Basaltic reservoirs are targeted for geological carbon sequestration because they are abundant in divalent orthosilicates such as olivine [(Mg,Fe)2SiO4] that can permanently trap CO2 through carbonation to mitigate the effects of climate change. Under reservoir temperature and pressure conditions, CO2 is a supercritical fluid that contains dissolved H2O, which adsorbs to mineral surfaces as nanometer thin films. In this study, the role of Fe(II) in the thin H2O film carbonation of olivine was investigated using in situ high-pressure infrared spectroscopy and ab initio thermodynamic calculations. Experiments were performed for 72 hours at 50 °C and 90 bar in both supercritical CO2 and supercritical CO2 with 1% H2 at a relative humidity of 79% on synthetic forsterite, fayalite, and two Mg/Fe mixture olivines, (MgxFe1−x)2SiO4 (x = 0.33 and 0.66). Increasing concentrations of Fe(II) in the olivine decreases carbonation rates and extents, but the addition of 1% H2 increases the level of carbonation in Fe(II)-rich olivines. These trends are explained according to the dependencies of olivine acidic solubilities, carbonate precipitation equilibria, and dissolution and growth kinetics on Fe(II) content, and by considering that Fe(II)-rich olivines can undergo competing oxidation reactions by O2 and H2O to form Fe(III)-containing oxides. Given that CO2 injected into the subsurface will be relatively oxic, the oxidation of Fe(II)-rich olivines could impact their carbonation potential in geological basalt reservoirs.

Role of Fe(II) carbonation in olivine carbonation in wet supercritical CO2

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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