Surface-Catalyzed Oxygen Exchange during Mineral Carbonation in Nanoscale Water Films

Properties of nanoconfined adsorbed H2O on mineral surfaces are distinct from those of bulk H2O, and this can lead to significant differences in reactivity. Here, we investigate how O-exchange between H2O and CO2 depends on the thickness of H2O films on the mineral, forsterite (Mg2SiO4), which at sufficient adsorbed H2O is highly reactive toward carbonation. Rates of O-exchange measured using O-isotopic tracers and infrared spectroscopy increase with adsorbed H2O concentration and are two orders of magnitude faster than those for inert substrates such as fumed silica (SiO2). Quantum chemical calculations demonstrate that O-exchange can be catalyzed through interactions with active Mg2+ sites that lower the barrier for carbonic acid formation. These active metal centers exist as Mg–bicarbonate surface complexes or dissolved Mg2+ with predominantly bicarbonate counterions, as evidenced by infrared and nuclear magnetic resonance spectroscopies. Intermolecular proton hopping to bicarbonate can form a carbonic acid complex that readily decomposes to CO2 and H2O, leading to O-isotope scrambling. Unlike fumed silica, we find no evidence that adsorbed H2O film structure dictates O-exchange rates. In contrast, it is mainly Mg–bicarbonate surface complexes and Mg2+ fully dissolved within the H2O films that catalyze O-isotope scrambling.