Kinetic and Thermodynamic Analysis During Dissimilatory γ-FeOOH Reduction: Formation of Green Rust 1 and Magnetite

In laboratory experiments, lepidocrocite reduction by the dissimilatory iron reducing bacteria, Shewanella putrefaciens , is known to generate extra-cellular iron (II-III) minerals as green rust (GR) or magnetite. However, the parameters controlling the formation of these minerals remain unclear. In order to identify these parameters, reduction experiments were designed to obtain either GR or magnetite with methanoate as electron source and lepidocrocite (γ-FeOOH) as the electron acceptor. The mineral products were monitored by XRD analyses, and the rate of reduction and E _h /pH evolution were assessed during GR and magnetite formation. The only difference between the two conditions lies with the inoculum size: magnetite is systematically produced by the treatments containing the lowest cell density (5 × 10 ⁸ CFU mL ⁻¹ ) and GR1(CO ₃ ²⁻ ) precipitated in the highest cell density (2 × 10 ⁹ CFU mL ⁻¹ ). We showed that 100 μ M of anthraquinone 2,6 disulfonate (AQDS) did not influence the nature of the biogenic minerals. In addition, based on thermodynamic calculations, we observed that the E _h /pH paths in the Pourbaix diagram depend on the nature of the minerals formed, i.e. GR1(CO ₃ ²⁻ ) or Fe ₃ O ₄ . However, the Pourbaix diagram cannot be used to forecast unambiguously the nature of these minerals. We propose that a close association of bacterial cells and γ -FeOOH particles occurs during the Fe(III) mineral reduction. We hypothesis that this aggregation influences the bio-reduction rate and is related to the nature of the biogenic mineral precipitated in the reduction media.

Keywords:

• Shewanella putrefaciens
• lepidocrocite
• bio-reduction
• green rust
• magnetite
• Pourbaix diagrams

Acknowledgments

The authors acknowledge Isabelle Bihannic of the UMR INPL-CNRS 7569, Nancy, France for the XRD analyses. We are grateful to Carrick Eggleston of the University of Wyoming, USA for encouragement and helpful discussion that led to improving the manuscript.