Transformation of 4-tert-octylphenol by UV irradiation and by an H2O2/UV process in aqueous solution

Abstract

The transformation of the organic pollutant 4-(1,1,3,3-tetramethylbutyl)phenol (4-tert-octylphenol; OP) upon irradiation at 253.7 nm and by hydroxyl radicals generated by the photolysis (λ_exc = 253.7 nm) of hydrogen peroxide in aqueous solution has been studied. The quantum yield of direct OP photolysis in pure aqueous solution was evaluated to be 0.058 ± 0.004 in aerated conditions ([O₂] = 272 µM). The rate of photoreaction depends on oxygen concentration; it increases with increasing [O₂]. 4-tert-Octylcatechol has been identified as one of the degradation products, together with a dimeric structure. The probable mechanism of OP photolysis involves photoejection of an electron from the singlet state, leading to the formation of the 4-tert-octylphenoxyl radical. In the presence of hydrogen peroxide, the degradation of octylphenol by hydroxyl radicals has been observed. The second-order rate constant was found to be (6.4 ± 0.5) × 10⁹ M⁻¹ s⁻¹ by direct measurement at various high concentrations of hydrogen peroxide and competitive kinetic measurements using atrazine as the competitor. The degradation products are 4-tert-octylcatechol, again, and 2-hydroxy-5-tert-octylbenzoquinone. The later product may arise from the oxidation of 4-tert-octylcatechol by hydrogen peroxide or from a subsequent reaction of hydroxyl radicals with 4-tert-octylcatechol. Kinetic modelling when using either purified water or natural water successfully simulated the elimination of 4-tert-octylphenol by UV and H₂O₂/UV processes.