Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: Formation of carbonate green rust in the presence of phosphate

Authors

Ravi K. Kukkadapu, Pacific Northwest National LaboratoryFollow
John M. Zachara, Pacific Northwest National Laboratory
James K. Fredrickson, Pacific Northwest National LaboratoryFollow
David W. Kennedy, Pacific Northwest National Laboratory

Date of this Version

2004

Citation

Geochimica et Cosmochimica Acta, Vol. 68, No. 13, pp. 2799–2814, 2004

Abstract

The reductive biotransformation of two Si-ferrihydrite coprecipitates (1 and 5 mole % Si) by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ~7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate, an electron shuttle, was present in the media. Experiments were performed without and with PO₄^3- (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO₄^{4 -} (Si) and P on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction and Mossbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl^- and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si and P content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of P whereas carbonate green rust GR(CO₃^2-) ([Fe^II _(6-x)Fe^III _x(OH)₁₂]^x+CO₃^2-)_0.5x · yH₂O) and vivianite [Fe₃(PO₄)₂ · 8H₂O], were formed when P was present. GR(CO₃^2-) dominated as a mineral product in samples with /L P. The Fe(II)/Fe(III) ratio of GR(CO₃^2-) varied with P concentration; the ratio was 2 in 1 mmol/L P and approached 1 with 4- and 10 mmol/L P. Green rust appeared to form by solid-state transformation of ferrihydrite. Media P and Si concentration dictated the mechanism of transformation. In the 1 mole % Si coprecipitate with 1 mmol/L P, an intermediate Fe(II)/Fe(III) phase with structural Fe(II) slowly transformed to GR with time. In contrast, when ferrihydrite contained more Si (5 mole %) and/or contained higher P (4 mmol/L), sorbed Fe(II) and residual ferrihydrite together transformed to GR. Despite similar chemistries, P was shown to have a profound effect on extent of ferrihydrite reduction and biotransformations while that of Si was minimal.