Role of Graphite\nElectrodes for Removal of Persistent\nContaminants in Electrochemical Systems with Chloride Salts as an\nElectrolyte: A Case Study of Disappearance of an Azo Dye

In this study, a Hoffman cell with a 4.28 M NaCl solution\nat 298\nK and initial pH ∼ 6 was operated at 0.1 A and ∼16 V\nto identify the main redox reactions on the surface of the graphite\nanode and cathode. The measured volumes of evolved gas and the concentration\nof free chlorine in the anode cell revealed that H<sub>2</sub>O, graphite,\nand Cl<sup>–</sup> were oxidized to form O<sub>2</sub>(g),\nCO<sub>2</sub>(g), and Cl<sub>2</sub>(g) at current efficiencies accurately\ndetermined. The recorded volume of gas at the cathode indicated that\nthe major reaction in this cell was the electrochemical (EC) reduction\nof H<sub>2</sub>O to form H<sub>2</sub>(g) at a current efficiency\nclose to unity. Experiments of consumption of tartrazine azo dye with\ngraphite electrodes were also carried out in reactors with divided\nand undivided EC cells at potentials ≥8 V, at different NaCl\nconcentrations (2.74, 5.13 × 10<sup>–3</sup> and 2.57\n× 10<sup>–3</sup> M) and currents (0.1 and 0.2 A). Tartrazine\nwas almost completely removed from the solution by both anodic oxidation\n(direct and indirect) and cathodic reduction and at times markedly\ndecreased as the concentration of NaCl and current increased. The\nkinetics of tartrazine consumption, pH change, and anode consumption\nunder all the considered conditions was properly described by a model\ninvolving heterogeneous and homogeneous reactions. The main innovative\naspect of the model was the suggested individual rate expressions\nfor the in situ electrogeneration of O<sub>2</sub>(g), CO<sub>2</sub>(g), and Cl<sub>2</sub>(g) on the surface of graphite anodes as a\nfunction of the concentration of Cl<sup>–</sup>. Scanning electron\nmicroscopy/energy-dispersive X-ray, total organic carbon, and spectrophotometric\nanalyses confirmed graphite oxide as one of the products of C(s) oxidation\nat the anode, and aromatic species, with CO<sub>2</sub>(g) and H<sub>2</sub>O in low yields, as products of the redox reactions of tartrazine.