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Mercury (Hg) bioavailability and geochemical cycling is affected by its partitioning between the aqueous and particulate phases. We applied a synchrotron-based X-ray fluorescence (XRF) microprobe to visualize and quantify directly the spatial localization of Hg and its correlations with other elements of interest on suspended particles from a Hg-contaminated freshwater system. Up to 175 μg g⁻¹ Hg is found on suspended particles, but less than 0.01% is in the form of methylmercury. Mercury is heterogeneously distributed among phytoplankton (e.g., diatoms) and mineral particles that are rich in iron oxides and natural organic matter (NOM). The diatom-bound Hg is mostly found on outer surfaces of the cells, suggesting passive sorption of Hg on diatoms. Our results indicate that localized sorption of Hg onto suspended particles, including diatoms and NOM-coated oxide minerals, may play an important role in affecting the partitioning, reactivity, and biogeochemical cycling of Hg in natural aquatic environments.

Microbial reduction of ferric iron [Fe(III)] is an important biogeochemical process in anoxic aquifers. Depending on groundwater pH, dissimilatory metal-reducing bacteria can also respire alternative electron acceptors to survive, including elemental sulfur (S⁰). To understand the interplay of Fe/S cycling under alkaline conditions, we combined thermodynamic geochemical modeling with bioreactor experiments using Shewanella oneidensis MR-1. The new 13-ID-E microprobe was used to obtain sulfur K-edge XANES spectra of the S-containing bioreactors to define the sulfur speciation. Under these conditions, S. oneidensis can enzymatically reduce S⁰ but not goethite (α-FeOOH). The HS^– produced subsequently reduces goethite abiotically. Because of the prevalence of alkaline conditions in many aquifers, Fe(III) reduction may thus proceed via S⁰-mediated electron-shuttling pathways.

Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3-9 that requires the presence of four distinct sites, with pK{sub a} values of 3.3 {+-} 0.2, 4.8 {+-} 0.2, 6.7 {+-} 0.4, and 9.4 {+-} 0.5, and site concentrations of 8.9({+-}2.6) x 10{sup -5}, 1.3({+-}0.2) x 10{sup -4}, 5.9({+-}3.3) x 10{sup -5}, and 1.1({+-}0.6) x 10{sup -4} moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1-200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.

Microbial reduction of Fe(III) oxides results in the production of Fe(II) and may lead to the subsequent formation of Fe(II)-bearing secondary mineralization products including magnetite, siderite, vivianite, chukanovite (ferrous hydroxy carbonate (FHC)), and green rust; however, the factors controlling the formation of specific Fe(II) phases are often not well-defined. This study examined effects of (i) a range of inorganic oxyanions (arsenate, borate, molybdate, phosphate, silicate, and tungstate), (ii) natural organic matter (citrate, oxalate, microbial extracellular polymeric substances [EPS], and humic substances), and (iii) the type and number of dissimilatory iron-reducing bacteria on the bioreduction of lepidocrocite and formation of Fe(II)-bearing secondary mineralization products. The bioreduction kinetics clustered into two distinct Fe(II) production profiles. 'Fast' Fe(II) production kinetics [19-24 mM Fe(II) d-1] were accompanied by formation of magnetite and FHC in the unamended control and in systems amended with borate, oxalate, gellan EPS, or Pony Lake fulvic acid or having 'low' cell numbers. Systems amended with arsenate, citrate, molybdate, phosphate, silicate, tungstate, EPS from Shewanella putrefaciens CN32, or humic substances derived from terrestrial plant material or with 'high' cell numbers exhibited comparatively slow Fe(II) production kinetics [1.8-4.0 mM Fe(II) d-1] and the formation of green rust. The results are consistent with a conceptual model whereby competitive sorption of more strongly bound anions blocks access of bacterial cells and reduced electron-shuttling compounds to sites on the iron oxide surface, thereby limiting the rate of bioreduction.

