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 HClO, 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.