Molecular dynamics simulations of uranyl adsorption and structure on the basal surface of muscovite

Abstract

Anthropogenic activities have led to an increased concentration of uranium on the Earth's surface and potentially in the subsurface with the development of nuclear waste repositories. Uranium is soluble in groundwater, and its mobility is strongly affected by the presence of clay minerals in soils and in subsurface sediments. We use molecular dynamics simulations to probe the adsorption of aqueous uranyl () ions onto the basal surface of muscovite, a suitable proxy for typically ultrafine-grained clay phases. Model systems include the competitive adsorption between potassium counterions and aqueous ions (0.1 M and 1.0 M UO₂Cl₂, 0.1 M NaCl). We find that for systems with the presence of potassium and uranyl ions, potassium ions dominate the adsorption phenomenon. Potassium ions adsorb entirely as inner sphere complexes associated with the ditrigonal cavity of the basal surface. Uranyl ions adsorb in two configurations when it is the only ion species present, and in a single configuration in the presence of potassium. The majority of adsorbed uranyl ions are tilted < 45° relative to the muscovite surface, and are associated with the Si₄Al₂ rings near the aluminium substitution sites.

Keywords::

• mica
• uranium
• surface charge
• electrolyte

Acknowledgements

We thank Franz Geiger and Sarah (Saslow) Gomez for helpful discussions and comments, and we appreciate the opportunity to examine their recent experimental data for uranyl–muscovite systems. This study was funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Geosciences Research Program. Sandia National Laboratories is a multi-programme laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.