Enhanced Removal of Sulfamethoxazole by a Fe(VI)/Redox Mediator System: Insights into the Key Role of Fe(V)

Ferrate [Fe(VI)] has been widely applied due to its multifunctional effects of flocculation, disinfection, and decontamination in wastewater treatment. To achieve an enhancement for Fe(VI) activity under alkaline conditions, 1-hydroxybenzotriazole (HBT), as an emerging redox mediator, promotes electron transfer to assist Fe(VI) to achieve more excellent decontamination. In this study, the introduction of HBT increases the degradation rate of sulfamethoxazole (SMX) from 53 to 98% at pH 8.0 within 5 min. Surprisingly, quenching and probe experiments indicated that active species are all high-valent iron species rather than radicals (HO^· and HBT^·) by the Fe(VI)/HBT process. Further research disclosed that the enhancement for SMX degradation with the Fe(VI)/HBT system was driven by the enhancement of Fe(VI) self-decomposition and the formation of more Fe(V) via single-electron transfer by HBT. Moreover, a kinetic model is developed for fitting SMX degradation to reveal that the dominant active species shifts from Fe(IV) to more active Fe(V), reaching nearly 100% SMX degradation as the HBT dosage increased. Furthermore, 12 SMX degradation intermediates are detected using an ultrahigh performance liquid chromatography quadrupole time-of-flight mass spectrometer, and three possible removal pathways are proposed, including oxidation, hydroxylation, and hydrolysis. More importantly, the Fe(VI)/HBT system manifests a great decontamination performance on the coexisting anions, cations, and actual water such as lake, river, and tap water. In summary, this work establishes a new system for the effective removal of pollutants and provides crucial information on the application of ferrate in wastewater treatment.