Low Fe(II) Concentrations Catalyze the Dissolution of Various Fe(III) (hydr)oxide Minerals in the Presence of Diverse Ligands and over a Broad pH Range.

Dissolution of Fe(III) (hydr)oxide minerals by siderophores (i.e., Fe-specific, biogenic ligands) is an important step in Fe acquisition in environments where Fe availability is low. The observed coexudation of reductants and ligands has raised the question of how redox reactions might affect ligand-controlled (hydr)oxide dissolution and Fe acquisition. We examined this effect in batch dissolution experiments using two structurally distinct ligands (desferrioxamine B (DFOB) and  N, N'-di(2-hydroxybenzyl)ethylene-diamine- N, N'-diacetic acid (HBED)) and four Fe(III) (hydr)oxide minerals (lepidocrocite, 2-line ferrihydrite, goethite and hematite) over an environmentally relevant pH range (4-8.5). The experiments were conducted under anaerobic conditions with varying concentrations of (adsorbed) Fe(II) as the reductant. We observed a catalytic effect of Fe(II) on ligand-controlled dissolution even at submicromolar Fe(II) concentrations with up to a 13-fold increase in dissolution rate. The effect was larger for HBED than for DFOB. It was observed for all four Fe(III) (hydr)oxide minerals, but it was most pronounced for goethite in the presence of HBED. It was observed over the entire pH range with the largest effect at pH 7 and 8.5, where Fe deficiency typically occurs. The occurrence of this catalytic effect over a range of environmentally relevant conditions and at very low Fe(II) concentrations suggests that redox-catalyzed, ligand-controlled dissolution may be significant in biological Fe acquisition and in redox transition zones.