Breaking the permeability-selectivity trade-off in loose nanofiltration: Zwitterionic layer-by-layer membranes for...

Resource recovery from textile effluents represents a pivotal objective within the circular economy, yet this process is significantly constrained by the membrane "trilemma" of balancing selectivity, permeability, and stability. In this study, we systematically investigated the dye/salt separation mechanism of layer-by-layer (LbL) membranes by modulating the bilayer numbers and terminal chemistry (polyethyleneimine/poly (sodium 4-styrenesulfonate) (PEI/PSS)). Our findings reveal that the separation performance is governed by non-sieving interactions with the terminal layer, a mechanism validated by Density Functional Theory (DFT) calculations, which showed a significantly stronger dye-polymer binding energy for PSS (-4.47 eV) than for PEI (-2.53 eV). To address persistent limitations in permeability and fouling resistance, we incorporated various zwitterionic moieties into the LbL architecture and conducted a comparative analysis among representative zwitterionic groups. The optimal SBMA modified-membrane effectively overcame the conventional trade-off, achieving an outstanding water permeability of 194.16 L·m⁻²·h⁻¹·bar⁻¹ while maintaining >99 % dye rejection. This design also imparted exceptional fouling resistance, evidenced by a 66 % flux recovery ratio and a 229% increase in the interfacial repulsive energy barrier. Notably, when treating real industrial wastewater, a two-stage process dramatically enhanced the dye/salt separation factor to nearly 1000. This work establishes a mechanism-driven strategy for designing high-performance membranes, offering a robust and scalable solution to key challenges in industrial wastewater reclamation.