Kinetic Modelling of Photoconversion of Phenol by a Photo-Fenton Reagent and UV

Abstract

Abstract This study analyses the oxidation of phenol using Fenton reagent in presence of ultraviolet light, enhancing the mineralization capacity of the treatment. Phenol oxidation generates reaction intermediates more toxic and refractory than phenol itself. Thus, the reaction must be carried out under harder conditions, leading these compounds to biodegradable acids that can be dumped into public sewages. Then, this paper considers a complete kinetic modelling of the phenol oxidation stages, differentiating the formation rate of species generated through ortho, meta and para pathways. The model is based on a mechanism of series reactions, wherein a first step degrades into dihydroxylated intermediates (catechol, resorcinol and hydroquinone). A second stage leads to tri-hydroxylated rings (pyrogallol) and benzoquinones (o-benzoquinone and p-benzoquinone), and both species may coexist in the reaction mixture. Next, if conditions are sufficiently oxidizing, they decompose into biodegradable organic acids (muconic, formic or acetic acids). In this mechanism, the parallel reactions, due to the presence of the metal catalyst, are considered. So, on one hand, there is a formation of metal complexes between species ortho-substituted and ferric ions from the catalyst. On the other hand, there is a precipitation of part of the initial catalyst as ferric hydroxide, due to the presence of ultraviolet light, which is determined by the operation pH. Studies have been performed on the selectivity of the oxidation in function of the reaction conditions. Then, when operating with pH = 3.0, oxidation was favoured via ortho-substituted compounds. Treatment is advisable within Pho = 100–300 mg/L, being necessary to apply molar ratios of oxidant above R = 6.0 mol H2O2/mol C6H5OH to degrade these intermediates. Moreover, kinetic constants of formation of generated species are estimated, supposing that follows an evolution comparable to first-order reactions. Analysis checks that these kinetic constants present some relation to the operating conditions. In this way, a mathematical estimation is presented, showing the dependence of the kinetics on the parameters controlling the process (pH, initial concentration of phenol, oxidant molar ratio, catalyst dosage and temperature). As a result, the behaviour of the system can be predicted within the considered range.