Accelerated Transformation of 6PPD to 6PPD-Q in Tire Wear Particles Driven by Roadway Manganese Oxides and Dry-Wet Cycles: Interfacial Catalysis Coupled with Climatic Stressors

This study reveals the synergistic mechanism whereby roadway manganese oxides and dry-wet cycling accelerate N-(1,3-dimethylbutyl)-N’-phenylenediamine (6PPD) oxidation to toxic 6PPD-quinone (6PPD-Q) in tire wear particles (TWPs). Three Mn-loading strategies (Mn²⁺ adsorption, MnO_x coating, in-situ δ-MnO₂ synthesis) coupled with simulated dry-wet cycles (12 h cycle: 10 h drying + 2 h rain spray) demonstrated that in-situ MnO_x-loaded TWPs (4.2 mg MnO_x/g) achieved the highest 6PPD-Q yield (3.48 mg/L), following sigmoidal kinetics (R² = 0.999). Key mechanisms include: 1) Wet phase: Mn(III) (↑650%) and triple excited states of dissolved organic matter (³DOM*) in water films mediated O₂•⁻ generation, attacking 6PPD to form 6PPD-OO•, which converted to 6PPD-Q via proton-coupled electron transfer (PCET); 2) Dry phase: Environmental Persistent Free Radicals (EPFRs) accumulated (g-factor shift: 2.0031 to 2.0041) and pre-activated Mn sites (Mn(III)* ↑ 90.29%); 3) Rewetting phase: Dissolved Mn(II)/Fe²⁺ triggered Fenton-like reactions, generating •OH (5.0 × 10⁹ spins/μL) to oxidize 6PPD. Scavenger experiments confirmed ROS (•OH/O₂•⁻) as critical bridges for Mn catalysis (6PPD-Q yield ↓ 90%). Fulvic acid (FA) derivatives prolonged 6PPD-Q half-life to 53.7 h via quinone complexation, enhancing environmental persistence. This study provides a climate-mineral regulation strategy for source control of 6PPD-Q, highlighting the critical need to manage roadway Mn-containing materials and dry-wet cycling impacts to mitigate aquatic ecosystem risks.