Dissolved organic matter inhibit the degradation process of polystyrene nanoplastic under utraviolet light

Nanoplastics (NPs) have higher surface energy and reactivity due to their nano-scale effects, but there is still uncertainty about the impact of dissolved organic matter (DOM) commonly in the environment on the photoaging process of NPs. This study investigates the ROS photogeneration process of polystyrene nanoplastics (PSNPs) in the presence of DOM under UV irradiation. Our results show that the addition of DOM inhibits the photoreactivity of PSNPs, as evidenced by a smaller reduction in particle size and a decreased abundance of oxygen-containing functional groups on the PSNPs surface. The inhibition mechanism involves light shielding and quenching effects, with lower singlet oxygen (¹O₂) and superoxide radical (O₂^•-) formation when DOM and PSNPs are mixed. The quenching of triplet states (³PSNPs* and ³DOM*) prevents efficient energy transfer to oxygen, thus lowering ¹O₂ formation. Additionally, O₂^•- reacts with DOM's C-H bonds to form hydroperoxide or peroxy radicals, further decreasing ROS production. The photogeneration mechanism of the interaction between PSNPs and DOM is proposed for the first time from the point of view of energy transfer and electron transfer. This study provides a deep understanding of the photoreactivity of PSNPs in natural water environment under UV irradiation and emphasizes the role of DOM.