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pH-responsive mixed-charge ultrafiltration membranes for adaptive control of single and mixed nanoplastic fouling
Summary
Scientists engineered a new type of water filter membrane that resists clogging from nanoplastics, tiny plastic particles too small to see that are increasingly found in drinking water. By coating the filter with a special material that switches its electrical charge depending on water conditions, they created a self-cleaning surface that blocked over 95% of nanoplastics while staying easier to rinse clean than standard filters. This matters because more effective, longer-lasting water filters could help reduce our exposure to nanoplastics, which scientists are still studying for potential health effects.
Nanoplastic (NP) fouling is an emerging barrier to the reliable application of ultrafiltration (UF) membranes, particularly in chemically complex waters where NP surface functionality governs deposition, cake formation, and cleaning performance. Herein, pH-responsive mixed-charge UF membranes were fabricated by plasma-induced graft copolymerisation of mono-2-(methacryloyloxy) ethyl succinate (MMES) and N-[3-(dimethylamino)propyl] methacrylamide (DMAPMA) onto poly(ethersulfone) (PES) to simultaneously regulate surface charge, hydration, and fouling reversibility. Unlike conventional single-charge or zwitterionic modifications, this approach enabled independent tuning of anionic and cationic moieties, generating composition-dependent and pH-switchable interfacial properties. ATR-FTIR, XPS, FESEM/EDX, AFM, WCA, streaming zeta potential, and MWCO analyses confirmed successful grafting, modified surface morphology, improved hydrophilicity, and pronounced charge switching from positive under acidic conditions to negative at neutral and alkaline conditions for the mixed-charge membranes. Among the fabricated membranes, the equimolar mixed-charge membrane (M1D1) provided the most favourable overall performance, combining a pure water flux of 138 LMH with NP rejection above 95 %. Fouling behaviour depended strongly on both membrane chemistry and NP functionality, with mixed-NP suspensions imposing the most severe fouling challenge. Under mixed-NP filtration, the neat PES membrane exhibited a first-cycle flux decline ratio of 36 %, whereas M2D1, M1D1, and M1D2 reduced this value to 15 %, 15 %, and 20 %, respectively, with hydraulic cleaning flux recovery ratios of 85-92 %. The M1D1 membrane also showed superior pH-responsive cleaning efficiency during sequential acid-alkali treatment, sustaining greater flux recovery than the neat PES membrane over repeated fouling-cleaning cycles. These findings position mixed-charge surface engineering as a promising route for next-generation UF membranes with improved resistance to NP fouling and enhanced operational cleaning efficiency.