Filler-size-gradient modification of asymmetric ultrafiltration membrane for simultaneous removal of multi-component pollutants from wastewater with reduced irreversible fouling

A hybrid system integrating adsorption, photocatalytic oxidation, and membrane ultrafiltration (UF) is regarded as an effective alternative to addressing the practical requirements of wastewater treatment. Herein, we proposed a size-gradient modification strategy to construct metal-free multifunctional UF membranes by incorporating small-sized resorcinol-formaldehyde resins (RF(s)) into the polyethersulfone (PES) membrane matrix and subsequently entrapping large-sized RF resins (RF(l)) in finger-like pores. The high specific surface area and abundant surface phenolic hydroxyl groups of RF(s) improved its interfacial compatibility with PES, enabling a higher filler loading with minimized interfacial defects. The filler/matrix interface with reduced defects lowered membrane resistance and narrowed pore size while increasing adsorption sites, which were further increased by in-pore entrapment of RF(l). When a methylene blue (MB)/humic acid (HA) mixed solution flowed through, the filler size differentiation synergistically optimized mechanisms for size-exclusion-based HA rejection and electrostatic-adsorption-based MB removal with a minor permeability loss, reducing their competition for electrostatic adsorption sites and achieving their simultaneous efficient removal from the mixed solution. The membrane performance was repeatedly restored through synergistic photocatalytic-Fenton reactions based on photocatalytic in situ H 2 O 2 production capacity of RF resins. Consequently, the defect-minimized membrane structure and the visible-light-driven cleaning capability enabled the membrane to achieve a flux recovery ratio exceeding 90%, while keeping the irreversible fouling resistance below 5.5% of the total resistance under various test conditions across multiple filtration–cleaning cycles.