Direct Nanoscale Observations of CO2 Sequestration during Brucite [Mg(OH)2] Dissolution

The dissolution and carbonation of brucite on (001) cleavage surfaces was investigated in a series of in situ and ex situ atomic force microscopy (AFM) experiments at varying pH (2-12), temperature (23-40 degrees C), aqueous NaHCO3 concentration (10(-5)-1 M), and PCO2 (0-1 atm). Dissolution rates increased with decreasing pH and increasing NaHCO3 concentration. Simultaneously with dissolution of brucite, the growth of a Mg carbonate phase (probably dypingite) was directly observed. In NaHCO3 solutions (pH 7.2-9.3,), precipitation of Mg carbonates was limited. Enhanced precipitation was, however, observed in acidified NaHCO3 solutions (pH 5, DIC approximate to 25.5 mM) and in solutions that were equilibrated under a CO2 atmosphere (pH 4, DIC approximate to 25.2 mM). Nucleation predominantly occurred in areas of high dissolution such as deep step edges suggesting that the carbonation reaction is locally diffusion-transport controlled. More extensive particle growth was also observed after ex situ experiments lasting for several hours. This AFM study contributes to an improved understanding of the mechanism of aqueous brucite carbonation at low temperature and pressure conditions and has implications for carbonation reactions in general.