Transformation of Hematite into Magnetite During Dissimilatory Iron Reduction—Conditions and Mechanisms

Abstract

Magnetite formation during the reduction of nanoparticulate hematite by Shewanella putrefaciens 200R is investigated in media of variable composition, at circumneutral pH and with lactate as electron donor. The relative rates of production of dissolved Fe(II) and Fe(III), aqueous speciation, plus chemical gradients control whether or not magnetite forms in the experiments. High bicarbonate concentrations result in the precipitation of magnetite, presumably by enhancing the non-reductive dissolution of hematite, hence causing the simultaneous production of soluble Fe(III) and Fe(II) in the incubations. Magnetite formation is inhibited when hematite dissolution is slowed down by adsorption of oxyanions (phosphate and sulfate) at the mineral surface, when the reduction of soluble Fe(III) is enhanced by increasing the cell density or adding an electron shuttle (AQS), or when aqueous Fe(II) is complexed by ferrozine. In experiments where hematite suspensions with and without bacteria are separated by a dialysis membrane, magnetite formation occurs mainly in the cell-free portion of the reaction system. Most likely, precipitation of magnetite is favored in the cell-free suspension because of a higher soluble Fe(III) to Fe(II) ratio. The formation of magnetite in the absence of cells further implies that its nucleation is not catalyzed by the bacterial surfaces.

Keywords:

• Biogenic magnetite
• biomineralization
• dissimilatory iron reduction
• Shewanella putrefaciens

Acknowledgments

We wish to thank S. van der Gaast for making the XRD measurements possible, M. J. Dekkers and T. A. T. Mullender for their help characterizing the thermomagnetic properties of the solids, J. D. Meeldijk and M. R. Drury for their assistance in TEM analyses, and two anonymous reviewers for their constructive comments. This research has been financially supported by the Dutch Organization of Scientific Research (NWO Pioneer Programme).

Notes

*Expressed per unit total volume suspension: in Experiments 2, 7, 9 and 10 the bacteria occupy half the suspension volume inside the dialysis bag.

*Refers to outside and inside the dialysis bag.