Kinetic Analysis and Transformation Pathways of Sulfamethoxazole Degradation in Water and Wastewater Under Electron Beam Irradiation

Sulfamethoxazole (SMX), a widely used antibiotic, persists in aquatic environments due to its resistance to conventional wastewater treatments. This work examined the breakdown of SMX in both purified water and urban wastewater through the application of electron beam irradiation (EBI). Experiments were conducted across doses of 0.5–3.0 kGy and varying pHs (2.70, 6.13, 9.00 and 11.10) and initial concentrations (5, 10, 15, 20 and 30 mg/L), and the role of reactive species was investigated with the help of scavengers. The results showed that SMX degradation followed pseudo-first-order kinetics and was most efficient at lower pH and concentrations. The scavenger experiments confirmed hydroxyl radicals as the dominant oxidizing agents responsible for SMX degradation, while wastewater constituents slightly inhibited the process. Nevertheless, over 99% SMX degradation was achieved at higher doses (1.5–3.0 kGy). TOC analysis revealed the partial mineralization of SMX, indicating the persistence of intermediate by-products despite high degradation efficiency. LC-MS analysis revealed multiple transformation products including hydroxylated sulfonamides and nitro-substituted derivatives, reflecting diverse degradation pathways. These results demonstrate that EBI is a highly effective laboratory-scale method for degrading SMX from water and wastewater, with promising potential for practical application.