Strong Photochemical Activity of Colored Microplastics Containing Cadmium Pigments: Mechanisms and Implications

Plastics used in daily life are often colored for esthetic and functional purposes. Nevertheless, little is known about the photochemical activity of colored microplastics and the associated risks that are ubiquitous in the environment. In this study, we report the strong photochemical activity of microplastics colored with cadmium pigments. These colored microplastics can be excited by photons within the solar spectrum (<514 nm), readily generating •OH, O₂^•–, and H₂O₂. Consequently, they can effectively degrade 17β-estradiol, achieving >91% degradation within 23 h under simulated solar exposure. Among microplastics colored with different cadmium pigments, those with a cadmium pigment S/Se ratio of 2:5 exhibited the highest photoactivity. This is attributed to the narrow band gap, fast charge separation, and efficient charge transfer of the microplastics, as suggested by the energy band, photocurrent, and electrochemical impedance results. Meanwhile, hazardous Cd²⁺ was leached from colored microplastics mainly owing to the oxidation of pigment lattices by photogenerated holes. Our results reveal that microplastics colored with photoactive inorganic pigments behave drastically differently from uncolored counterparts. This highlights the importance of considering pigments as a critical factor for better assessing the environmental fate and risks of colored microplastics and plastic products.