Accelerated transformation and enhanced gastrointestinal toxicity of tire wear particles during photo-chlorination mediated by reactive species.

Elevated concentrations of microplastic in surface water and wastewater necessitate understanding their transformation during disinfection and subsequent discharge. This process occurs inevitably when microplastics enter disinfection systems or are discharged from wastewater treatment plants. However, the behavior of tire wear particles (TWPs), an important and complex microplastic component, remains poorly understood during photo-chlorination. Here, we show that photo-chlorination significantly accelerates TWP transformation and degrades the released organic matter compared to dark-aging, photo-aging, or chlorination alone. Specifically, photo-chlorination induces extensive physical damage, surface oxidation, and chlorine addition in TWPs while simultaneously consuming released organic matter to produce a broader diversity of byproducts. The photolysis of free chlorine generates hydroxyl radicals (HO•) and O3, which primarily drive TWP oxidation, while reactive chlorine species (RCS) and HO• induce chlorination reactions. The oxidative effects of HO• and O3 on TWPs and released organic matter subsequently enhance RCS-mediated degradation of these organic compounds. Critically, photo-chlorinated TWPs and their transformation products exhibit enhanced cytotoxicity toward human gastric epithelial cells, as well as intensified proliferative toxicity and nutrient uptake inhibition in mouse small intestinal organoids. These findings provide new insights into the environmental behavior of TWPs and highlight their potential toxicological implications in impacted water systems.