Free energy of adsorption of water and metal ions on the [1014] calcite surface.

We calculated the free energy profiles of water and three metal ions (magnesium, calcium, and strontium) adsorbing on the [1014] calcite surface in aqueous solution. The approach uses molecular dynamics with parametrized equations to describe the interatomic forces. The potential model is able to reproduce the interactions between water and the metal ions regardless of whether they are at the mineral surface or in bulk water. The simulations predict that the free energy of adsorption of water is relatively small compared to the enthalpy of adsorption calculated in previous papers. This suggests a large change in entropy associated with the water adsorption on the surface. We also demonstrate that the free energy profile of a metal ion adsorbing on the surface correlates with the solvent density and that the rate of formation of an innersphere complex depends on overcoming a large free energy barrier, which is mainly electrostatic in nature. Furthermore, comparison among the rates of desorption of magnesium, calcium, and strontium from the calcite surface suggests that magnesium has a much lower rate of desorption due to its strong interactions with both water and the surface.