Decolorization and mineralization of Sunset Yellow FCF azo dye by anodic oxidation, electro-Fenton, UVA photoelectro-Fenton and solar photoelectro-Fenton processes

The decolorization and mineralization of 100mL of 290mgL^-1 Sunset Yellow FCF (SY) azo dye at pH 3.0 were studied by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF), UVA photoelectro-Fenton (PEF) and solar photoelectro-Fenton (SPEF). Trials were performed in a one-compartment cell equipped with a boron-doped diamond (BDD) anode and a carbon-PTFE air-diffusion cathode. Organics were removed by hydroxyl radical (OH) formed: (i) at the BDD anode from water oxidation, (ii) in the bulk from Fenton's reaction between added Fe²⁺ and generated H₂O₂ at the cathode and (iii) from the photolysis of Fe(OH)²⁺ species by UV light. The most powerful method was SPEF, achieving an almost total mineralization more rapidly than PEF due to the higher UV intensity of sunlight, which quickly photolyzes Fe(III)-carboxylate complexes that cannot be destroyed by OH in EF. However, SY was completely decolorized at similar rate by EF, PEF and SPEF. The little oxidation action of OH at the BDD anode yielded a slow decolorization and mineralization in AO-H₂O₂. The effect of current density on all treatments was examined. The azo dye decay always followed a pseudo-first-order reaction. It was more rapidly removed than decolorized, indicating that colored aromatic products are involved in the decolorization process. A total of 14 aromatic products and 34 hydroxylated derivatives, including benzenic, naphthalenic and phthalic acid compounds, were detected by LC-MS. Generated carboxylic acids like tartronic, oxalic, formic and oxamic were identified by ion-exclusion HPLC. The viability of SPEF at industrial scale was demonstrated using a solar pre-pilot plant with a Pt/carbon-PTFE air-diffusion cell coupled with a compound parabolic collectors (CPCs) photoreactor. In this plant, the treatment of 10L of 290mgL^-1 SY at pH 3.0 between 33.2 and 77.6mAcm^-2 gave total decolorization and 91-94% mineralization in short time. A plausible general reaction sequence for SY mineralization involving all oxidation products detected was proposed.