Tailored nanofiltration membrane for enhanced performance by surface modification using zwitterionic piperazine

Nanofiltration (NF) membranes are indispensable for removing hazardous contaminants such as dyes and salts from industrial wastewater, yet their performance is often bottlenecked by the inherent permeability-selectivity trade-off and membrane fouling. Zwitterionic surface modification offers a means to overcome these limitations by promoting hydrophilicity and introducing charge-mediated transport. Here we synthesized a pre-formed zwitterionic piperazine (ZW-PIP) via a one-pot ring-opening of piperazine with 1,3-propanesultone, and directly grafted it onto polyamide films through residual acyl chloride groups. Combined low-field nuclear magnetic resonance (LF NMR) and molecular dynamics (MD) simulations demonstrated that the grafted zwitterionic moieties stabilized a hydration layer and facilitated water and chloride-ion diffusion (>90%), while sterically excluding sulfate. The resulting membrane exhibited a pure water permeance of 29.2 L∙m^-2∙h^-1∙bar^-1. Crucially, this enhanced permeability was achieved while maintaining the exceptional salt-rejection performance for Na₂SO₄ (>97.0%). This high selectivity persisted even under challenging conditions of elevated salt concentration (8.0 g/L) and operating pressure (8.0 bar). The membrane exhibited robust rejection of various dyes (>96%), sustained stability, and potent antifouling (BSA, FRR 89%) and antibacterial (>99.97%) properties. Collectively, this work offers a mechanistic basis for designing zwitterionic NF membranes with high permeability, selectivity, and fouling resistance for treating complex hazardous wastewater.