Mechanism and Kinetics of Ferrous Iron Oxidation and Ferric Iron Reduction in Simulated Natural Waters: Impact of light and pH

Abstract

The redox chemistry of iron in natural aquatic systems is of great interest due to its significance to the bioavailability of iron, a critical micronutrient to all living organisms. Natural organic matter (NOM) plays a significant role in iron redox transformations, mainly due to the interaction of redox-active organic moieties present in NOM with Fe(II) and Fe(III). In order to improve our understanding of iron redox transformations in natural waters, the oxidation and reduction kinetics of nanomolar concentrations of iron were investigated. Particular attention has been given to the role of quinone groups in iron redox transformations by comparing results obtained in Suwannee River fulvic acid (SRFA) and pure hydroquinone solutions. A kinetic modelling approach that facilitates the analysis and understanding of the mechanism of iron redox transformations under various conditions has been used extensively throughout this work. The results show that hydroquinone-like moieties are intrinsically present in SRFA and reduce Fe(III) under acidic and circumneutral pH conditions. These hydroquinone-like moieties oxidize to form long-lived semiquinone-like moieties on irradiation that are capable of oxidizing Fe(II) under acidic and circumneutral pH conditions; they also act as Fe(III) reductant under circumneutral pH conditions. pH was shown to play a critical role in controlling the rate of iron redox transformations, mainly by affecting Fe(II) oxidation kinetics. In the presence of light, ligand-to-metal charge transfer (LMCT) is the main Fe(III) reduction pathway under acidic and circumneutral pH conditions. It is further shown that while short-lived photo-generated peroxyl-like radicals play an important role in Fe(II) oxidation under acidic conditions, Fe(II) oxidation is mainly driven by dioxygen and/or semiquinone-like radicals under circumneutral pH conditions. The presence of divalent calcium ions was shown to impact iron redox transformation kinetics in non-irradiated and irradiated SRFA solutions mainly due to changes in iron speciation as a result of the competition between iron and calcium for the binding sites on SRFA. Overall, the results show that NOM affects iron availability, not only by providing organic ligands that enhance iron solubility, but also by facilitating redox transformations of iron by redox active groups that directly participate in iron redox transformations.