Efficient Removal of Sulfamethoxazole in Electro-Oxidation System with Boron-Doped Diamond Anode and Electrolyte NaCl: Degradation Mechanisms

In recent years, the pollutant sulfamethoxazole (SMX) that is widely used in medical therapy has been frequently detected in different water systems. Thereby, it is necessary to develop green and effective advanced oxidation strategies, especially the electro-oxidation process. In this study, an electro-oxidation system featuring a boron-doped diamond (BDD) anode and NaCl as the supporting electrolyte was implemented to effectively remove sulfamethoxazole (SMX) without the addition of external oxidants. The operational parameters were optimized using the response surface methodology with a pH 7.5, current density of 4.44 mA/cm2, and NaCl concentration of 20 mmol/L. The optimization significantly enhanced the degradation efficiency of SMX to obtain 100% removal in 5 min. Results of scavenging and chemical probe experiments indicated the presence of hydroxyl radicals (center dot OH) and chlorine radicals (Cl center dot), with the latter primarily forming between the reaction of Cl- and center dot OH. A competition experiment further revealed the relative oxidative contribution of Cl center dot of 38.6%, highlighting its significant role in the degradation process. Additionally, ion chromatography analysis confirmed the presence of Cl center dot without the formation of harmful by-products such as ClO4-, affirming the environmentally friendly nature of the system. The toxicity of the degradation by-products was also assessed. The application of current was investigated to explore the influence of coexistence ions as well as repeatability. Overall, this work highlighted the effectiveness of the electro-oxidation system for the degradation of organic pollutants in saline wastewater, demonstrating the significance of optimization of operational parameters for efficient and sustainable environmental remediation.