Homogeneous oxidation kinetics of aqueous ferrous iron at circumneutral pH

Oxidation of aqueous Fe(II) was investigated at circumneutral pH and 23°C in the absence of ligands (other than H₂O, OH^–, and Cl^–) and catalysts (e.g., microbes or solids surfaces). Enzymes (superoxide dismutase and catalase) were used as specific catalytic probes to determine whether superoxide and hydrogen peroxide are intermediates in oxygen reduction by Fe(II). The kinetic evidence suggests that Fe(II) and D.O. react in a termolecular transition state complex, the reaction produces hydrogen peroxide (probably without intermediation by superoxide), and Fe(II) and H₂O₂ react in a termolecular reaction or in a two-step sequence of bimolecular reactions. The rate data permit modeling the overall Fe(II) oxidation reaction at pH7.0 with a rate law that has non-integer orders with respect to Fe(II)] and OH^–].