An interlayer-based positive charge compensation strategy for the preparation of highly selective Mg2+/Li+ separation nanofiltration membranes

The Mg2+/Li+ screening selectivity determines the practical application of nanofiltration technology in lithium extraction from salt-lake brines and industrial lithium production. In this work, a membrane structure design strategy based on the interlayers was developed to prepare nanofiltration (NF) membranes with high Mg2+/Li+ separation selectivity and high permeability. Through investigating the influence of the interlayer charge on the structure and performance of the NF membrane, the interlayer for the preparation of the NF membrane with high Mg2+/Li + separation selectivity was screened. Relevant characterizations indicate that the positively charged interlayer can endow the NF membrane with appropriate pore size and surface charge density to achieve high Mg2+/Li+ separation selectivity through the synergistic effect of size sieving and Donnan exclusion. Moreover, the pore size of the NF membrane is controlled by the release rate of the piperazine (PIP) from the interlayer, while the surface charge density of the NF membrane is jointly determined by the charge of the interlayer and PA layer. The performance evaluation suggests that the NF membrane prepared with the best screened interlayer has a Mg2+/Li+ separation factor up to 88.6 while maintaining a high permeability (22.5 L m−2 h−1·bar−1), which is better than that of the most membranes reported in the literature. Overall, this work is expected to promote the practical application of NF membranes in lithium extraction from salt-lake brines and industrial lithium production.