Aquatic Thermal and Photochemical Reactivity of N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), N-Isopropyl-N′-phenyl-p-phenylenediamine (IPPD), and 6PPD-quinone

A ubiquitously used tire rubber antidegradant, 6PPD (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine), and its toxic ozonation product, 6PPD-quinone (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine quinone), have become recognized as important environmental pollutants since 6PPD-quinone (6PPD-Q) was identified as the likely cause of decades of mass Coho salmon kills. The reactivity of 6PPD, 6PPD-Q, and similar phenylenediamines requires study to better understand their environmental fate. This study explores the aquatic reactivity of 6PPD, N-isopropyl-N′-phenyl-1,4-phenylenediamine (IPPD), and 6PPD-Q through thermal and photochemical pathways using both steady-state photochemistry and time-resolved laser spectroscopy techniques. 6PPD was found to rapidly degrade in the dark, with its degradation rate being highly dependent on the pH, temperature, and oxygen concentrations. IPPD behaves similarly to 6PPD. In contrast, 6PPD-Q is much more stable in the dark. All three chemicals are degraded via direct photochemistry. Regarding indirect photochemistry, ³CDOM* plays a role in the degradation of 6PPD and IPPD but not 6PPD-Q, while ¹O₂ does not play a significant role for any of the compounds. Reaction rate constants are reported as well as 6PPD-Q molar yields from 6PPD, which were minimal for all aqueous pathways examined. 6PPD-Q may have a longer environmental lifetime as there are fewer degradation pathways. This research will help us to better understand and control these chemicals in the environment.