MoO2 co-catalytic Fe3+/periodate for tetracycline degradation: Key role of Fe/Mo cycling and high-valent iron (Fe(IV)) generation

Homogeneous Fenton-like catalysts with a single redox site have a rate-limiting step in the oxidant activation process, limiting their application in wastewater treatment. In this study, the addition of MoO2 not only accelerated the cycling of Fe3+/Fe2+ in the Fe3+/periodate system, but also generated more active species to degrade tetracycline (TC). The MoO2/Fe3+/periodate system rapidly eliminated TC within 10 min, and its removal rate was 3.68 times higher than that of the Fe3+/periodate process. The degradation system was able to achieve high degradation efficiency despite the interference of pH (2.95–9.04), inorganic anions (Cl−, NO3−, SO42−) and humic acid (HA) in the aqueous environment. Non-radical including high-valent iron (Fe(IV)) and singlet oxygen (1O2) played a dominant role in the degradation of TC. Self-degradation of Fe(IV) and the subsequent Fenton-like processes indirectly trigger the production of •OH and O2•−. XPS analysis demonstrated that the conversion of Mo(IV) to Mo(V)/Mo(VI) facilitated the Fe3+/Fe2+ cycle. Subsequently, Mo(VI) could be reduced to realize the regeneration of Mo(IV) while completing the conversion of O2•− to 1O2. In addition, after five cycles of reaction, MoO2 showed superb stability and recyclability. This case expanded a new direction for the study of the production and transformation of active species in the inorganic co-catalyst-assisted Fe3+/periodate system.