PNNL

Abstract

The reactivity of Ångstroms to nanometers thick water films that readily form on minerals in humidified environments is not well understood because their properties diverge from those of bulk water. In this study, we investigated carbonation of forsterite (Mg2SiO4) in humidified supercritical CO2 (50 °C, 90 bar), which serves as a model system for understanding subsurface reactivity during carbon storage operations in basalt terrains. Specifically, we correlated the transition from carbonate surface complexation to carbonate precipitation to the minimum H2O coverage required for the onset of forsterite dissolution and ions to diffuse. In situ attenuated total reflection and transmission infrared spectroscopies showed that forsterite begins to dissolve and magnesite (MgCO3) precipitation occurs at greater than 1.5 monolayer (ML) of H2O. In situ electrical impedance spectroscopy demonstrated that ion diffusion accelerates beyond 1.3 ML of H2O, while molecular dynamics simulations indicated that this onset of diffusion is due to an abrupt decrease in the free energy barriers for lateral mobility of outer-spherically adsorbed Mg2+. Our cumulative results suggest that CO2 conversion to carbonates is dissolution and mass transport limited below about 1.5 ML of adsorbed H2O.