Weaken aqueous monomers to thicken the polyamide layer of nanofiltration membranes with a high carboxyl density

For thickening the polyamide (PA) active layer, the weakened aqueous monomer, piperazine-2-carboxylic acid (PIP–COOH), was selected to explore a new way of fabricating the thick and negative nanofiltration (NF) membranes via classic interfacial polymerization (IP). The presence of electron-withdrawing carboxyl on the PIP-COOH greatly reduced the reactivity to totally reserve ample time and space, especially at the early stage of the IP and in the incipient polymeric structure, respectively, for PIP-COOH continuously diffusing forward to extend the reaction zone. Furthermore, the extra introduction of carboxyl, belonging to PIP-COOH, significantly increased the negative charge density of the resultant membranes. The optimal membrane exhibited the high rejection (∼92.4 %) of Na 2 SO 4 and the effective removal of perfluorinated compounds (PFCs) (>80.0 %) while maintaining moderate water permeance of 5.2 L m −2  h −1 bar −1 and excellent durability. It was validated repeatedly that the sufficient supply of aqueous PIP-COOH ensured the thickness of active layer increased at least two-fold, most up to ∼200 nm.