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Iron Redox Transformations in Continuously Photolyzed Acidic Solutions Containing Natural Organic Matter: Kinetic and Mechanistic Insights

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journal contribution
posted on 19.02.2016, 00:25 by Shikha Garg, Chao Jiang, Christopher J. Miller, Andrew L. Rose, T. David Waite
In this work, the various pathways contributing to the formation and decay of Fe­(II) in photolyzed acidic solutions containing Suwannee River fulvic acid (SRFA) are investigated. Results of experimental and computational studies suggest that ligand to metal charge transfer (LMCT), superoxide-mediated iron reduction and interaction with reduced organic species that are present intrinsically in SRFA each contribute to Fe­(III) reduction with LMCT the most likely dominant pathway under these conditions. Fe­(II) oxidation occurs as a result of its interaction with a variety of light-generated species including (i) short-lived organic species, (ii) relatively stable semiquinone-like organic species, and (iii) hydroperoxy radicals. While not definitive, a hypothesis that the short-lived organic species are similar to peroxyl radicals appears most consistent with our experimental and modeling results. The semiquinone-like organic species formed during photolysis by superoxide-mediated oxidation of reduced organic moieties are long-lived in the dark but prone to rapid oxidation by singlet oxygen (1O2) under irradiated conditions and thus play a minor role in Fe­(II) oxidation in the light. A kinetic model is 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 natural organic matter and light.