Promoted photodegradation of cadmium pigment-embedded microplastics: Role of reactive microenvironment

The environmental fate of microplastics, largely derived from plastic fragmentation, is strongly influenced by their photodegradation behavior. While polymer degradation has been widely studied, less attention has been paid to the role of additives, particularly pigments used in colored plastics, in modulating these processes. This study investigates microplastics colored with cadmium pigments in three representative host polymers-polystyrene (PS), polypropylene (PP), and polyethylene (PE). The reactive microenvironment generated by the pigment includes electrons, holes, and protons through photo-induced reactions. Under this environment, PS was the most affected and exhibited accelerated degradation, attributed to proton attack at benzylic positions facilitated by its conjugated aromatic structure. Furthermore, we propose an additional pathway, where the photo-induced electron reduces the proton-induced carbocation to a radical, which will subsequently undergo oxidation reactions. Photodegradation, promoted by the pigment, correlates with its photodissociation and the concurrent release of Cd²⁺. This effect is due to surface morphological changes that increase pigment exposure to irradiation. These findings demonstrate that pigment-polymer interactions significantly reshape degradation pathways. Notably, the accelerated degradation could increase the release of hazardous additives into water bodies, thus amplifying the environmental risks and revealing deficiencies in existing risk management strategies associated with microplastics.