Insight into the roles of Cl− for the degradation of Acid Red 14 in an electrochemical advanced oxidation system: Mechanisms and DFT studies

Textile wastewater was typically characterized by high concentrations of chloride ions, with Acid Red 14 (AR14) representing a significant pollutant. Nevertheless, research into the degradation of AR14 by electrochemical advanced oxidation processes (EAOPs) had largely overlooked the influence of chloride ions. In this regard, the EC-Chlorine-MMO/Ti system was designed, and the degradation mechanism of AR14 was subjected to meticulous investigation. The adopted anode was composed of Ti/IrO 2 -Ta 2 O 5 (mixed metal oxide, MMO). It demonstrated superior efficiency in degrading AR14 within a sodium chloride (NaCl) electrolyte, registering an apparent kinetic constant rate approximately 16.76 times greater than that observed within a sodium perchlorate (NaClO 4 ) electrolyte. The optimal chloride ion concentration was identified as 0.04 mol/L. Besides, alkaline conditions were not conducive to the degradation of AR14. It was noteworthy that the electrochemical byproducts chlorite (ClO 2 − ), chlorate (ClO 3 − ), perchlorate (ClO 4 − ), and trihalomethane (THMs) were not produced in the solution treated by the EC-Chlorine-MMO/Ti system. Electron paramagnetic resonance (EPR) spectroscopy, probe, and quenching experiments identified that hydroxyl radicals ( • OH), superoxide radicals (O 2 •− ), and dichloride radicals/chlorine monoxide radicals (Cl 2 •− / ClO • ) were the major reactive species. Of these, • OH played a pivotal role in AR14 degradation. Additionally, the three principal degradation pathways of AR14 were deduced by LC-MS analysis, UV–vis spectra, density-functional theory (DFT) calculations, and excitation-emission matrix (EEM) fluorescence spectroscopy. Furthermore, the generation mechanism of reactive species and the proposed mechanism of AR14 degradation by the EC-Chlorine-MMO/Ti system were presented. The study made a significant contribution to the understanding of the electrochemical reactions occurring in textile wastewater and provided valuable insights into the rational control of electrolytic conditions for the treatment of saline wastewater.