Treatment of natural dye wastewaters by electrochemical oxidation: Comparative analysis of anode materials and optimization for reuse

This study evaluates the feasibility of electrochemical oxidation for treating real wastewater from the eco-dyeing of linen fabric with a natural dye extracted from Punica granatum (pomegranate peel), aiming to enable its reuse in new dyeing processes. The untreated wastewater exhibited a yellowish colour, chemical oxygen demand (COD) of 500 mg L^-1, low biodegradability (0.24), and moderate acute ecotoxicity toward Daphnia magna (2.3 toxic units), confirming the need for advanced treatment. Comparative electrooxidation experiments were conducted using boron-doped diamond (BDD) and different titanium-based mixed metal oxide anodes. The BDD anode exhibited the highest pollutant removal efficiency, achieving over 90 % COD reduction and significant mineralization. The optimal operating conditions were identified at 50 A m^-2 and an applied charge of 0.72 kC (≈4 h treatment), yielding a colorless effluent with COD below 400 mg L^-1 and an energy consumption of 5.4 kWh m^-3. Under these conditions, the biodegradability index increased to 0.35, and ecotoxicity decreased to 2.0 toxic units. Prolonging the treatment time enhanced COD removal and mineralization. However, it also led to higher energy demand and a rise in toxicity. The effluent treated for 4 h, reused as a 50:50 blend with freshwater, produced fabrics with negligible color difference (ΔE < 1) and excellent fastness, meeting industrial standards and, thus, representing the most balanced compromise between pollutant removal, energy efficiency, and dyeing performance.