Natural Organic Matter (NOM), including humic and fulvic acids, are heterogeneous organic macromolecules that play important roles in various biogeochemical processes including redox transformations of trace metals and pollutants. As a result, redox properties of humic and fulvic acids have received considerable interest in recent years. Our interest mainly lies in understanding the redox transformations of Fe in the presence of NOM since these processes control Fe availability which in turn controls primary productivity in many open oceans and coastal upwelling regimes. Compared to the ferric (Fe(III)) state, the reduced ferrous (Fe(II)) state of Fe is more soluble and more weakly complexed by NOM under circumneutral pH such that reduction of Fe(III) is critical for Fe acquisition by microorganisms in oxygenic water.  Our work shows that reduced hydroquinone-like organic group natively exist in SRFA and act as important reductants of Fe(III) under acidic conditions in the dark[1]. Furthermore, on irradiation these reduced hydroquinone-like organic groups oxidize to form semiquinone-like moieties that are capable of oxidizing Fe(II) on time scales that are similar to that of Fe(II) oxygenation under circumneutral pH conditions. These Fe(II)-oxidizing species, formed during photolysis by superoxide-mediated oxidation of hydroquinone-like moieties, are long-lived in the dark. This has strong implications in natural sunlit environments where humic or fulvic-type NOM is present under mildly acidic conditions with steady state Fe(II) concentrations far less than would be the case if Fe(II) oxidation were controlled by the presence of either oxygen or H2O2 alone. Based on our experimental results, a kinetic model has been developed that adequately describes all aspects of the experimental data obtained, and which is capable of predicting Fe(II) oxidation rates and Fe(III) reduction rates in the presence of photolyzed SRFA solution.References:Garg, S.; Ito, H.; Rose, A. L.; Waite, T. D., Mechanism and Kinetics of Dark Iron Redox Transformations in Acidic Previously Photolyzed Acidic Natural Organic Matter Solutions Environ. Sci. Technol 2013,  47, 1861-1867.

Redox transformations of Fe in the presence of pre-photoloysed SRFA solutions

248th National Meeting (2014)

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