Photoaging-Driven Transformation of Tire Wear Particles: Unraveling the Spatiotemporal Dynamics of Persistent Free Radicals and Their Phytotoxic Impact

Tire wear particles (TWPs) undergo intricate weathering that significantly influences their environmental behavior. However, the spatiotemporal evolution of environmentally persistent free radicals (EPFRs) and their associated phytotoxicity remain poorly understood. This study addresses these knowledge gaps through a 60-day lettuce cultivation experiment conducted in soils amended with both pristine and photoaged TWPs of different particle sizes. Our results reveal a photoaging-dependent response in EPFR concentrations, peaking at moderate exposure (equivalent to 280 days of sunlight) before declining with extended aging (equivalent to 605 days of sunlight). Comparatively, stable EPFRs demonstrate higher persistence than reactive EPFRs. EPFR stability also exhibits strong particle size dependence, with TWP diameters <100 μm showing faster decay than larger particles (100-425 μm) under extended simulated irradiation (280 → 605 days). Comprehensive analyses of plant physiological parameters and antioxidant responses, combined with EPFR quenching experiments, demonstrate that the phytotoxicity of TWPs to lettuce is predominantly mediated by EPFR concentration dynamics and photoaging-induced reactive oxygen species (ROS) production. Interestingly, smaller TWPs show decreased ROS generation owing to restricted soil aeration but induce comparable or greater toxicity to lettuce and soil microorganisms than larger particles, highlighting particle size as a critical regulator of TWP toxicity beyond oxidative mechanisms.