Influence of surface passivation and water content on mineral reactions in unsaturated porous media: Implications for brucite carbonation and CO2 sequestration

Abstract

The evolution of mineral reactive surface area is an important control on the progress of carbon mineralization reactions that sequester anthropogenic CO2. Dry conditions in unsaturated porous media and the passivation of reactive surface area by secondary phase precipitation complicate predictions of reactive surface during carbon mineralization reactions. Unsaturated brucite [Mg(OH)2] bearing column experiments were used to evaluate the effects of water saturation and hydrous Mg-carbonate precipitation on reaction of brucite with 10% CO2 gas streams at ambient conditions. We demonstrate that a lack of available water severely limits reaction progress largely due to the requirement of water as a reactant to form hydrated Mg-carbonates. The precipitation of a poorly crystalline carbonate phase in the early stages of the reaction does not significantly hinder brucite dissolution, as the carbonate coating remains sufficiently permeable. It is postulated that the conversion of this phase to substantially less porous, crystalline nesquehonite [MgCO3·3H2O] results in passivation of the brucite surface. Although a mechanistic model describing the passivating effect of nesquehonite remains elusive, reactive transport modeling using MIN3P-DUSTY confirms that conventional geometric surface area update models do not adequately reproduce observed reaction progress during brucite carbonation, while an empirically based model accounting for surface passivation is able to capture the transient evolution of CO2 uptake. Both water limits and surface passivation effects may limit the efficiency of CO2 sequestration efforts that rely on the conversion of mafic and ultramafic rock to carbonate minerals.