The incorporation of radionuclides into mineral hosts may strongly influence the concentration and migration of radioactive contaminants in the subsurface. Because of this potential to sequester radioactive species such as uranyl (UO22+), neptunyl (NpO2+), and radium, I am interested in the thermodynamics and the resulting structures of radionuclide incorporated minerals, which may be directly applicable to the engineering of permeable reactive barriers to impede these elements or to the development of predictive models that calculate the mobility of metal contaminants in surface and near-surface environments.
In one study, I am using density functional theory to calculate the energetics, structure, and electronic configuration of uranyl and neptunyl incorporation into sulfate (gypsum, anhydrite, anglesite, celestine, barite) and carbonate minerals (calcite, aragonite, cerussite, strontianite, witherite). Incorporation reactions involve both solid, periodic mineral phases as well as hydrated ions in solution; therefore, we must use two different types of quantum mechanical models. With a new approach developed by current and former members of our research group, periodic and cluster computational methods are combined as separate chemical equations in which each species is calculated under the same parameters.
In another study, I am looking at the thermodynamic properties of mixing (entropy, enthalpy, and Gibbs free energy) calculated from molecular mechanics for the (Ra,Ba)SO4 solid solution. The natural affinity between radium and barite has remained a topic of interest since the early days of radiochemistry at the end of the nineteenth century, and more recently, the (Ra,Ba)SO4 solid solution has been observed as a by-product in multiple industrial operations, including uranium ore processes, oil production, and phosphoric acid manufacturing. Despite the connection between radium and barite, relatively little thermodynamic or structural data has been published for radium sulfate or the (Ra,Ba)SO4 solid solution. In this study, empirical forcefields are used to calculate the entropy, enthalpy, and free energy of mixing. Preliminary results indicate that the Ra-O interatomic potential is described by a Buckingham potential, though consequential conclusions from the thermodynamic evaluation of the (Ra,Ba)SO4 solid solution are forthcoming.