Phase evolution and mesostructural transformation of multi-metal carbonates under CO2 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.