Degradation of Sulfamethoxazole in Secondary Wastewater Based on Persulfate Activated by Citric-Acid-Complexed Ferrous Ion under Sunlight

The narrow pH application range and lower utilization of ferrous ions (Fe(II)) restrict the application of Fe(II)/persulfate (PS) technology. In this paper, simulated sunlight and citric acid (Cit) as a chelator were introduced in an Fe(II)/PS system to overcome the drawbacks and enhance the degradation of typical antibiotic sulfamethoxazole (SMX) in secondary wastewater. The degradation kinetics, mechanism, and influence factors of SMX in a sunlight/Fe(II)/Cit/PS system and a sunlight/Fe(II)/Cit system as a comparable system were investigated. The removal efficiency of SMX can reach 71.15% and 85.25% in the sunlight/Fe(II)/Cit system and sunlight/Fe(II)/Cit/PS system with 0.1 mM Fe(II), 0.6 mM Cit, and 1 mM PS. The increase of Fe(II) concentration in both systems proved that sunlight promoted the regeneration of Fe(II) from the ferric ion chelates. However, the Fe(II) concentration decreased after 30 min in the sunlight/Fe(II)/Cit/PS system because of the decomposition of Cit. Radical quencher experiments indicated that SO4<middle dot>-, <middle dot>OH, and O2<middle dot>- contributed 2.48%, 88.43%, and 6.91% to the removal of SMX, respectively. Electron paramagnetic resonance spectra also proved the formation of <middle dot>OH and O2<middle dot>-. The degradation of SMX was proposed to proceed via isomerization, cleavage of S-N bond, and hydroxylation. Overall, the sunlight/Fe(II)/Cit/PS process can be used as an advanced treatment technology for antibiotics in municipal wastewater.