Adsorption of Trihydroxamate and Catecholate Siderophores on α‐Iron (Hydr)oxides and their Dissolution at pH 3.0 to 6.0

Abstract

The adsorption of desferrioxamine (DFOB), acetyl desferrioxamine (DFOD1), and a catecholate derivative (DFOMTA)] on goethite (α‐FeOOH) and hematite (α‐Fe2O3) and mineral dissolution behavior at pH 3.0 to 6.0 were studied to yield an understanding of the mechanisms of siderophore complexation on α phases and also to study how variations in siderophore molecular structure, as modified by proton activity, affect siderophore effectiveness at releasing Fe from different soil minerals. Wet‐chemistry experiments were conducted to react 1 g L−1 goethite or hematite with 240 μmol L−1 siderophores. Iron release values were found to be as high as 350 ± 20 μmol L−1, with no evidence of mineral transformation as determined by x‐ray diffraction analyses. The proton activity strongly influenced Fe release, but no correlations between Fe concentrations and pH were found. The siderophore molecular structure influenced Fe release; the effectiveness for releasing Fe decreased in the order DFOMTA > DFOD1 > DFOB. Kinetics and Fe siderophore stability constants in solution data trends correlated possibly because the formation of Fe–siderophore complexes limits dissolution for both α‐phase trihydroxamate and catecholate siderophores. Incongruency between dissolution kinetics and siderophore adsorption data trends was explained by siderophores triggering dissolution mechanisms that may not require surface adsorption. It is plausible then that the mineralogical composition of the soil could, in part, regulate the production of siderophores by microorganisms and plants. Based on Fe release data for hematite, it was concluded that siderophores contribute to the Fe pool in soil solutions with decreasing proton activity. Thus, the role of siderophores in releasing structural Fe becomes increasingly important as mineral solubility becomes limiting. By contrast, based on Fe release for goethite, the contribution of siderophores to the acquisition of Fe in goethitic soil profiles becomes less pronounced with decreases in the proton activity of the soil solution.