How the Structure of Aluminum Goethite Affects Iron Release

Aluminum substitution for Fe in iron(hydr)oxides often occurs in nature and is known to affect mineral weathering, Fe cycling and Fe bioavailability because it alters crystal growth rate, domain morphology, surface area, and other structural properties that influences the rates and mechanisms of mineral biodissolution and iron mobility. Of particular interest is how the structural changes of goethite associated with aluminum substitution (Fe_1-xAl_xOOH) influence solubilization of Fe by siderophores, organic ligands secreted by bacteria to selectively complex Fe(III) in iron-limiting situations. This study examines the effect of crystallinity in a series of synthetic aluminum goethites (x = 2, 4, 6, and 10) on the adsorption of desferrioxamine-B (DFO-B) and oxalate and on the release kinetics of iron by these two ligands at pH 5. At low [DFO-B] and irrespective of Al-content, a poor relation between adsorption of DFO-B and the release kinetics of iron is observed, giving a strong indication that the surface density of the ligand and soluble iron concentration alone do not suffice to explain mineral dissolution kinetics. Attenuated total reflectance-Fourier transformed infrared spectroscopy analysis of the aluminum goethite samples show shifting of the central band in the 400-500 cm^-1 region to higher frequencies with increasing Al substitution, a signature of oxygen displacement. The shifting of the bands at 400 cm^-1 occurs at a greater extent than those observed at > 800 cm^-1, which confirms replacement of iron by aluminum, because of increased O^2-- O^2- repulsion. A consequence is the protonation of more stoichiometric oxygens in the structure and the formation of more stable hydroxyl units. Hydroxyl deformation and water bending vibrations indicate there are energetic constraints on the detachment of structural hydroxyl units during mineral dissolution. X-ray absorption (XANES and Al-EXAFS) spectra provided evidence for changes in the orientation and the structure of the unit cell with increasing aluminum content. These findings support the idea that structural aberrations associated with aluminum insertion in goethite bring about redistribution of OH groups in various crystal faces, and affect the dislocation of iron structural units following weathering.