Photolysis of p-phenylenediamine rubber antioxidants in aqueous environment: Kinetics, pathways and their photo-induced toxicity

The widespread use of rubber antioxidants, especially p-phenylenediamines (PPDs), has raised increasing concerns about their risk assessment. However, there is a notable lack of research on their transformation products (TPs). Photolysis, influenced by active components, plays a significant role in the environmental fates of PPDs. This study investigated four emerging PPDs (N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), N, N′-diphenyl-p-phenylenediamine (DPPD), N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD), and N-cyclohexyl-N′-phenyl-p-phenylenediamine (CPPD)) through a combination of experiments (photolysis kinetics, quenching experiments, acute toxicity test to Vibrio Fischeri (V. fischeri) and identification of photolytic products) and theoretical calculations. The results revealed different pathways for indirect photolysis mediated by the hydroxyl radicals (•OH) and singlet oxygen (1O2) of DPPD and IPPD under simulated sunlight irradiation. The effects of dissolved organic matter (DOM) and fulvic acid (FA) on the rates of photolysis of PPDs highlighted the complex interactions among the molecular structure, light absorption properties, and environmental variables. Quenching for reactive oxygen species (ROS) reduced photo-induced toxicity, whereas the addition of DOM and FA increased it, suggesting the crucial role of ROS in the formation of more toxic photolytic products. The study of photolysis pathways and the evaluation of the health risks provide a comprehensive understanding of the environmental effects of these pollutants.