Crystallographic controls on uranyl binding at the quartz/water interface

Abstract

Molecular dynamics methods were used to simulate UO₂(OH)₂⁰ binding to pairs of oxo sites (O_S) on three low-index planes of α-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)_nO_S] (90% for n = 1 and 10% for n = 2 over 5 ns production runs). Binuclear bidentate coordination, [UO₂(OH)₂(H₂O)_n(O_S)₂], was however favored on the (010) (99% for n = 0 and 1% for n = 1) and the (101) (72% for n = 0 and 28% for n = 1) planes. These results underscore a predominant four-coordinated equatorial shell for U when complexed to the quartz/water interface. Potential of mean force calculations uncovered a diversity of metastable outer- and inner-sphere complexes at local energy minima up to ∼0.4 nm from the surface. These calculations point to 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 (001) (3.7 kJ mol⁻¹) < (101) (5.6 kJ mol⁻¹) < (010) (6.5 kJ mol⁻¹). A general binding mechanism involves (1) formation of monodentate outer-sphere complexes, (2) removal of oxo-bound waters, (3) formation of one (monodentate), then two (bidentate) direct U–O_S bonds (inner-sphere), and (4) expulsion of excessive waters from the equatorial shell of U.