Phase evolution and mesostructural transformation of multi-metal carbonates under CO ₂ exposure via neutral-pH co-precipitation

Multi-metal carbonates (MMCs) containing Mn²⁺, Fe²⁺, Ca²⁺, and Mg²⁺ were synthesised via a Na₂CO₃-assisted co-precipitation process at 50°C under mildly alkaline pH (∼9.5) conditions. Na₂CO₃ acted as both the carbonate source and a mild pH buffer, facilitating the formation of complex carbonate and hydroxide phases. Structural analysis (XRD, FTIR, and XPS) revealed that MMCs are composites of single and binary carbonates, layered double hydroxides (LDHs), and hydromagnesite. Upon CO₂ exposure, selective dissolution of CaMn(CO₃)₂ and hydromagnesite occurred without reprecipitation of CaCO₃ or MgCO₃ phases due to acidic pH (<6), leading to the enrichment of MnCO₃ and M²⁺Fe³⁺-LDHs. This transformation was accompanied by a mesostructural reorganisation, as evidenced by an increase in specific surface area from 29.3 to 46.3 m²/g and a reduction in system pH. These results provide fundamental insights into phase-selective transformations induced by CO₂ interaction with MMCs synthesised under mildly alkaline carbonate precipitation conditions, thereby clarifying the underlying transformation pathways and mesostructural evolution.