Optimized loose nanofiltration membranes via synergistic polyphenol and surfactant for efficient dye/salt separation

The treatment of textile dyeing wastewater, containing high concentrations of dyes and salts, remains a critical environmental challenge. There is growing interest in loose nanofiltration membranes (LNMs) for their ability to selectively separate dyes from salts, yet conventional fabrication strategies are often constrained by the inherent trade-off between permeability and selectivity, as well as difficulties in scalable production. This study developed a novel LNM via one-step interfacial polymerization (IP) strategy by synergistically incorporating tannic acid (TA) and cetyltrimethylammonium bromide (CTAB) into the aqueous piperazine phase. This synergy is proposed to arise from the complementary roles of the two additives: TA extends polymer chains to enlarge membrane pores and enhance surface negative charge, while CTAB acts as an interfacial regulator to reduce selective layer thickness and modulate IP kinetics. Optimized membranes containing 0.8 g·L⁻¹ TA and 0.4 wt% CTAB exhibited a significantly thinner selective layer (36.03 nm) and loose pore size (0.646 nm) compared to the control, resulting in a remarkable pure water permeance of 44.77 L·m⁻²·h⁻¹·bar⁻¹, surpassing that of the pristine polyamide membrane by a factor of nearly two. The membrane also demonstrated excellent selectivity, with 97.5% rejection of Direct Fast Blue B2RL and over 94% transmission of NaCl. A strong negative zeta potential (−45 mV at pH 7) enabled Donnan-effect-dominated dye retention while facilitating salt permeation. Molecular dynamics simulations revealed that CTAB competitively adsorbs at the water–organic interface, moderating the IP kinetics, while TA promotes homogeneous water dispersion within the membrane, modulating chain packing and enhancing pore size upon hydration. Long-term filtration tests confirmed stable performance over 24 h under dye/salt mixture. This study introduces a facile, scalable, and economical strategy for fabricating high-performance LNMs, offering both mechanistic insights and practical potential for resource-oriented treatment of textile wastewater.