Selective Separation of Inorganic and Organic Carbonates in Aqueous Solutions by Reverse Osmosis (RO) and Nanofiltration (NF) Membranes.

This study presents a systematic comparison of commercial reverse osmosis (RO) and nanofiltration (NF) membranes for the separation of inorganic and organic carbonates from aqueous solutions, providing insight into the roles of membrane pore structure and surface charge in governing separation mechanisms. Membrane molecular weight cut-off (MWCO), zeta potential, and thermal stability were characterized and correlated with separation performance. The RO membrane exhibited a lower MWCO and a more negative surface charge than the NF membrane, resulting in superior rejection of both inorganic and organic carbonates. For inorganic carbonates, rejection increased with pH owing to enhanced carbonate ionization and stronger electrostatic repulsion, reaching 91-98% for the RO membrane compared with 60-95% for the NF membrane. In contrast, the rejection of neutral organic carbonates was governed primarily by steric exclusion, with the RO membrane achieving approximately 80% rejection, whereas the NF membrane exhibited negligible removal. These findings demonstrate the combined influence of membrane pore size and surface charge on carbonate separation and provide practical guidance for selecting commercial membranes for efficient carbonate removal in water treatment applications.