Thin Water Films Enable Low-Temperature Magnesite Growth Under Conditions Relevant to Geologic Carbon Sequestration

Injecting supercritical CO2 (scCO(2)) into basalt formations for long-term storage is a promising strategy for mitigating CO2 emissions. Mineral carbonation can result in permanent entrapment of CO2; however, carbonation kinetics in thin H2O films in humidified scCO(2) is not well understood. We investigated forsterite (Mg2SiO4) carbonation to magnesite (MgCO3) via amorphous magnesium carbonate (AMC; MgCO3 center dot xH(2)O, 0.5 < x < 1), with the goal to establish the fundamental controls on magnesite growth rates at low H2O activity and temperature. Experiments were conducted at 25, 40, and 50 degrees C in 90 bar CO2 with a H2O film thickness on forsterite that averaged 1.78 +/- 0.05 monolayers. In situ infrared spectroscopy was used to monitor forsterite dissolution and the growth of AMC, magnesite, and amorphous SiO2 as a function of time. Geochemical kinetic modeling showed that magnesite was supersaturated by 2 to 3 orders of magnitude and grew according to a zero-order rate law. The results indicate that the main drivers for magnesite growth are sustained high supersaturation coupled with low H2O activity, a combination of thermodynamic conditions not attainable in bulk aqueous solution. This improved understanding of reaction kinetics can inform subsurface reactive transport models for better predictions of CO2 fate and transport.

Automated summary

The study investigated the carbonation of forsterite (Mg2SiO4) to magnesite (MgCO3) in thin H2O films in humidified scCO2, aiming to understand the fundamental controls on magnesite growth rates at low H2O activity and temperature. Experimental results showed that magnesite growth was driven by sustained high supersaturation and low H2O activity, conditions not typically found in bulk aqueous solutions. This improved understanding of reaction kinetics can enhance subsurface reactive transport models for better predictions of CO2 fate and transport.