The conversion of metal ions to metals particles contributes the photoaging of microplastics in aquatic environments

Metals ions enriched on the surface of Microplastics (MPs) may undergo redox reactions under UV irradiation to promote its photoaging. However, there is a paucity of information on studies of the aging behavior of MPs mediated by different metal ions. This study aims to investigate the effects of three different metal ions (Ag+, Cu2+ and Al3+) on the photoaging properties of polypropylene (PP) to enhance our comprehension of their aging in water contaminated by heavy metals. Physicochemical analyses, like the ratio of O/C and carbonyl index (CI), suggested that metal ions accelerated the aging of PP. The correlation model was established between CI and aging time to confirm that the aging sequence of PP in three metal ions solution was Ag+-PP>Cu2+-PP>Al3+-PP. Electron Paramagnetic Resonance (EPR) results showed metal ions enhanced the generation of reactive oxygen species (center dot O-2(-), HO center dot and O-1(2)) to accelerate aging through free radical pathways. Electronic exchange capability (EEC) and X-Ray Photoelectron Spectroscopy (XPS) confirmed metal particles were on PP, leading to the e-bridge for electronic relays that altered the EEC of PP and accelerated its protonation process. It needs to note the surface plasmon resonance (SPR) effect of metal nanoparticles may also accelerated the photoaging of MPs in UV-vis illumination. For the first time, the study establishes a connection between different valence states metal ion and PP aging, shedding light on interfacial and free radical aging process of MPs. The finding also proposes the potential e-bridge or SPR effect of metal particles may exist in accelerating photoaging reaction of PP in special water bodies contaminated with heavy metals. The environmental implications and ecological risks of this phenomenon warrant greater consideration.