Dissolution Kinetics of Hausmannite in the Presence of the Siderophore, DFO-B

Siderophores are organic ligands with a very high affinity for Fe(III) that have been shown to dissolve iron-(III) bearing minerals, thus increasing the bioavailability of this nutrient. Recent work also has shown that siderophores are capable of forming stable Mn (III) complexes in aqueous environments and that they can promote the dissolution of Mn (III)-bearing minerals such as manganite (γ -MnOOH). The goal of this study was to investigate the dissolution kinetics of the mineral, hausmannite, in the presence of desferrioxamine B (DFO-B), a common trihydroxamate siderophore, synthesized by fungi and bacteria, which may play a role in the reduction of soluble Mn (III) to Mn^{2^+}. Hausmannite, a mixed-valence Mn (II, III) oxide mineral with the formula Mn₃O₄, is a proposed intermediate in the oxidation of Mn^{2^+} to thermodynamically stable MnO₂. Hausmannite particles were synthesized and characterized by X-ray diffraction, BET specific surface area, and SEM imaging. The extent and rate of dissolution were measured as a function of pH and ligand concentration in batch reactors at 25 ± 3 ° C. Total Mn concentrations were measured by ICP-AES, while complexed-Mn (III) was measured using UV-Vis spectrophotometry. In the absence of the ligand, the greatest amount of total Mn in solution was observed at pH ≈ 5. There was a sharp decrease in the extent of dissolution at pH > 5, however, and no appreciable dissolution occurred at pH > 8. In the presence of DFO-B, ligand-promoted dissolution dominated at pH > 8, with more than 90 % of the ligand complexed within 1 hour after addition. Below pH ≈ 8, a complex combination of reductive, ligand-promoted, and proton-promoted dissolution was observed. Our study revealed that complex mechanisms are involved in the DFOB-promoted dissolution of hausmannite, with several dissolution pathways occurring simultaneously. The results presented herein may have implications for mineral weathering, the biogeochemical cycling of Fe and Mn in the environment, and the availability of these nutrients to plants and microorganisms.