Interfacial photochemical transformation of the rubber antioxidant IPPD on soil minerals: Kinetics, mechanisms, and...

N-isopropyl-N'-phenyl-1,4-phenylenediamine (IPPD) is a critical rubber antioxidant and a promising alternative to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD). Its photochemical fate at soil-mineral interfaces remains insufficiently elucidated, particularly regarding the mineral-dependent transformation behaviors. In this study, the phototransformation of IPPD on the surfaces of silica (SiO₂) and montmorillonite (MMT) was systematically investigated under simulated sunlight irradiation. IPPD underwent rapid degradation following pseudo-first-order kinetics, with significantly different transformation rates and pathways observed between SiO₂ and MMT systems. The degradation process was notably regulated by surface pH, moisture content, natural organic matter, and coexisting ions. Mechanistic investigations via quenching experiments and electron paramagnetic resonance (EPR) spectroscopy demonstrated that direct photolysis was the dominant pathway in the SiO₂ system, whereas reactive oxygen species (¹O₂ and •OH) governed the photodegradation process in the MMT system. Multiple transformation products (TPs) were generated in both systems, with IPPD quinone (IPPD-Q) identified as a common intermediate. Notably, Vibrio fischeri bioluminescence assays revealed a distinct "toxicity peak" during the photolysis process, and computational toxicology further suggested that specific TPs exhibited enhanced acute toxicity, carcinogenicity, and endocrine-disrupting potential compared to the parent compound IPPD. These findings clarify the critical role of mineral types in governing the interfacial photochemical behavior, environmental persistence, and hazard of IPPD, and provide compelling support for the inclusion of matrix-specific transformation products in the environmental risk assessments of rubber additives.