Thermodynamic properties of the uranyl carbonate minerals roubaultite, fontanite, widenmannite, grimselite, cejkaite and bayleyite

In this paper, the fundamental thermodynamic functions of six important uranyl carbonate minerals, roubaultite, fontanite, widenmannite, grimselite, cejkaite and bayleyite, are computed using first principles solid-state methods based on Periodic Density Functional Theory, from their energy-optimized crystal structures determined in previous works. These properties are obtained within a wide range of temperature (250-800 K) and are employed in order to derive the thermodynamic functions of formation of these minerals in terms of the elements. The resulting temperature-dependent functions of formation are merged with the thermodynamic functions of other prominent uranyl-containing minerals, also determined using theoretical methods, to determine a rich set of thermodynamic functions of reaction for a series of chemical reactions relating these mineral phases. The influence of the presence of hydrogen peroxide in many of these reactions is also investigated. These additional minerals include uranyl oxide hydrates, hydroxides, peroxides, silicates, sulfates and another uranyl carbonate mineral (rutherfordine) and, therefore, a detailed and wide-ranging view of the relative thermodynamic stability of uranyl minerals is afforded. Unexpectedly, the uranyl tricarbonate minerals, grimselite, cejkaite and bayleyite, are shown to be by far the most stable phases within the full range of temperature considered and under the presence and absence of hydrogen peroxide. Furthermore, the analysis of the solubility products of the considered uranyl carbonate minerals, obtained from the Gibbs free energies of the dissolution reactions, reveals that the widespread belief of the great solubility of these minerals is not supported. Except for roubaultite and widenmannite, all these minerals are sparingly soluble. As a consequence, the development of accurate temperature-dependent thermodynamic functions of an even larger number of uranyl carbonate minerals is mandatory for the simulation of the migration of uranium from nuclear waste repositories, uraninite deposits and uranium contaminated sites, as well as for the study of the paragenetic sequence of uranyl minerals arising from the oxidative dissolution processes occurring in uraninite ore deposits and corrosion of spent nuclear fuel.