Degradation of Evans Blue diazo dye by electrochemical processes based on Fenton's reaction chemistry

Abstract A solution with 0.245 mM of the diazo dye Evans Blue and 0.50 mM Fe²⁺ as catalyst of pH 3.0 was comparatively degraded by electrochemical processes based on Fenton's reaction chemistry like electro-Fenton (EF), photoelectro-Fenton (PEF) with a 6 W UVA light and solar photoelectro-Fenton (SPEF). Electrolytic trials were made in a 100 mL stirred tank reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode at constant current density. Organics were destroyed by ·OH produced at the anode surface from water oxidation and in the bulk from Fenton's reaction between added Fe²⁺ and H₂O₂ generated at the cathode. Evans Blue decay obeyed a pseudo-first-order kinetics and was much faster than solution decolorization due to the formation of colored aromatic products. The mineralization rate rose in the sequence EF < PEF < SPEF. Almost total mineralization was rapidly achieved in SPEF at current density ≥66.7 mA cm^-2 because of the potent UV radiation from sunlight. Up to 19 aromatic intermediates and 16 hydroxylated derivatives including diazo, monoazo, biphenylic, benzenic, naphthalenic and phthalic acid compounds were detected by LC-MS. The SPEF process was performed in a 10 L flow plant with a Pt/air-diffusion cell coupled to a CPC photoreactor in order to confirm its viability at industrial scale. 88% mineralization with 42% current efficiency and 2.13 kWh kg^-1 DOC energy consumption were obtained after 300 min of treatment at 55.4 mA cm^-2. Nine short-linear carboxylic acids were identified as final products, oxalic, formic and oxamic acids being the most persistent. The photodecarboxylation of Fe(III)-carboxylate complexes explained the good oxidation ability of PEF and SPEF. The initial S of the diazo dye was transformed into SO₄^2- ion, whereas its initial N was mineralized to NO₃^- ion but largely lost as N-volatile products.