Solar Photoelectro-Fenton Degradation of Acid Orange 7 Azo Dye in a Solar Flow Plant: Optimization by Response Surface Methodology

The experimental variables to optimize the solar photoelectro-Fenton (SPEF) degradation of 10 L of Acid Orange 7 (AO7) azo dye in 0.05 M Na₂SO₄ of pH 3.0 have been determined from a central composite rotatable design and response surface methodology. Trials were performed with a solar flow plant with a Pt/air-diffusion reactor generating H₂O₂ and a solar compound parabolic collector photoreactor under an UV irradiation intensity of about 31 W m⁻². Optimum variables of 4.0 A, 0.75 mM Fe²⁺, and 150 mg L⁻¹ total organic carbon (TOC) of the azo dye were found for 90 % color and 70 % TOC removals. Under optimized conditions, total decolorization was achieved in 300 min, but AO7 decay obeyed a pseudo-first-order kinetics to disappear in 80 min, pointing to the generation of recalcitrant colored aromatic products. After 360 min of electrolysis, TOC was reduced by 96 %, with 64 % mineralization current efficiency and 133 kWh kg⁻¹ TOC energy consumption. The final solution contained carboxylic acids like tartronic, oxalic, oxamic, and in larger proportion, formic. An almost total mineralization was achieved by SPEF due to the persistence of Fe(III)-formate complexes to be photolyzed by sunlight. The initial N of the azo dye was mainly released as NH₄⁺.