Environmental behavior and the degradation of tire-rubber-related pollutant 6PPD-quinone in soils

Abstract N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone (6PPD-Q), a transformation product of the tire rubber antioxidant N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine, has recently been identified as a contaminant of emerging concern due to its acute aquatic toxicity and widespread occurrence. However, its behavior in terrestrial environments remains insufficiently understood. To address this gap, this study investigates the environmental degradation and behavior of 6PPD-Q in three soils with contrasting physicochemical properties through photodegradation, leaching, and adsorption–desorption experiments. Under simulated sunlight, 6PPD-Q exhibited varying degradation kinetics, with sandy soils showing the most rapid photodegradation. Organic- and clay-rich soils demonstrated slower breakdown and greater degradation driven by microbial activity under dark conditions. The soil column leaching test revealed extremely limited vertical mobility, with more than 90% of 6PPD-Q retained within the top 6 cm of soil after artificial rainfall treatment, suggesting a low potential for groundwater contamination. Adsorption–desorption experiments confirmed strong binding to soil organic matter and clay fractions, with less than 10% of the initially sorbed 6PPD-Q desorbed after 48 h. Collectively, these results demonstrate that 6PPD-Q persists primarily in surface soils, where its fate is governed by the interplay between photolytic processes and sorption-driven stabilization. From an applied perspective, the prolonged surface retention of 6PPD-Q raises ecological and agronomic concerns, including chronic exposure risks for soil invertebrates, potential plant uptake, and secondary mobilization during rainfall. This study provides critical mechanistic insights into the environmental fate of 6PPD-Q and highlights the importance of soil-specific assessments for the robust ecological risk evaluation of tire-derived pollutants.