Effects of Sediment Iron Mineral Composition on Microbially Mediated Changes in Divalent Metal Speciation: Importance of ferrihydrite
Dissimilatory metal reducing bacteria (DMRB) can influence geochemical processes that subsequently affect the speciation the speciation and lability of metallic contaminants within natural environments. Most investigations into the effect of DMRB on sediment geochemistry utilize various synthetic oxides as the FeIII source (e.g. ferrihydrite, goethite, hematite, hydrous ferric oxide), providing for well-controlled experiments. However, these materials do not necessarily emulate the actual mineralogical composition of natural systems, nor do they account for the effect of sediment mineralogy on microbial activity and/or microbially induced geochemical processes. Our experiments with a divalent metal (ZnII) indicate that, while sediment mineralogy may have little effect on the net rate of microbial iron reduction, it does impact the resultant speciation of reduced iron and sediment associated transition metals. These data demonstrate that microbial reduction of synthetic goethite carrying previously sorbed ZnII increased both [ZnII-aq] and the proportion of sorbed ZnII that is insoluble in 0.5 M HCl. Microbial reduction of FeIII in similarly treated iron-bearing clayey sediment (Fe-clay) and hematite sand had no similar effect. Moessbauer spectroscopy data indicate that small amounts of ferrihydrite present in the synthetic VHSA goethite are preferentially consumed during FeIII reduction, a process that may result from FeII-driven conversion of ferrihydrite to goethite. Microbial reduction of Fe-clay did not permanently alter iron speciation within the Fe-clay. Zinc k-edge XAS data collected for ZnII previously sorbed to VHSA goethite and Fe-clay indicate that microbial FeIII reduction altered ZnII bonding in fundamentally different ways for VHSA goethite and Fe-clay. In VHSA goethite, XANES data indicate ZnO6 octahedra in both sterile and reduced samples. EXAFS data indicate 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 with 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-clay, XANES data indicate that microbial FeIII reduction transforms Zn-O polyhedra from octahedral to tetrahedral coordination and leads to an increased degree of multiple scattering. EXAFS data indicate formation of a ZnCl2 moiety that may also incorporate some Zn-O bonds in the microbially reduced sample. These data indicate that, while many sedimentary iron minerals are easily reduced by DMRB, the geochemical effects of microbial FeIII reduction are highly dependent on sediment iron speciation. Thus, one must consider the effects of sediment mineralogy when investigating the effect of microbial processes on trace metal geochemistry.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15016878
- Report Number(s):
- PNNL-SA-39354; GCACAK; 3155; KP1301030; TRN: US200516%%1034
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 69, Issue 7; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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