Biogenic iron mineralization accompanying the dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium

Dissimilatory iron-reducing bacteria (DIRB) couple the oxidation of organic matter or H-2 to the reduction of iron oxides. The factors controlling the rate and extent of these reduction reactions and the resulting solid phase s are complex and poorly understood. Batch experiments were conducted with amorphous hydrous ferric oxide (HFO) and the DIRE Shewanella putrefaciens, strain CN32, in well-defined aqueous solutions to investigate the reduction of HFO and formation of biogenic Fe(II) minerals. Lactate-HFO solutions bu ffered with either bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPE S) containing various combinations of phosphate and anthraquinone-2,6-disul fonate (AQDS), were inoculated with S. putrefaciens CN32, AQDS, a humic aci d analog that can be reduced to dihydroanthraquinone by CN32, was included because of its ability to function as an electron shuttle during microbial iron reduction and as an indicator of pe. Iron reduction was measured with time, and the resulting solids were analyzed by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diff raction (SAED). In HCO3- buffered medium with AQDS, HFO was rapidly and ext ensively reduced, and the resulting solids were dominated by ferrous carbon ate (siderite). Ferrous phosphate (vivianite) was also present in HCO3- med ium containing P, and fine-grained magnetite was present as a minor phase i n HCO3- medium with or without P. In the PIPES-buffered medium, the rate an d extent of reduction was strongly influenced by AQDS and P. With AQDS, HFO was rapidly converted to highly crystalline magnetite whereas in its absen ce, magnetite mineralization was slower and the final material less crystal line. In PIPES with both P and AQDS, a green rust type compound [Fe(6-x)Fe- II(x)(II)(OH)(12)](x+)[(A(2-))(x/2) . yH(2)O](x-) was the dominant solid ph ase formed; in the absence of AQDS a poorly crystalline product was observe d. The measured pe and nature of the solids identified were consistent with thermodynamic considerations. The composition of aqueous media in which mi crobial iron reduction occurred strongly impacted the rate and extent of ir on reduction and the nature of the reduced solids. This, in turn, can provi de a feedback control mechanism on microbial metabolism. Hence, in sediment s where geochemical conditions promote magnetite formation: two-thirds of t he Fe(III) will be sequestered in a form that may not be available for anae robic bacterial respiration. Copyright (C) 1998 Elsevier Science Ltd.