Evidence of Fenton-like reaction with active chlorine during the electrocatalytic oxidation of Acid Yellow 36 azo dye with Ir-Sn-Sb oxide anode in the presence of iron ion

Abstract

The degradation of 2.5 L of Acid Yellow 36 solutions at pH 3.0 by electro-oxidation (EO) has been studied in a flow plant with a reactor containing an Ir-Sn-Sb oxide anode and a stainless steel cathode. The anode was prepared onto Ti by the Pechini method and characterized by SEM-EDX and XRD. It showed a certain ability to electrocatalyze both, the generation of adsorbed ¿OH from water oxidation in sulfate medium and, more largely, the production of active chlorine in a mixed electrolyte containing Cl- ion. The EO treatment of the dye solution in the latter medium led to a rapid decolorization because active chorine destroyed the colored by-products formed, but color removal was much slower in pure NaClO4 or Na2SO4 due to the limited formation of ¿OH. In contrast, greater mineralization was obtained in both pure electrolytes since the by-products formed in the presence of Cl- became largely persistent. The effect of liquid flow rate, current density and dye content on the EO performance in the mixed electrolyte was examined. The drop of absorbance and dye concentration obeyed a pseudo-first-order kinetics. Interestingly, the decolorization rate, dye concentration decay and TOC removal were enhanced upon catalysis with 1.0 mM Fe2+. Such better performance can be accounted for by the formation of ¿OH in the bulk from the electro-Fenton-like reaction between electrogenerated HClO and added Fe2+. Even larger mineralization was achieved by the photoelectro-Fenton-like process upon irradiation of the solution with UVA light due to photolysis of some refractory intermediates. Maleic and acetic acids were detected as final short-chain linear carboxylic acids. The loss of Cl- and the formation of ClO3-, ClO4-, SO42-, NO3- and NH4+ were evaluated as well.