Photo-Fenton treatment of tire wear particles: structural evolution, mineralization, and the kinetic bottleneck of silica fillers

Tire wear particles (TWPs) are a major source of aquatic microplastic pollution. Because conventional wastewater treatment plants struggle to fully mitigate their continuous release, Advanced Oxidation Processes (AOPs) are increasingly investigated as advanced water treatments for their elimination. This study evaluates the degradation of cryogenic milled tire tread (CMTT) particles of a passenger car tire, a reliable TWP proxy, using a homogeneous photo-Fenton process under ambient conditions to elucidate the structural evolution and mass transfer limitations of filler-loaded elastomers. Initial characterization revealed a silica-rich matrix containing leachable organic additives, including vulcanization accelerators and antiozonants. The photo-Fenton system demonstrated high efficiency, achieving a greater than 90% organic mass reduction and an 88% total organic carbon (TOC) mineralization in 24 h. Furthermore, thermal and spectroscopic analyses of the solid residues indicated selective polymer degradation, suggesting that natural rubber (NR) is consumed significantly faster than the synthetic rubber (SBR/BR) fraction. Kinetic analysis showed that TWP organic conversion was well described by an ash layer diffusion-controlled Shrinking Core Model. Because the highly aggregated silica filler is non-oxidizable, it accumulates as the organic rubber degrades, forming a rigid physical barrier that maintains the initial macroscopic particle size. This continuously thickening silica layer impedes intraparticle diffusion of hydroxyl radicals, making it the primary kinetic bottleneck of the system. Ultimately, while the photo-Fenton treatment successfully mineralizes the polymeric fraction of TWPs, the specific inorganic filler formulation dictates the overall oxidation rate and leaves behind a persistent silica-rich shell. These findings suggest that while AOPs hold potential for advanced water treatment, their overall efficiency may be constrained by the persistent inorganic filler matrix. Furthermore, the applied photo-Fenton system could serve as a complementary accelerated aging tool to help simulate oxidative weathering and better understand the long-term degradation pathways of tire wear particles in aquatic ecosystems.