Kinetic Modelling of Toxic Compounds Generated during Phenol Elimination in Wastewaters

Abstract

This work focuses on the application of Advanced Oxidation Processes (AOPs) to purify phenolic effluents, which when combined with innovation and effectiveness contribute towards the preservation of natural resources and environmental quality. Phenols are typical pollutants found in wastewaters from different industries such as petrochemical, pharmaceutical, resin manufacturing, olive-oil, pesticides, dyeing production or cosmetic industry. Nevertheless due to their toxicity and harmfulness, they cannot be poured to water systems. Phenol has been chosen as a model compound in this study because it is usually a component of industrial wastewaters. Furthermore phenol-like compounds are generated by intermediate compounds in the oxidation pathway of higher molecular weight aromatic hydrocarbons.Phenol can be effectively removed but this does not imply that the all toxicity levels of the effluent have been diminished, since the oxidation pathway passes through more toxic products. Before each AOP application, the reaction intermediates must be identified in order to determine the influence they have on operating conditions, and subsequently establish the criteria for the complete destruction of toxic compounds. These studies will assist in the cost-effective operation of industrial setups, and also in the design of integrated processes for wastewater treatment.The main objectives of this work are, first to identify and measure toxic intermediates and, second, to develop a kinetic model for phenol degradation based on a mechanism in series. Chemical pathways in the phenol oxidation consider its hydroxylation to catechol, hydroquinone and resorcinol. These dihydroxylated rings are later oxidised to quinone-like compounds. These quinone-like compounds break into short-chain acids, like acetic, formic and oxalic acids. In order to propose a reaction pathway several runs were carried out at room temperature controlling the operational pH throughout the reaction. Experiments were carried out under different operating conditions, and the decay curves of phenol and aromatic intermediates were monitored, using doses of oxidant and catalyst until R=40 mol oxidant/mol phenol and [Fe(II)]=20 mg/L. Kinetic parameters were rightly fitted to the kinetic model, predicting successfully the reaction rates of each toxic compounds.