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Quantifying the critical regulator role of irradiation-enhanced electric field in nanoplastics aggregation with heavy metals
Summary
Researchers quantified how UV irradiation enhances the breakdown of microplastics by acting as a critical regulator of photodegradation rates in natural waters. The study improves models predicting how quickly microplastics fragment into nanoplastics under sunlight exposure.
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 (E) 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 Zn, Cd, Caand Mg 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 E from 0.55 to 3.49×10 V/m within 48 h. The measured E 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 E 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 E-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 Zn/Cd 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.