Preparation of hybrid nanofiltration membranes based on Schiff base reaction for hard water softening

To obtain positively charged layer and achieve high divalent cation rejection, a novel nanofiltration (NF) membrane was successfully fabricated by utilizing polyethyleneimine (PEI), newly fabricated polyacrolein (PA) and amine silane coupling agent γ-aminopropyl triethoxysilane (APTES). The PA was synthesized via radical polymerization to avoid the migration and volatilization of small molecule in the typical coating process, making NF membranes even with great stability. On the one hand, the PEI crosslinked with PA through Schiff base reaction; on the other hand, the in-situ hydrolysis self-polycondensation of APTES generated loose networks with Si–O–Si bonds which provided the permeation channel for water and accordingly boosted the water permeance. The water permeance of the optimal PEI/PA/APTES membrane could reach 16.9 L m −2  h −1 ·bar −1 and the Mg 2+ and Ca 2+ rejections were 98.8 % and 97.3 %, respectively, better than those of commercial NF membranes, breaking up the “trade off” effect. This study prepared a high-performance PEI/PA/APTES hybrid NF membrane oriented for hard water softening in tap water and brackish water via the non-interfacial macromolecule crosslinking coating method.