Sunlight-Induced Transformation of Tire Rubber Antioxidant N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) to 6PPD-Quinone in Water

The huge consumption of the tire rubber antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) has resulted in pervasive contamination in aquatic environments. More importantly, the transformation product of 6PPD, i.e., 6PPD-quinone (6PPD-Q), is raising increasing concerns due to its high toxicity to aquatic organisms. However, whether and how 6PPD-Q can be formed from 6PPD in aquatic environments remain unknown. Herein, we reported the transformation of 6PPD to 6PPD-Q in water under sunlight irradiation and elucidated the underlying mechanisms. In result, the transformation of 6PPD to 6PPD-Q in water was confirmed with a noteworthy molar yield of ∼1.01% within 90 min at pH 7.0 under simulated sunlight irradiation. The experimental observations, including transient and stable intermediate identification and reactive species quenching, evidenced that the transformation pathways started with photoexcitation of 6PPD and followed by generation of hydroxyl intermediates to form 6PPD-Q. This study revealed that sunlight-induced transformation of 6PPD could be an important origin of 6PPD-Q in aquatic environments, providing significant insights to the potentially underestimated ecological risks of 6PPD.