Mechanistic insights into electric field-driven specific ion effects on nanoplastics aggregation in heavy metal...

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^2 + (1.6 mM) < Cu²⁺ (4.6 mM) < Cd²⁺ (13.9 mM) < Zn²⁺ (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.