Fabrication of high performance Mg2+/Li+ nanofiltration membranes by surface grafting of quaternized bipyridine

A new monomer (quaternized bipyridine, QBPD) was synthesized to modify the interfacially polymerized polyethyleneimine (PEI) membranes. The QBPD monomer, containing binary charge sites and amine groups (NH2), was anchored to the pristine PEI membrane through the “amine ∼ acyl chloride” amidation reaction. Pure water flux of the QBPD membrane is ∼ 96.6 L m−2h−1 while the MgCl2 rejection is maintained at 92%±3. Noteworthy, the flux is ∼ 2.8 times as high as that of the pristine PEI membrane (∼34.0 L m−2h−1 at 0.6 MPa). Meanwhile, separation performance of the QBPD membrane remains stable in 162 h operation time at 0.6 MPa. When filtrating 50:1 Mg2+/Li+ mixture, the flux of the QBPD membrane is 81.6 L m−2h−1, and the Mg/Li ratio in the permeate was reduced to 8.5:1. Collectively, the QBPD membrane shows good performance compared with recent nanofiltration membranes for Mg2+/Li+ separation, exhibiting considerable potential for lithium extraction from salt lakes with high Mg2+/Li+ ratio.