Thermal Oxidation Increases Dissolved Carbon Leaching through Altered Microplastic Structural Chemistry and Accelerated Release Kinetics

We assessed the leaching potential of dissolved carbon (DC) from polyvinyl chloride (PVC) and polyethylene terephthalate (PET) microplastics (MPs), selected for their differing polymeric structure and response to thermal oxidation. Although thermal oxidation alters the chemical structure and reactivity of MPs, the mechanisms influencing DC leaching remain poorly understood. We quantified physicochemical changes resulting from thermal oxidation at 100 and 250 °C followed by leachate kinetic modeling. Thermal oxidation decreased chlorine (Cl) concentration by 96% in PVC, increased carbonyl index from 0.397 to 0.504 in PET, and increased oxidation index from 0.151 to 0.506 in PVC. No significant DC leaching occurred in PET; however, leaching increased 15-fold in PVC 250 °C compared to nonoxidized PVC. Leaching kinetics for all PVC oxidation conditions were best fit to the biphasic model, although goodness of fit decreased with thermal oxidation, with adjusted R2 values decreasing from 0.95 in nonoxidized PVC to 0.85 to 0.82 in PVC exposed to 100 and 250 °C, respectively. These findings highlight the distinct chemical fingerprint and unique leaching behavior of thermally oxidized MPs, providing new insights about their release of DC at burn sites or following wildland-urban interface fire events.