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.