Plasma coupled with ultrasonic for degradation of organic pollutants in water: Revealing the generation of free radicals and the dominant degradation pathways

This study explores the synergistic effects of dielectric barrier discharge (DBD) plasma coupled with ultrasound (US) in the degradation of organic pollutants in water. The optimal degradation conditions were determined to be 100 W ultrasonic power, 190 V DBD plasma voltage, and neutral pH. The combination of DBD plasma and US significantly enhances the generation of reactive species (·OH, ·O₂^-, and ¹O₂), leading to a 97.3 % degradation efficiency of methyl orange (MO), which is 17.3 % higher than DBD plasma alone. Electron spin resonance (ESR) identified ·OH, ·O₂^-, and ¹O₂ in DBD/US system, and radical scavengers were employed to elucidate their roles. The degradation process was assessed through changes in pH, conductivity, TOC and COD, with characterization and analysis via UV-Vis and LC-MS. By combining LC-MS with density functional theory (DFT) calculations, nine intermediates were identified and three degradation pathways dominated by free radicals were established. Additionally, the DBD/US system treated wastewater containing sulfamethoxazole (SMX), sulfadiazine (SDZ), tetracycline (TC), and ciprofloxacin (CIP), achieving degradation rates exceeding 80 %. These findings suggest that the DBD/US coupled system holds promising potential for treating organic pollutant wastewater.