Kinetics of [FeII(edta)] Oxidation by Molecular Oxygen Revisited. New Evidence for a Multistep Mechanism

A detailed mechanistic study of the reaction of [FeII(edta)] (edta = ethylenediaminetetraacetate) with molecular oxygen was conducted and the oxidation kinetics were investigated as a function of [FeII], [O2], pH, temperature, and pressure. The observed kinetic data in the presence of an excess of [FeII(edta)] can be accounted for in terms of a four-step mechanism: reaction of (I) [FeII(edta)H2O]2- with O2 by a substitution-controlled process to form [FeII(edta)O2]2-; (II) electron transfer to form an FeIII−superoxo species; (III) subsequent substitution on a second [FeII(edta)H2O]2- anion followed by electron transfer to give [FeIII(edta)−O22-−FeIII(edta)]4-; and (IV) a fast decomposition yielding the monomeric [FeIII(edta)]- and H2O2, which rapidly reacts with [FeII(edta)]. Rate and activation parameters for some of these steps are reported and discussed with reference to available literature data.