Due to its widespread occurrence in natural, engineered, and industrial settings, calcite is amongst the most thoroughly investigated minerals in the scientific literature. It has also become a mainstay focus of studies aimed at understanding mineral nucleation, growth, and dissolution pathways. Like many reactions at the mineral-water interface, the kinetics of ion attachment and detachment are often correlated with the rates of water exchange on ions in bulk solution; however, the characteristics of the corresponding reactions occurring at the mineral-water interface remain elusive. In this study, we have performed umbrella sampling and metadynamics simulations of calcium ion detachment from obtuse kink sites on the calcite (104) surface. The energy landscapes produced by both methods indicate that the rate limiting step corresponds to the dissociation of the first of three Ca – O bonds that anchor the calcium atom to the kink site. To investigate the mechanism of ion detachment and determine how it differs from ordinary water exchange, we have performed forward flux sampling simulations of water exchange (in solution and at the kink site) and of the initial Ca – O bond dissociation step at the kink site. A preliminary analysis of the reactive trajectories indicates that whereas water exchange on calcium most often occurs as a consequence of the ion being overcoordinated by water, the initial ion detachment step also involves overcoordination by ligands on the mineral surface.

Kinetics of water exchange and calcium detachment at the calcite-water interface

ACS Spring 2022

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