Reverse dual ionic liquid polymerization strategy to construct TA/IL sandwich composite nanofiltration membranes with enhanced Mg2+/Li+ separation

Thin-film composite (TFC) nanofiltration (NF) membranes generally face a challenging balance between permeability and selectivity when separating saline water with a high Mg 2+ /Li + ratio, and are also vulnerable to fouling and operational instability during prolonged operation. This study presents a dual ionic liquid (IL) mediated reverse interface polymerization (RIP) strategy. A hydrophilic polar intermediate layer is generated on the PES substrate via the co-deposition of tannic acid (TA) and [HEMIm][BF 4 ]. Subsequently, the organic phase containing TMC is first introduced on the surface of this intermediate layer, and then the PEI aqueous phase doped with hydrophobic IL ([C 6 mim] [Tf 2 N]) is introduced to trigger the RIP reaction. TIL-PIL membranes with rapid hydration behavior, a high positive surface charge (IEP pH = 9.5), excellent antifouling properties, and a uniform pore-size distribution were constructed for Mg 2+ /Li + separation. The separation factor (SF) of the membrane attained 88 at a feed Mg 2+ /Li + mass ratio of 300, while maintaining a high salt water permeation flux of 13.58 LMH. Throughout the permeation process, the Mg 2+ /Li + ratio diminished from 300 in the feed to 1.2 during permeation. Moreover, the membrane demonstrated exceptional long-term stability throughout a continuous 100 h filtration experiment, with the Mg 2+ rejection rate consistently surpassing 98.5%. Compared with the RNFm membranes prepared on PES substrates without IL incorporation, the TIL-PIL membranes exhibit significantly improved antifouling performance.