Direct oxidation of sulfonamides on carbon anode surface at low current density: Effect of R substituents and adsorption ability

The extensive use of sulfonamides (SAs) in the treatment of respiratory and bacterial infections for humans and animals have triggered their ubiquitous occurrence in environment. Anodic oxidation processes have prominent potential in degrading SAs from wastewater. Herein, carbon cotton (CC) and graphite plate (GP) are applied as anodes to degrade SAs with various R substitutions at low current density (j, 0.42-5 mA/cm2, current intensity 2.5-30 mA). At j = 1.67 mA/cm², degradation efficiency of sulfathiazole, sulfadiazine, sulfisoxazole, sulfamethoxazole, sulfapyridine, and sulfamethazine reached 100, 90.1, 81.6, 92.8, 84.2, and 90.8 % at CC, and 100, 81.5, 54.2, 79.3, 83.2 and 79.1 % at GP, respectively. Most of the degraded SAs have been mineralized in both reaction systems. Sulfaguanidine, without heterocycle in R substituent, was more refractory to degrade at CC and GP surface even at higher j. The quenching experiments and DFT calculation disclosed that direct electron transfer (DET) process took a fundamental role on SAs decomposition. The surface sorbed [•OH] was responsible for mineralization of SAs. The two anodes can efficiently degraded sulfathiazole in different high-salinity wastewater samples at low j. Low-energy consumption and excellent mineralization efficiency endowed CC and GP with promising application for electrochemical oxidation of SAs from wastewater.