Aged Polystyrene Microplastics Accelerate the Photo-Reduction of Chromium(VI)

Microplastics (MPs) and hexavalent chromium (Cr(VI)) are typical environmental pollutants, yet their interactions in aquatic systems remain poorly understood. This study investigates the mutual influence between Cr(VI) and both virgin and aged polystyrene microplastics (PS-MPs) under light conditions. Concentration kinetics revealed that the total chromium concentration remained stable across all systems, while Cr(VI) concentrations decreased over time, indicating that PS-MPs accelerate the reduction of Cr(VI) to Cr(III). Conversely, it had been found that Cr(VI) promoted the aging of PS-MPs, and this was evidenced by an increase in surface roughness and the generation of oxygen-containing functional groups. Cr(VI) led to a rise in the O/C ratio and carbonyl index, providing additional evidence for the aging of PS. Two-dimensional correlation spectroscopy (2D-COS) elucidated that under Cr(VI) exposure, the order of functional group alterations in PS and aged PS exhibited an opposite trend. Additionally, three-dimensional fluorescence spectroscopy revealed distinct changes in the fluorescence characteristics of leached substances from aged and pristine PS, both with and without Cr(VI), under light and dark conditions. These results furnish innovative understandings of environmental behavior and risks associated with the co-occurrence of MPs and heavy metals, highlighting the complex interplay between Cr(VI) and PS-MPs in aquatic environments.