Copolymer properties regulation enables selectivity-retained permeability enhancement in positively charged nanofiltration membranes

Positively charged nanofiltration (NF) membranes hold distinct advantages in water softening and heavy-metal removal. However, they often suffer from insufficient permeability. In this study, a cationic monomer, methacryloxyethyltrimethyl ammonium chloride (DMC), and a hydrophilic monomer, 3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate (DMAPS), were employed to synthesize a copolymer (SCA), which was applied to fabricate positively charged NF membranes via a coating-crosslinking strategy. Compared with the control, NF–SCAs exhibited progressively enhanced water permeability with increasing DMAPS content. However, the sulfonate groups of DMAPS partially shielded the quaternary ammonium groups of DMC, thereby reducing the zeta potential of NF–SCAs. Among them, NF-SCA-2 achieved the most balanced performance, exhibiting a water permeance of 18.58 L m⁻² h⁻¹ bar⁻¹, 9.68-fold that of the control, and maintaining rejection rates of 96.08 % for MgCl₂ and 95.43 % for CaCl₂. In addition, NF-SCA-2 maintained over 90 % rejection for heavy-metal ions including Fe³⁺, Zn²⁺, and Cu²⁺. Operating-condition tests confirmed that NF-SCA-2 retained stable rejection and permeability across feed temperature (20–40 °C), pressure (2–10 bar), and ion concentration (1000–3000 mg L⁻¹) ranges. Mechanistic analysis indicated that the regulation of copolymers properties induced by DMAPS were the primary factors for the enhanced performance of NF-SCA-2. Notably, NF-SCA-2 exhibited unusually reinforced rejection with the feed ion concentration increased, which was attributed to the anti-polyelectrolyte effect of DMAPS. Specifically, feed ions neutralized the opposite charges of DMAPS, thereby allowing more complete expression of the cationic nature of DMC. This work provides a new approach to the design of positively charged NF membranes and contributes to overcoming their intrinsic performance limitations.