PNNL

Abstract

Molecular dynamics methods were used to simulate UO2(OH)20 binding to pairs of oxo sites on three low-index planes of ?-SiO2 in contact with water. Differences in binding site distributions on the (001), (010) and (101) planes produced distinct sets of stable U inner-sphere species. Steric constraints prevented bidentate coordination to the (001) surface, resulting in a mononuclear monodentate complex, [UO2(OH)2(H2O)nOS] (n=1 64% and n=2 28% of the 5 ns production runs). Binuclear bidentate coordination, [UO2(OH)2(H2O)n(OS)2], was however favored on the (010) (n=0 93% and n=1 7%) and the (101) (n=0 76% and n=1 24%) planes. These results imply that oxo sites and OH groups of UO2(OH)20 can force the U equatorial shell to a tetrahedral configuration. Potential of mean force calculations also uncovered a diversity of metastable outer- and inner-sphere complexes at local energy minima up to ~0.5 nm from the surface. These calculations underscored important differences in both energetic requirements and mechanisms for the approach of UO2(OH)20 to different quartz surfaces. Binding strengths are affected by binding site distribution, steric freedom, U hydration and OH orientation, and increase in the order (101) (11-13 kJ/mol)