Physical and chemical effects of H2O on mineral carbonation reactions in supercritical CO2

Mineral carbonation through reaction with supercritical CO2 (scCO2) is the ultimate pathway to permanent carbon storage for geological sequestration. Whether and how much H2O is required for the scCO2-mineral interaction to proceed readily, and the role of H2O in the reaction have been actively researched topics. We designed and built a novel in situ Raman reactor to study the carbonation product during brucite [Mg(OH)2]-scCO2 interaction, and investigated the role of free H2O by comparing the products in neat, H2O, and formamide (FM) conditions. We introduced FM to provide a physical polarity environment similar to that of H2O but without the chemical protonation effect. We further evaluated the role of structural H2O by conducting experiments with periclase (MgO). The in situ Raman analysis revealed the occurrence of brucite carbonation both in the presence of H2O and FM, demonstrating the effect of polarity in the reaction; in contrast, carbonation of periclase only occurred in H2O-saturated scCO2. The lack of carbonation of periclase with the presence of FM was attributed to the difficulty of magnesite mineralization, as CaCO3 was able to form in the CaO-FM-scCO2 system. Post experimental X-ray diffractometry and Fourier-transform infrared spectrometry of the products showed that brucite carbonation yielded nesquehonite and hydromagnesite in H2O and FM, respectively; whereas periclasescCO2 interaction in H2O produced a mixture of the two products. Rate calculations suggested that the carbonation reaction was much faster in H2O; nonetheless, 40-50% carbonation was still achieved in the Mg (OH)2-FM-scCO2 system after 330 h. Overall, our results showed that the brucite-scCO2 reaction could proceed without H2O under certain conditions - the polarity effect of H2O/FM was large enough to break the Mg-OH and MgO-H bonds and promote the carbonation process.