Collective molecular-scale carbonation path in aqueous solutions with sufficient structural sampling: From CO2 to CaCO3

The carbonation reaction is essential in the global carbon cycle and in the carbon dioxide (CO2) capture. In oceans (pH 8.1) or in synthetic materials such as cement or geopolymers (pH over 12), the basic pH conditions affect the reaction rate of carbonation. However, the precipitation of calcium or magnesium carbonates acidifies the environment and, therefore, limits further CO2 capture. Here, we investigate how pH influences carbonation pathways in neutral and basic solutions at the atomic scale using reactive molecular simulations coupled with enhanced sampling methods from CO2 to calcium carbonate (CaCO3). Two distinct CO2 conversion pathways are identified: (1) CO2 hydration: CO2+H2O⇌H2CO3⇌HCO3-+H+⇌CO32-+2H+ and (2) CO2 hydroxylation: CO2+OH-⇌HCO3-⇌CO32-+H+. The CO2 hydration pathway occurs in both neutral and basic aqueous solutions, but reactions differ significantly between the two pH conditions. The formation of the CO32- is characterized by a markedly high free energy barrier in the neutral solution. The CO2 hydroxylation pathway is only found in basic solutions. Notably, the CO2 molecule exhibits a pronounced energetic preference for reacting with hydroxide ions (OH-) rather than with water molecules, resulting in significantly reduced free energy barriers along the CO2 hydroxylation pathway. The reaction rate estimation suggests that the CO2 hydroxylation path is the most favorable carbonation pathway in the basic solution. Once the CO32- anion is formed in the presence of alkali-earth (e.g., Ca2+ and Mg2+) cations, carbonate formation can proceed.