Comparison of electrochemical advanced oxidation processes for the treatment of complex synthetic dye mixtures

Many industries use complex synthetic organic dye mixtures in their dyeing processes, producing wastewater effluents with complex dye mixtures. In this study, electrochemical advanced oxidation processes like electrochemical oxidation (ECO), ECO with electrogenerated H₂O₂ (ECO-H₂O₂), electro-Fenton (EF), and photo electro-Fenton (PEF) with UVC irradiation were compared to discolor and mineralize a mixture of Methyl Orange, Methyl Red, and Toluidine Blue O with a boron-doped diamond anode and a gas-diffusion cathode. While ECO and ECO-H₂O₂ required long times to achieve a complete discoloration, the dyes were quickly degraded by EF and PEF due to the oxidation with ^∙OH formed from Fenton's reaction. Almost complete mineralization was rapidly achieved by PEF for a solution with 424 mg O₂ L⁻¹ of chemical oxygen demand (COD) at a current density of 66.6 mA cm⁻² due to the synergistic action of the UVC irradiation. Additionally, the treated effluent was evaluated using the DFZ (Deutsche Farb Zah) parameter for water condition, offering a new perspective and distinguishing this work from previous studies focused on electrified technologies for dye removal. The same behavior has been found for the mixture of such dyes in an industrial effluent upon PEF treatment, even showing faster discoloration and COD reduction as result of the additional removal with active chlorine formed from Cl⁻ oxidation. Ion-exclusion HPLC analysis of the treated synthetic effluent revealed the formation of final carboxylic acids like malic, acetic, oxalic, and formic. Released nitrate, ammonia, and nitrite were detected as nitrogenated by-products from the initial N of the dyes.