Neglected but Crucial Role Played by Rainwater in the Photodegradation of Plastic

Plastic pollution poses a worldwide environmental threat, yet limited attention has been paid to its terrestrial fate. Although rainwater has low photochemical activity, its long-term effect on the photodegradation of plastic waste has predominantly remained unexplored. In this study, poly(vinyl chloride) bags, polypropylene masks, polystyrene foam, and polyethylene bags were exposed to sunlight, with or without rainwater, for 12 months under field conditions. The results revealed that the plastics exposed to rainwater released enormous amounts of organic carbon and severely aged microplastics in the first few months. Rainwater-exposed plastics also exhibited strong oxidation, and rainwater altered the order of functional group changes of commercial plastics (except polystyrene foam), as evidenced by two-dimensional correlation spectroscopy (2D-COS) analysis. To provide more direct evidence of the relationship between rainwater exposure and the photodegradation of plastic/microplastic, a laboratory experiment was conducted. The laboratory experiments confirmed the promoting effect of rainwater on photodegradation. Moreover, a stronger signal of reactive oxygen species was detected with an electron paramagnetic resonance spectrometer on microplastics exposed to rainwater residues compared with those not exposed to rainwater, which highlights the significance of rainwater as a crucial external source of reactive oxygen species (ROS). Overall, these findings provide essential insights into the effect of rainwater on plastic photodegradation.