Sunlight mediated cadmium release from colored microplastics containing cadmium pigment in aqueous phase

The plastics industry is the biggest consumer of cadmium pigments. Little is known about the leaching of hazardous Cd²⁺ from colored microplastics containing cadmium pigment in aquatic systems. Here, we reported the release of Cd²⁺ from different sized microplastics containing cadmium pigment in aqueous phase under simulated sunlight. The release of Cd²⁺ was caused by the photo-dissolution of cadmium pigment. The release kinetics is highly size-dependent. It was relatively low for microplastics with size larger than 0.85 mm but increased significantly with decreasing size for microplastics smaller than 0.85 mm. The polymer matrix was oxidized during light exposure, leading to lower average molecular weight and the formation of oxygen-containing groups. Part of the polymer matrix was degraded into soluble organic carbon under simulated sunlight, resulting in continuous Cd²⁺ release from the pigment particles embedded in the polymer. The polymer degradation rate is also highly size-dependent. The degradation of the polymer matrix and the release of Cd²⁺ were intertwined. Cadmium leaching from microplastics from a commercial product containing cadmium pigment was confirmed in water under simulated sunlight. This work suggests that the photochemical processes of inorganic pigments will lead to the release of heavy metals from colored plastic debris.