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)(H(2)O)(6)(2+) with H(2)O(2) yields OH(•) radicals or Fe(IV)=O(aq). It is now known that this reaction proceeds via the formation of the intermediate complex (H(2)O)(5)Fe(II)(O(2)H)(+)/(H(2)O)(5)Fe(II)(O(2)H(2))(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)(H(2)O)(6)(2+) + Fe(III)(OOH)(aq) → Fe(IV)=O(aq) + Fe(III)(aq). In the presence of ligands, the process often observed is L(n)(H(2)O)(5−n)Fe(II)(O(2)H) → L(•)(+) + L(n−1)Fe(III)(aq). Thus, in the presence of small concentrations of HCO(3)(−) i.e., in biological systems and in advanced oxidation processes—the oxidizing radical formed is CO(3)(•)(−). 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)(H(2)O)(5−n)M(III/II)(O(2)H(−)), 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.