In this paper, we present a theoretical study of the free-energy changes of water-layer formation at a mineral in contact with water-saturated supercritical CO2 (scCO2) by density functional-based molecular dynamics simulations (AIMD). Feldspar minerals, as typified by the Ca-end member Anorthite, serve as prototypical model systems to assess reactivity and ion mobility in solid/water-bearing supercritical fluid (WBSF) interfaces. Recent X-ray measurements provide detailed information on water-film formation and cation vacancies at these surfaces [1]. Molecular dynamics simulations based on density functional theory allow for the evaluation of reactivity and condensed phase dynamics on equal footing. We report on the structure, dynamics and reaction energetics of water nucleation and surface aggregation, carbonation and Ca mobilization under geologic carbon sequestration scenarios (T=323 K and P=90 bar). Our results indicate a strong enthalpic affinity of water towards the O-terminated anorthite (001) surface. However, entropy partially hinders film formation. Carbonation reactions readily occur at electron-rich terminal Oxygen sites adjacent to cation vacancies when in contact with supercritical CO2. Cation vacancies of this type easily form in the presence of a water layer that enthalpically favors Ca2+ solvation, mobilization and extraction. Ramifications of these results are discussed with respect to facile carbonation reactions in feldspar-rich reservoirs targeted for CO2 storage and sequestration. References 1. Fenter, P.; Sturchio, N. C. Progress in Surface Science 2004, 77, 171.

Structure dynamics and reactivity of water-saturated supercritical CO2 and anorthite interface from ab initio molecular dynamics

249th National Meeting (2015)

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