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

Efficient separation of Mg 2+ 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 Mg 2+ and Li + from the MgCl 2 /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 Mg 2+ ions were almost completely rejected (no Mg 2+ detected by an ICP-OES with a determination limit of 0.01 ppm). A high permeance of Li + (0.55 molm −2 h −1 ) and a stable solution flux of 75 ± 2 Lm −2 h −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 Mg 2+ on the membrane surface, leading to the high rejection for Mg 2+ . 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 Mg 2+ /Li + , and would have a huge potential in the extraction of high-pure Li + salt from salt lakes.