Guanidyl-incorporated nanofiltration membranes toward superior Li+/Mg2+ selectivity under weakly alkaline environment

Positively charged nanofiltration membranes hold great potential in Li+/Mg2+ separation, but their application in alkaline solutions, such as alkaline salt-lake brines has not been reported. Herein, inspired by the arginine's unique structure and biological function, we explore superstrong basic triaminoguanidine (TG) molecule as aqueous monomer to fabricate positively charged polyamide nanofiltration membranes for Li+/Mg2+ separation. With the Li+/Mg2+ mass ratios of 1:50 and 1:120, at pH 9.6, the separation factors (SLi,Mg) of membranes reach 31.8 and 20.9. Meanwhile, at pH 5.6, the SLi,Mg are 46.4 and 36.4, which are the ever-reported highest value and such high SLi,Mg can be well retained in the pH range of 5.6–8.6. The superior Li+/Mg2+ selectivity stems from the superhigh protonation capacity of guanidine groups in a broad pH range.