Mechanistic insights into electric field-driven specific ion effects on nanoplastics aggregation in heavy metal co-contaminated aquatic systems

The concurrent pollution of nanoplastics and heavy metals poses emerging threats to relevant aquatic systems. To elucidate the interfacial mechanisms governing nanoplastics-heavy metal aggregation, specifically clarify the unreconized role of electric fields on these processes, systematic investigation of carboxyl-modified polystyrene nanoplastics (PNs) aggregation mediated by Pb²⁺, Cu²⁺, Cd²⁺ and Zn²⁺ were conducted. Quantitative analyses revealed distinct specific ion effects on PNs aggregation, as evidenced by critical coagulation concentrations (CCC: Pb (1.6 mM) < Cu (4.6 mM) < Cd (13.9 mM) < Zn (16.6 mM)) and effect magnitudes intensifying proportionally to electric field strength. Further theoretical calculations and correlation analyses demonstrated that the strong interfacial electric field (-1.23 ×10⁸ V/m) at PNs surfaces induces polarization-enhanced induction force and polarization-induced covalent bonding between heavy metal cations and O on the PNs surface. These electric field-dependent interactions contributed more than 41 % to PNs aggregation, accounting for the observed specific ion effects, corroborated by FTIR and XPS results. Our findings provide a paradigm shift in understanding nanoplastics fate, offering critical insights for predictive modeling and remediation strategies of multi-pollutant in relevant watersheds.