Application of anodic oxidation, electro-Fenton and UVA photoelectro-Fenton to decolorize and mineralize acidic solutions of Reactive Yellow 160 azo dye

Abstract

The degradation of 100 cm3 of a solution with 0.167 mmol dm-3 Reactive Yellow 160 (RY160) azo dye in sulfate medium of pH 3.0 has been comparatively studied by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF). Trials were carried out with a stirred tank reactor equipped with a boron-doped diamond (BDD) anode and an air-diffusion cathode for H2O2 production, upon addition of 0.50 mmol dm-3 Fe2+ as catalyst in EF and PEF. The solution was slowly decolorized by AO-H2O2 because of the low rate of reaction of the azo dye and its colored products with hydroxyl radicals generated at the BDD anode from water oxidation. The color loss was enhanced in EF by the larger oxidation ability of hydroxyl radicals produced in the bulk from Fenton's reaction between added Fe2+ and generated H2O2, whereas the solution was more rapidly decolorized by PEF owing to the additional generation of hydroxyl radicals from the photolysis of Fe(III)-hydroxy complexes by UVA light. The relative mineralization ability of the processes also increased in the sequence AO-H2O2 < EF < PEF. The PEF method was the most powerful due to the synergistic oxidation action of hydroxyl radicals and UVA irradiation, yielding 94% mineralization after 360 min at 100 mA cm-2. The influence of current density and RY160 concentration on the performance of all processes was assessed. Final carboxylic acids like maleic, fumaric, tartronic, acetic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. All these acids were totally removed by PEF, but the formation of small amounts of other highly recalcitrant products impeded the total mineralization. Chloride, sulfate, ammonium and, to a smaller extent, nitrate ions were released to the solution from the heteroatoms of the azo dye in all cases.