Electrocoagulation of Congo Red dye-containing wastewater: Optimization of operational parameters and process mechanism

Abstract

Abstract The electrocoagulation (EC) of Congo red dye-containing wastewater was investigated, focusing on the optimization of operational parameters and process mechanism. The EC was conducted using iron electrodes and utilizing Congo Red (CR) dye as a model pollutant. Response surface methodology (RSM) was used to investigate the effect of operational parameters (initial dye concentration, pH and inter-electrode distance) and optimize the process. To provide insight into the process mechanism, quantum chemical analysis and FTIR were utilized. The process achieved 89% and 97% COD and color removal respectively under optimum conditions (1000 mg/L initial dye concentration, pH 3 and 3 cm inter-electrode distance). The electrical energy required to achieve the optimum COD removal was estimated to be about 36.9 kJ. The quantum analysis showed that CR with HOMO energy of −5.58 eV and Fe3+ LUMO energy of −15.09 eV exhibit electron donating and electron accepting tendency respectively. In addition, Fe3+ shares its LUMO with the HOMO of HO– due to the lower energy gap of −19.47 eV, which leads to the formation of monomeric coagulant such as Fe(OH)3 and the subsequent polymeric flocculants including Fe2(OH)24+ and Fe6(OH)153+. The FTIR analysis of the sludge indicates the presence of coagulant species i.e. Fe(OH)3, Fe(OH)2+, Fe(OH)2+ and Fe2(OH)24+ that are generated from the EC. The findings of this study show that quantum chemical analysis and RSM are suitable for investigating the mechanism and optimization of EC as applied to textile effluents.