Heterogeneous ozone oxidation of p-phenylenediamine antioxidants

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q), a transformation product formed from atmospheric ozone oxidation of 6PPD antioxidant, has recently been recognized as an emerging contaminant due to the potential adverse impacts on the environment and human health. While there have been many studies on environmental 6PPD, the atmospheric fate of other commonly used PPDs including N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), N-phenyl-N'-cyclohexyl-p-phenylenediamine (CPPD), and N,N'-diphenyl-p-phenylenediamine (DPPD), remain largely unknown. Here, the heterogeneous ozone oxidation of these three surface-bound PPDs were explored by investigating the reaction kinetics, chemical mechanisms, and hazard properties. The measured heterogeneous rate constants for IPPD, CPPD, and DPPD were (2.28 ± 0.19) × 10^-16, (2.38 ± 0.17) × 10^-16 and (1.77 ± 0.26) × 10^-16 cm³ molecule^-1 s^-1, respectively, equivalent to atmospheric lifetimes of less than 9.9, 9.5, and 12.7 h. The short lifetimes of PPDs suggest their high reactivity toward ozone at atmospherically relevant levels. 72 PPD transformation products were identified from the heterogeneous reactions, many of which have not previously been reported. Furthermore, hazard assessment results indicate that 49 transformation products possess persistent, bioaccumulative, or toxic properties, which warrants attention when assessing the environmental risks of PPDs. Together, the results not only highlight the importance of a comprehensive kinetic and mechanistic investigation of the atmospheric transformation of PPDs, but also offer new insights for prioritizing PPD transformation products for future risk evaluation.