Fouling-resistant ionic graft-polyamide nanofiltration membrane with improved permeance for lithium separation from MgCl2/LiCl mixtures

Selective recovery of lithium from Salt Lake brines using environmentally friendly technologies is increasingly demanded to overcome the shortage of lithium. However, satisfying high separation precision is challenging for most polymer nanofiltration membranes. Besides, bestowing high permeance together with improved membrane fouling remains a challenge for the widespread implementation of membrane processes. Herein, a new tri-quaternary ammonium-based ionic liquid (TQAIL) electrolyte monomer is synthesized and utilized for the surface modification of nascent polyamide active layer to fabricate IL-modified polyamide nanofiltration (NF) membrane for Mg2+/Li+ separation. Molecular dynamics simulations confirm that TQAIL modification creates nanoscale structural heterogeneity leading to enhanced surface hydrophilicity and reduces internal resistance through the membrane selective layer. The TQAIL membrane exhibits improved Li+/Mg2+ selectivity and improved water permeability of 26.11 L m−2 h−1 bar−1, which is fourfold that of unmodified membrane accompanied with a 100-h stable continuous nanofiltration. Moreover, the membrane displays good anti-biofouling behaviour with excellent antimicrobial properties. This study offers new insights into how surface functionalization using multication side chains can remarkably improve ion separations by molecularly tuning ion-membrane electrostatic interactions in polymeric membranes.