Reactions of Diphenylamine with OH Radicals in the Environment: Theoretical Insights into the Mechanism, Kinetics, Temperature, and pH Effects

Diphenylamine (DPL) has been widely utilized in industrial chemicals, but its degradation by HO^• radicals in the environment has not been fully studied yet. The present study uses quantum chemical calculations to evaluate the reaction of DPL with HO^• radicals in atmospheric and aqueous environments. The results showed that, in the atmosphere, the diphenylamine reacted with the HO^• radical rapidly, with an overall rate constant of 9.24 × 10¹¹ to 1.34 × 10¹¹ M^–1 s^–1 and a lifetime of 0.17 to 1.55 h at 253–323 K. The calculated overall rate constant in water (k_overall = 1.95 × 10¹⁰ M^–1 s^–1, pH = 3–14) is in excellent agreement with the experimental value (k_overall = 1.00 × 10¹⁰-1.36 × 10¹⁰ M^–1 s^–1). The HO^• + DPL reaction in water could occur following the hydrogen transfer (15.4%), single electron transfer (41.6%), and radical adduct formation (41.7%) mechanisms, clarifying that addition products were not exclusive products. Nevertheless, variations in temperature and pH within aqueous environments had an impact on the mechanisms, kinetics, and degradation products of the reaction of DPL with HO^• radicals.