Comparative decolorization of monoazo, diazo and triazo dyes by electro-Fenton process

The decolorization process of the monoazo Acid Orange 7, diazo Acid Red 151 and triazo Direct Blue 71, with similar aromatics and functional groups, has been comparatively studied by electro-Fenton. Solutions of 100 cm³ of each azo dye in 0.05 mol dm^-3 Na₂SO₄ with 0.5 mmol dm^-3 Fe²⁺ of pH 3.0 were electrolyzed in an undivided BDD/air-diffusion cell under galvanostatic conditions. Organics were oxidized with hydroxyl radicals formed at the BDD anode from water oxidation and mainly in the bulk from Fenton's reaction between cathodically generated H ₂O₂ and added Fe²⁺. A simple parameter so-called initial decolorization rate was used to analyze the behaviour of the azo dyes at the beginning of the process. This parameter decreased with increasing initial azo bonds concentration due to the oxidation of more organic matter with similar amounts of hydroxyl radicals. It rose linearly with current density by the greater production of these oxidants, even when similar color removal was found at long electrolysis times, indicating that the process was mass-transfer controlled. The initial decolorization rate became lower as the number of azo bonds in the molecule increased owing to their smaller reactivity with hydroxyl radicals. Reversed-phase HPLC of electrolyzed solutions revealed the total removal of all azo dyes following a pseudo first-order kinetics with rate constants that showed the same trends as those predicted by initial decolorization rates. The pseudo first-order rate constants for decolorization obtained from absorbance decay also showed similar tendencies, but they were not useful to describe the comparative total decolorization of azo dye solutions because of the slower and parallel destruction of colored conjugated intermediates formed during the electro-Fenton treatment.