New kinetic analysis of the Fenton reaction: Critical examination of the free radical – chain reaction concept

Using [H₂O₂] in the molar range, the reaction with Fe²⁺ has two phases: in the first rapid phase, only a small fraction of the total O₂ is evolved; the bulk of the gas is formed in a slow second phase. In interpretations based on the free radical model of Barb et al., the first phase has been identified with the ‘Fenton reaction’ (reaction of Fe²⁺with H₂O₂), while the second with catalytic decomposition of H₂O₂ by Fe³⁺ ions. This interpretation is not correct. A new analysis of the model shows that (1) it is a chain reaction having no termination steps and (2) the ‘Fenton part’ alone consists of two phases. It starts with rapid evolution of O₂ via a five-membered chain reaction (first phase). When [Fe²⁺] becomes low, evolution of O₂ continues in a three-membered chain reaction at a greatly reduced rate (second phase). In later stages of the second phase, Fe³⁺ catalysis contributes to O₂ evolution. Thus, the amount of O₂ formed in the rapid phase cannot be identified with the total amount formed in the ‘Fenton reaction’ but only with that formed in its first phase. Computer simulations of O₂ evolution based on the model of Barb et al. and rate constants show a definite dependence of this quantity on the initial [H₂O₂] – in contrast to the experimentally found independence. More satisfactory, but not complete, agreement with measured data could be reached in simulations using a non-radical model. Some of the difficulty has been due to the determination of the exact position of the end of the first phase. The transition between the two phases of the reaction occurs in a short, but finite time interval. It has been shown that the quantity ‘total amount of O₂ evolved in the Fenton reaction’ (subtracting the part due to Fe³⁺catalysis) is not accessible to experimental determination nor to theoretical calculation.