Quantifying the critical regulator role of irradiation-enhanced electric field in nanoplastics aggregation with heavy metals

The co-occurrence of UV-irradiated nanoplastics and heavy metals (HMs) in aquatic environments raises significant environmental concerns, yet the irradiation-induced surface electric field (E₀) enhancement on nanoplastics and its implications for colloidal stability remain unclear. By integrating experimental characterization with theoretical modeling, this study systematically compared the aggregation kinetics of pristine and aged PET-NPs (polyethylene terephthalate nanoplastics) mediated by Zn²⁺, Cd²⁺, Ca²⁺and Mg²⁺ using dynamic light scattering (DLS), spectroscopic analyses, interaction forces calculation and density functional theory (DFT) simulations. UV irradiation facilitated surface oxidation of PET-NPs, significantly amplifying E₀ from 0.55 to 3.49×10⁸ V/m within 48 h. The measured E₀ intensities exhibited strong positive correlations with both the critical coagulation concentration (CCC) across all cation systems (r > 0.90, p < 0.05) and the CCC differences (ΔCCC) among different cations (r > 0.90, p < 0.05). These findings identified E₀ as a key regulator enhancing colloidal stability of PET-NPs and governing specific ion effects on their aggregation during irradiation. Further calculations revealed that, beyond Coulombic forces, two critical but previously unrecognized E₀-dependent interactions occurred between HM cations and surface oxygen atoms (primarily on the COH groups): polarization-enhanced induction force (PEIF) and polarization-induced covalent bonding (PICB). These interactions contributed nearly 40 % to PET-NPs aggregation in Zn²⁺/Cd²⁺ systems after 10-h of irradiation. These results were strongly validated by FTIR and XPS analyses. Our findings provide fundamental mechanistic insights into the sedimentation dynamics of photoaged nanoplastics and establish a theoretical framework for assessing the environmental risks associated with combined nanoplastics-HMs pollution in aquatic ecosystems.