Degradation of the azo dye Acid Red 1 by anodic oxidation and indirect electrochemical processes based on Fenton's reaction chemistry. Relationship between decolorization, mineralization and products

Solutions of 236 mg dm−3 Acid Red 1 (AR1), an azo dye widely used in textile dying industries, at pH 3.0 have been comparatively treated by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) at constant current density (j). Assays were performed with a stirred tank reactor equipped with a Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode for H2O2 generation from O2 reduction. The main oxidizing agents were hydroxyl radicals produced at the anode from water oxidation in all methods and in the bulk from Fenton's reaction between generated H2O2 and 0.5 mmol dm−3 Fe2+ in EF and PEF. For each anode, higher oxidation power was found in the sequence AO-H2O2 < EF < PEF. The oxidation ability of the BDD anode was always superior to that of Pt. Faster and similar decolorization efficiency was achieved in EF and PEF owing to the quicker destruction of aromatics with hydroxyl radicals produced in the bulk. The PEF process with BDD was the most potent method yielding almost total mineralization due to the additional rapid photolysis of recalcitrant intermediates like Fe(III)-carboxylate complexes under UVA irradiation. The increase in j always enhanced the decolorization and mineralization processes because of the greater production of hydroxyl radicals, but decreases the mineralization current efficiency. A total of 11 aromatic intermediates, 15 hydroxylated compounds, 13 desulfonated derivatives and 7 short-linear carboxylic acids were identified. NH4+, NO3− and SO42− ions were released during azo dye degradation. From the products detected, a comprehensive reaction sequence for AR1 mineralization is proposed. The relationship between decolorization, mineralization and products formed is finally discussed.