Iron (IV) Formation and the pH Dependent Kinetics of the Fenton Reaction

Despite its widespread importance for biological and environmental chemistry and decades of study, the mechanism underlying the Fenton reaction is still a matter of some controversy. To elucidate the pH dependence of this complex reaction, a new kinetic model is developed to explain the increase in rate and mechanistic shift that occurs from acidic to neutral conditions. This mechanism originated from a re-analysis of a previously proposed model, which neglected explicit iron speciation, leading to unrealistic rate constants. Accounting for speciation suggests a much faster formation rate of Fe(IV), which is estimated to be on the order of 10⁶ M^-1 s^-1. Expanding on prior kinetic studies that include speciation under acidic conditions, we propose a unified kinetic model that captures the pH-dependent rate acceleration in Fe(II) oxidation by H₂O₂, which is a significant step toward resolving the long-standing mechanistic ambiguity of Fenton chemistry.