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 (Mg 2 SiO 4 ) exposed to 90 atm supercritical carbon dioxide at 35-65 °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 Mg 2+ 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 ∼1.4 monolayers, from ∼0.92 to ∼0.64 nm, inhibited reaction rates by up to 97%, promoted anhydrous Mg-carbonate (magnesite, MgCO 3 ) precipitation, and more than doubled the apparent activation energy of carbonation. The transport simulations suggest that four monolayers are required to enable sufficiently facile Mg 2+ diffusion, helping explain previously observed water film thickness-dependent reactivity thresholds. Mineral carbonation reactivity trends and thresholds in nanoconfined water films delineated with in situ X-ray diffraction and molecular simulations.