Transformation Products of Tire Rubber Antioxidant 6PPD in Heterogeneous Gas-Phase Ozonation: Identification and Environmental Occurrence

6PPD, a tire rubber antioxidant, poses substantial ecological risks because it can form a highly toxic quinone transformation product (TP), 6PPD-quinone (6PPDQ), during exposure to gas-phase ozone. Important data gaps exist regarding the structures, reaction mechanisms, and environmental occurrence of TPs from 6PPD ozonation. To address these data gaps, gas-phase ozonation of 6PPD was conducted over 24-168 h and ozonation TPs were characterized using high-resolution mass spectrometry. The probable structures were proposed for 23 TPs with 5 subsequently standard-verified. Consistent with prior findings, 6PPDQ (C18H22N2O2) was one of the major TPs in 6PPD ozonation (similar to 1 to 19% yield). Notably, 6PPDQ was not observed during ozonation of 6QDI (N-(1,3-dimethylbutyl)-N '-phenyl-p-quinonediimine), indicating that 6PPDQ formation does not proceed through 6QDI or associated 6QDI TPs. Other major 6PPD TPs included multiple C18H22N2O and C18H22N2O2 isomers, with presumptive N-oxide, N,N '-dioxide, and orthoquinone structures. Standard-verified TPs were quantified in roadway-impacted environmental samples, with total concentrations of 130 +/- 3.2 mu g/g in methanol extracts of tire tread wear particles (TWPs), 34 +/- 4 mu g/g-TWP in aqueous TWP leachates, 2700 +/- 1500 ng/L in roadway runoff, and 1900 +/- 1200 ng/L in roadway-impacted creeks. These data demonstrate that 6PPD TPs are an and class of contaminants in environments.

Automated summary

It was found that 6PPD, a tire rubber antioxidant, can form a highly toxic quinone transformation product (6PPDQ) during exposure to gas-phase ozone. Gas-phase ozonation of 6PPD was conducted and the ozone transformation products (TPs) were characterized. The major TPs included 6PPDQ, multiple isomers of C18H22N2O and C18H22N2O2, with presumptive N-oxide, N,N'-dioxide, and orthoquinone structures. The concentrations of these TPs were quantified in environmental samples, such as tire tread wear particles, aqueous leachates, roadway runoff, and roadway-impacted creeks.