A mechanistic study of the oxidation of phenol by OH/NO2/NO3 in aqueous solution

Abstract

The flash photolysis of nitrate anions at λ = 248 nm and peroxydisulfate anions at λ = 351 nm was used to study the oxidation process of phenol by OH/NO₂ and NO₃/NO₂in aqueous solution under different experimental conditions. Two different mononitrophenols (isomer ortho- and para-) and a dihydroxy derivative (catechol) were identified as the main reaction products by means of HPLC-DAD-ED technique and their yields of formation where directly compared with the initial radical concentrations of OH, NO₂ and NO₃, respectively. Product yields were determined for different pH and as a function of the number of laser photolysis pulses down to single pulse experiments, where yields of 70 nM o-nitrophenol, 60 nM p-nitrophenol and 10 nM catechol have been obtained starting with [OH]₀ = [NO₂]₀ = 160 nM leading to the formation of 50 nM of NO₃ radicals at pH = 0.5 and T = 298 K. From single pulse experiments under varying conditions a mechanism involving (i) initial attack of phenol by OH and (ii) by initial attack of NO₃ followed by the reaction of the intermediate with NO₂ is suggested. Absolute product yields are determined. For OH/NO₂ the o-/p-nitrophenol product ratio is ρ = 1.5 whereas for the NO₃/NO₂ reaction sequence it is ρ = 1.0. Implications for tropospheric chemistry are discussed.