Ferryl for real. The Fenton reaction near neutral pH

Abstract

According to the literature, the Fenton reaction yields HO˙ and proceeds with 53 M−1 s−1 at 25 °C and low pH. Above pH 5, the reaction becomes first-order in HO−, and oxygen atom transfer has been detected, which indicates formation of oxidoiron(2+), FeO2+. These observations, and the assumption that the intermediate [FeHOO]+ decays approximately iso-energetically to FeO2+, allow one to estimate an Gibbs energy of formation FeO2+ of +15 ± 10 kJ mol−1, from which follows the one-electron E°′(FeO2+, H2O/[Fe(HO)2]+) = +2.5 ± 0.1 V and the two-electron E°′(FeO2+, 2H+/Fe2+, H2O) = +1.36 ± 0.05 V, both at pH 7. In the presence of HCO3−, formation of FeCO3(aq) occurs which may facilitate formation of the [FeHOO]+ intermediate, and leads to CO3˙−. At pH 7, the product of the Fenton reaction is thus FeO2+, or CO3˙− if HCO3− is present.