<p>Heterogeneous high-valent iron-oxo species (Fe^IV = O) are known as reactive oxygen species with specific oxygen-atom transfer capabilities, yet the characterization of their presence and kinetic behaviors in Fenton or Fenton-like systems remains challenging under conventional techniques. Herein we develope a operando technique through the integration of the stopped-flow (SF) technique with rapid-scan attenuated total reflection (ATR) IR spectroscopy. This operando technique allows for dynamic monitoring of heterogeneous Fenton reactions, including the variations and kinetics of reactants and intermediates with a maximum time resolution of 60 ms. Supported by this technique, our investigation of the peroxymonosulfate (PMS)-based Fenton-like reaction on an Fe-N₄ single-atom catalyst (Fe-N-C) not only confirms the formation of surface Fe^IV = O but also provides the experimental identification of its lifetime on the timescale of seconds (3.96 s) through spectral kinetics analysis. Additionally, we uncover an oxygen exchange between Fe^IV = O and water molecules, further substantiating the notion of Fe^IV = O as a long-lived reactive species in aqueous systems. Furthermore, based on the spectroscopic results, leveraging the specific reactivity of Fe^IV = O towards aqueous As^III along with its extended lifetime, we achieve effective preferential removal of As^III even in complex wastewater environments using Fe-N-C/PMS system.</p>