Ultrahigh-efficient separation of Mg2+/Li+ using an in-situ reconstructed positively charged nanofiltration membrane under an electric field

Efficient separation of Mg2+ and Li + remains a major challenge in the extraction of lithium from salt lake brine. In this work, an electro-nanofiltration (NF) process was employed to separate Mg2+ and Li+ from the MgCl2/LiCl feed solution under a current density of 0–2.83 mA cm−2 and at an operating pressure of 5 bar. The surface chargeability of the NF membrane under an electric field (EF) was investigated in detail by combination of the experiment and theoretical simulation. It was found that Mg2+ ions were almost completely rejected (no Mg2+ detected by an ICP-OES with a determination limit of 0.01 ppm). A high permeance of Li+ (0.55 molm−2h−1) and a stable solution flux of 75 ± 2 Lm−2h−1 were achieved. The separation performance was far better than the results reported in the literature until now. It was believed that high positive charges on the negative-charged NF membrane surface were in-situ reconstructed and enhanced with the increase in current density due to the complexion and enrichment of Mg2+ on the membrane surface, leading to the high rejection for Mg2+. The dehydration of Li + ions with a low hydration energy under EF promoted the permeance of Li+. This work provides a practical route for efficient separation of Mg2+/Li+, and would have a huge potential in the extraction of high-pure Li+ salt from salt lakes.