Photoreductive Dissolution of Iron(III) (Hydr)oxides in the Absence and Presence of Organic ligands: Experimental Studies and Kinetic Modeling

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)-O-center dot radicals and subsequently to H2O2. The formation of (OH)-O-center dot radicals at the surface was confirmed by photodissolution experiments conducted in the presence of (OH)-O-center dot 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 13, the photoreductive dissolution of lepidocrocite, ferrihydrite, and to a lesser extent hydrous ferric oxide may also proceed at sea water pH.