Silicate Carbonation in Supercritical CO2 Containing Dissolved H2O: An in situ High Pressure X-Ray Diffraction and Infrared Spectroscopy Study

Abstract

Technological advances have been significant in recent years for managing environmentally harmful emissions (mostly CO2) resulting from combustion of fossil fuels. Deep underground geologic formations are viewed as reasonable options for long-term storage of CO2, but mechanisms controlling the stability of rocks and minerals in contact with injected supercritical fluids containing water are relatively unknown. In this paper, we discuss mineral transformation reactions occurring between supercritical CO2 containing water and the silicate minerals forsterite (Mg2SiO4), wollastonite (CaSiO3), and enstatite (MgSiO3). We utilized newly developed in situ high pressure x-ray diffraction (HXRD) and in situ infrared (IR) spectroscopic capabilities to examine the mineral transformation reactions. Forsterite and enstatite were selected as they are important minerals present in igneous and mafic rocks. Wollastonite, classified as a pyroxenoid (similar to a pyroxene), was chosen as a suitably fast-reacting proxy for examining silicate carbonation processes. The experiments were conducted under modest pressures (90 to 160bar), temperatures between 35° to 70° C, and varying concentrations of water dissolved in the scCO2. Under these conditions, scCO2 contains up to 3,500ppm dissolved water. Forsterite carbonation products identified by in situ HXRD included nesquehonite and magnesite. Wollastonite produced calcite and no detectable crystalline hydrated carbonates. In contrast, enstatite was the least reactive, based on in situ HXRD data that contained no detectable crystalline carbonation. Based on in situ IR spectroscopic measurements, mineral surface hydration processes are critical for these reactions. Thicker water films were associated with forsterite and wollastonite compared to enstatite. Carbonation was evidenced by the appearance and growth of asymmetric C-O stretching bands of carbonate precipitates (1400 to 1550 cm-1), and carbonation extents were correlated to water-film thicknesses. Overall, these fundamental studies are beginning to illustrate processes controlling carbonation rates and potentials of silicate minerals.