Photon-induced redox chemistry on pyrite promotes photoaging of polystyrene microplastics

The mineral particles in sediment could affect polystyrene microplastics (PS-MPs) prosperity through physical and chemical interactions. Pyrite with semiconducting properties is the most abundant metal sulfide mineral in the sediments of lake and river mouths. The widespread sunlight and the coexistence of PS-MPs and pyrite in lake or river water due to frequently water fluctuation is a typical photoaging environment for PS-MPs. The oxidation of reactive oxygen species (ROS) generated from pyrite would degrade the PS-MPs in theory. However, researches about photoaging of PS-MPs mediated by pyrite are paucity. Here, we investigated the photoaging process of PS-MPs affected by pyrite under simulated light condition. Remarkably, surface morphology of PS-MPs mediated by pyrite was broken. And the oxygen-containing functional group of PS-MPs increased, as revealed by Fourier Transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and contact angle test. 2D-COS analysis showed photoaging of PS-MPs with pyrite happened in the following order: C-H > C=C > C=O > C-O > OH. The photoaging of PS-MPs and transformation of intermediate were accelerated by ROS (O₂·^-, ·OH and ¹O₂) generated from pyrite. The free ·OH may play a major role in the promotion. Because the interfacial ROS reactions on pyrite surface were limited due to the electrostatic repulsion between pyrite and PS-MPs. The study explored photoaging behavior of PS-MPs accelerated by pyrite, which could be helpful for understanding photon-induced redox chemistry on PS-MPs via widespread sulfide metal minerals on earth's surface and providing further information to assess potential risks of PS-MPs.