Revealing the fundamental role of MoO2 in promoting efficient and stable activation of persulfate by iron carbon based catalysts: Efficient Fe2+/Fe3+ cycling to generate reactive species.

Electron-rich iron sites are the main sites for iron-based catalysts to activate persulfate (PS) to generate reactive species, while blocked Fe/Fe cycling usually reduces the catalytic performance of iron-based materials and hinders the generation of reactive species in the reaction. To solve the bottleneck, we synthesized an iron-carbon nanocomposite catalyst loaded with MoO (Fe/Mo-CNs). The promotion of MoO on the Fe/Fe cycle in the system allowed Fe/Mo-CNs to exhibit excellent catalytic performance and environmental adaptability. The degradation rate of bisphenol S (BPS) by the Fe/Mo-CNs/PS system was significantly increased to 0.080 min compared with the iron-carbon based catalyst/persulfate system, and the degradation efficiency of BPS was maintained at around 85% after four cycles. Density functional theory (DFT) calculations showed that the introduction of MoO reduced the reaction energy barrier of persulfate activated by catalysts to produce reactive species, especially promoted the production of more high valent iron (Fe(IV)). Fe(IV) and reactive oxygen species (SO·, ·OH, ·O and O) worked together on the efficient degradation of BPS. In addition, the test of an automatic circulating degradation plant had proved that Fe/Mo-CNs had a good practical application prospect. BPS was mainly degraded by ring cleavage and O=S=O bond cleavage, and the toxicity of BPS and its intermediates were also evaluated. This work clarifies the mechanism of improving the catalytic performance of heterogeneous iron-based catalysts by MoO in sulfate radical-based advanced oxidation processes (SR-AOPs), providing a new idea for solving the blockage of Fe/Fe cycle in SR-AOPs.