ROS-mediated photoaging pathways of nano- and micro-plastic particles under UV irradiation

Reactive oxygen species (ROS) generation is considered as an important photoaging mechanism of microplastics (MPs) and nanoplastics (NPs). To elucidate the ROS-induced MP/NP aging processes in water under UV365 irradiation, we examined the effects of surface coatings, polymer types and grain sizes on ROS generation and photoaging intermediates. Bare polystyrene (PS) NPs generated hydroxyl radicals (center dot OH) and singlet oxygen (O-1(2)), while coated PS NPs (carboxyl-modified PS (PS-COOH), amino-modified PS (PS-NH2)) and PS MPs generated fewer ROS due to coating scavenging or size effects. Polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate and polycarbonate MPs only generated center dot OH. For aromatic polymers, center dot OH addition preferentially occurred at benzene rings to form monohydroxy polymers. Excess center dot OH resulted in H abstraction, C-C scission and phenyl ring opening to generate aliphatic ketones, esters, aldehydes, and aromatic ketones. For coated PS NPs, center dot OH preferentially attacked the surface coatings to result in decarboxylation and deamination reactions. For aliphatic polymers, center dot OH attack resulted in the formation of carbonyl groups from peracid, aldehyde or ketone via H abstraction and C-C scission. Moreover, O-1(2) might participate in phenyl ring opening for PS NPs and coating degradation for coated PS NPs. This study facilitates understanding the ROS-induced weathering process of NPs/MPs in water under UV irradiation.