Enhanced coagulation of nanoplastics: impact of complex water matrices on nanoplastics removal

Coagulation is a promising approach for nanoplastic (NP) removal from water. However, its performance under complex water matrix conditions remains poorly understood. This study investigated the influence of surface-water matrix components, namely humic acid and clay, on NP removal during enhanced coagulation using ferric ions (Fe 3+ ). Fluorescent polystyrene sulphate (PS-OSO 3 − ) nanoplastics were treated in synthetic surface water containing environmentally relevant concentrations of humic acids and clay. Component-specific quantification of nanoplastics, clay and humic acid was achieved using flow cytometry, inductively coupled plasma-optical emission spectroscopy and liquid chromatography-organic carbon detection respectively, enabling independent assessment of their removal and interactions. Individual, binary, and multicomponent systems were systematically investigated to evaluate the independent and combined effects of the matrix components on nanoplastic removal. Our results demonstrate that clay and nanoplastics exhibited similar coagulation behaviour across all experimental conditions, indicating that clay removal may serve as a practical surrogate for predicting nanoplastic removal. In contrast, humic acid significantly inhibited nanoplastic removal at low coagulant concentrations (2–5 ppm), whereas effective nanoplastic removal (>98%) was achieved at 10 ppm coagulant regardless of matrix composition. Humic acid removal showed no correlation with nanoplastics removal, suggesting that different mechanisms govern the removal of these components. These findings improve understanding of coagulation under surface water conditions and provide practical guidance for optimizing nanoplastics removal from complex water matrices.