UV-Electrochemical Catalytic Oxidation of Quinoline-Containing Wastewater Using an IrO2-Based Anode: COD Reduction, Degradation Kinetics, and Transformation Pathway

Quinoline is a refractory nitrogen-containing pollutant commonly found in coal chemical, coking, pharmaceutical, and dye wastewater. Although advanced oxidation processes have shown potential for quinoline removal, an integrated understanding of pollutant removal, COD reduction, degradation kinetics, and intermediate transformation in UV-electrochemical systems remains limited. In this study, simulated quinoline-containing wastewater was treated using a dual-wavelength UV-electrochemical catalytic oxidation system equipped with a commercial Ti/IrO2 anode and a stainless-steel cathode. The effects of current density, electrolysis time, NaCl concentration, and initial pH on quinoline degradation were systematically investigated, while COD reduction, degradation kinetics, and intermediate transformation were further evaluated by HPLC and GC-MS. Under a current density of 30 mA/cm2, NaCl concentration of 0.5 mol/L, near-neutral initial pH, and reaction time of 90 min, quinoline and COD removal efficiencies reached 87.2% and 84.7%, respectively. Further increases in current density or reaction time produced only limited additional improvement under the investigated conditions. Control experiments showed higher quinoline removal in the combined process than in either individual process. Kinetic analysis showed that quinoline degradation could be adequately described by an apparent pseudo-first-order model. HPLC and GC-MS analyses indicated a proposed transformation pathway involving hydroxylation and quinolinone formation, followed by cleavage of the aromatic and heterocyclic structures and subsequent oxidation to small-molecule organic acids. These results provide an integrated evaluation of quinoline removal, COD reduction, kinetic behavior, and intermediate transformation, thereby providing a basis for further optimization of UV-electrochemical treatment of quinoline-containing wastewater.