Toxic effects of environmentally persistent free radicals (EPFRs) on the surface of tire wear particles on freshwater biofilms: The alleviating role after sewage-incubation-aging

The aquatic ecological risks posed by the surface-active components of tire wear particles (TWPs) are not fully understood. This study aimed to determine the acute (24 h exposure) aquatic toxicity effects of TWPs on freshwater biofilms in terms of total organic carbon (TOC), chlorophyll-a (Chl-a) abundance, quantum yield (ФM), and adenosine triphosphate (ATP). Three types of TWP were tested: TWPs produced via the typical wear of tires and roads (i.e., rolling friction (R-TWPs) and sliding friction (S-TWPs)) and cryogenically milled tire treads (C-TWPs). The results showed that the surface structural properties of the three TWPs differed significantly in morphology, bare composition, functional groups, and surface-active components (environmental persistent free radicals). The exposure of biofilms to the TWPs increased TOC and ATP at low concentrations (1 mg L−1) but inhibited them at high concentrations (50 mg L−1). All TWP types inhibited biofilm photosynthesis (reduced Chl-a and ФM) and altered the community structure of algae to varying degrees; in addition, the toxicity mechanisms of the TWPs contributed to the accumulation of reactive oxygen species and cell membrane (or cell-wall) fragmentation, leading to lactate dehydrogenase release. S-TWPs were the most toxic because their surface carried the highest environmental persistent free radicals. R-TWPs were the second most toxic, which was attributed to their smaller particle size. The toxicity of all TWPs was tested after sewage incubation aging. The results showed that the toxicity of all TWPs reduced as the sewage covered their surface components and active sites. This process also reduced the differences in toxicity among the TWPs. This study filled a research gap in our understanding of aquatic toxicity caused by the surface structural properties of tire microplastics and has implications for the study of microplastic biotoxicity mechanisms. © 2023 Elsevier Ltd