Date of this Version
Geochimica et Cosmochimica Acta, Vol. 68, No. 13, pp. 2799–2814, 2004
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 PO43- (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO44 - (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(CO32-) ([FeII (6-x)FeIII x(OH)12]x+CO32-)0.5x · yH2O) and vivianite [Fe3(PO4)2 · 8H2O], were formed when P was present. GR(CO32-) dominated as a mineral product in samples with <4 mmol/L P. The Fe(II)/Fe(III) ratio of GR(CO32-) 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.