Mg2+/Li+ separation by electric field assisted nanofiltration：the impacts of membrane pore structure, electric property and other process parameters

An electric field assisted nanofiltration (ENF) process was used for Mg 2+ /Li + efficient separation from a MgCl 2 /LiCl solution and a simulated salt-lake brine. The influence of membrane pore structure, electric property, and other process parameters on the Mg 2+ /Li + separation performance in the ENF was systematically investigated. The structural coefficient ( λ ) of NF membrane was introduced to quantify the membrane pore structure including pore size, pore length and porosity. Results suggested that membrane pore structure, instead of membrane pore size, had a critical role on the ENF separation performance. Direct current electric field (DC-EF) markedly promoted Mg 2+ /Li + separation. However, the effect of alternating current EF (AC-EF) with different frequencies was nominal. The synergistic effect between λ and DC-EF on Mg 2+ /Li + separation was also demonstrated. A low-concentration feed with suitable Mg 2+ /Li + mass ratio and pH value, and low transmembrane pressure were beneficial to Mg 2+ /Li + separation. For 2000 ppm MgCl 2 /LiCl feed solution with a Mg 2+ /Li + mass ratio of 20, a nearly complete rejection to Mg 2+ and a high Li + flux simultaneously achieved; correspondingly, a separation factor of Li + over Mg 2+ ( S Li,Mg ) reached above 7000, which is 2-3 orders of magnitude higher than that of pure NF membrane. For a simulated 10,000 ppm brine solution, a S Li,Mg value was up to above 50. The study suggested that ENF process witnessed simple and efficient separation performance for Li + extraction from salt lakes. • The electric-field assisted nanofiltration (ENF) process had an ultra-high Mg 2+ /Li + separation performance. • The structural coefficient ( λ ) of NF membrane played a key role in the ENF process. • Direct current electric field (DC-EF) markedly promoted Mg 2+ /Li + separation. • There was a synergistic effect between DC-EF and λ on Mg 2+ /Li + separation.