Combustion of textile monoazo, diazo and triazo dyes by solar photoelectro-Fenton: Decolorization, kinetics and degradation routes

The viability of the electrochemical combustion of the monoazo Acid Orange 7, the diazo Acid Red 151 and the triazo Disperse Blue 71 by solar photoelectro-Fenton (SPEF) has been demonstrated. Comparative trials were made by electrolyzing 10L of 50mgL^-1 of dissolved organic carbon of each azo dye in 0.05M Na₂SO₄ with 0.50 mM Fe²⁺ of pH 3.0 using a solar flow plant equipped with a Pt/air-diffusion filter-press cell coupled to a CPC photoreactor. Organics are oxidized by OH radicals formed from water oxidation at the Pt anode and in the bulk from the Fenton's reaction between Fe²⁺ and H₂O₂ generated at the cathode, whereas the mineralization was enhanced by the photolysis of intermediates by UV light from sunlight that irradiates the CPC photoreactor. The decay of all the azo dyes was followed by reversed-phase HPLC and always obeyed a pseudo-first-order reaction, being more rapid than the decolorization of the corresponding solutions due to the formation of colored products. The mineralization rate decreased in the order Acid Orange 7>Disperse Blue 71> Acid Red 151. Up to 97% mineralization was achieved for the former compound, remaining short-linear carboxylic acids in the final solution. In contrast, 90-92% mineralization was reached for the other two azo dyes and their final solutions contained undetected products more recalcitrant than carboxylic acids. The effect of current density on each degradation process was examined. Similar results were found using either a power supply or a photovoltaic panel to provide the same current density to the cell, corroborating the use of autonomous solar flow plants for SPEF. Aromatic products and hydroxylated derivatives were identified by LCMS and short-linear carboxylic acids were quantified by ion-exclusion HPLC. The formation of SO₄^2-, NO₃^- and NH₄⁺ ions, along with the loss of volatile N-products, was confirmed by ion chromatography.