Electrochemical decolourisation of dispersed indigo on boron-doped diamond anodes

In denim production the decolourisation of intensively coloured, indigo-particulate containing waste water is a factor of major environmental concern. Successful anodic decolourisation of solutions containing 0.29 mM dispersed indigo and 0.070 M Na 2SO 4 could be achieved on boron-doped diamond electrodes. Current densities were varied from 0.36 to 80 mA cm − 2 . Relative current efficiency decreases with increased current density from 43% to less than 1% at 80 mA cm − 2 . At higher concentration of dispersed indigo of 25.1 mM, increased relative current efficiency is observed. Diffusion limited current density for decolourisation of 0.292 M indigo dispersion can be calculated with 0.166 ± 0.007 mA cm − 2 . At the experimental conditions studied, peroxodisulfate, formed as by product of the electrolysis, was found to be ineffective for dyestuff destruction. Experiments in presence of small amounts of chloride proved, that the observed decolourisation of the dispersed indigo cannot be attributed to hypochlorite, formed by anodic oxidation of chloride. While anodic decolourisation of a soluble reactive dye proceeds rapidly, commercial vat dyes resist to oxidative anodic attack at the conditions chosen. Bleach experiments of dyed fabric failed, which supports the model of indigo oxidation in the diffusion layer of the anode.