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

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