Evidence for Carbonate Surface Complexation during Forsterite Carbonation in Wet Supercritical Carbon Dioxide

Continental flood basalts are attractive formations for geologic sequestration of carbon dioxide because of their reactive divalent-cation containing silicates, such as forsterite (Mg₂SiO₄), suitable for long-term trapping of CO₂ mineralized as metal carbonates. The goal of this study was to investigate at a molecular level the carbonation products formed during the reaction of forsterite with supercritical CO₂ (scCO₂) as a function of the concentration of H₂O adsorbed to the forsterite surface. Experiments were performed at 50 °C and 90 bar using an in situ IR titration capability, and postreaction samples were examined by ex situ techniques, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), focused ion beam transmission electron microscopy (FIB-TEM), thermal gravimetric analysis mass spectrometry (TGA-MS), and magic angle spinning nuclear magnetic resonance (MAS NMR). Carbonation products and reaction extents varied greatly with adsorbed H₂O. We show for the first time evidence of Mg-carbonate surface complexation under wet scCO₂ conditions. Carbonate is found to be coordinated to Mg at the forsterite surface in a predominately bidentate fashion at adsorbed H₂O concentrations below 27 μmol/m². Above this concentration and up to 76 μmol/m², monodentate coordinated complexes become dominant. Beyond a threshold adsorbed H₂O concentration of 76 μmol/m², crystalline carbonates continuously precipitate as magnesite, and the particles that form are hundreds of times larger than the estimated thicknesses of the adsorbed water films of about 7 to 15 Å. At an applied level, these results suggest that mineral carbonation in scCO₂ dominated fluids near the wellbore and adjacent to caprocks will be insignificant and limited to surface complexation, unless adsorbed H₂O concentrations are high enough to promote crystalline carbonate formation. At a fundamental level, the surface complexes and their dependence on adsorbed H₂O concentration give insights regarding forsterite dissolution processes and magnesite nucleation and growth