Electrochemical Reduction Prior to Electro-Fenton Oxidation of Azo Dyes: Impact of the Pretreatment on Biodegradability

The aim of this work was to study the degradation of three azo dyes, Orange II, Methyl red and Biebrich Scarlet by electro-Fenton and the effect of the electrochemical pretreatment on the biodegradability of the solutions. The electrochemical pretreatment showed that an electrochemical reduction on the carbon felt electrode was mainly responsible for the decolorization of the azo dyes. Indeed, the electrochemical behaviour of the azo dyes highlighted their electroactivity; electrolysis with and without ferric ions led to the same decolorization yield, namely 99 % at 15 min for Methyl red, and stable chemical oxygen demand (COD) values were recorded during decolorization. In a second step and owing to the absence of by-product electroactivity in reduction, the formation of hydroxyl radicals by the Fenton reaction led to the oxidation of by-products from the electrochemical reduction. It was illustrated by the decrease recorded for the COD values. The results also showed that the azo bond cleavage occurring during the electrochemical reduction was not sufficient to significantly reduce recalcitrance, as shown from biological oxygen demand (BOD) 5 /COD ratio examination below the limit of biodegradability (0.4). Contrarily, a positive trend was recorded for Orange II during the electro-Fenton reaction, with a BOD 5 /COD ratio of 0.81 after 28 h of pretreatment.