Hydrogen peroxide mediated oxidation of ferrous iron and associated production of hydroxyl radicals in natural aquatic systems

摘要

The oxidation kinetics of ferrous iron, Fe(II), in natural aquatic systems is of interest due to both its importance to the biogeochemistry of iron, a critical micronutrient, and also due to the formation of reactive oxygen species (ROS) during these oxidation reactions. The most important oxidants of Fe(II) are O2 and H2O2, with the H2O2-mediated oxidation process of particular interest due to the potential formation of hydroxyl radical, HO•, a powerful oxidizing agent, during this process.The impact of natural organic matter (NOM) upon the H2O2-mediated oxidation of Fe(II) has been explored under both circumneutral freshwater and seawater conditions, with addition of NOM resulting in markedly slower Fe(II) oxidation rates compared to inorganic Fe(II). The formation of less-readily oxidized Fe(II)-NOM complexes is proposed to explain this, with the results quantitatively described by a kinetic model incorporating such complexes. The ability of NOM to complex Fe(II) is lower in seawater due to both ionic strength effects and the presence of cations such as Mg2+ and Ca2+ which compete with Fe(II) for NOM binding-sites, as such, the impact of NOM in seawater conditions was much lower, with the same kinetic model applicable after a decrease in the effective formation constant between Fe(II) and NOM.The question of whether HO• is formed during Fe(II) oxidation at circumneutral pH was explored using phthalhydrazide as a probe for HO•, with this method extensively characterized and validated for quantitative measurements in natural systems. Initial studies employed the well-characterized ligands EDTA, DTPA and citrate, where it was shown that at pH 8 HO• was only formed when Fe(II) was organically complexed; when inorganic Fe(II) is oxidized by H2O2 a species other than HO• is formed. Similar results were found when Fe(II) is complexed by NOM, with the HO• production data able to be modelled using the same kinetic model developed to describe the Fe(II) oxidation process if it is assumed that only Fe(II)-NOM complexes yield HO•. Overall, under circumneutral conditions Fe(II)-NOM complexes, although only slowly-oxidized by H2O2, yield HO•, whereas the more-rapidly oxidized inorganic Fe(II) species produce some other unidentified intermediate.