Surface Speciation of Brucite Dissolution in Aqueous Mineral Carbonation: Insights from Density-Functional Theory Simulations

Aqueous mineral carbonation of brucite is an important mineralization route for carbon capture and sequestration. Prerequisite to mineral carbonation are the simultaneous CO2 absorption and brucite dissolution which imply, in the first place, the formation and release in the liquid phase of CO32–, HCO3–, Mg2+, MgOH+, and MgHCO3+ ions. To gain insights on the nature of adsorption sites and resulting surface complexes, the affinity of water and of dissolved species for pristine and partially dissolved brucite (001) cleaved surfaces in aqueous mineral carbonation has been investigated using density-functional theory (DFT) simulations. The species’ affinity for uptake by the brucite (001) surface is predicted to obey the trend: Mg2+ > MgHCO3+ > MgOH+ > HCO3– > CO32–, whereas the surface acid/base behavior controls affinity following the order: dehydroxylated (001) surface > deprotonated (001) surface > neutral and protonated (001) surfaces. Covalent bonds have been predicted for the following (charge-determining) ion-(001) brucite surface sites: CO32– ≡ dehydroxylated site, HCO3– ≡ dehydroxylated site, MgOH+ ≡ dehydroxylated/deprotonated sites, MgHCO3+ ≡ dehydroxylated/(de)­protonated sites, and Mg2+ ≡ neutral/(de)­protonated/dehydroxylated sites. Congruent dissolution of (001) brucite surface leads to a diverse population of coordination-deficient Mg and O centers which are more active to form covalently bonded surface complexes with aqueous CO32–, HCO3–, Mg2+, MgOH+, MgHCO3+ as compared to the undissolved surface. However, although affinity of the altered surfaces for dissolved ions increases conspicuously, the same affinity trend is predicted for the dissolving surfaces as compared to the pristine (001) brucite surface