Impact of brine composition on the selective removal of Ca²⁺ and Mg²⁺ via carbonate formation

Seawater-based mineral carbonation or CO₂ mineralization is a promising carbon capture and utilization (CCU) technology for achieving carbon neutrality and resource circulation. However, carbonates have a low conversion rate, and their crystal structures and precipitate sizes are difficult to control, thereby limiting the competitiveness of CCU technology. This study explored a direct carbonation method that sequentially carbonated ions in seawater reverse osmosis brine using Na₂CO₃ as a CO₂ carrier to address these challenges. Moreover, the effects of ion concentration and interactions on the carbonate precipitation mechanism were analyzed. Depending on the concentration (1000–2592 ppm), the presence of Mg²⁺ ions induced either amorphous CaCO₃ or aragonite crystal structures during Ca²⁺ carbonation. For example, aragonite precipitation occurred when the Mg²⁺ concentration exceeded the theoretical threshold of 11.96 mol%. These findings suggest a novel Ostwald ripening pathway for CaCO₃ that differs from conventional CaCO₃ precipitation. Furthermore, by controlling the ion concentration and aging time, the crystal structures of MgCO₃ and CaCO₃ could be transformed into spherical and petal-like shapes. Our proposed direct carbonation process successfully enabled the selective precipitation of ions from brine. Thus, by analyzing the effect of ion concentration on carbonate precipitation, our study contributes to the broader applicability of CCU technology for various wastewater sources.