Experiments and geochemical modeling of CO ₂ sequestration during hydrothermal basalt alteration

The interaction of CO ₂-rich waters with basaltic glass was studied experimentally and using reaction path and kinetic modeling in order to improve our understanding on the integrated effects of temperature, acid supply and reaction progress on the fluid composition, secondary mineralogy and mass of CO ₂ mineralization during progressive water-rock interaction. Hydrothermal batch type experiments were carried out at 75, 150 and 250°C and ~80 to 270mmol/kg initial dissolved CO ₂ concentrations. At 75°C, the pH was buffered at ~4.5 with Ca, Mg, Fe and Si being incorporated into Ca-Mg-Fe carbonates and chalcedony. At 150 and 250°C, the pH increased from ~5.5 to >6 with Ca being incorporated into calcite and Mg, Fe, Al and Si into smectites and/or chlorite depending on temperature. Comparison between the experimental results and the reaction path models indicated that the basalt alteration and sequestration of CO ₂ depend on temperature. The mineralization of chalcedony at 75°C inhibited the formation of clays resulting in Ca, Mg and Fe being available for carbonate mineralization. At 150 and 250°C, Mg and Fe were predominantly incorporated into clays limiting the availability of Mg and Fe, resulting in calcite being the only carbonate forming. Comparison of the reaction path model with kinetic simulations yielded a similar modeled secondary mineralogy, and additionally showed that the transition of the formation of Ca-Mg-Fe carbonates at <150°C to calcite at <150°C was not solely related to temperature dependent mineral properties, but also related to the faster reaction kinetics observed at <150°C. This was especially reflected in the basaltic glass and smectite reaction rates. According to mass balance calculations using the reaction path simulations, the maximum amount of CO ₂ mineralized was reached at 75°C and pH of ~5.5, indicating that CO ₂ sequestration is favorable at <100°C. The experimental results and geochemical models were compared with data on natural waters and suggest that during progressive CO ₂-water-basalt interaction, the water composition and reaction stoichiometry were closely reproduced by mineral-solution equilibria at 75 to 250°C and were controlled by temperature, pH and reaction progress. In turn, the pH was determined by the input of CO ₂ and amount of basalt dissolution.