Roles of different intermediate active species in the mineralization reactions of phenolic pollutants under a UV-A/C photo-Fenton process

Abstract

The roles of different intermediate oxidative species, such as singlet oxygen, triplet oxygen, superoxide radical anions and hydroperoxy or hydroxyl radicals, were studied in the mineralization reactions of solutions containing a mixture of three phenolic compounds (namely gallic, p-coumaric and protocatechuic acids). The degradation process that was developed was a ferrioxalate-induced artificial ultraviolet (UV)-A/C photo-Fenton system. Reactions were carried out in an UV reactor consisting of two 50-W UV-C lamps and two 75-W UV-A lamps. The roles played by the oxidative active species in various stages of the process were determined by running the reaction in the presence of different scavenging agents. Analyses were conducted within a multivariate experimental design combined with neural networks that included the following variables: initial concentrations of tert-butyl alcohol, 1,4-benzoquinone, sodium azide and potassium iodide. The response function was the %TOC removal after 15 and 60min of reaction. Under optimal conditions (e.g., in the absence of scavenging agents), 88% TOC removal of the phenolic mixture could be achieved in 60min at 28°C. Superoxide radical anions and triplet oxygen were the main oxidants in earlier stages of the mineralization of this phenolic-mixture solution, whereas singlet oxygen, hydroxyl and hydroperoxy radicals played more significant roles in later stages. Tert-butyl alcohol was the most significant scavenger affecting inhibition of the mineralization reaction, as it trapped almost all of the intermediate active species.