Mechanism and kinetics of dark iron redox transformations in previously photolyzed acidic natural organic matter solutions

Stable organic species produced on irradiation of Suwannee River Fulvic Acid (SRFA) are shown to be important oxidants of Fe(II) in aqueous solutions at acidic pH, with rate constants substantially larger than those for oxygenation of Fe(II) under the same conditions. These Fe(II)-oxidizing species, which are formed during photolysis by superoxide-mediated oxidation of reduced organic moieties that are present intrinsically in SRFA, are long-lived in the dark but prone to rapid oxidation by singlet oxygen (¹O₂) under irradiated conditions. The intrinsic reduced organic species are able to reduce Fe(III) at acidic pH. Although the exact identities of the organic Fe(II) oxidant and the organic Fe(III) reductant are unclear, their behavior is consistent with that expected of semiquinone and hydroquinone-like moieties respectively. A kinetic model is developed that adequately describes all aspects of the experimental data obtained, and which is capable of predicting dark Fe(II) oxidation rates and Fe(III) reduction rates in the presence of previously photolyzed natural organic matter.