Effective removal of Orange-G azo dye from water by electro-Fenton and photoelectro-Fenton processes using a boron-doped diamond anode

Coupled electrochemical advanced oxidation processes like electro-Fenton (EF) and photoelectro-Fenton (PEF) were evaluated for the treatment of an aqueous solution containing 100 mg L^-1 total organic carbon of the low biodegradable azo dye Orange-G (Acid Orange 10) using a boron-doped diamond (BDD)/air-diffusion cell. It was confirmed the existence of synergic effects between UVA light photo-oxidation and/or hydroxyl radicals (·OH) formed from water oxidation at the BDD anode and the Fenton reaction between added Fe²⁺ and H₂O₂ produced at the air-diffusion cathode. A rapid discoloration of the solution was always obtained, mainly attributed to the oxidation of the azo dye with ·OH generated from Fenton's reaction in EF and PEF. The color loss followed a pseudo-first-order kinetics, controlled by the applied current. The dye mineralization was enhanced with increasing current due to the greater production of ·OH and finally, short-linear carboxylic acids like oxalic and oxamic were pre-eminently accumulated. The Fe(III) complexes of these acids were slowly removed by ·OH in EF and rapidly photodecomposed by UVA light in PEF up to 98% mineralization. Sulfate and nitrate ions were accumulated in the medium during both EF and PEF treatments. The more powerful coupled PEF process is then able to efficiently degrade streams contaminated with Orange G.