Membrane fouling behaviors of microplastic in coagulation-ultrafiltration process: Role of surface functional groups

Global water contamination by persistent microplastics (MPs) poses severe public health risks, yet the influences of MP surface properties on membrane fouling in coagulation-ultrafiltration (CUF) systems remain poorly understood. This study examines the impact of MP surface functional groups on membrane fouling behavior during the CUF process. Functionalized polystyrene (PS) with –COOH and –NH₂ groups was employed to assess their effects on filtration performance and fouling mechanisms. Filtration flux test demonstrated that both PS-COOH and PS-NH₂ effectively mitigate flux decline. PS-COOH significantly mitigates fouling by enhancing charge neutralization and flocculation with polymeric aluminum chloride (PAC), while PS-NH₂ reduces fouling by providing additional binding sites for –COOH groups in sodium alginate (SA) and promoting the formation of larger aggregates through electrostatic interactions, resulting in the development of more substantial and stable flocs. Thermodynamic analysis further confirmed that PS-COOH exhibits a higher repulsion energy barrier (6.19 × 10⁷ kT) than PS-NH₂ (3.79 × 10⁷ kT), effectively delaying the adhesion and accumulation of foulants on the membrane surface. Filtration model analysis showed that pore clogging dominated the early fouling stages, followed by cake filtration. MPs affected fouling layer compactness, leading to fouling progression reduction, particularly with PS-COOH. This study represents the first systematic exploration of how surface functional groups of MPs influence membrane fouling mechanisms in CUF systems. These findings offer novel strategies for optimizing CUF processes in microplastic-contaminated water treatment.