Wavelength-Dependence of Photoreductive Dissolution of Lepidocrocite (γ-FeOOH) in the Absence and Presence of the Siderophore DFOB

Abstract

Photoreductive dissolution of lepidocrocite (gamma-FeOOH) in the presence/absence of the siderophore desferrioxamine B (DFOB) was investigated at different wavelengths. At pH 3 in the absence of DFOB, Fe(II) formation rates normalized to the photon flux increased with decreasing wavelengths below 515 nm, consistent with enhanced Fe(II) formation at lower wavelengths by photolysis of surface Fe(III)-hydroxo groups or by surface scavenging of photoelectrons generated in the semiconducting bulk. In the presence of DFOB at pH 3, photoreductive dissolution rates, normalized to the photon flux, increased more strongly with decreasing wavelengths below 440 nm. We hypothesize that acid-catalyzed hydrolysis of DFOB generates degradation products that form photoreactive surface complexes leading to an increase in photodissolution rates at low pH. At pH 8 in the presence of DFOB, normalized photodissolution rates had a maximum in the spectral window 395-435 nm and were significantly smaller at lower wavelengths, suggesting that adsorbed DFOB is directly involved in the reduction of surface Fe(III) by a light-induced ligand-to-metal charge-transfer reaction within the surface Fe(III)-DFOB complex. The strong response in the visible light suggests that photoreductive dissolution of iron (hydr)oxides promoted by siderophores with hydroxamic acid groups may occur deep into in the euphotic zone of oceans.