Regulatory role of natural organic matter in magnetic capture of functionalised polystyrene nanoplastics: Heteroaggregation, attachment performance and life cycle sustainability.

Natural organic matter (NOM) influences the interactions between nanoplastics and magnetic particles; however, the mechanisms governing NOM-mediated attachment of functionalised nanoplastics during magnetic capture remain unclear. This study evaluated the capture of pristine polystyrene (PS) and carboxylated PS (PS-COOH) nanoplastics on magnetically modified lignite (MML) in NOM-containing waters. Mechanistic analyses revealed that nanoplastic attachment was controlled by collision efficiency and interfacial interactions rather than by NOM adsorption alone. Capture was enhanced by heteroaggregation promotion at low NOM concentrations and inhibited by surface-site masking and electrosteric stabilisation at high NOM concentrations. Nanoplastic attachment followed pseudo-first-order kinetics and Langmuir-type saturation behaviour, with PS-COOH exhibiting stronger attachment affinity than pristine PS. Attachment efficiency measurements, Derjaguin-Landau-Verwy-Overbeek analysis and X-ray photoelectron spectroscopy confirmed that nanoplastic capture exhibited a concentration-dependent transition, shifting from enhanced to suppressed as NOM concentration increased. Artificial neural network-SHapley additive explanation analysis identified nanoplastic concentration as the dominant factor, followed by NOM concentration. During continuous-flow experiments using real wastewater, MML achieved over 90% nanoplastic removal. Life cycle assessment identified acid consumption and electricity demand as key environmental hotspots. This study provides mechanistic insights into NOM-regulated magnetic nanoplastic capture and supports sustainable remediation strategies in complex aquatic environments.