Effect of anions on electrochemical degradation of azo dye Carmoisine (Acid Red 14) using a BDD anode and air-diffusion cathode

Solutions of 2.5 L with 209.3 mg L-1 of the azo dye Carmoisine in 0.050 M SO42-, ClO4-or Cl-have been comparatively treated by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2) and electro-Fenton (EF) with 0.5 mM Fe2+ as catalyst at constant current density. Experiments were made using a recirculation flow plant containing a reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode to allow H2O2 generation. The dye and its oxidation products were oxidized by hydroxyl radical and/or HClO formed at the anode from water or Cl-oxidation, respectively, in EO-H2O2, as well as by hydroxyl radical produced in the bulk from Fenton's reaction between added Fe2+ and generated H2O2 in EF. In both methods, the decolorization process was always much faster in Cl-medium because of the quick oxidation of colored compounds by HCIO, being enhanced by increasing current density and Cl- concentration. The solutions with SO42-or ClO4- were more rapidly decolorized in EF due to the higher oxidation power of hydroxyl radicals in the bulk. Regarding the overall decontamination, a poor and similar mineralization of about 50% was obtained by EO-H2O2 at 480 min in all the supporting electrolytes at 100 mA cm(-2). The comparative EF treatments were always much more powerful, being SO42- the most favorable medium leading to 76% mineralization with the lowest energy consumption. Up to 15 aromatic products were detected by GC-MS and short-linear carboxylic acids like tartronic, oxalic, oxamic and formic were quantified by ion-exclusion HPLC. The large persistence of Fe(III)-oxalate complexes accounted for the partial mineralization of the Carmoisine solution in EF. Nitrate and sulfate were the major ions released during the mineralization process. (C) 2014 Elsevier B.V. All rights reserved.