Surface transformation thermodynamics of alkaline earth carbonates using first-principles calculations

Abstract

Alkaline Earth (AE) carbonates are ubiquitous minerals with important roles in both industry and the environment. For example, CaCO₃ helps regulate biogeochemical cycles and the amount of CO₂ present in the atmosphere through cycles of dissolution and re-precipitation. Knowing the atomistic response of carbonates to changes in their chemical environment, such as by surface termination, composition, or pH, will allow for better controls of both naturally occurring processes and industrial applications. Here first-principles methods were used to investigate the initial stages of carbonate dissolution by comparing surface exchanges and ion release processes from the aragonite mineral phase. The focus was on computing the thermodynamics of CaCO₃, SrCO₃, and BaCO₃ surface transformations and to link key features in surface structure to changes in the thermodynamics of ion release. The overall cycle of surface cation exchange, subsurface atom release, and subsequent surface healing was predicted to occur for a wide range of pH values. Surface bicarbonate reorganization, measured as a change in vertical height, dictated in part the thermodynamics of subsurface release, and it was shown that subsurface anion release was the most energetically costly part of the thermodynamic analyses. Moreover, these cycles resulted in thermodynamically stable defect structures that i) may be a better starting point for creating more accurate models of CO₂ release and sequestration in aqueous environments, ii) could be used to guide future spectroscopic analyses, and iii) lead to improvements in technologies as diverse as nanomedicine, sensing, and water treatment.