Optimization of electrocoagulation removal of a mixture of three azo dyes: spectrophotometric colour characteristics for best operating conditions

Abstract

In the present study, a mixture of three azo dyes viz. methyl orange, congo red and acid blue-113, was treated using electrocoagulation. Experiments were carried out to optimize the process parameters, namely, pH (5–9), current density (40–100 A m⁻²), treatment time (10–30 min) and electrode type (aluminium, iron or stainless steel) using flexible design structure by combining mixture components and process factors. Under the optimum conditions, a maximum colour removal of 99% and a COD removal efficiency of 81.9% were obtained for the mixture of dyes (33.3 mg L⁻¹ each) using stainless-steel electrodes with a specific energy consumption of 87 kW h kg⁻¹ COD. ANOVA model statistics showed that the interactions of electrode type with all the dyes were highly significant. The current density demonstrated a significant effect on methyl orange and Congo red removal, whereas treatment time was only significant for removal of Congo red dye. The pH was not found to be a significant process factor for the removal of the mixture of dyes and may be kept within the range of pH 5–9. Colour characterization using spectrophotometric methods at different wavelengths for estimating hue, tinge and luminance is required to check the effectiveness of electrocoagulation treatment of dye mixtures. A thorough literature review of twenty different studies to propose the optimal conditions is the strength of this study. Multi-dimensional convex hull plots have been prepared to show the effect of current density on colour removal efficiency, in which the size of the circle represents current density, the colour of the circle represents electrode type, and each circle is labelled with the corresponding reference on a single graph, which provides an efficient way to visualize the best operating conditions. However, the efficiency of the electrodes to remove colour and COD is largely dependent on the nature of the dye and the concentration of each dye in the mixture.