Degradation of acidic aqueous solutions of the diazo dye Congo Red by photo-assisted electrochemical processes based on Fenton's reaction chemistry

The degradation of solutions with 0.260mM of the diazo dye Congo Red at pH 3.0 has been studied by electrochemical advanced oxidation processes (EAOPs) like anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and photoelectro-Fenton (PEF) with a 6W UVA light. Experiments were made in a 100mL stirred tank reactor with a boron-doped diamond (BDD) anode and an air-diffusion cathode at constant current density (j). In these systems, organics were mainly destroyed by OH formed at the anode surface from water oxidation and/or in the bulk from Fenton's reaction between added Fe²⁺ and cathodically generated H₂O₂. The oxidation power of the EAOPs increased in the sequence AO- H₂O₂<EF<PEF. Almost total mineralization was attained after 360min of PEF at j≥66.7mAcm^-2 due to the parallel photolytic action of UVA light. In all the EAOPs, increasing j enhanced the degradation process, but with a loss of mineralization current efficiency and higher energy consumption. Congo Red decay always obeyed a pseudo-first-order kinetics. The study of the Congo Red degradation in a 2.5L solar flow plant with a Pt/air-diffusion cell confirmed the viability of the solar PEF (SPEF) treatment at industrial scale. Optimum conditions were found for 0.260mM of Congo Red with 0.50mM Fe²⁺ at 100mAcm^-2, yielding almost total mineralization in 240min with about 49% current efficiency and 0.45kWh (g DOC)^-1 energy consumption. LC-MS analysis of treated solutions allowed the identification of 21 aromatic intermediates and 13 hydroxylated derivatives, including diazo, monoazo, biphenyl, benzene, naphthalene and phthalic acid compounds. Tartatic, tartronic, acetic, oxalic and oxamic acids were detected as final carboxylic acids in all the EAOPs. The fast photodecarboxylation of the Fe(III)-carboxylate complexes explained the higher oxidation ability of the photo-assisted methods of PEF and SPEF. The initial N of the dye was mainly lost as N-volatile products and mineralized to NO₃^- ion and in lesser extent to NH₄⁺ ion, whereas its initial S was converted into SO₄^2- ion.