Remineralization of ferrous carbonate from bioreduction of natural goethite in the Lorraine iron ore (Minette) by Shewanella putrefaciens

• Bioreduction extent of natural iron ore is higher than synthetic goethite.
• Bacterial iron reduction of the aged iron ore produced ferrous carbonate.
• The neo-formed ferrous carbonate could be a precursor of siderite and supersede the inter-oolitical cement of iron ore.

Bacterial iron oxide reduction has been extensively studied over recent decades with the aim of improving knowledge of Fe-bearing mineral transformations. Chemically synthesized Fe(III) oxides such as ferrihydrite or goethite have mainly been used as iron oxide models but very few studies have focused on natural oxides. The scope of our work was to evaluate the ability of iron-reducing bacteria to transform iron ore and to identify the nature and outcome of the reduced phases. For this purpose, Lothringen (minette), the oolitic iron ore type found in the Lorraine area (North-East of France), was incubated with Shewanella putrefaciens CIP 80.40 as a model iron-reducing bacteria. Chemical and mineralogical analyses (ferrozine assay, X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, Mössbauer spectroscopy, transmission electron microscopy) were performed on both aged (i.e. iron ore oxidized by air during ~ 80 years of mining exploitation) and intact iron ores, before and after bioreduction in anoxic conditions. The oolites of intact iron ore were composed of goethite (α-FeOOH), with siderite (FeCO3) and phyllosilicates as cement. Oolites of the aged iron ore contained hematite (α-Fe2O3), as well as goethite and significantly less siderite. We observed that 26% and 20% of goethite was bio-reduced for aged and intact iron ore respectively. No other Fe(III) phase was significantly reduced. Natural iron oxides constituting the iron ore can now be considered more available for iron-reducing bacteria than chemically synthesized oxides. The nano-crystallinity of iron ore goethite and the presence of sorption sites for Fe(II) were suggested as explanations for this discrepancy. Finally, a poorly crystalline ferrous carbonate was the main Fe(II) phase formed during the bio-reduction process of the iron ore (aged and intact). In the context of iron mines, the neo-formed ferrous carbonate could be a precursor of siderite and could recreate the diagenetic links and the inter-oolitic cohesion lost by the iron ore during its oxidizing process. The long-term stability of iron ore pillars would therefore be ensured.