Advances in solar photoelectro-Fenton: Decolorization and mineralization of the Direct Yellow 4 diazo dye using an autonomous solar pre-pilot plant

Here, an overview on the advances in solar photoelectro-Fenton (SPEF) is initially presented to show that it is the more potent electrochemical advanced oxidation process based on Fenton's reaction chemistry to remove organic pollutants from waters, due to the synergistic action of generated hydroxyl radicals and solar irradiation. As a novel advance for SPEF, an autonomous solar pre-pilot plant is proposed to make an energetically inexpensive process that can be viable at industrial level. The plant of 10 dm³ capacity contained a Pt/air-diffusion cell with 90.2 cm² electrode area, coupled to a solar compound parabolic collectors (CPCs) photoreactor of 1.57 dm³ irradiation volume and to a solar photovoltaic panel that provides a maximum average current of 5.0 A. The oxidation ability of this plant was assessed by studying the degradation of Direct Yellow 4 (DY4) diazo dye, which involved the predominant destruction of organics by OH formed from Fenton's reaction between H₂O₂ generated at the cathode and added Fe²⁺, along with the photolysis of Fe(III)-carboxylate complexes with sunlight in the CPCs photoreactor. The effect of Fe²⁺ and dye contents as well as current on decolorization rate, substrate decay and mineralization rate was examined. About 96-97% mineralization was rapidly attained using 0.50 mmol dm^-3 Fe²⁺ and up to 0.32 mmol dm^-3 DY4 at 5.0 A. The DY4 decay always obeyed a pseudo-first-order kinetics. Eleven aromatic products, twenty two hydroxylated derivatives and nine short-linear carboxylic acids were identified as intermediates. The Fe(III) complexes of most acids were rapidly removed, pre-eminently photolyzed by sunlight, except those of acetic and oxamic acids that were slowly destroyed. The initial N of the dye was mainly released as NH₄⁺ ion and its initial S was lost as SO₄^2- ion. A plausible reaction sequence for DY4 mineralization involving all the detected products was finally proposed.