Molecular dynamics study of aqueous uranyl interactions with quartz (010)

Molecular dynamics simulations were used to study the structure and dynamics of the uranyl ion and its aquo, hydroxy, and carbonato complexes in bulk water and near the hydrated quartz (010) surface, All simulations were performed in the constant (NVT) ensemble with three-dimensional periodic boundary conditions, and a slab technique was used to model the quartz-water interface. The uranyl coordination shell exhibits pentagonal bipyramidal symmetry, with carbonate and hydroxide ions readily replacing water molecules in the first shell. Radial distribution functions of the hydroxy and carbonato complexes are characterized by a consistent splitting in the equatorial shell, caused by the close proximity of hydroxide and carbonate oxygen atoms. Average U-O distances are 2.31-2.35 Angstrom for hydroxide ions, 2.35-2.39 Angstrom for carbonate ions, and 2.49-2.55 Angstrom for water molecules. Two protonation states of the quartz surface were considered for adsorption simulations: singly protonated and partially deprotonated. Surface complexes formed only when the initial uranyl position was close to the surface; otherwise, a diffuse species was observed. Outer-sphere surface complexes formed at the singly protonated surface and are characterized by hydrogen bonding between a coordinating water molecule and the surface. Inner-sphere surface complexes formed at the partially deprotonated surface, with water and surface oxygen atoms equidistant to the uranium atom. In both types of surface complex, splitting of the equatorial shell of the uranyl ion was due to the presence of hydroxide or carbonate ions in the first coordination shell.