Impact of natural organic matter on H2O2-mediated oxidation of Fe(II) in a simulated freshwater system

The oxidation of Fe(II) by H2O2 has been studied in the presence of Suwannee River fulvic acid, a standard form of natural organic matter, by adding inorganic Fe(II) to solutions containing both H2O2 and fulvic acid and monitoring the total Fe(II) concentration using a luminol chemiluminescence method. At pH 8.4 and in the absence of competing metals, Suwannee River fulvic acid significantly retards the rate of Fe(II) oxidation due to gradual formation of a species that is oxidized more slowly than inorganic Fe(II) by both O2 and H2O2. It is suggested that rapid formation of a weak Fe(II)–fulvic acid complex that is not readily oxidized by H2O2 is the cause of the reduction in the initial oxidation rate, and that the subsequent further reduction in oxidation rate is a result of the formation of a second type of Fe(II)–fulvic acid complex that is resistant to both O2 and H2O2 oxidation. A kinetic model has been developed that supports this conceptual model. The results demonstrate that, under certain conditions, natural organic matter may stabilize Fe(II) in the presence of elevated H2O2 concentrations, significantly increasing the lifetime of ferrous iron and reducing the flux of hydroxyl radicals produced through this oxidation pathway.