A proposed solution for the long-term storage of carbon dioxide (CO2) is the reaction of basaltic minerals, abundant in divalent-metal silicates, with CO2 to form carbonate minerals. During injection of anhydrous CO2 at depth, the gas turns into a supercritical fluid (scCO2). Water in the host rocks humidifies the scCO2, and the fluid likely remains undersaturated with respect to water in the near term. Thus, silicate mineral carbonation takes place on mineral surfaces in contact with wet scCO2, within Å-to-nm thick films of adsorbed water. Mineral carbonation under these conditions has been reported only by a limited number of studies that indicate formation of tens-of-nm thick silica-rich coatings that slow down carbonation significantly. The molecular mechanism of how these coatings form and the role water (as relative humidity, RH) plays during the process remain unexplored. Here, we measure reaction rates during the carbonation of the basalt mineral forsterite (Mg2SiO4), in humid scCO2 (50 °C, 90 bar) and characterize the mineral surface during the reaction. Using in situ high-pressure infrared (IR) spectroscopy, current results demonstrate that forsterite dissolution slows significantly after approximately 24 hours of reaction. 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS) show that this slow-down in carbonation is associated with a silica-rich layer that is only a few nm thick, as well as secondary precipitates of amorphous silica. Future work by our group aims to further discern the impact of water layer thickness and silicate composition on the formation of this carbonation-passivating layer. Results from this and related studies connect previous literature on mineral carbonation reactions in bulk water to the thin-water-film conditions likely encountered near CO2 injection sites in the near-term. Further, precise models of basalt reactivity may improve the efficiency of currently planned CO2 mineralization efforts.

Basalt mineral carbonation: Formation of silica-rich surface layers when water is limited

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