Interlayer–polymerization coupling via g-C₃N₄/PEI interlayer for high-performance nanofiltration membranes

Interfacial polymerization (IP)-based nanofiltration (NF) membranes often suffer from the permeability–selectivity trade-off due to uncontrolled monomer diffusion and nonuniform polyamide (PA) layer formation. In this work, a g-C₃N₄/polyethyleneimine (PEI) hybrid interlayer was constructed on a polyethersulfone substrate via vacuum-assisted deposition to regulate interfacial polymerization and enhance sulfate separation performance for nanofiltration-related water softening applications. The synergistic interaction between g-C₃N₄ nanosheets and PEI effectively suppressed nanosheet aggregation, enlarged interlayer spacing, and improved interfacial compatibility, thereby promoting the formation of an ultrathin and highly cross-linked PA selective layer. The optimized TFNi-2 membrane exhibited a high water flux of 65.8   L·m⁻²·h⁻¹ together with a Na 2 SO 4 rejection of 96.8% at 0.4   MPa, demonstrating excellent permeability and selective rejection toward multivalent ions. Structural characterizations confirmed that the g-C₃N₄/PEI interlayer regulated amine diffusion behavior and suppressed nonuniform PA formation, resulting in a compact and defect-suppressed selective layer with an MWCO of approximately 600   Da. In addition, the highly permeable interlayer generated a favorable “gutter effect” that reduced transport resistance and facilitated water permeation. The optimized membrane also exhibited satisfactory chlorine resistance and long-term operational stability. This work provides new insights into interlayer-regulated interfacial polymerization for the development of high-performance nanofiltration membranes for sulfate removal and water softening applications.