Construction of crown ether-based polyamide composite nanofiltration membrane with high Mg2+ transportation barrier for efficient Mg2+/Li+ separation

Nanofiltration (NF) membrane with high ion sieving efficiency is desired for Mg 2+ /Li + separation. To achieve this aim, a crown ether functional layer with a high Mg 2+ transportation barrier was constructed by grafting water-soluble diazo-18-crown-6 ether (DA18C6) monomer on the piperazine (PIP)-based polyamide NF membrane surface via secondary interfacial polymerization (SIP). Molecular simulations revealed that the crown ether monomer has a stronger interaction force with Mg 2+ (−355.9 kcal/mol) than Li + (−110.6 kcal/mol) due to its higher bonding energy, thereby reducing the diffusion rate of Mg 2+ . The ion separation experiment showed that the PIP-TMC-DA18C6 (5min) NF membrane demonstrated a better Mg 2+ rejection of 87.7 % compared with the pristine PIP-TMC NF membrane of 76.1 %. Besides, the Li + shows a higher mean square displacement (MSD) value in PIP-TMC-DA18C6 NF membrane than PIP-TMC NF membrane, resulting in a lower Li + rejection of 34.1 %. Furthermore, the PIP-TMC-DA18C6 (5 min) NF membrane also shows an excellent separation factor of 18 under the Mg 2+ /Li + ratio of 50:1. This work demonstrated that the crown ether-based composite NF membrane exhibits a good performance for Mg 2+ /Li + separation.