Fabrication of highly positively charged nanofiltration membranes by novel interfacial polymerization: Accelerating Mg2+removal and Li plus enrichment

Li+, a vital green energy source, is mainly extracted from salt lakes, which also contain high concentration of Mg2+. Therefore, there is an imminent need for separation of Li+ and Mg2+. Recent innovations highlight the great potential of positively charged nanofiltration (NF) membranes for separating Mg2+ and Li+. Typically, positively charged NF membranes are prepared by the interfacial polymerization approach, where the most commonly applied aqueous phase monomer is polyethyleneimine (PEI). In this work, a new class of polyallyl-amine (PAA)-based NF membrane with a high density of positive charge, was developed to achieve high perm-selectivity for separating Mg2+ and Li+. The results indicated that the abundant primary amine (-NH2) groups on the PAA molecular chain endowed the NF membrane with a highly hydrophilic and positively charged polyamide layer. The resulting NF membrane exhibited superior Mg2+/Li+ separation ability with a high separation factor SMg2+/Li+ of 82.8 and a water permeance of 7.39 L/(m2hbar). It is also noteworthy that the SMg2+/Li+ value is more than 4 times higher than that of the previously reported PEI-based membrane. Moreover, our work also provides a new potential candidate for the future development of positive charged NF membranes.