New Insights into Calcite Dissolution Mechanisms under Water, Proton, or Carbonic Acid-Dominated Conditions

Carbonate minerals are ubiquitous in nature, and their dissolution impacts many environmentally relevant processes including preferential flow during geological carbon sequestration, pH buffering with climate-change induced ocean acidification, and organic carbon bioavailability in melting permafrost. In this study, we advance the atomic level understanding of calcite dissolution mechanisms to improve our ability to predict this complex process. We performed high pressure and temperature (1300 psi and 50 °C) batch experiments to measure transient dissolution of freshly cleaved calcite under H₂O, H⁺, and H₂CO₃-dominated conditions, without and with an inhibitory anionic surfactant present. Before and after dissolution experiments, we measured dissolution etch-pit geometries using laser profilometry, and we used density functional theory to investigate relative adsorption energies of competing species that affect dissolution. Our results support the hypothesis that calcite dissolution is controlled by the ability of H₂O to preferentially adsorb to surface Ca atoms over competing species, even when dissolution is dominated by H⁺ or H₂CO₃. More importantly, we identify for the first time that adsorbed H⁺ enhances the role of water by weakening surface Ca-O bonds. We also identify that H₂CO₃ undergoes dissociative adsorption resulting in adsorbed HCO₃^- and H⁺. Adsorbed HCO₃^- that competes with H₂O for Ca acute edge sites inhibits dissolution, while adsorbed H⁺ at the neighboring surface of CO₃ enhances dissolution. The net effect of the dissociative adsorption of H₂CO₃ is enhanced dissolution. These results will impact future efforts to more accurately model the impact of solutes in complex water matrices on carbonate mineral dissolution.