Smart NF membrane featuring a close-gate top layer (PAA) and an open-gate sublayer (SiO2/P4VP/PSf): Overcoming the permeability-rejection trade-off in DOX separation

In this study, a dual pH-responsive nanocomposite membrane was developed by integrating three complementary pH-responsive agents within a ternary architecture, enabling synergistic regulation of doxorubicin (DOX) separation. To this end, Poly(4-vinylpyridine) (P4VP) (open-gate mechanism) was incorporated into the substrate, Poly(acrylic acid) (PAA) (closed-gate mechanism) was introduced in the selective layer, and pH-sensitive silicon dioxide (SiO 2 ) nanoparticles were embedded in the substrate. The fabricated membranes (SiTj, with i=P4VP concentration and j=PAA concentration) were characterized using various analytical techniques. The membrane structure, surface properties, pH responsiveness, and separation performance were systematically characterized. Filtration results indicated that raising the P4VP concentration increased permeability by 166% (S5T3 vs. S0T3) and rejection by 32%, though accompanied by a 37% rise in fouling. In contrast, increasing the PAA content (S3T1 vs. S3T5) enhanced permeability by 40% and rejection by 16%, while fouling decreased by 22%. Increasing pH induced surface gate closure, enhancing rejection, and substrate gate opening, improving permeability. The combined effects of membrane-DOX electrostatics and pH-responsive gating minimized fouling, resulting in the best performance for the S4T4 at pH 10, with a permeability of 81 LMH/bar, and 99% rejection. Compared to the S0T1 at pH 7, permeability increased by 153%, and rejection by 57%. The membrane exhibited stable flux behavior and reversible pH-responsive permeability switching over six successive filtration-cleaning cycles. In addition, no detectable Si leaching was observed. These findings demonstrate that the designed dual-gate ternary pH-responsive membrane exhibits self-regulating behavior, enabling high permeability together with strong rejection in pharmaceutical wastewater treatment.