Dissolution mechanisms of gypsum, bassanite, and anhydrite: A molecular dynamics simulation approach

Calcium sulfate has one of three hydration states, CaSO₄∙ x H₂O where x equals 0 (anhydrite), 0.5 (bassanite), or 2 (gypsum). Despite numerous investigations of their dissolution in aqueous environments, relatively little is known about the mechanisms at the atomic scale. Here, we shed light on these mechanisms through molecular dynamics simulations of selected surfaces of all three hydrated forms. Umbrella Sampling is used to determine the Potential of Mean Force and to calculate dissolution energy barriers from atomically smooth surfaces with or without one neighboring vacancy and from anhydrite kink sites. The force profiles for Ca²⁺ and SO₄²⁻ reveal intermediate steps prior to complete solvation and indicates that the energy barriers are impacted by the mineral's hydrated state, the detaching ion, and any neighboring surface vacancy. Water adsorption on anhydrite and bassanite is influenced by the type of vacancy present, with the SO₄²⁻ vacancies promoting surface hydration.