Nanoconfined geometries, including pores, fractures, and grain boundaries, are ubiquitous in environmental and technologic environments and consequently exert a strong influence over the fluid-rock interactions they host. Understanding reactions in these types of environments is important because nanopores may comprise the bulk of the reactive surface area in porous geologic media. Forsterite carbonation kinetics in ~1 nm interfacial water films at 35-90 °C were monitored with high-pressure (90 atm) in situ X-ray diffraction. We report how Ångstrom-scale thickness changes in quasi-two-dimensional interfacial water films influence the rates, pathways, and apparent activation energies of Mg-carbonate precipitation. Specifically, the apparent activation energy of magnesite precipitation (50-90 °C) was determined to be anomalously-low, at only ~35 kJ/mol. This experimentally-derived result suggests that nanoconfined Mg2+ adopts a hydration configuration that mimics that of aqueous Ca2+, in energetics, if not necessarily in structure. Additionally, molecular dynamics simulations of Mg2+ diffusion were conducted for 3-10 monolayer (ML)-thick water films on the (010) forsterite surface. These results will also be placed in a quantitative kinetic context of 15 additional olivine carbonation studies, revealing the temperature-dependence of carbonation reaction rates and mechanisms. The evolution of lattice parameters during the growth and assembly of nanocrystalline magnesite will also be discussed. Potential of mean force calculations revealed a significant decrease in the free energy barrier for the transition to a mobile outer-sphere complex that indicates at least 4 MLs are needed to allow for the diffusion of Mg2+ across the forsterite surface. In agreement with experimental-determinations, MD simulations therefore predict the presence of a reactivity threshold between 3 and 4 ML required for the continuous and rapid formation of Mg-carbonates. Although the general phenomenon of water film thickness-dependent reactivity thresholds in wet supercritical CO2 has been known for over a decade for steel corrosion and rock carbonation, the mechanistic origin of these reactivity thresholds were previously unknown. Overall, our results provide a better understanding of rates, mechanisms, and solute transport thresholds controlling physicochemical mineral transformations at complex interfaces in rocks, soil, and other porous media.<br/>

Origin of mineral carbonation kinetics trends and reactivity thresholds in nanoscale water films

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

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.