Decreasing Hygroscopicity Slows Forsterite Carbonation under Low-Water Conditions

A fundamental understanding of processes that slow divalent metal silicate carbonation is important for developing effective strategies to durably store carbon dioxide and mitigate atmospheric CO₂. This study presents a detailed investigation of a passivation effect unique to low-water conditions during the carbonation of forsterite (Mg₂SiO₄) and highlights the importance of hygroscopicity in influencing metal silicate carbonation. Integrated in situ and ex situ experimental results showed that the decrease in the carbonation rate of forsterite observed after ∼10 h in humid supercritical CO₂ (50 °C, 90 bar) correlates with a reduction in water film thickness, and in particular, weakly hydrogen bonded adsorbed water that facilitates ion transport. We attribute the decrease in thickness to a drop in the concentrations of hygroscopic Mg²⁺, MgHCO₃⁺, and HCO₃^- ions within the film as the predominate forsterite carbonation product evolves from amorphous magnesium carbonate (AMC) to magnesite (MgCO₃). When more soluble AMC is present, hygroscopic ion concentrations are higher, drawing more water from the supercritical phase to the forsterite surface. Carbonation rates are faster because thicker water films can better mobilize ions to growing carbonates. In contrast, when less soluble magnesite predominates, hygroscopic ion concentrations are lower, water films are thinner, and carbonate rates are slower.