Loose nanofiltration membranes prepared by rapid co-deposition of HMMP-1 and dopamine for dye/salt highly selective separation

Loose nanofiltration membranes (LNMs) prepared by co-deposition of polyphenols and amine-rich polymers has been considered as an effective method for the efficient treatment of textile wastewater. However, the dye/salt selectivity remains unsatisfactory, due to the low free volume with wide distribution caused by the stacking and tight entanglement of polymeric chain segments. A high dye/salt selective LNM was prepared for the first time by rapid co-deposition of high-valent metal-induced microporous polymer framework (HMMP-1) and dopamine (DA). HMMP-1 with high specific area and suitable pore size can provide additional transfer channels for water/salt, breaking the limitation of free volume generated by polymer stacking and further improving perm-selectivity. In addition, the amine-rich HMMP-1 with a flexible polymer framework can be covalently linked to DA by Michael addition/Schiff base reaction, effectively avoiding the selectivity reduction caused by poor interfacial compatibility. The results showed that the pure water permeance of the optimal membrane (LNM-2) was 57.22 LMH·bar−1, which was 0.65 times higher than that of LNM-1 (without HMMP-1). In addition, LNM-2 exhibited excellent dye/salt selectivity for dye/salt mixtures with different concentrations, reaching >47 (CR/NaCl) and >29 (CR/Na2SO4), respectively, which is more than twice of the conventional membranes reported so far. Moreover, the dye/salt selectivity of LNM-2 still maintained at 28 (CR/Na2SO4) and 55 (CR/NaCl) respectively for 12 h, when salt concentration was 10 g·L−1, which fully demonstrated the good stability of LNM-2 and the superiority of HMMP-1 for the design of LNM.