Enhanced Oxidation of Organic Compounds by the Ferrihydrite–Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

Previous studies mostly held that the oxidation capacity of ferrate depends on the involvement of intermediate iron species (i.e., Fe^IV/Fe^V), however, the potential role of the metastable complex was disregarded in ferrate-based heterogeneous catalytic oxidation processes. Herein, we reported a complexation-mediated electron transfer mechanism in the ferrihydrite–ferrate system toward sulfamethoxazole (SMX) degradation. A synergy between intermediate Fe^IV/Fe^V oxidation and the intramolecular electron transfer step was proposed. Specifically, the conversion of phenyl methyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO₂) suggested that Fe^IV/Fe^V was involved in the oxidation of SMX. Moreover, based on the in situ Raman test and chronopotentiometry analysis, the formation of the metastable complex of ferrihydrite/ferrate was found, which possesses higher oxidation potential than free ferrate and could achieve the preliminary oxidation of organics via the electron transfer step. In addition, the amino group of SMX could complex with ferrate, and the resulting metastable complex of ferrihydrite/ferrate would combine further with SMX molecules, leading to intramolecular electron transfer and SMX degradation. The ferrate loss experiments suggested that ferrihydrite could accelerate the decomposition of ferrate. Finally, the effects of pH value, anions, humic acid, and actual water on the degradation of SMX by ferrihydrite–ferrate were also revealed. Overall, ferrihydrite demonstrated high catalytic capacity, good reusability, and nontoxic performance for ferrate activation. The ferrihydrite–ferrate process may be a green and promising method for organic removal in wastewater treatment.