PFAS-Induced Charge Regulation and Aggregation in Polystyrene Nanoplastic Colloids

The co-occurrence of the smallest fraction of plastic debris (nanoplastics, NPLs) together with perfluoroalkyl and polyfluoroalkyl substances (PFASs) represents a major emerging environmental and health risk. Although their combined presence in aquatic systems is increasingly recognized, the physicochemical mechanisms governing their colloidal and interfacial behavior remain poorly understood. Here, the particle charge and aggregation mechanism of polystyrene NPLs were comprehensively studied in the presence of C8 PFASs differing in head groups. In oppositely charged systems, PFAS adsorption on NPLs leads to charge neutralization and reversal at appropriate mass ratios. Rapid particle aggregation was observed near the isoelectric points, where electrical double layers were absent. In contrast, at low and high PFAS concentrations, the development of substantial particle charge gives rise to electrostatically stabilized colloids. The dispersion stability was sensitive to the ionic strength at partial PFAS coverage of the NPL surface, while it was insensitive at high extent of PFAS adsorption. The results provide a unique physicochemical insight into the interfacial and aggregation processes relevant in aqueous systems containing both NPLs and PFASs in various environmental compartments.