High performance Mg2+/Li+ separation membranes modified by a bis-quaternary ammonium salt

The separation of lithium (Li + , 3.8 Å) and magnesium (Mg 2+ , 4.3 Å) ions represents the main challenge for lithium extraction from salt lakes with high magnesium-to‑lithium (Mg 2+ /Li + ) ratio. A bis -quaternary ammonium salt (abbreviated as QEDTP) was designed to modify Polyethyleneimine (PEI) polyamide nanofiltration membrane. Water flux of the modified PEI membrane is 126.9 ± 1.6 L·m −2 ·h −1 (LMH) under 6 bar hydraulic pressure , which is 4.4 times as high as the pristine PEI membrane (25.8 ± 0.3 LMH). Meanwhile, the MgCl 2 rejections of both membranes are maintained >95%. Nanofiltration performance of the QEDTP membrane is stable in 180 h continuous nanofiltration test at 6 bar pressure. The water flux and Mg 2+ /Li + selectivity ( S Mg2+/Li+ ) of the QEDTP modified membrane in filtrating 2000 ppm Mg 2+ /Li + mixture (Mg 2+ /Li + ratio: 120:1, 6 bar) are 112.9 ± 1.6 LMH and 15.6 ± 0.3, which is one of the best Mg 2+ /Li + separation performance considering both the flux and selectivity.