Electro-oxidation of indigo blue aqueous solution for COD and color removal: RSM optimization and pilot-scale study

Abstract

In this study, the removal of indigo dye, which is used in the denim industry, by using the electro-oxidation method was investigated. Experiments were carried out using Sn/Sb/Ni-Ti anode and Pt-Ti cathode electrodes of indigo dye aqueous solution with an initial concentration of 100 mg/L, which was prepared synthetically in the laboratory. In the first stage of the study, appropriate support electrolyte (NaCl, KCl, Na2SO4, K2SO4) detection investigations were carried out. As a result of the trials, NaCl electrolyte was chosen as the supporting electrolyte type because it is both affordable and has high removal efficiency compared to other electrolytes. Color and COD removal were established according to the effective factors (pH, current density, supporting electrolyte concentration and electro-oxidation time) for the electro-oxidation method using the central composite design (CCD) based response surface methodology (RSM). The design consists of four components and seven central experiments, each with three levels. According to the RSM result, ideal conditions were pH 11, current density 51.01 mA/cm(2), supporting electrolyte concentration 1565.66 mg/L and duration 120 min. In ideal conditions, COD removal was determined as 97.43% and color removal as 98.93%, while energy consumption was determined as 17.49 kWh/kg COD. In the last stage of the study, pilot-scale tests were carried out in a spherical polyethylene (PE) tank with a bottom diameter of 65 cm and a height of 157 cm. Color and COD removal efficiencies obtained from the pilot scale under optimum conditions were calculated as 95.26% and 92.16%, respectively.