Thermodynamic properties of selected uranium compounds and aqueous species at 298.15 K and 1 bar and at higher temperatures; preliminary models for the origin of coffinite deposits

Abstract

Thermodynamic values for 110 uranium-bearing phases and 28 aqueous uranium solution species (298.15 K and 1 bar) are tabulated based upon evaluated experimental data (largely from calorimetric experiments) and estimated values. Molar volume data are given for most of the solid phases. Thermodynamic values for 16 uranium-bearing phases are presented for higher temperatures in the form of and as a supplement to U.S. Geological Survey Bulletin 1452 (Robie et al., 1979). The internal consistency of the thermodynamic values reported herein is dependent upon the reliability of the experimental results for several uranium phases that have been used as secondary calorimetric reference phases. The data for the reference phases and for those phases evaluated with respect to the secondary reference phases are discussed. A preliminary model for coffinite formation has been proposed together with an estimate of the free energy of formation of coffinite. Free energy values are estimated for several other uranium-bearing silicate phases that have been reported as secondary uranium phases associated with uranium ore deposits and that could be expected to develop wherever uranium is leached by groundwaters. I: Evaluation of experimental and estimated thermodynamic data, and properties at 298.15 K and 1 bar (10 5 Pascals). Preliminary models for the origin of coffinite deposits. Introduction Several reviews of the thermodynamic properties of uranium bearing phases and aqueous species have been published recently (e.g., Wagman, et al., 1981; Lemire and Tremaine, 1980; Langmuir, 1978; and Cordfunke and O'Hare, 1978). These reviews draw upon the same experimental data and, in general, the same interpretation of the experimental data for those phases evaluated in two or more of these studies. Although there has been moderate experimental activity on phases bearing uranium in the last twenty years, many of the naturally occurring phases have not been studied. Where data are lacking it has been necessary to estimate thermodynamic properties and where the experimental data is sparse or poorly defined, it has been necessary to make simplifying assumptions in order to calculate thermodynamic values. This report contains a review of some of the experimental data and interpretations for uranium phases and aqueous species that may be of importance in evaluating the origin of uranium ore bodies or the evolution of such ore bodies, in developing programs for the solution mining of low grade uranium deposits or mine dumps, in the study of radioactive waste and in the containment of such waste, and finally, that may be of importance in reactions within breeder reactors. The thermodynamic data listed in Tables 1 and 2 represent, to the extent possible, an internally consistent data set that has been evaluated and developed by the stepwise or sequential method. The majority of data are tied to the enthalpy of formation of 1)303 as given by Huber and Holley (1969) or to secondary standards such as -U03, U02Cl2» UC14, and U02. Consequently, few cross links are available within the system that would allow the use of the simultaneous regression approach of Haas and Fisher (1976) to identify erroneous data. Where a reaction involving a reference phase is found to be in error, all thermodynamic data based upon that reaction must be corrected. In order to facilitate that process, a brief description of the reaction scheme for each phase is provided in this study along with a reference to the experimental study that contains a detailed description of the thermochemical cycle. Ancillary thermodynamic values required in the thermochemical cycles cited in this study are from Robie et al. (1979) unless otherwise stated. The atomic weights and physical constants are also taken from Robie et al. Triuranium octaoxide is the primary reference compound in this network. Selection of this material was predicated by its use as the reference phase for 7-1103 and U02Cl2» the two phases to which most other Table 1. Thermodynamic properties of selected uranium bearing phases at 298.15 K and 1 bar. Estimated values are enclosed in parentheses Uncertainties are listed below values Name ^ formula and (references) J/(mol-K) Molar volume 3 cm Enthalpy of formation kJ/mol Gibbs free energy of formation kJ/mol Oxides, hydroxides and multiple oxides Uraninite: U0 2 (133;133;133) Triuranium heptaoxide (alpha): U 3 0 ? (158;74;) Triuranium heptaoxide (beta): U 3 0 ? (158;74;49) Tetrauranium ennanoxide: U 4 0 9 (120;116;49) Triuranium octaoxide: U 3 0 g (157;45;23) Hydrated uranium dioxide: UO '2H 0 (crystals) (;;166) UO -2H 0 (amorphous) (;;166) Uranium peroxide dihydrate: UO -2H 0 (;152;30) Studite: U0 4 '4H 2 0 (;152;30) Uranyl trioxide (gamma): U0 3 (156;133;23) Uranyl trioxide (beta): U0 3 (156;45;34) Uranyl trioxide (alpha): U0 3 (156;45;25) Uranyl trioxide (delta): UO( ; ; 25 ) 77.03 0.24 247.65 0.75