Unveiling the protective role of biofilm formation on the photoaging of microplastics

In natural water, microplastics (MPs) inevitably undergo microbial colonization to form biofilm, while the effect of biofilm formation on the photoaging of MPs remains unknown. This study systematically investigated the photoaging behavior of disposable box-derived polypropylene (PPMPs) and polystyrene (PSMPs) mediated by formed biofilm in water. After incubating in Weihe water samples for 105 d, the biofilm was validated to form in PPMPs and PSMPs, with higher formation in PSMPs. In particular, biofilm formation inhibited the photoaging of PPMPs and PSMPs, with an 11.1% and 50.6% decrease in photo-oxidation compared to their virgin counterparts after 20 d of ultraviolet (UV) irradiation. Moreover, the photoaging route of MPs such as the reaction priority of functional groups (e.g., C–H, C=O, and benzene ring) was altered, indicating the important role of biofilm formation in the photoaging process of MPs. The protective effect of the biofilm was mainly caused by the role of optical light filters that absorbed UV energy by sacrificing themselves. Also, as the main attacker on MPs, the production of reactive oxygen species (ROS, mainly ¹O₂ and •OH) was inhibited by biofilm, which was mainly responsible for the decreased photoaging of MPs. This study revealed the important role of biofilm in the photoaging process of MPs and suggested a higher resistance of (micro)plastics in natural water than that in the laboratory using pure MPs, which contributed to a deeper understanding of the environmental fate and pollution of MPs.