New Insight into a Fenton-like Reaction Mechanism over Sulfidated β-FeOOH: Key Role of Sulfidation in Efficient Iron(III) Reduction and Sulfate Radical Generation.

Sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal; however, whether this method benefits Fenton-like reactions or not and the possible mechanism are not well understood. In this study, we revealed that surface sulfidation can greatly promote the heterogeneous Fenton activity of β-FeOOH (FeS@β-FeOOH) by 40 times, in which not only the OH formation was enhanced but also SO as a new oxidation species was generated. Moreover, their contribution to metronidazole (MTZ) degradation was 52.5 and 37.1%, respectively. In comparison, almost no HO/O was detected in the FeS@β-FeOOH/HO system. These results were different from some previously reported Fenton counterparts. Based on the characterization and probe experiments, sulfur species, including S, S, and S, as an electron donor and electron shuttle were responsible for efficient conversion of Fe(III) into Fe(II) other than via the Haber-Weiss mechanism, leading to excellent OH generation via a Fenton-like mechanism. Most importantly, HSO can be generated from SO oxidized by OH, and its scission into SO was not dependent on the extra electric potential or Fe-O-S(IV) intermediate. These findings provided new insight for utilizing sulfidation to improve the activity of iron-based Fenton catalysts.