Critical Impact of Colored Pigments on the Long-Term Photoaging of Polyethylene Microplastics in Coastal Seawater Environments

In this work, we developed the long-term photoaging of polyethylene microplastics (PE MPs) in coastal seawater ecosystems, focusing on pigment-dependent effects. After 0.5 g of MP was exposed to 20 mL of coastal seawater and underwent 12-day simulated ultraviolet (UV) irradiation (UV365 = 10 mW/cm2), the intensity of crystalline region peaks in the PE MPs decreased in the following order: red (6.11%) > blue (2.11%) > green (0.45%), corresponding to the increase in carbonyl index after UV aging (red (298.6%) > blue (376.4%) > green (192.9%)). This disparity can be attributed to the susceptibility of the red pigment adhered to the surface of MPs, which is more prone to absorb UV light photons under sunlight irradiation, leading to the generation of higher levels of oxidative free radicals, including hydroxyl radicals (·OH, red: 7.67 × 10–15 M; green: 3.43 × 10–15 M; blue: 5.34 × 10–15 M) and superoxide anions (O2·–, red: 120.62 ± 9.31 μM; green: 66 ± 1.32 μM; blue: 95.97 ± 0.88 μM), thereby accelerating the photoaging of MPs during prolonged exposure in coastal seawater. Regarding color, which is regarded as a significant yet often overlooked factor influencing MP phototransformation, the obtained findings herein provide methodological strategies to elucidate the formation and ecological risks associated with MPs and nanoplastics in marine environments.