Emerging investigator series: ion diffusivities in nanoconfined interfacial water films contribute to mineral carbonation thresholds

The dynamics and reactivity of nanoconfined fluids play critical roles across a wide range of environmental and technological systems, though reaction mechanisms and kinetics are not well understood. The carbonation kinetics of forsterite (Mg2SiO4) exposed to 90 atm supercritical carbon dioxide at 35-65 degrees C and 85-100% relative humidity (RH) was monitored with in situ X-ray diffraction, and partner molecular dynamics simulations were used to describe the free energy landscape of Mg2+ adsorption and diffusion on forsterite surfaces covered in water films 3-10 monolayers thick. The collective findings reveal how decreasing the water film thickness by similar to 1.4 monolayers, from similar to 0.92 to similar to 0.64 nm, inhibited reaction rates by up to 97%, promoted anhydrous Mg-carbonate (magnesite, MgCO3) precipitation, and more than doubled the apparent activation energy of carbonation. The transport simulations suggest that four monolayers are required to enable sufficiently facile Mg2+ diffusion, helping explain previously observed water film thickness-dependent reactivity thresholds. Environmental significance Thin adsorbed water films are ubiquitous examples of nanoconfined environments, whose reactivity have broad implications for natural and engineered processes. This study uses coupled experiments and molecular modelling to determine Mg-silicate carbonation kinetics and energetics of Mg2+ diffusion as a function of water film thickness. 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.