Positive microporous polymer nanofiber interlayer tailored polyamide nanofiltration membranes for Li+/Mg2+ separation

Construction of positively charged nanofiltration membranes is essential for extracting lithium ions from salt-lake brine to meet the growing demand of the new energy industry. However, conventional membranes suffer from low water permeability due to the large thickness of the fabricated positive-charged polyamide (PA) layer, which increases transport resistance. To address this challenge, a porous interlayer was introduced to regulate the formation of the PA layer and accelerate water transport. Specifically, water-dispersible conjugated microporous polymer (CMP) fibers were synthesized and blended with positively charged polyethyleneimine (PEI) to form a porous, positively charged interlayer. The PEI in this interlayer further participated in the interfacial polymerization process, tailoring the PA morphology and converting the originally negative surface charge to positive. As a result, a dual positively charged membrane was obtained, featuring an enlarged surface area in the PA top layer and a porous structure in the interlayer. Benefiting from the synergistic effects of the Donnan exclusion and reduced transport resistance, the membrane exhibited outstanding separation performance in purifying lithium from a Mg 2+ /Li + mixed solution (mass ratio 20), achieving a water permeance of 11.3 L m −2  h −1 ⋅bar −1 and a separation factor of 43.8. Furthermore, the membrane maintained stable performance during a 10-day continuous operation, demonstrating strong promise for practical applications.