Degradation of sulfamethoxazole in high-salinity wastewater by low-temperature plasma: efficiency, mechanism, and toxicity assessment

The treatment of high-salinity organic wastewater, characterized by complex composition and poor biodegradability, poses a significant challenge. This study investigated the degradation of a typical antibiotic, sulfamethoxazole (SMX), in NaCl-rich wastewater using a low-temperature plasma (LTP) system. The effects of operational parameters, including SMX initial concentration, input voltage, initial pH, and salt content (NaCl), on degradation efficiency were systematically evaluated. Results indicated that lower SMX concentration (15 mg/L), higher input voltage (200 V), and acidic/neutral conditions favored SMX degradation. The presence of low-concentration salt ions (e.g., 5 g/L NaCl) enhanced degradation compared to a salt-free environment, primarily by improving solution conductivity and generating chlorine-active species, whereas excessive salinity inhibited the process. Electron spin resonance (ESR) and radical quenching experiments confirmed that reactive oxygen species (ROS), particularly singlet oxygen ( 1 O 2 ), hydroxyl radicals (·OH), and superoxide radicals (·O 2 − ), played crucial roles in SMX decomposition. The degradation pathways, elucidated via liquid chromatography-mass spectrometry (LC-MS) and density functional theory (DFT) calculations, involved nitration, hydroxylation, deamination, ring-opening, and coupling reactions. Ecological Structure Activity Relationship (ECOSAR) model predictions demonstrated a significant reduction in the ecological toxicity of SMX and its transformation products towards fish, daphnia, and green algae after LTP treatment. This work confirms the potential of LTP technology for the effective and environmentally friendly treatment of antibiotic-contaminated high-salinity wastewater.