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

The ash photolysis of nitrate anions at lambda = 248 nm and peroxydisulfate anions at lambda = 351 nm was used to study the oxidation process of phenol by OH/NO2 and NO3/NO2 in 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 rho = 1.5 whereas for the NO3/NO2 reaction sequence it is rho = 1.0. Implications for tropospheric chemistry are discussed.