Elucidation of the oxidation mechanisms and pathways of sulfamethoxazole degradation under Fe(II) activated percarbonate treatment.

Fe(II) activated sodium percarbonate (SPC) process (SPC/Fe(II)) could efficiently remove sulfamethoxazole (SMX) in the aqueous phase, and has the potential in groundwater remediation. However, the degradation mechanisms, especially the degradation products and pathways till now have remained unclear. In the present study, intermediate products were identified using high resolution liquid chromatography coupled with ion trap and time-of-flight mass spectrometry (LCMS-IT-TOF). Nine intermediate products were identified, six of which have not yet been reported during the oxidation of SMX. The oxidation mechanisms involved hydroxyl substitution, the cleavage of sulfonamide bond, isoxazole ring opening and a rearrangement following the loss of the SO2-group. Based on the identified intermediate products, the degradation pathways of SMX by SPC/Fe(II) process were illustrated. Fenton's reaction after the dissolution of SPC was proposed as the main reaction mechanisms, which was checked and confirmed by radical species detection tests and radical species scavenging studies. The results showed that although both O2- and HO were present in SPC/Fe(II) system, HO was dominant in the system while O2- was seldom involved in the degradation of SMX. These findings provided useful information and supported the application of this advanced oxidation process for antibiotics elimination in the groundwater.