Ecotoxicity of three typical tire wear particles to periphytic biofilms: The potentiating role after natural water-incubation-aging.

Tire wear particles (TWPs), abundant in the aquatic environment, pose potential ecological risks, yet their implications have not been extensively studied. Rolling friction TWPs, sliding friction TWPs (S-TWPs) and cryogenically milled tire treads were used as research objects to study the ecotoxicity and difference of the above materials before and after aging in natural water (AS-TWPs) to the periphytic biofilm. The results showed that there were significant differences in the microstructure, surface elements, size, functional groups and environmentally persistent free radicals (EPFRs) of the three TWPs. After aging in natural water, the properties of the three TWPs mentioned above showed homogenization, but the EPFRs and reactive oxygen species (ROS) yield were different. After exposure to TWPs (10 mg L-1), total organic carbon and adenosine triphosphate decreased significantly (p < 0.05), and the production of extracellular polymeric substances (EPS) in the periphytic biofilm increased, in which the content of humic-like substance and proteins (tryptophan protein and humic acid-like substances) increased obviously. The increment of TB-EPS was higher than that of LB-EPS, and S-TWPs and AS-TWPs had the strongest promoting effect on EPS secretion. In addition, 10 mg L-1 TWPs caused massive cell death in the periphytic biofilm, which was more obvious in the S-TWPs and AS-TWPs exposure group. The toxic mechanism of TWPs promotes intracellular ROS accumulation and leads to the release of lactate dehydrogenase, which was attributed to the formation of EPFRs on the surface of TWPs and an increase in EPFRs intensity after aging in natural water. TWPs at environmentally relevant concentrations (0.1 mg L-1) had no biological toxicity to periphytic biofilms. This study fills the gap in the study of the surface structure characteristics of TWPs on the toxicity of periphytic biofilms, and is of great significance to the study of the aquatic toxicity mechanism of TWPs.

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.