Mechanistic insights into electrocoagulation-driven removal of polystyrene nanoplastics from urban treated wastewater

The occurrence of nanoplastics in urban treated wastewater represents an emerging environmental risk due to their high mobility, bioavailability, and documented toxicity toward aquatic organisms. Conventional wastewater treatment processes are largely ineffective at retaining nanoplastics, enabling their continuous discharge into receiving water bodies. In this study, electrocoagulation (EC) using aluminum and iron electrodes was evaluated as a tertiary treatment for the selective removal of polystyrene nanoplastics (PS-NPs, 175 nm, 20 mg L^-1) from a simulated urban treated wastewater matrix, at current densities ranging from 1 to 10 mA cm^-2. Complete removal of PS-NPs was achieved under all operating conditions, as evidenced by >99% turbidity reduction and the full elimination of the TOC fraction associated with nanoplastics, while dissolved organic matter remained unaffected. Aluminum electrodes exhibited faster removal kinetics at low current density (1 mA cm^-2), whereas both materials showed comparable performance at higher current densities. Mechanistic analysis combining metal dissolution profiles, zeta potential evolution, coagulant-pH speciation diagrams, X-ray diffraction, and transmission electron microscopy demonstrated that nanoplastics destabilization and capture are governed by sweep flocculation driven by in-situ generated metal (oxy)hydroxides. The untreated wastewater exhibited significant acute toxicity toward Aliivibrio fischeri (EC50 = 30%), which was completely suppressed within 20 min of electrocoagulation treatment for both electrode materials, without the need for oxidative degradation or mineralization. To the best of the authors' knowledge, this is the first study to evaluate electrocoagulation for nanoplastics removal in a complex, realistic wastewater matrix and the first to directly link physicochemical removal performance to the abatement of nanoplastics-driven acute ecotoxicity. These findings highlight electrocoagulation as a robust and selective polishing step capable of eliminating nanoplastics-associated toxicity from treated wastewater effluents.