Hematite/selenium disulfide hybrid catalyst for enhanced Fe(III)/Fe(II) redox cycling in advanced oxidation processes

Regeneration of Fe(II) is a key issue for heterogeneous advanced oxidation processes (AOPs) using iron-based catalysts. Herein, a hybrid catalyst was developed from α-Fe₂O₃ and SeS₂ to enhance the Fe(III)/Fe(II) redox cycling in both hydrogen peroxide (H₂O₂) system and persulfate (PS) system. The regeneration of Fe(II) was evidenced by the increased Fe(II)/Fe(III) ratio in the used catalyst (205.8% in the H₂O₂ system or 125.4% in the PS system), compared to 68.4% in the fresh hybrid catalyst Fe/Se-3. Methyl orange was used as a model pollutant to evaluate the degradation performance of the hybrid catalyst. Owing to the promotion of Fe(II) regeneration, Fe/Se-3 achieved a pollutant removal efficiency of 100.0% in 12 min in both systems, significantly higher than that with pure α-Fe₂O₃ (33.9 ± 3.6% in the H₂O₂ system or 30.7 ± 2.8% in the PS system). The dominant active species were identified as hydroxyl radicals in the H₂O₂ system and sulfate radicals in the PS system. In the proposed mechanism, soluble and surface-bound Fe species are provided by α-Fe₂O₃ to activate H₂O₂ or PS to radicals, and SeS₂ participates in the reactions via Se(IV) reducing Fe(III) to Fe(II) and S atoms being released through protonation to expose more active Se sites.