Comparative degradation of the diazo dye Direct Yellow 4 by electro-Fenton, photoelectro-Fenton and photo-assisted electro-Fenton

The degradation of 100 ml of a 200 mg l^-1 Direct Yellow 4 (DY4) solution in 0.05 M Na₂SO₄ with 0.5 mM Fe²⁺ of pH 3.0 has been comparatively studied by electro-Fenton (EF), photoelectro-Fenton (PEF) with UVA light and a novel photo-assisted EF (PA-EF). The latter method consists of the application of EF for a given time, followed by UVA illumination alone. Electrolytic experiments were made with a boron-doped diamond (BDD) anode and an air-diffusion cathode generating H₂O ₂ at constant current density. In this system, oxidant ^̇OH was produced at the BDD surface from water oxidation and in the bulk from Fenton's reaction. The DY4 solution was rapidly decolorized in EF and PEF due to the action of ^̇OH. The PEF process yielded an almost total mineralization of DY4, being more powerful than EF that only allowed partial mineralization. The proposed PA-EF was as potent as PEF if the initial electrolysis was prolonged until the production of intermediates that can be mineralized by UVA light. At 33.3 mA cm^-2, the best PA-EF was found by stopping the electrolysis and starting UVA irradiation at 120 min. The PA-EF process was always more economic than PEF and even less expensive than EF at high current density. Oxalic and oxamic acids were detected as ultimate by-products by ion-exclusion HPLC. The Fe(III) complexes of both acids were slowly mineralized with ^̇OH in EF, but rapidly photolyzed by UVA light in PEF and PA-EF. In these processes, the photolysis of N-intermediates produced oxamic acid. NH4+ ion was released in much larger proportion than NO3- ion, but the major part of the initial N was lost as volatile N-compounds. The photolytic removal of Fe(III)-oxalate complexes and N-intermediates accounts for the high effectiveness of the PEF and PA-EF processes.