What Are the Oxidizing Intermediates in the Fenton and Fenton-like Reactions? A Perspective †

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The Fenton and Fenton-like reactions are of major importance due to their role as a source of oxidative stress in all living systems and due to their use in advanced oxidation technologies. For many years, there has been a debate whether the reaction of FeII(H2O)62+ with H2O2 yields OH radicals or FeIV=Oaq. It is now known that this reaction proceeds via the formation of the intermediate complex (H2O)5FeII(O2H)+/(H2O)5FeII(O2H2)2+ that decomposes to form either OH radicals or FeIV=Oaq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of FeIIIaq, the complex FeIII(OOH)aq is formed. This complex reacts via FeII(H2O)62+ + FeIII(OOH)aq → FeIV=Oaq + FeIIIaq. In the presence of ligands, the process often observed is Ln(H2O)5−nFeII(O2H) → L+ + Ln−1FeIIIaq. Thus, in the presence of small concentrations of HCO3 i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO3. It is evident that, in the presence of other transition metal complexes and/or other ligands, other radicals might be formed. In complexes of the type Ln(H2O)5−nMIII/II(O2H), the peroxide might oxidize the ligand L without oxidizing the central cation M. OH radicals are evidently not often formed in Fenton or Fenton-like reactions.

Original languageEnglish
Article number1368
Issue number7
StatePublished - Jul 2022


  • CO
  • Fe=O
  • OH
  • pH effect
  • reactive oxidizing species


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