20
StatePublished - Oct 1997
Externally publishedYes

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Uranium
hydrolysis
Hydrolysis
uranium
oxidation
Oxidation
Fluids
fluid
Equilibrium constants
Temperature
Redox reactions
temperature
diagram
Equations of state
Minerals
uraninite
Buffers
Solubility
hydrothermal system
Thermodynamics

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

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title = "Uranium in geologic fluids: Estimates of standard partial molal properties, oxidation potentials, and hydrolysis constants at high temperatures and pressures",
abstract = "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).",
author = "Everett Shock and Sassani, {David C.} and Heidi Betz",
year = "1997",
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T2 - Estimates of standard partial molal properties, oxidation potentials, and hydrolysis constants at high temperatures and pressures

AU - Shock, Everett

AU - Sassani, David C.

AU - Betz, Heidi

PY - 1997/10

Y1 - 1997/10

N2 - 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).

AB - 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).

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