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 (Fe3S4@β-FeOOH) by 40 times, in which not only the •OH formation was enhanced but also SO4•– 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 HO2•/O2•– was detected in the Fe3S4@β-FeOOH/H2O2 system. These results were different from some previously reported Fenton counterparts. Based on the characterization and probe experiments, sulfur species, including S2–, S0, and Sn2–, 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, HSO5– can be generated from SO32– oxidized by •OH, and its scission into SO4•– was not dependent on the extra electric potential or Fe–O2–S(IV) intermediate. These findings provided new insight for utilizing sulfidation to improve the activity of iron-based Fenton catalysts.