Within a particular mineral group (e.g., simple oxides, orthosilicates, carbonates), the bulk dissolution rate of a given mineral often correlates with the kinetics of water exchange on its constituent metal ions in solution. Some investigations now hypothesize that the reactivity of specific sites on a mineral surface may also scale with the local kinetics of water exchange. Indeed, molecular dynamics simulations have shown that the slowest rates of water exchange observed along step edges on calcite (104) are similar to observed rates of step advance during growth. Nonetheless, the mechanistic aspects of the water exchange and ion detachment reactions are likely to differ, as the former is an elementary process that results in no net change to the mineral surface, and the latter is a complex chain reaction that results in the transfer of an ion to solution. Simulation studies have only recently begun to directly investigate the mechanisms of ion attachment (i.e., creation of kink sites) at the calcite (104)-water interface. In this study, we report preliminary free energy landscapes for the detachment of calcium and carbonate ions from kink sites on the calcite (104) surface. The free energy maps are then used to identify key intermediate states which serve to constrain forward flux sampling rate calculations. Our initial findings suggest that in contrast to water exchange on Ca<sup>2+</sup>, which occurs via over-coordination of the ion by water on the obtuse edge, ion detachment requires simultaneous over-coordination of Ca<sup>2+</sup> by oxygen atoms sourced from carbonate ions in the mineral and from solvent molecules in solution.

Simulated detachment of ions from calcite (104): Insights from metadynamics and forward flux sampling

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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