Additive impact on early-stage magnesium carbonate mineralisation

Carbon capture and utilisation has attracted significant interest due to increasing concerns about global warming. Mineral trapping via MgCO₃ precipitation is a promising strategy, though restricted by the slow rate of magnesite (MgCO₃) formation and high temperatures needed to avoid the formation of hydrated minerals. Amorphous magnesium carbonate (AMC) is a transient phase, determining the characteristics of the final crystalline MgCO₃ phase(s). Research has focused on accelerating MgCO₃ formation using additives, but their modus operandi is not completely understood. Here, AMC titration experiments were conducted at constant pH, monitoring solution transmittance, conductivity, and species size evolution to clarify the effect of citrate on the initial steps of MgCO₃ precipitation. We demonstrate that citrate, similar to more complex additives, alters the hydration of free ions relative to ion associates, thereby destabilising prenucleation ion associates and delaying AMC nucleation. The system is thus forced to go through liquid–liquid separation before the formation of the solid, resulting in amorphous and crystalline phases with lower water content, which are more stable and efficient for C storage, having a positive impact on the cost of CO₂ mineralisation.