Porous organic polymer interlayers modulated nanofiltration membranes for ultra-permselective Li+ /Mg2+ separation

The ion-ion precise separation is of paramount importance but presents significant technical challenges. Achieving such an accurate separation utilizing nanofiltration (NF) membranes demands ion-selective channels with appropriate pore geometry structure and charge property. In this work, thinner polyamide (PA) NF membranes with narrower pore size distribution than conventional NF membranes were fabricated via the porous organic polymer (POP) interlayer-mediated interfacial polymerization. The dual-rigid-distorted POP interlayers were incorporated in situ onto the substrates through azo-coupling reactions. The POP interlayers exhibited a high-density adsorption capacity of amine monomers and acted as a space manipulator that regulated their diffusion behavior into the oil phase, enabling their reaction with the acyl chloride. The synergistic effects engendered a uniform PA active layer and impeded further growth of the membrane, compared with the typical PA membranes, facilitating the generation of a thin and a narrow pore size distribution PA active layer with a nanostripe surface. The prepared NF membranes showed an outstanding ion-ion separation factor of Li+/Mg2+ (up to 78.56), almost 18 times that of the pristine NF membrane (4.32), along with a two-fold increase in water permeance. Furthermore, the newly developed NF membranes demonstrate long-term stability under operational pressure, thus highlighting the promising potential of these NF membranes for extracting Li+ from salt-lake brines.