Atomistic Simulations
of Calcium Uranyl(VI) Carbonate
Adsorption on Calcite and Stepped-Calcite Surfaces
- Publication date
- Publisher
Abstract
Adsorption of actinyl ions onto mineral surfaces is one
of the
main mechanisms that control the migration of these ions in environmental
systems. Here, we present computational classical molecular dynamics
(MD) simulations to investigate the behavior of U(VI) in contact with
different calcite surfaces. The calcium-uranyl-carbonate [Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>] species is shown to display
both inner- and outer-sphere adsorption to the flat {101̅4}
and the stepped {314̅8} and {31̅2̅16} planes of
calcite. Free energy calculations, using the umbrella sampling method,
are employed to simulate adsorption paths of the same uranyl species
on the different calcite surfaces under aqueous condition. Outer-sphere
adsorption is found to dominate over inner-sphere adsorption because
of the high free energy barrier of removing a uranyl–carbonate
interaction and replacing it with a new uranyl–surface interaction.
An important binding mode is proposed involving a single vicinal water
monolayer between the surface and the sorbed complex. From the free
energy profiles of the different calcite surfaces, the uranyl complex
was also found to adsorb preferentially on the acute-stepped {314̅8}
face of calcite, in agreement with experiment
