Transformation Product Formation upon Heterogeneous Ozonation of the Tire Rubber Antioxidant 6PPD (<i>N</i>-(1,3-dimethylbutyl)-<i>N</i>′-phenyl-<i>p</i>-phenylenediamine)

Abstract

Transformation products (TPs) originating within tire tread wear particles (TWPs) are likely pervasive contaminants of roadway environments although their formation, fate, and risks are poorly characterized. Here, we investigated TP formation occurring during heterogeneous reaction of gas-phase ozone with the common tire rubber antioxidant 6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine); exposures included both pure compound and TWP rubbers. Oxidative transformation occurred during ozonation (∼360 ppbv), with up to 81% of 6PPD mass reacting over 6 h. Conversion of 6PPD to 6PPD-quinone (6PPDQ)─a recently reported highly toxic TP─was confirmed at a 9.7% molar yield for pure 6PPD and a 0.95% molar yield for 6PPD present within TWPs, representing likely minima over these time scales and conditions. Leveraging high-resolution mass spectrometry, we identified 19 probable 6PPD-derived TPs in both ozonated 6PPD and TWP samples, underscoring formation of diverse TPs from this antioxidant. By screening environmental samples, nine 6PPD-derived TPs were detected within roadway runoff. The data confirm that when tire rubber antioxidants react with ozone, as intended, they form and release various TPs to surrounding environments.