Molecular-scale mechanisms of carbonate mineralization in nanoscale water films in geothermal reservoir

The urgent need to mitigate anthropogenic CO₂ emissions necessitates the advancement of large-scale, permanent carbon sequestration technologies. Geothermal systems, with their unique thermo-hydro-chemical conditions, are increasingly recognized as promising environments for CO₂ mineralization, with porous carbonate reservoirs serving as ideal storage formations. At the mineral-fluid interface, nanoscale water films act as a critical reactive microenvironment. These films exhibit pronounced size and confinement effects regarding thickness, ionic composition, and pH, which fundamentally dictate mineral dissolution, nucleation, and phase transformation. However, the molecular mechanisms governing cation mobilization and the associated rate-limiting steps within these nanoconfined films remain poorly understood. In this study, we developed a computational framework integrating Density Functional Theory (DFT) and Ab Initio Molecular Dynamics (AIMD), coupled with enhanced sampling techniques to capture rare events, such as proton transfer and ligand exchange, at the electronic level. Using calcite and dolomite as representative carbonate phases, we constructed slab models for the (104) and (110) surfaces. By systematically varying the water film thickness, we simulated the transition from molecular monolayers to continuous thin films. We investigated the heterogeneous reaction mechanisms of CO₂ and H₂O on these surfaces, elucidating the cation de-coordination pathways, rate-limiting steps, and the characteristics of transient intermediates. Our quantitative evaluation reveals that nanoconfinement introduces a unique free-energy landscape for mineral dissolution. Specifically, the highly structured water layers in ultra-thin films significantly modulate the solvation shells of Ca²⁺/Mg²⁺ ions, leading to a thickness-dependent shift in activation barriers. Furthermore, the simulations demonstrate that elevated geothermal temperatures and increased ionic strength synergistically facilitate cation mobilization by lowering the activation enthalpy. We also identified that specific ligand adsorption promotes the formation of inner-sphere complexes, which destabilize surface lattice sites and accelerate dissolution. Notably, the (110) surface exhibits higher kinetic reactivity than the (104) plane due to its lower coordination environment and higher density of reactive sites. These findings provide a robust mechanistic bridge between molecular-scale interfacial processes and macro-scale mineralization kinetics, offering critical theoretical insights for optimizing carbon sequestration efficiency in geothermal reservoirs.