UC Irvine

Carbonation of natural earth-abundant and synthetic metal silicates promises scalable solutions to permanently store CO2. With enigmatic observations of enhanced reactivity in wet CO2-rich fluids, understanding the kinetics proves critical in designing secure and economical geological carbon sequestration and concrete technologies. Here, we use atomistic simulations, density functional theory, and free energy calculation techniques to probe the nature of physicochemical processes at the rock-water-CO2 interface. We choose nanoporous calcium-silicate-hydrate (C-S-H) and forsterite (Mg2SiO4) as model metal silicate surfaces that are of significance in the cement chemistry and geochemistry communities, respectively. We show that while a nanometer-thick interfacial water film persists at undersaturated conditions consistent with in situ infrared spectroscopy, the phase behavior of water-CO2 mixture changes from its bulk counterpart depending on the surface chemistry and nanoconfinement. We also observe enhanced solubility at the interface of water and CO2 phases, that could amplify CO2 speciation rate. Through free energy calculations, we show that CO2 could be found in a metastable state near the C-S-H surface, which can potentially react with surface water and hydroxyl groups to form carbonic acid and bicarbonate. These findings support the explicit consideration of nanoconfinement effects in reactive and non-reactive pore-scale processes. To investigate reactions at the solid-liquid interface, we develop Mg/C/O/H ReaxFF parameter sets for two environments: an aqueous force field for magnesium ions in solution and an interfacial force field for minerals and mineral–water interfaces. Then, we leverage reactive and non-reactive molecular simulations to probe the elementary reaction steps involved in the interaction of bicarbonate with metal silicate surfaces. We observe that a reverse proton transport between the bicarbonate and surface hydroxides drives carbonate production and surface metal carbonate complexation in agreement with in situ spectroscopy measurements. The resultant carbonate can also contribute to the ligand-enhanced dissolution that appears to be slightly favorable over carbonate-unassisted dissolution. We also discuss the potential implications of metal carbonate complex formation and dissolution on lowering the growth’s configurational entropy penalty and the rise of interfacial carbon mineralization pathways.