Influence of the supporting electrolyte on the electrolyses of dyes with conductive-diamond anodes

In this work, electrolyses of solutions containing one anthraquinonic (Acid Blue 62) and three azoic (Reactive Red 141, Direct Black 22, and Disperse Orange 29) synthetic dyes were carried out using conductive-diamond anodes. From the obtained results, clearly the oxidation was largely influenced by the supporting electrolyte used: (i) in the presence of chloride ions, the primary mechanism is mediated electrooxidation in solution with formation of a significant amount of intermediates; (ii) in the presence of sulfate ions, the dyes are directly mineralized by hydroxyl radicals. In every condition, the synthetic dyes were completely removed from the solutions. The kinetics of the electrooxidation process was mass-transport controlled, although the proper treatment of such dye solutions was assured with energy consumptions below 60 kWh m−3.

► The supporting electrolyte plays a key role in the oxidation mechanism of the dyes.
► The anthraquinonic dye AB 62 and the azoic dye RR 141 are completely mineralized.
► In chloride media, mediated oxidation leads to a significant amount of intermediates.
► In sulfate media, the dyes are directly mineralized to CO2.
► Mass transfer controls the process kinetics. Energy consumptions below 60 kWh m−3 are required.