Differential Morphological and Physicochemical Responses of Polyvinyl Chloride and Polyamide-12 Micro- and Nanoplastics to Fenton Oxidation

Oxidative aging of micro- and nanoplastics (MNPs) alters their physicochemical properties and may influence their environmental persistence and biological interactions. Here, we investigated the effects of Fenton oxidation on 5µm sized polyvinyl chloride (PVC) and polyamide-12 (PA-12) MNPs using a multimodal characterization approach. Particles were treated with a Fenton reagent mixture (1 mM FeSO₄, 10 mM H₂O₂) at 40 °C for 2 hours to simulate oxidative conditions relevant to advanced water treatment and processes. Helium Ion Microscopy revealed that pristine PVC particles were aggregated and spherical, while oxidized PVC exhibited extensive aggregation and surface roughening. In contrast, PA-12 maintained a largely dispersed morphology post-oxidation, with moderate increases in surface texture. Dynamic light scattering measurements showed that the hydrodynamic diameter of PVC increased from 300–400 nm (pristine) to 700–900 nm (oxidized), with polydispersity index broadening and a new minor population emerging at 80–90 nm, indicating aggregation and polymer fragmentation. Oxidized PA-12 showed a stable size profile with minor peak shifts. X-ray photoelectron spectroscopy revealed a 150.0% increase in surface oxygen on PVC (from 8.2% to 20.5%) and a 126.5% increase on PA-12 (from 11.3% to 25.6%). PVC showed a 17.1% reduction in chlorine and PA-12 experienced a 62.9% nitrogen loss, consistent with dechlorination and amide degradation. Attenuated total reflectance Fourier-transform infrared spectroscopy confirmed these chemical transformations, showing new bands for hydroxyl (O–H stretch, ~3400 cm⁻¹), C–O (1000–1200 cm⁻¹), and possible CO₂-related species (~2300 cm⁻¹), alongside diminished amide II signals in oxidized PA-12. These findings demonstrate that Fenton oxidation induces significant morphological and chemical transformation in PVC and PA-12, with PVC exhibiting greater aggregation and surface restructuring, while PA-12 maintains greater colloidal stability but undergoes substantial surface oxidation and nitrogen loss. These oxidative transformations may enhance the surface reactivity, inflammatory potential, bioavailability, and ultimately influence the biological uptake, fate, and transport of MNPs in human exposure pathways, particularly through inhalation or ingestion. Our findings highlight the need to consider oxidation processes, such as those used in advanced water and wastewater treatment, in health risk assessments of plastic particles in both ambient and engineered settings.