Decoding surface chemistry effects on polystyrene nanoplastic fouling in plasma-grafted PES membranes with distinct functional groups

Nanoplastic (NP) fouling remains a key challenge for ultrafiltration (UF) membranes, yet systematic, controlled comparisons of how membrane functional groups govern NP-membrane interactions are still limited. Here, the role of membrane surface chemistry was systematically investigated using a controlled comparative framework based on plasma-grafted polyethersulfone (PES) UF membranes tailored with structurally comparable methacrylate monomers bearing distinct terminal functionalities, including mono-2-(methacryloyloxy)ethyl succinate (MMES, -COOH), N-[3-(dimethylamino)propyl] methacrylamide (DMAPMA, -N(CH 3 ) 2 ), and [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SBMA, zwitterionic). Comprehensive characterisations supported successful surface grafting while preserving the membrane substructure. Fouling and separation were assessed using polystyrene (PS) NPs having different surface chemistries, i.e., PS, PS-COOH and PS-NH 2 , revealing strong charge- and hydration-dependent behaviours. The SBMA-g-PES membrane exhibited the best overall performance, achieving NP rejection above 95%, a high pure water flux of 142 LMH, and a flux recovery ratio of 95%, compared with 137 LMH and 58% flux recovery for the neat PES membrane. This enhancement was attributed to a dense hydration layer and smoother surface morphology. Beyond SBMA, the MMES-g-PES membrane showed strong resistance to negatively charged PS-COOH fouling, whereas DMAPMA-g-PES most effectively mitigated fouling by positively charged PS-NH 2 , highlighting clear structure-function correlations. Multi-stage Hermia modelling showed that surface modification suppressed early-stage pore-blocking and substantially slowed subsequent cake-layer growth, with SBMA-g-PES exhibiting the lowest long-term cake-filtration constants, approximately 36-48 times lower than neat PES across the tested NP types. This study provides mechanistic insights into charge-driven NP-membrane interactions and informs the design of antifouling UF membranes for organic contaminant removal.