Oxidation byproduct formation by electro-oxidation of textile dye wastewater: Occurrence, characterization and mitigation

Electrochemical oxidation (EO) is a promising approach for treating textile dye wastewater and is currently used in industrial applications, but the widespread addition of NaCl to enhance treatment efficiency may lead to the formation of harmful oxidation byproducts (OBPs). This study evaluates volatile and semi-volatile OBP formation during EO of textile dyes using boron-doped diamond (BDD), iridium (Ir), and iridium-ruthenium (Ir/Ru) electrodes. Two model dyes were studied: Orange II (OD), a non-halogenated dye, and eosin yellowish (EY), a brominated dye. NaCl addition increased dye degradation and reduced energy demand but significantly elevated DBP formation, primarily chloroform from OD, as well as dibromochloromethane and bromoform from EY. Total OBP formation exceeded 250 and 1000 µg L −1 for OD and EY after 90 min of electrolysis, respectively. While Ir/Ru electrodes produced more reactive chlorine species, BDD electrodes achieved greater degradation rates and OBP formation, suggesting electrochemical-initiated dye degradation is a key mechanism for OBP formation. Mitigation strategies were investigated, including the use of Na₂SO₄ and tetra-amido macrocyclic ligand (TAML). While Na₂SO₄, in place of NaCl, completely mitigated OBP formation, dye degradation rates significantly decreased. TAML enabled comparable dye degradation at lower NaCl concentrations while reducing OBP yields by over 35%. These findings present one of the first studies to report and characterize the formation OBPs during the EO of dye wastewater and highlight the tradeoffs between treatment efficiency and byproduct risk during industrial dye effluent treatment.