Iron isotope fractionation during dissolution of goethite (alpha-FeOOH) was studied in laboratory batch experiments. Proton-promoted (HCl), ligand-controlled (oxalate dark), and reductive (oxalate light) dissolution mechanisms were compared in order to understand the behavior of iron isotopes during natural weathering reactions. Multicollector ICP-MS was used to measure iron isotope ratios of dissolved iron in solution. The influence of kinetic and equilibrium isotope fractionation during different time scales of dissolution was investigated. Proton-promoted dissolution did not cause iron isotope fractionation, concurrently demonstrating the isotopic homogeneity of the goethite substrate. In contrast, both ligand-controlled and reductive dissolution of goethite resulted in significant iron isotope fractionation. The kinetic isotope effect, which caused an enrichment of light isotopes in the early dissolved fractions, was modeled with an enrichment factor for the 57Fe/ 54Fe ratio of -2.6 per thousandth between reactive surface sites and solution. Later dissolved fractions of the ligand-controlled experiments exhibit a reverse trend with a depletion of light isotopes of approximately 0.5 per thousandth in solution. We interpret this as an equilibrium isotope effect between Fe(III)-oxalate complexes in solution and the goethite surface. In conclusion, different dissolution mechanisms cause diverse iron isotope fractionation effects and likely influence the iron isotope signature of natural soil and weathering environments.

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This study investigated the kinetics of the photoreductive dissolution of various iron(III)(hydr)oxide phases, lepidocrocite (gamma-FeOOH), ferrihydrite, and hydrous ferric oxide, in the absence of organic ligands as a function of pH in deaerated and aerated suspensions. Photoreductive dissolution of lepidocrocite and ferrihydrite only occurred below pH 6. Under oxic conditions, we observed both the formation of aqueous Fe(II) and H2O2 during photoreductive dissolution of lepidocrocite and ferrihydrite at pH 3. These experimental findings are consistent with the light-induced reduction of surface Fe(III) at the (hydr)oxide surface and the concomitant oxidation of surface-coordinated water or hydroxyl groups, leading to surface Fe(II) and *OH radicals and subsequently to H2O2. The formation of *OH radicals atthe surface was confirmed by photodissolution experiments conducted in the presence of *OH radical scavengers. Kinetic modeling of the experimental data suggests that the relevant pathway for the formation of H2O2 is the reoxidation of surface lattice Fe(II) by molecular oxygen. This study furthermore shows that in the presence of strong iron binding ligands such as siderophores, specifically desferrioxamine B, the photoreductive dissolution of lepidocrocite, ferrihydrite, and to a lesser extent hydrous ferric oxide may also proceed at seawater pH.

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