Role of Graphite Electrodes for Removal of Persistent Contaminants in Electrochemical Systems with Chloride Salts as an Electrolyte: A Case Study of Disappearance of an Azo Dye

Abstract

In this study, a Hoffman cell with a 4.28 M NaCl solution at 298 K and initial pH ∼ 6 was operated at 0.1 A and ∼16 V to identify the main redox reactions on the surface of the graphite anode and cathode. The measured volumes of evolved gas and the concentration of free chlorine in the anode cell revealed that H₂O, graphite, and Cl^– were oxidized to form O₂(g), CO₂(g), and Cl₂(g) at current efficiencies accurately determined. The recorded volume of gas at the cathode indicated that the major reaction in this cell was the electrochemical (EC) reduction of H₂O to form H₂(g) at a current efficiency close to unity. Experiments of consumption of tartrazine azo dye with graphite electrodes were also carried out in reactors with divided and undivided EC cells at potentials ≥8 V, at different NaCl concentrations (2.74, 5.13 × 10^–3 and 2.57 × 10^–3 M) and currents (0.1 and 0.2 A). Tartrazine was almost completely removed from the solution by both anodic oxidation (direct and indirect) and cathodic reduction and at times markedly decreased as the concentration of NaCl and current increased. The kinetics of tartrazine consumption, pH change, and anode consumption under all the considered conditions was properly described by a model involving heterogeneous and homogeneous reactions. The main innovative aspect of the model was the suggested individual rate expressions for the in situ electrogeneration of O₂(g), CO₂(g), and Cl₂(g) on the surface of graphite anodes as a function of the concentration of Cl^–. Scanning electron microscopy/energy-dispersive X-ray, total organic carbon, and spectrophotometric analyses confirmed graphite oxide as one of the products of C(s) oxidation at the anode, and aromatic species, with CO₂(g) and H₂O in low yields, as products of the redox reactions of tartrazine.