Photoaging of Typical Microplastics as Affected by Air Humidity: Mechanistic Insights into the Important Role of Water Molecules

Recent studies showed that land is the most important sink for microplastics (MPs); however, limited information is available on the photoaging processes of land surface MPs that are exposed to the air. Herein, this study developed two in situ spectroscopic methods to systematically explore the effect of air humidity on MP photoaging using a microscope of Fourier transform infrared spectroscopy and a laser Raman microscope, which were equipped with a humidity control system. Polyethylene microplastics, polystyrene microplastics, and poly(vinyl chloride) microplastics (PVC-MPs) were used as model MPs. Our results showed that relative humidity (RH) could significantly influence the MP surface oxygen-containing moieties generated from photo-oxidation, especially for PVC-MPs. As the RH level varied from 10 to 90%, a decrease in the photogenerated carbonyl group and an increase in the hydroxyl group were observed. This could be attributed to the involvement of water molecules in the production of hydroxyl groups, which subsequently inhibited carbonyl generation. Moreover, the adsorption of coexisting contaminants (i.e., tetracycline) on photoaged MPs exhibited strong RH dependence, which could be assigned to the varied hydrogen bonding between tetracycline carbonyls and aged MP surface hydroxyls. This study reveals a ubiquitous but previously overlooked MP aging route, which may account for the changed MP surface physiochemical properties under solar irradiation.