Pathways Contributing to the Formation and Decay of Ferrous Iron in Sunlit Natural Waters. In: Tratnyek PG, Grundl TJ, Haderlein SB, editors. Aquatic Redox Chemistry. Washington: American Chemical Society; 2011. pp. 153-76. [DOI: 10.1021/bk-2011-1071.ch008]

Is Superoxide-Mediated Fe(III) Reduction Important in Sunlit Surface Waters?

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O₂^•-) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O₂^•- does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O₂^•- and LMCT. The reduction of AFO by O₂^•- occurs following the dissolution of AFO, and hence, the contribution of O₂^•- to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O₂^•-, there is no involvement of O₂^•- in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

Impact of pH on Iron Redox Transformations in Simulated Freshwaters Containing Natural Organic Matter

The impact of the pH of natural waters on the various pathways contributing to the formation and decay of Fe(II) in the presence of Suwannee River Fulvic Acid (SRFA) is investigated in this study. Our results show that thermal Fe(III) reduction occurs as a result of the presence of hydroquinone-like moieties in SRFA with the rate of Fe(III) reduction by these entities relatively invariant with change in pH in the range 6.8-8.7. The Fe(II) oxidation rate in the dark is controlled by its interaction with O₂ and increases with increase in pH with the overall outcome that the steady-state Fe(II) concentration in the dark is strongly affected by solution pH. On irradiation, a portion of the hydroquinone-like moieties present are oxidized to form semiquinones that are capable of reducing Fe(III) and/or oxidizing Fe(II) under circumneutral pH conditions. The extent of photogeneration of semiquinones on irradiation of SRFA and the persistence of these radicals increases significantly with decrease in pH. Due to the higher concentration and longevity of these organic moieties under low pH conditions, the impact of pH on steady-state Fe(II) concentration is less pronounced in previously irradiated SRFA solution compared to that observed in dark SRFA solution. Under irradiated conditions, the rates of Fe transformation (including both Fe(II) oxidation and Fe(III) reduction) are nearly independent of pH. While ligand-to-metal charge transfer (LMCT) is the dominant pathway for photochemical Fe(III) reduction, Fe(II) oxidation under irradiated conditions mainly occurs as a result of interaction with O₂, semiquinones and other short-lived oxidants. Overall, our data supports the conclusion that, as a result of the contribution from photogenerated organic moieties to Fe redox transformations, the steady-state Fe(II) concentration in irradiated surface waters containing natural organic matter may not be impacted significantly by changes in pH.

Iron Redox Transformations in the Presence of Natural Organic Matter: Effect of Calcium

The effects of calcium on iron redox transformations in acidic nonirradiated and irradiated Suwannee River Fulvic Acid (SRFA) solutions are investigated in this study. Our results reveal that, even though calcium is redox-inert, it affects the redox transformations of iron with increased Fe(III) reduction rates under irradiated conditions and decreased Fe(II) oxidation rates in the dark in the presence compared to the absence of calcium. While the exact mechanism via which the Fe(III) reduction rate under irradiated conditions is impacted by calcium addition is not clear, the formation of more photolabile weakly complexed Fe(III)SRFA is most consistent with our experimental results. An observed decline in the Fe(II) oxidation rate in nonirradiated and previously irradiated SRFA solutions with the addition of calcium can be rationalized by formation of more weakly bound Fe(II) and Fe(III). The higher Fe(III) reduction rates and lower Fe(II) oxidation rates in the presence compared to the absence of calcium will help to maintain higher concentrations of Fe(II) thereby increasing the bioavailability of iron in calcium-containing waters. On the basis of our experimental results, we have developed a mathematical model that well describes the iron redox transformations mediated by SRFA in calcium-containing waters under irradiated and nonirradiated conditions.

Hydroquinone-Mediated Redox Cycling of Iron and Concomitant Oxidation of Hydroquinone in Oxic Waters under Acidic Conditions: Comparison with Iron-Natural Organic Matter Interactions

Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3-5 in the air-saturated and partially deoxygenated solution are examined here. Our results show that 1,4-hydroquinone reduces Fe(III) in acidic conditions, generating semiquinone radicals (Q(•-)) that can oxidize Fe(II) back to Fe(III). The oxidation rate of Fe(II) by Q(•-)increases with increase in pH due to the speciation change of Q(•-) with its deprotonated form (Q(•-)) oxidizing Fe(II) more rapidly than the protonated form (HQ(•)). Although the oxygenation of Fe(II) is negligible at pH < 5, O2 still plays an important role in iron redox transformation by rapidly oxidizing Q(•-) to form benzoquinone (Q). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron-Suwannee River fulvic acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution are rapidly oxidized by dioxygen, while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen, with the result that steady-state semiquinone concentrations in SRFA solutions are 2-3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in the steady-state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states, with iron turnover frequencies in solutions containing SRFA being 10-20 times higher than those observed in solutions of 1,4-hydroquinone.

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 ((1)O(2)) 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.