Hydroxyl Radical Production by H2O2-Mediated Oxidation of Fe(II) Complexed by Suwannee River Fulvic Acid Under Circumneutral Freshwater Conditions

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Abstract
The Fenton reaction, the oxidation of ferrous iron by hydrogen peroxide (H2O2), is typically assumed to be a source of hydroxyl radical (HO•) in natural systems, however, formation of HO• in this process is strongly dependent upon solution pH and the ligand environment, with HO• only formed when Fe(II) is organically complexed. In this study we examine the formation of HO• when Fe(II)–NOM complexes are oxidized by H2O2 using phthalhydrazide as a probe for HO•. We demonstrate that HO• formation can be quantitatively described using a kinetic model that assumes HO• formation occurs solely from the reaction of Fe(II)–NOM complexes with H2O2, even though this reaction is sufficiently slow to play only a negligible role in the overall oxidation rate of total Fe(II). As such, NOM is seen to play a dual role in circumneutral natural systems in stabilizing Fe(II) toward oxidation by H2O2 while enabling the formation of HO• through this oxidation process.

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