Site-specific structure and energetics of uranyl adsorption at hydrated muscovite (001) surfaces probed by classical molecular dynamics simulations

A new series of molecular dynamics (MD) simulations of the Cs⁺ and UO₂²⁺ adsorption on the basal (001) surface of muscovite from relatively low concentration aqueous solutions demonstrates both ions are preferably adsorbed near the centers ditrigonal cavities on this surface. A more realistic random distribution of isomorphic Al/Si substitutions in the muscovite structure allowed us to identify three types of surface adsorption sites somewhat differing in their composition, structure, and local charge distribution. UO₂²⁺ ions are preferably adsorbed as outer-sphere surface complexes with one H₂O molecule in their hydration shell acting as a bridge to the surface. Adsorption free energies for UO₂²⁺ ions at each of the three different adsorption sites were obtained via potential of mean force (PMF) calculations. These adsorption free energy profiles as a function of distance along the direction normal to the surface indicate that UO₂²⁺ ions at two the adsorption sites exhibit only stable OS surface complexes with energy minimum around −26 kJ/mol. At the same time at the third adsorption site, that was not observed and studied in earlier simulations, an inner-sphere surface complex is identified with an energy minimum almost twice as deep as the outer-sphere complexes at the two other adsorption sites. Significant agreement of the present results with available experimental data and results of other MD simulations is also demonstrated, while the sources of observed disagreements and inconsistencies are identified and discussed in detail.