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.