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

The Fenton reaction, the oxidation of ferrous iron by hydrogen peroxide (H2O2), is typically assumed to be a source of hydroxyl radical (HO center dot) in natural systems, however, formation of HO center dot in this process is strongly dependent upon solution pH and the ligand environment, with HO center dot only formed when Fe(II) is organically complexed. In this study we examine the formation of HO center dot when Fe(II)-NOM complexes are oxidized by H2O2 using phthalhydrazide as a probe for HO center dot. We demonstrate that HO center dot formation can be quantitatively described using a kinetic model that assumes HO center dot 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 center dot through this oxidation process.