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 (Fe ₃ S ₄ @β-FeOOH) by 40 times, in which not only the ^• OH formation was enhanced but also SO ₄ ^•- 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 ₂ ^• /O ₂ ^•- was detected in the Fe ₃ S ₄ @β-FeOOH/H ₂ O ₂ system. These results were different from some previously reported Fenton counterparts. Based on the characterization and probe experiments, sulfur species, including S ^2- , S ⁰ , and S _n ^2- , 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 ₅ ^- can be generated from SO ₃ ^2- oxidized by ^• OH, and its scission into SO ₄ ^•- was not dependent on the extra electric potential or Fe-O ₂ -S(IV) intermediate. These findings provided new insight for utilizing sulfidation to improve the activity of iron-based Fenton catalysts.