Nanofiltration membranes with ion-induced swelling for elevated water permeance in water treatment

Nanofiltration (NF) membranes are widely used in water treatment and desalination. One commonly observed limitation of these membranes is the reduced water permeance in the presence of salts. Here, we introduce diol-based solvents into interfacial polymerization (IP) to form polyamide-type NF membranes. The high viscosity and low surface tension of the solvents reduced the concentration of piperazine (PIP) at the interface, resulting in membranes with a reduced thickness (82.7 to 30.2 nm), lower surface roughness (20.9 to 6.2 nm) as well as higher water permeance (11.1 to 21.0 L.m⁻².h⁻¹.bar⁻¹) compared to original NF membrane, without significantly sacrificing salt rejection ability (97.0 % rejection to Na₂SO₄). More interestingly, the crosslinking degree of the polyamide layer prepared by diol-based solvents was lower, giving rise to higher charge density and swelling in salt solutions. Thus, higher water permeance in treating saline water was observed, and the membranes demonstrated effective separation of organic molecules from salts at elevated water permeance and lower fouling extent in concentrating antibiotics from the realistic pharmaceutical wastewater stream.