Iron is a critical micronutrient in natural waters. In natural waters, iron exists principally in two oxidation states, namely, as ferric (Fe(III) and/or ferrous (Fe(II) iron. The rates of transformation of Fe between these oxidation states generally control Fe availability in natural waters. In this work, we investigated the mechanism of Fe redox transformation occurring in the presence of natural organic matter (NOM). In particular, we determined the rate and mechanism of (i) Fe(III) reduction in dark and irradiated conditions and (ii) Fe(II) oxidation in dark and irradiated condition using a comprehensive experimental data set. We also investigated the associated NOM oxidation occurring as a result of interaction with reactive oxygen species (ROS such as hydroxyl radicals) generated on Fe redox transformation and/or NOM photochemistry. The presentation will highlight the main findings of our studies, in particular, (i) The presence of native reduced organic groups in NOM which oxidizes to form stable organic specie on irradiation. The reduced organic group present intrinsically in NOM is capable of reducing Fe(III), while the oxidized organic specie formed on irradiation is a strong Fe(II) oxidant. (ii) Photogeneration of short-lived organic moieties, that is capable of oxidizing Fe(II), on irradiation of NOM. (iii) The influence of pH on the lifetime of the photogenerated organic moeties that are involved in Fe(II) oxidation.  (iv) No significant influence of pH on the steady-state Fe(II) concentration (and hence Fe bioavailability), which is traditionally assumed to vary significantly with pH due to changes in Fe(III) solubility and Fe(II) oxygenation rate with variation in pH. (v) The influence of Fe(III) speciation (iron oxides versus organically-complexed Fe(III)) on the main pathways of Fe(III) reduction. (vi) Hydroxyl radical generation on Fe(III) photoreduction and the associated oxidation of NOM present.

Photochemical transformation of Fe in the presence of natural organic matter (NOM): Implications to Fe availability and NOM transformation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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