Non-activated peroxymonosulfate oxidation of sulfonamide antibiotics in water: Kinetics, mechanisms, and implications for water treatment

Despite that sulfate radical-based activated peroxymonosulfate (PMS) oxidation processes (e.g., UV/PMS, Co2+/PMS, etc.) have been widely applied for decontamination, the direct oxidation of organic contaminants by PMS per se is less known. This contribution reports that certain contaminants, such as sulfonamides (SAs), are amendable to direct oxidation by PMS without activation. Using sulfamethoxazole (SMX) as a representative, kinetics and density functional theory (DFT)-based computational methods were applied to elucidate the underlying mechanisms and pathways through which SMX was transformed by direct PMS oxidation. High resolution mass spectrometry (HR-MS) coupled with high performance liquid chromatography (HPLC) analyses using authentic standards were adopted to qualifying and quantifying SMX transformation products. Our results reveal that nonradical oxidation of SMX by PMS was initiated by formation of a transition state complex between PMS molecule and amino functional group of SMX. Such reaction was assisted by two water molecules, which significantly reduced energy barrier. Direct PMS oxidation of SMX led to the formation of N4-hydroxyl-sulfamethoxazole (N4-OH-SMX), 4-nitroso-sulfamethoxazole (4-NO-SMX), and 4-nitro-sulfamethoxazole (4-NO2-SMX), sequentially. Implications of PMS oxidation with SAs to water treatment were further evaluated by investigating the effects of PMS dosage, pH, and natural water matrices. While PMS has a potential to transform a suite of SAs with similar structures (SMX, sulfisoxazole, sulfamethizole, sulfapyridine, sulfadiazine, and sulfachloropyridazine), the formation of potential hazardous nitroso- and nitro-byproducts should be scrutinized before this technology can be safely used for water and wastewater treatment.