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

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 Fe^II(H₂O)₆²⁺ with H₂O₂ yields OH^• radicals or Fe^IV=O_aq. It is now known that this reaction proceeds via the formation of the intermediate complex (H₂O)₅Fe^II(O₂H)⁺/(H₂O)₅Fe^II(O₂H₂)²⁺ that decomposes to form either OH^• radicals or Fe^IV=O_aq, depending on the pH of the medium. The intermediate complex might also directly oxidize a substrate present in the medium. In the presence of Fe^III_aq, the complex Fe^III(OOH)_aq is formed. This complex reacts via Fe^II(H₂O)₆²⁺ + Fe^III(OOH)_aq → Fe^IV=O_aq + Fe^III_aq. In the presence of ligands, the process often observed is L_n(H₂O)_5−nFe^II(O₂H) → L^•+ + L_n−1Fe^III_aq. 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 L_n(H₂O)_5−nM^III/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.