Reconstruction of the polyamide film in nanofiltration membranes via the post-treatment with a ternary mixture of ethanol-water-NaOH: Mechanism and effect

Flexible regulation of the membrane performance via the reconstruction of structural and physicochemical features is a promising technical route for the modification of nanofiltration (NF) membranes. Here we present a facile but effective post-treatment method with a ternary mixture of Ethanol-Water-NaOH (EWN). It was demonstrated the polyamide (PA) film in the nascent NF membrane could be effectively reconstructed by the EWN which consisted of 1 mol/L NaOH, 70% (v/v) ethanol, and 30% (v/v) ultrapure water. The EWN post-treatment greatly improved the water permeability (almost 100%) while maintaining high Na2SO4 rejection (~95.9%), which made membrane separation performance closer to the “upper bound” related to the “trade-off” between water permeability and Na2SO4 rejection. This improvement was attributed to the “etching effect” brought by the synergistic interaction between NaOH and ethanol, which partially broke the polyamide chains, thus reducing the PA film thickness from 43.1 nm to 18.5 nm, decreasing the water contact angle from 38° to 28°, and decreasing the zeta potential from −34.4 mV to −69.1 mV. In addition, the long-term stability testing up to 60 h verified the good durability of the post-treated PA film. The performance improvement of different commercial NF membranes confirmed the general applicability of EWN post-treatment.