Routes for the electrochemical degradation of the artificial food azo-colour Ponceau 4R by advanced oxidation processes

Abstract

The performance of three electrochemical advanced oxidation processes, namely electro-oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) for the treatment of aqueous solutions of the food azo dye Ponceau 4R in an undivided cell with a BDD anode and an air-diffusion cathode was compared in terms of colour, dye concentration and total organic carbon (TOC) removals. PEF treatments in ultrapure water with Na2SO4 were performed to assess the effect of current density, as well as supporting electrolyte and dye concentrations. At 100 mA cm-2, solutions of 130 mL of 254 mg L-1 of the dye in 0.05 M Na2SO4 became colourless and totally mineralized after 50 and 240 min, respectively, which can be explained by the synergistic action of BDD(¿OH) at the anode surface and homogeneous ¿OH formed in the bulk from Fenton's reaction promoted in the presence of Fe2+ catalyst. Furthermore, UVA photons induced the continuous Fe2+ regeneration and photolytic decomposition of refractory intermediate complexes. In that aqueous matrix, the cleavage of the dye molecules proceeded through several reaction routes to yield N-containing and non-N-containing derivatives with one or two aromatic rings, short-chain aliphatic carboxylic acids and inorganic ions. Oxalic and oxamic acids and sulfate ions were accumulated at different rates in EO-H2O2, EF and PEF. The three methods allowed the progressive decontamination of Ponceau 4R solutions in a real water matrix even without the addition of electrolyte, although complete TOC abatement after 360 min at 33.3 mA cm-2 was only ensured by the iron-catalyzed PEF process.