HR: 08:00h
AN: B21A-01 INVITED     [PDF]
TI: Reductive Dissolution of Iron Oxides and Iron-Rich Clays Enhanced by Sulfate-Reducing Bacteria
AU: * Zhang, C L
EM: zhang@srel.edu
AF: University of Georgia, Savannah River Ecology Laboratory, Drawer E, Aiken, SC 29802 United States
AB: Iron oxides and iron-rich clays are abundant in low-temperature sedimentary environments where sulfate-reducing bacteria are also present. This study summarizes our research on reductive dissolution of ferrihydrite, goethite, hematite, magnetite, and a nontronite clay by {\it Desulfovibrio} spp. strain G-20 and strain G-11. The goal was twofold: (1) to understand the enzymatic processes of iron reduction by sulfate-reducing bacteria (SRB) using iron as the sole electron acceptor and (2) to determine whether iron reduction from the oxides and clays could be enhanced by biogenic H$_{2}$S through an enzymatic process during sulfate reduction. In the iron-oxide experiments without sulfate, iron reduction by G-20 averaged about 4.5% of total iron for ferrihydrite, goethite, and hematite. The reduction of magnetite, however, was about threefold higher (13.3%). The maximum biomass of G-20 gained during iron reduction was also highest in the magnetite culture, suggesting that reduction of magnetite may have stimulated the growth of G-20. In the presence of sulfate, iron reduction was dramatically enhanced in all cultures (>70%). In inorganic experiments using Na$_{2}$S, less than 4% total iron was reduced from goethite or hematite and about 19% was reduced from magnetite. The enhanced reduction of iron during sulfate reduction may have resulted from enzymatic activity of the SRB or through the chelation of solids with organic acids and other organic molecules. Transmission electron microscopy (TEM) showed shortened and thinned goethite and hematite crystals during sulfate and iron reduction. The magnetite crystals, on the other hand, were disintegrated extensively. For the nontronite experiments using G-11, iron reduction from the clay was about 10% of total structural Fe(III) in the absence of sulfate but reached 29% in the presence of sulfate. Abiotic iron reduction using Na$_{2}$S, on the other hand, was ca. 7.5% of total structural Fe(III). Analyses of TEM and X-ray diffraction revealed significant changes in structure and composition of the clay during its dissolution by G-11. Overall, this study demonstrates that sulfate-reducing bacteria can dramatically enhance the dissolution of iron oxides and iron-rich clays, thus accelerating the transformation of these minerals in sulfate-rich environments.
DE: 0400 Biogeosciences
DE: 0614 Biological effects
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
DE: 4803 Bacteria
SC: Biogeosciences [B]
MN: 2003 Fall Meeting