A Kinetic and ESR Investigation of Iron(II) Oxalate Oxidation by Hydrogen Peroxide and Dioxygen as a Source of Hydroxyl Radicals

Abstract

The reaction of FeII oxalate with hydrogen peroxide and dioxygen was studed for oxalate concentrations up to 20 mM and pH 2–5, under which conditions mono- and bis-oxalate comlexes (FeII(ox) and FeII(ox)22-) and uncomplexed Fe2+ must be considered. The reaction of FeII oxalate with hydrogen peroxide (Fe2+ + H2O2 → Fe3+ + *OH + OH−) was monitored in continuous flow by ESR with t-butanol as a radical trap. The reaction is much faster than for uncomplexed Fe2+ and a rate constant, k = 1 × 104 M-1 s−1 is deduced for FeII(ox). The reaction of FeII oxalate with dioxygen is strongly pH dependent in a manner which indicates that the reactive species is FeII(ox)22-, for which an apparent second order rate constant, k = 3.6 M−1 s−1, is deduced. Taken together, these results provide a mechanism for hydroxyl radical production in aqueous systems containing FeII complexed by oxalate. Further ESR studies with DMPO as spin trap reveal that reaction of FeII oxalate with hydrogen peroxide can also lead to formation of the carboxylate radical anion (CO2*-), an assignment confirmed by photolysis of FeIII oxalate in the presence of DMPO.