Microstructural Response of Variably Hydrated Ca-rich Montmorillonite to Supercritical CO2.

First-principles molecular dynamics simulations were carried out to explore the mechanistic and thermodynamic ramifications of the exposure of variably hydrated Ca-rich montmorillonites to supercritical CO2 and CO2-SO2 mixtures under geologic storage conditions. In sub- to single-hydrated systems (⩽1W), CO2 intercalation causes interlamellar expansion of 8-12%, while systems transitioning to 2W may undergo contraction ( ∼7%) or remain almost unchanged. When compared to ∼2W hydration state, structural analysis of the ⩽1W systems, reveals more Ca-CO2 contacts and partial transition to vertically confined CO2 molecules. Infrared spectra and projected vibrational frequency analysis imply that intercalated Ca-bound CO2 are vibrationally constrained and contribute to the higher frequencies of the asymmetric stretch band. Reduced diffusion coefficients of intercalated H2O and CO2 (10-6-10-7 cm2/s) indicate that Ca-montmorillonites in ∼1W hydration states can be more efficient in capturing CO2. Simulations including SO2 imply that ∼0.66 mmol SO2/g clay can be intercalated without other significant structural changes. SO2 is likely to divert H2O away from the cations, promoting Ca-CO2 interactions and CO2 capture by further reducing CO2 diffusion (10-8 cm2/s). Vibrational bands at ∼1267 or 1155 cm-1 may be used to identify the chemical state (oxidation states +4 or +6, respectively) and the fate of sulfur contaminants. [ABSTRACT FROM AUTHOR]