Insights into the mechanism of nitrate salt-mediated MgCO₃ formation

Nitrate salt-mediated carbonation of MgO feedstocks has been extensively studied for CO₂ absorption, but the underlying mechanism remains poorly understood. This study investigates the nucleation and growth of magnesite (MgCO₃) in the presence of nitrate salts (NaNO₃ and KNO₃) to resolve discrepancies in the carbonation mechanism. Using a simplified sample preparation method, we propose that nitrate salts catalyze MgCO₃ formation by (i) lowering the activation energy for brucite (Mg(OH)₂) dehydroxylation, (ii) stabilizing Mg²⁺-CO₃^2- ion pairs in nanolayers of water, and (iii) acting as structural nucleation sites due to their crystallographic similarity to magnesite. In situ thermogravimetric analyses (TGA-DSC) reveal that the carbonation reaction initiates at ∼300 °C, with an exothermic nucleation peak at ∼311 °C, indicating that nitrate salts enhance the formation of stable carbonate intermediates. This revised mechanism, where magnesite nucleation occurs via water-mediated diffusion of carbonate ions and structural templating by nitrate salts, refines our understanding of MgCO₃ crystallization and offer new insights for catalytic carbon mineralization.