Decolorization and mineralization of Allura Red AC azo dye by solar photoelectro-Fenton: Identification of intermediates

• Allura Red AC azo dye degraded by EF and SPEF in a 2.5 L flow plant.
• Rapid decolorization and almost total mineralization by SPEF with a Pt/air-diffusion cell.
• Fast total decolorization but poor mineralization achieved under EF conditions.
• Identification of 16 aromatic intermediates and 11 aliphatic carboxylic acids.
• Release of sulfate and nitrate ions, with a large proportion of volatile N-derivatives.

The degradation of 2.5 L of Allura Red AC solutions in sulfate medium containing 0.50 mM Fe2+ has been studied by solar photoelectro-Fenton (SPEF) using a flow plant equipped with a Pt/air-diffusion cell and a solar photoreactor. Comparative electro-Fenton treatment yielded rapid total decolorization but poor mineralization, since most products were slowly destroyed by OH formed from Fenton’s reaction between Fe2+ and H2O2 generated at the air-diffusion cathode. In contrast, the potent action of UV radiation from sunlight in SPEF allowed the rapid photolysis of recalcitrant intermediates, thus giving rise to a quick mineralization. Sulfate and nitrate ions, along with a large proportion of volatile N-derivatives, were always released. The increase in current density and decrease in azo dye concentration accelerated the decolorization and mineralization in SPEF, although lower current efficiency and greater specific energy consumption were obtained. The most cost-effective SPEF treatment was found for 460 mg L−1 azo dye in 0.05 M Na2SO4 at 50 mA cm−2, which yielded 95% mineralization with 81% current efficiency and 8.50 kW h m−3. No significant effect of sulfate concentration was found. Up to 16 aromatic intermediates and 11 short-chain carboxylic acids, including oxalic and oxamic as the most persistent ones, were detected by GC–MS and HPLC. The large oxidation ability of SPEF can be explained by the quick photolysis of Fe(III)-oxalate complexes and other undetected intermediates.