Electro-Fenton and photoelectro-Fenton degradation of indigo carmine in acidic aqueous medium

Acidic aqueous solutions containing concentrations of the dye indigo carmine up to 0.9 g l−1 have been degraded by environmentally friendly electrochemical methods such as electro-Fenton and photoelectro-Fenton using an undivided cell with a Pt or boron-doped diamond (BDD) anode and an O2-diffusion cathode at 35.0 °C. The great oxidation ability of these systems is due to the large production of hydroxyl radical (OH) at the anode surface from water oxidation and in the medium mainly from Fenton's reaction between catalytic Fe2+ and cathodically generated H2O2. Complete mineralization is feasible using electro-Fenton with a BDD anode and 1.0 mM Fe2+ and when 1.0 mM Fe2+ and 0.25 mM Cu2+ are combined as catalysts in photoelectro-Fenton with a Pt anode. The first method yields similar degradation rate in the pH range 2.0–4.0, whereas the second one is more potent up to 0.44 g l−1 of the dye at pH 3.0. Mineralization is enhanced with increasing current density and initial dye concentration. The indigo carmine decay always follows a pseudo zero-order reaction. Isatin 5-sulfonic acid, indigo and isatin are detected as aromatic products, which are degraded to oxalic and oxamic acids. The electro-Fenton process with BDD yields the destruction of Fe3+-oxalate and Fe3+-oxamate complexes by OH at the anode surface. In the presence of Fe2+ and Cu2+ the photoelectro-Fenton process with Pt involves the photolysis of Fe3+-oxalate complexes under the action of UVA light, while competitive Cu2+-oxalate and Cu2+-oxamate complexes are mineralized with OH produced by Fenton's reaction. The nitrogen of the dye is mainly converted into NH4+.