The Use Of Models To Understand The Aqueous Chemistry Of Uranium

Uranium chemistry has been extensively studied in industrial (typically for the reprocessing of nuclear fuels) or environmental (typically for waste or pollution management) waters. Most of the aqueous reactions are sufficiently fast that modelling and predictions, derived from thermodynamics based on Gibbs energies of reaction (Delta(r)G) or equivalently from equilibrium constants of complexation and hydrolysis, solubility products and standard redox potentials, should be reliable. Measured Delta(r)G values are usually converted into Gibbs energies of formation (Delta(f)G) in thermochemical data bases, using auxiliary data. We discuss briefly how and where to obtain and use these (Delta(r)G and Delta(f)G) values, and the corresponding thermodynamic basis as developed for solution chemistry - including solid solutions. We show that correlations between these numerical values or with physical parameters give a comprehensive view of chemical reactivity, which can be used for better qualitative understanding and tentative predictions. The geometries of the chemical species are useful for this purpose, but it is difficult to obtain experimental geometries in solution. We emphasize that these geometries can now be obtained by molecular modelling, even though molecular modelling in aqueous solutions is more difficult than in the gas or solid phases. Several recent studies in which the hydration of actinide ions has been modelled by quantum chemistry, and by classical and quantum molecular modelling, are described. These studies enable us to explain trends in physical and chemical properties across the actinide series, including the anomalous properties of Pa: other classical analogies and rules of thumb help us to understand and tentatively predict the chemical behaviour of uranium and its chemical analogues.