Fabrication of monovalent/divalent anionic salt separation membranes by polydopamine-assisted deposition of metal-induced microporous polymer as an interlayer

Nanofiltration (NF) technology represents a promising approach for separating monovalent and divalent anionic salts. However, the separation layer of traditional thin-film composite (TFC) NF membranes is relatively thick and loose, resulting in unsatisfactory water permeance and separation performance. In this study, a novel high-valent metal-induced microporous polymer, HMMP-2, was synthesized. Then, the polydopamine (PDA)/HMMP-2 interlayer was constructed by depositing HMMP-2 on the polysulfone membrane via PDA-assisted deposition. The interlayer had a hydrophilic and rough surface, enabling the retention of more piperazine monomers to regulate the interfacial polymerization (IP) process. Subsequently, an ultrathin (thickness reduced from 100.41 ± 5.22 nm to 30.02 ± 4.48 nm) and dense polyamide separation layer was fabricated via IP reaction on the interlayer. Additionally, the hydrophilicity of the interlayer combined with the suitable pore size of HMMP-2 synergistically enhanced transmembrane water transport, significantly improving the water permeance of the NF membrane. The optimized membrane, NF-PH(5), demonstrated superior water permeance of 20.93 ± 1.13 L m⁻² h⁻¹·bar⁻¹, representing a 193.53 ± 15.18 % enhancement compared with the control membrane (NF0). The NaCl/Na₂SO₄ separation factor of NF-PH(5) reached 62.88 ± 5.40, showing an increase of 91.42 ± 4.92 % over NF0. Moreover, NF-PH(5) exhibited superior perm-selectivity in the mixed-salt solution compared with NF0. The fabricated NF membrane with a PDA/HMMP-2 interlayer also demonstrated excellent operational stability. This study presents novel insights for the rational design of hybrid interlayers to regulate IP processes and fabricate high-performance monovalent/divalent anionic salt separation membranes.