The effect of different operational parameters on the electrooxidation of indigo carmine on Ti/IrO₂-SnO₂-Sb₂O₃

The electrochemical oxidation of indigo carmine (IC) is evaluated on a Dimensionally Stable Anode (DSA) made up of Sb₂O₃ doped Ti/IrO₂-SnO₂ (Ir-Sn-Sb). Particular efforts are devoted to investigate different electrochemical parameters including current density, IC concentration, initial pH of the solution, and the content of multiple ions (Cl^-, SO₄^-2 or HCO₃^-) typically found in textile dyeing wastewater. The IC removal efficiency is calculated using the following physicochemical parameters: color removal (611 nm), Chemical Oxygen Demand (COD mg L^-1 O₂), Total Organic Carbon (TOC mg L^-1C) and Average Oxidation State (AOS). The results reveal that IC oxidation is enhanced in the presence of chlorides due to the action of active chlorine species (Cl₂-active) electrogenerated on the anode surface. In this medium, the initial oxidation rate of IC was higher at pH 2 than any other value including natural pH of 6.62 and 10. Both the increase of the chloride concentration in the electrolyte and the rise of current density expedite the color removal with a lower energetic consumption. Optimum treatment conditions were found using 0.25 mol L^-1 NaCl at pH 2 and applying 9.375 mA cm^-2. Under these conditions, a total color removal is achieved, along with 77.5% COD, 24% TOC and the rise of AOS value to 2.9. High performance liquid chromatography analysis and mass spectrometry provide insights concerning the first degradation stages, involving the attacks of electrogenerated HClO in the double bond C=C chromophore group of IC, thus, generating isatin-5-sulfonic acid (m/z 226) as the main reaction intermediate. This compound is subsequently removed to form aliphatic byproducts. COD removal and the final AOS value suggest that the electrochemical process transforms the IC to biodegradable intermediates, which can be eventually eliminated in a subsequent biological treatment.