Ionic interactions drive SC[4]A to regulate polyamide networks for achieving high-efficiency Li+/Mg2+ separation

The selective separation of lithium and magnesium by conventional nanofiltration (NF) membranes remains a major challenge for lithium extraction from salt-lake brine. This difficulty results from the extremely fast and disordered nature of the interfacial polymerization process, which hampers precise control over the pore size of the separation layer. In this study, to utilize the hollow internal cavity of the macrocycle, calix[4]arene was directly sulfonated to prepare hydrophilic sulfonated calix[4]arene (SC[4]A), which was incorporated as an aqueous-phase additive into the piperazine (PIP) solution. NF membranes were then fabricated via interfacial polymerization (IP) on a self-made polysulfone (PSf) substrate. The results demonstrated that SC[4]A effectively accelerated the kinetics of the IP reaction. More importantly, SC[4]A, which was ionically bonded into the polyamide (PA), served as the additional mass transfer channel and formed a separation layer with a smaller free volume, narrower pore size distribution, and enhanced negative surface charge. The prepared TFC-0.5 membrane exhibited a Li + /Mg 2+ separation factor as high as 28.90, a 2.19-fold increase over the pristine membrane. Additionally, the incorporation of SC[4]A improved the hydrophilicity of the membrane surface, imparting superior antifouling properties to the modified membrane. By precisely regulating the IP process and tailoring the free volume of the separation layer, this study provides a new strategy for developing high-performance lithium‑magnesium separation NF membranes.