The tailoring of nanofiltration membrane structure for mono/divalent anions separation via precisely adjusting the reaction site distance

To improve the selectivity of nanofiltration membranes (NFMs) for multi/monovalent anions, including SO42−/Cl−, a strategy to moderately amplify the pore size while maintaining chargeability of the membranes was proposed. Four diamine monomers based on piperazine ring, with reaction site distance increased from 2.8 Å to 5.1 Å, were adopted and the effect of the monomer structure especially the reaction site distance on the structure and performance of the NFMs was studied. Position annihilation spectroscopy and molecular dynamics simulation were adopted to provide thorough characterization of the separation layer of the NFMs. The results showed that NFMs prepared by monomers with moderately increased reaction site distance displayed a moderately amplified pore size, which could facilitate the penetration of Cl− and thus improve SO42−/Cl− separation ability. The Na2SO4/NaCl selectivity got 10.0 with water permeability up to 18.5 L m−2 h−1 bar−1, exceeding the upper-bound of permeation flux and Na2SO4/NaCl selectivity of membranes prepared by traditional interfacial polymerization.