Photoaging of Terrestrial Plastic Pollution: A Process Affected by Precipitation

Rainwater is the primary water source with which terrestrial plastic pollutants interact. Not only could active substances in rainwater generate additional reactive oxygen species but water could also react with the photoaging intermediates of plastic. Precipitation and evaporation lead to repeated shifts between the solid–liquid and solid–gas interfaces during photoaging. To investigate the impact of these interfaces on photoaging, polyvinyl chloride, polypropylene, polystyrene, and polyethylene microplastics were exposed to UVA, UVB, or UVC radiation in common rainwater (CR), rainwater residue (RR), or rainwater-free (RF) treatments. Seasonal field exposure to the corresponding commercial plastics was also conducted. FT-IR spectroscopy was utilized to analyze the chemical changes in both microplastics and commercial plastics. Compared with RF and RR, CR exposure advanced the relative time of carbonyl production in the photoaging process. A model based on local precipitation and radiation data successfully predicted the carbonyl index of field-exposed commercial plastics and highlighted the importance of alternating interface transitions. The increase in the carbonyl index due to each wet–dry cycle was 1.013–5.460 times greater than that of plastics not exposed to rainwater. These findings indicate that plastics undergo different photochemical reactions on different interfaces, and alternating interface transitions can accelerate the photoaging of plastic.