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 (NH 2 ), 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 −2 h −1 while the MgCl 2 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 −2 h −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 Mg 2+ /Li + mixture, the flux of the QBPD membrane is 81.6 L m −2 h −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 Mg 2+ /Li + separation, exhibiting considerable potential for lithium extraction from salt lakes with high Mg 2+ /Li + ratio.