A review of the nanofiltration membrane for magnesium and lithium separation from salt-lake brine

Nanofiltration (NF) membrane with small pore size and charged properties has attracted more attention recently in lithium extraction from salt-lake brine for addressing the escalating demand for lithium resources. Different kinds of NF membrane have been developed to accomplish the high efficiency Mg2+ and Li+ separation. To enhance comprehension of NF membranes in Mg2+ and Li+ separation, we propose a critical review focusing on the preparation method and Mg2+ and Li+ separation performance. According to the separation mechanism for Mg2+ and Li+, the NF membranes were defined as five types: (1) negatively charged NF membrane, (2) positively charged NF membrane, (3) mix-charged NF membrane, (4) size-sieving NF membrane and (5) bionic NF membrane. It is pointed out that the commercially available NF membrane with negatively charged property should be designed with narrow pore size distribution and the biggest pore size not more than the hydration diameter of Mg2+ to improve the Mg2+ and Li+ selectivity. Besides, the space distribution of charges in separation layer is also critical for Mg2+ and Li+ separation. This kind of NF membrane is still regarded as the mainstream membrane materials for Mg2+ and Li+ separation. Even though the positively charged NF membrane improves the rejection of Mg2+, it also increases the retention of Li+ at the same time, which leads to low Li+ permeability and further reduces the Mg2+ and Li+ separation efficiency. Besides, the positively charged NF membrane is easily polluted by nature organic matters with negatively charged property extensively existing in salt-lake brine. The hollow fiber NF membrane fabricated via layer-by-layer (LBL) assembly of polyelectrolytes should be further developed to achieve commercial application due to their high separation efficiency, high packing density, acid/ alkali/chlorine resistance, and good antifouling capacity. This review provides insight into the optimization and future development direction of the NF membrane design for Mg2+ and Li+ separation.