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 multifunctionaleffects of flocculation, disinfection, and decontamination in wastewatertreatment. To achieve an enhancement for Fe(VI) activity under alkalineconditions, 1-hydroxybenzotriazole (HBT), as an emerging redox mediator,promotes electron transfer to assist Fe(VI) to achieve more excellentdecontamination. In this study, the introduction of HBT increasesthe degradation rate of sulfamethoxazole (SMX) from 53 to 98% at pH8.0 within 5 min. Surprisingly, quenching and probe experiments indicatedthat active species are all high-valent iron species rather than radicals(HO & BULL; and HBT & BULL;) by the Fe(VI)/HBT process.Further research disclosed that the enhancement for SMX degradationwith the Fe(VI)/HBT system was driven by the enhancement of Fe(VI)self-decomposition and the formation of more Fe(V) via single-electrontransfer by HBT. Moreover, a kinetic model is developed for fittingSMX degradation to reveal that the dominant active species shiftsfrom Fe(IV) to more active Fe(V), reaching nearly 100% SMX degradationas the HBT dosage increased. Furthermore, 12 SMX degradation intermediatesare detected using an ultrahigh performance liquid chromatographyquadrupole time-of-flight mass spectrometer, and three possible removalpathways are proposed, including oxidation, hydroxylation, and hydrolysis.More importantly, the Fe(VI)/HBT system manifests a great decontaminationperformance on the coexisting anions, cations, and actual water suchas lake, river, and tap water. In summary, this work establishes anew system for the effective removal of pollutants and provides crucialinformation on the application of ferrate in wastewater treatment.