A carbon-14 radiotracer-based study on the photo-transformation of polystyrene nanoplastics in water versus in air
Nanoplastics (<1 μm plastics debris) pollution, derived from degradation of larger plastic debris or direct release from cosmetic or cleaning products, becomes an emerging concern in the environment. In this study, we quantitatively investigated the degradation and mineralization of polystyrene (PS) nanoplastics under ultraviolet (UV) radiation at 254 nm using 14C radioisotope tracer technology. 14C-polystyrene (PS) nanoplastics were synthesized from 14C-styrene. Moreover, to study the role of water during the photo-degradation of PS nanoplastics, 14C-PS nanoplastics were exposed to UV radiation in air or suspended in water. The X-ray photoelectron spectroscopy (XPS) results showed that after 48 h of UV irradiation, C−O groups formed on the surface while no significant change was observed from Fourier-transform infrared spectroscopy (FTIR) analysis, indicating that short-term photo-oxidation only happen on the thin surface layer of the PS nanoplastics. The molecular weight (MW) of the PS nanoplastics increased in air after the irradiation, suggesting cross-linking of the PS chains, while it did not show significant changes in the presence of water. The mineralization of the PS nanoplastics was higher in water (17.1 ± 0.55%) than in air (6.17 ± 0.1%). A significant amount (11.0 ± 0.1%) of by-products with small MW was detected in the water during UV irradiation, much higher than that being washed out from the surface of nanoplastics exposed in air. The higher photo-reactivity in water suggests that the mechanisms underlying the photo-transformation of the PS nanoplastics in the two matrices could be different. The present study provided the first evidence of photo-degradation of PS nanoplastics in aqueous surroundings.