The Photodegradation Process of PP Plastics in Tidal Flat Environments: The Role and Mechanism of Chloride Ions

Mudflats are sites of plastic accumulation and a source of microplastics. Moreover, light and chloride ions are important factors affecting plastic aging and microplastic formation in mudflat zones. In this study, the role and mechanism of Cl- in the photodegradation process of polypropylene (PP) in the mudflat environment were investigated by laboratory simulations and field verification. The results showed that the degradation rate constant of PP plastic in the presence of Cl- is greater than that without Cl- and that with increasing photodegradation time, the carbonyl index of PP increases by 1.59 times from 0.22 to 0.35, and the oxygen-carbon ratio increases by 1.3 times, SEM images revealed μm-scale cracks on the surface of PP microplastics, and characterization methods such as EPR and XPS revealed that Cl- is involved in the plastic aging process and that chlorination reactions inside the form of plastics C-Cl bonds and contribute to the generation of OH radicals, thus affecting aging. The presence of Cl- in the field-aged PP plastics is evidence of interfacial chemical reactions that affect the photodegradation of the plastics, which suggests that Cl- influences the environmental degradation of PP plastics.