Water Activity as a Mechanistic Control on CO₂ Mineralization in Basalt

While rapid CO₂ mineralization in basalt has been demonstrated at both laboratory and field scales, the existing studies predominantly treat water as a bulk reaction or a transport medium. Parameters such as injection volume, fluid composition, and water-rock ratio have been investigated, yet the physical state of water at the basalt surface, particularly the level of water activity required to initiate and sustain fast carbonation remains unquantified. Thus, there remains much room for assessment in the water activity conditions required to trigger the fastest and most CO₂ mineralization yield on basaltic rocks. 

This study systematically quantifies water activity as an independent control on CO₂ mineralization kinetics and uptake capacity in basaltic materials. By identifying threshold and optimal water activity regimes, the study aims to understand the mechanism of how the spatial distribution of water and effective surface area jointly influence basalt carbonation. 

Basalt samples will be mechanically powderized with a controlled particle size to vary surface area. Water conditions will be regulated by varying relative humidity and liquid water availability in an environmental chamber. The level of exposure to water ranges from humid air to CO₂-saturated solution infusion, allowing direct comparison between gas–solid carbonation pathways and water-mediated dissolution–precipitation mechanisms. The CO₂ exposure chamber can be used to regulate gas composition and environmental conditions. CO₂ uptake will be quantified in real-time using flux-based measurements. Post-reaction products are analyzed with TGA-MS to determine the reaction efficiency. 

Overall, CO₂ mineralization is expected to show a strong dependence on water activity, with minimal uptake under dry conditions and maximum under high-humidity, non-flooded regimes, consistent with recent observations. Increased effective surface area is expected to enhance both kinetics and total uptake, with water mediating gas–solid reactions most effectively. By constraining the role of surface water films and dissolution–precipitation dynamics in basalt carbonation, this study provides new mechanistic insights relevant to optimizing field-scale CO₂ mineralization strategies in basaltic formations.