The tire antioxidant 6PPD transforms into the toxic 6PPD-quinone (6PPDQ, e.g., LC50 < 1 μg/L for sensitive aquatic species) via ozonation. However, its fate in ozone-free aqueous systems remains poorly understood. This study systematically deconvolutes the kinetics of 6PPD transformation into 6PPDQ and 4-hydroxydiphenylamine (4-HDPA) under various environmental stressors. Our results showed that direct 6PPDQ formation in water was negligible (fAB < 5%). Instead, 6PPD predominantly hydrolyzes to 4-HDPA under dark, alkaline (pH 9) conditions, whereas solar irradiation and natural organic matter (NOM) accelerate alternative degradation pathways. Because 4-HDPA is more prevalent and persistent in dark aquatic environments, monitoring strategies should broaden their focus beyond 6PPDQ to comprehensively evaluate the environmental fate and mass balance of these major transformation products.
Kim et al. (Sat,) studied this question.