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^•) 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.