Kenneth M. Kemner

A low-temperature gas-flow total electron yield detector for XAFS measurements

Abstract A design of a low temperature total electron yield (TEY) detector for XAFS is described. Transmission and TEY XAFS data obtained at 80 K on a Au film, GaAs (100) single crystal (TEY) and powder (transmission) are presented and compared. We find excellent agreement between the data obtained using these two techniques for both Au and GaAs.

Biotic and Abiotic Reduction of Goethite (α-FeOOH) by Subsurface Microorganisms in the Presence of Electron Donor and Sulfate

To better understand dissimilatory iron and sulfate reduction (DIR and DSR) by subsurface microorganisms, we investigated the effects of sulfate and electron donors on the microbial goethite (α-FeOOH) reduction. Batch systems were created 1) with acetate or glucose (donor), 2) with goethite and sulfate (acceptor), and 3) with aquifer sediment (microbial source). With 0.2 mM sulfate, goethite reduction coupled with acetate oxidation was limited. However, with 10 mM sulfate, 8 mM goethite reduction occurred with complete sulfate reduction and x-ray absorption fine-structure analysis indicated the formation of iron sulfide. This suggests that goethite reduction was due to the sulfide species produced by DSR bacteria rather than direct microbial reaction by DIR bacteria. Both acetate and glucose promoted goethite reduction. The rate of goethite reduction was faster with glucose, while the extent of goethite reduction was higher with acetate. Sulfate reduction (10 mM) occurred only with acetate. The results suggest that glucose-fermenting bacteria rapidly stimulated goethite reduction, but acetate-oxidizing DSR bacteria reduced goethite indirectly by producing sulfides. This study suggests that the availability of specific electron donor and sulfate significantly influence microbial community activities as well as goethite transformation, which should be considered for the bioremediation of contaminated environments.

A pH‐Dependent X‐Ray Absorption Spectroscopy Study of U Adsorption to Bacterial Cell Walls

Metal mobility in subsurface water systems involves the complex interaction of the metal, the fluid, and the mineral surfaces over which the fluid flows. This mobility is further influenced by metal adsorption onto bacteria and other biomass in the subsurface. To better understand the mechanism of this adsorption as well as its dependence on the chemical composition of the fluid, we have performed a series of metal adsorption experiments of aqueous uranyl (UO2)2+ to the gram‐positive bacterium B. subtilis in the presence and absence of carbonate along with X‐ray Absorption Spectroscopy (XAS) to determine the binding structures at the cell surface. In this paper we demonstrate an approach to the XAS data analysis which allows us to measure the partitioning of the adsorption of uranium to hydroxyl, carboxyl/carbonato, and phosphoryl active sites at the cell surface.

The role of nanopores on U(VI) sorption and redox behavior in U(VI)-contaminated subsurface sediments

Most reactive surfaces in clay-dominated sediments are present within nanopores (pores of nm dimension). The behavior of geological fluids and minerals in nanopores is significantly different from those in normal non-nanoporous environments. The effect of nanopore surfaces on U(VI) sorption/desorption and reduction is likely to be significant in clay-rich subsurface environments. Our research results from both model nanopore system and natural sediments from both model system (synthetic nanopore alumina) and sediments from the ORNL Field Research Center prove that U(VI) sorption on nanopore surfaces can be greatly enhanced by nanopore confinement environments. The results from the project provide advanced mechanistic, quantitative information on the physiochemical controls on uranium sorption and redox behavior in subsurface sediments. The influence of nanopore surfaces on coupled uranium sorption/desorption and reduction processes is significant in virtually all subsurface environments, because most reactive surfaces are in fact nanopore surfaces. The results will enhance transfer of our laboratory-based research to a major field research initiative where reductive uranium immobilization is being investigated. Our results will also provide the basic science for developing in-situ colloidal barrier of nanoporous alumina in support of environmental remediation and long term stewardship of DOE sites.

Investigation of the Transformation of Uranium under Iron-Reducing Conditions: Reduction of UVI by Biogenic FeII/FeIII Hydroxide (Green Rust)

The recent identification of green rusts (GRs) as products of the reduction of FeIII oxyhydroxides by dissimilatory iron-reducing bacteria, coupled with the ability of synthetic (GR) to reduce UVI species to insoluble UO2, suggests that biogenic green rusts (BioGRs) may play an important role in the speciation (and thus mobility) of U in FeIII-reducing environments. The objective of our research was to examine the potential for BioGR to affect the speciation of U under FeIII-reducing conditions. To meet this objective, we designed and executed a hypothesis-driven experimental program to identify key factors leading to the formation of BioGRs as products of dissimilatory FeIII reduction, to determine the key factors controlling the reduction of UVI to UIV by GRs, and to identify the resulting U-bearing mineral phases. The results of this research significantly increase our understanding of the coupling of biotic and abiotic processes with respect to the speciation of U in iron-reducing environments. In particular, the reduction of UVI to UIV by BioGR with the subsequent formation of U-bearing mineral phases may be effective for immobilizing U in suboxic subsurface environments. This information has direct applications to contaminant transport modeling and bioremediation engineering for natural or enhanced in situ remediationmorexa0» of subsurface contamination.«xa0less

Aqueous Complexation Reactions Governing the Rate and Extent of Biogeochemical U(VI) Reduction

The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. ? Elucidate the controls on the rate and extent of contaminant reactivity. (2) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.