Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate

The bioavailability of Fe(III) in oxic soils, sediments, and surface waters at near-neutral pH is limited by the low solubility and slow dissolution rates of iron oxides and hydroxides. In order to overcome this low iron solubility, many microorganisms and roots of graminaceous plants exude highly Fe(III)-specific, low-molecular weight (0.5 to 1.5 kDa) ligands, the a group of compounds called siderophores [1-5]. Hundreds of structurally distinct siderophores are known, typically with ligating catecholate, carboxylate, α-hydroxycarboxylate, or hydroxamate functional groups [2,6]. Most siderophores are hexadentate and form 1:1 Fe(III)-complexes [7-10]. Desferrioxamine-B (DFOB), presented in Figure 1, is an example of a cationic (pH<8) trihydroxamate siderophore found in both terrestrial and marine systems [11]. Sub-micromolar siderophore concentrations have been observed in soil solutions [12]. The role of siderophores in enhancing the bioavailability of iron depends on its specificity for iron. For example, even in soils in equilibrium with calcium carbonate, where otherwise strong competition between Ca and iron for complexation would be expected, the dominant soluble species of the iron specific DFOB is the free, fully protonated ligand.An important function of siderophores in biological iron acquisition is the acceleration of iron oxide dissolution [13]. Organic ligands and their complexes can adsorb onto iron oxides by forming inner-sphere and/or outer-sphere surface complexes. For simplicity, the term "outer-sphere" will be used here to include surface species which are sorbed by H-bonding with protonated surface hydroxyl groups or by hydrophobic interactions with adsorbed surfactant molecules. The adsorption of the ligand by formation of inner-sphere complexes between the ligand and structural iron at the oxide surface is usually considered to be the first step of a ligand-promoted dissolution mechanism [14]. This conclusion has been drawn based on observations