Impacts of environmentally persistent free radicals on the denitrification toxicity of photoaged tire wear particles in estuarine sediments

The widespread detection of tire wear particles (TWPs) in estuaries has raised concerns about their potential environmental hazards. However, knowledge of photoaging-induced environmentally persistent free radicals (EPFRs) formation on TWPs in estuarine environments and their impact on sediment denitrification remains limited. This study investigated the formation of EPFRs on TWP during photoaging in estuarine environments and evaluated their effects on sediment denitrification and nitrous oxide (N₂O) accumulation. Sixty days of illumination increased EPFR concentration on TWPs by 373%, with the generated EPFRs persisting in sediments for over 20 days. Exposure to pristine TWP (PTWP) reduced denitrification rates by 10.3 ± 5.6% and increased N₂O accumulation by 18.3 ± 4.5%. Further exposure to photoaged TWP (ATWP) under 10–60 days of illumination expanded denitrification suppression and N₂O accumulation to 28.1 ± 7.1–42.5 ± 6.6% and 18.8 ± 4.3–31.7 ± 4.6%, respectively. EPFRs exacerbated the accumulation of reactive nitrogen species in sediment and compromised the antioxidant systems. Structural equation modeling confirmed that EPFRs indirectly suppressed denitrification rates by directly impairing microbial processes involved in carbon metabolism and electron transfer. This study is the first to report that the formation of EPFRs enhances the negative effects of ATWP on the sediment’s nitrogen cycle, offering valuable insights for assessing the ecological risks associated with TWP.