Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite
D. CRAIG COOPER,1 ANDREW L. NEAL,2 RAVI K. KUKKADAPU,3
DALE BREWE,4 AARON CODY,5 and FLYNN W. PiCARDAL5
1Geosciences
Research, Idaho National Laboratory. P.O. Box 1625, MS 2107, Idaho Falls,
ID 83415-2107, USA
2Savannah River Ecology Laboratory, University of Georgia,
Drawer E, Aiken, SC 29802, USA
3Environmental Molecular Science Laboratory, P.O. Box 999,
MS K8-96, Richland, WA 99352, USA
4Argonne National Laboratory, Bldg. 435E Sector 20,9700 S.
Cass Ave., Argonne, IL 60439, USA
5School of Public and Environmental Affairs, Indiana University,
1315 E. 10th St., Bloomington, IN 47405, USA
(Received February 10, 2004; accepted in revised form September 10, 2004)
Abstract-Dissimilatory metal reducing bacteria (DMRB)
can influence geochemical processes that affect the speciation and mobility
of metallic contaminants within natural environments. Most investigations
into the effect of DMRB on sediment geochemistry utilize various synthetic
oxides as .the Felli source (e.g., ferrihydrite, goethite, hematite).
These synthetic materials do not represent .the mineralogical composition
of natural systems, and do not account for the effect of sediment mineral
composition on microbially mediated processes. Our experiments with a
DMRB (Shewanella putrefaciens 200) and a divalent metal (ZnII)
indicate that, while complexity in sediment mineral composition may not
strongly impact the degree of "microbial iron reducibility,"
it does alter the geochemical consequences of such microbial activity.
The ferrihydrite and clay mineral content are key factors. Microbial reduction
of a synthetic blend of goethite and ferrihydrite (VHSA-G) carrying previously
adsorbed ZnII increased both [ZnII-aq] and the proportion
of adsorbed Znn that is insoluble in 0.5 M HCl. Microbial reduction of
Felli in similarly treated iron-bearing clayey sediment (Fe-K-Q) and hematite
sand, which contained minimal amounts of ferrihydrite, had no similar
effect. Addition of ferrihydrite increased the effect of microbial Felli
reduction on ZnII association with a 0.5 M HCl insoluble phase
in all sediment treatments, but .the effect was inconsequential in the
Fe-K-Q. Zinc k-edge X-ray absorption spectroscopy (XAS) data indicate
.that microbial Felli reduction altered Znll bonding in fundamentally
different ways for VHSA-G and Fe-K-Q. In VHSA-G, ZnO6 octahedra
were present in both sterile and reduced samples; with a slightly increased
average Zn-O coordination number and a slightly higher degree of long-range
order in the reduced sample. This result may be consistent wi.th enhanced
ZnII substitution within goethite in the microbially reduced
sample, though these data do not show .the large. increase in the degree
of Zn-O-metal interactions expected to accompany this change. In Fe-K-Q,
microbial Fell reduction transforms Zn-O polyhedra from octahedral to
tetrahedral coordination and leads to the formation of a ZnCl2
moiety and an increased degree of multiple scattering. This study indicates
.that, while many sedimentary iron minerals are easily reduced by DMRB,
the effects of microbial Fell reduction on .trace metal geochemistry are
dependent on sediment mineral composition.
Copyright
@ 2005 Elsevier Ltd
SREL Reprint #2826
Cooper, D. C., A. L. Neal, R. K. Kukkadapu, D. Brewe, A. Coby and F. W. Picardal. 2005. Effects of sediment iron mineral composition on microbially mediated changes in divalent metal speciation: Importance of ferrihydrite. Geochimica et Cosmochimica Acta 69:1739-1754.