Biogenic Fe{sup II} phases (magnetite, green rust, siderite, vivianite, etc.) provide a reservoir of reducing capacity in many subsurface environments that may contribute to the reduction of contaminants such as U{sup VI}. We have examined the uptake and reduction of U{sup VI} in the presence of biogenic green rust (BioGR), magnetite (BioMAG), and siderite (BioSID) formed during the reduction of Fe{sup III} oxides by Shewanella putrefaciens CN32. Within 48 h, total solution-phase U{sup VI} concentrations decreased from 500{mu}M to 1.5{mu}M, 392{mu}M, and 472{mu}M in the U-BioGR, U-BioMAG, and U-BioSID systems, respectively. Analysis of the samples by U L{sub III} extended X-ray absorption fine structure spectroscopy (EXAFS) indicated that despite a stoichiometric excess of Fe{sup II}, no more than 6% of U{sup VI} was reduced to U{sup IV} in the U-BioSID system, and no more than 22% of U{sup VI} was reduced in the U-BioMAG system. For comparison, in the U-BioGR system, >99% of U{sup VI} was reduced to U{sup IV}. Uptake of U{sup VI} by BioGR and BioMAG was accompanied by formation of nanoparticulate uraninite. The U EXAFS data for the U-BioSID system were consistent with partial U{sup VI}/U{sup IV} substitution for Fe{sup II} in the surface layer of siderite particles and adsorption of U{sup IV}.

The bioreduction of U(VI) to U(IV) affects uranium mobility and fate in contaminated subsurface environments and is best understood in Gram-negative model organisms such as Geobacter and Shewanella spp. This study demonstrates that U(VI) reduction is a common trait of Gram-positive Desulfitobacterium spp. Five different Desulfitobacterium isolates reduced 100 {mu}M U(VI) to U(IV) in <10 days, whereas U(VI) remained soluble in abiotic and heat-killed controls. U(VI) reduction in live cultures was confirmed using X-ray absorption near-edge structure (XANES) analysis. Interestingly, although bioreduction of U(VI) is almost always reported to yield the uraninite mineral (UO{sub 2}), extended X-ray absorption fine structure (EXAFS) analysis demonstrated that the U(IV) produced in the Desulfitobacterium cultures was not UO{sub 2}. The EXAFS data indicated that the U(IV) product was a phase or mineral composed of mononuclear U(IV) atoms closely surrounded by light element shells. This atomic arrangement likely results from inner-sphere bonds between U(IV) and C/N/O- or P/S-containing ligands, such as carbonate or phosphate. The formation of a distinct U(IV) phase warrants further study because the characteristics of the reduced material affect uranium stability and fate in the contaminated subsurface.

One method for the in situ remediation of uranium contaminated subsurface environments is the removal of highly soluble U(VI) from groundwater by microbial reduction to the sparingly soluble U(IV) mineral uraninite. Success of this remediation strategy will, in part, be determined by the extent and products of microbial reduction. In heterogeneous subsurface environments, microbial processes will likely yield a combination of U(IV) and U(VI) phases distributed throughout the soil matrix. Here, we use a combination of bulk X-ray absorption spectroscopy (XAS) and micro-focused XAS and X-ray diffraction to determine uranium speciation and distribution with sediment from a pilot-scale uranium remediation project located in Oak Ridge, TN.

Elucidation of complex biogeochemical processes and their effects on speciation of U in the subsurface is critical for developing remediation strategies with an understanding of stability. We have developed static microcosms that are similar to bioreduction process studies in situ under laminar flow conditions or in sediment pores. Uranium L{sub 3}-edge X-ray absorption near-edge spectroscopy analysis with depth in the microcosms indicated that transformation of U{sup VI} to U{sup IV} occurred by at least two distinct processes. Extended X-ray absorption fine structure (EXAFS) analysis indicated that initial U{sup VI} species associated with C- and P-containing ligands were transformed to U{sup IV} in the form of uraninite and U associated with Fe-bound ligands. Microbial community analysis identified putative Fe{sup III} and sulfate reducers at two different depths in the microcosms. The slow reduction of U{sup VI} to U{sup IV} may contribute the stability of U{sup IV} within microcosms at 11 months after a decrease in bioreducing conditions due to limited electron donors.

During the past few decades, the use of electron microscopy approaches - many developed by Terry Beveridge - to probe the physiology of microorganisms has become a mainstay in fields including microbiology, human health, and geomicrobiology. Recent developments of third-generation synchrotron X-ray sources and X-ray-based microscopy approaches for studying microbial systems have proved their utility as complements to the very powerful approaches regularly employed by electron microscopists. In addition, in recent geomicrobiological studies, researchers have begun to take advantage of the strengths of each technique by using the superior spatial resolution of the electron microscope (relative to the X-ray microscope) and the superior elemental sensitivity of the X-ray microscope (relative to the electron microscope), along with the ability of the X-ray microscope to spatially probe the chemical speciation of elements. The benefits of integrating these two nanoprobes for investigating the same microenvironments within a geomicrobial system are far superior to those of independent studies separately employing each probe.

Because of the importance of probing molecular-scale chemical and physical structure of environmental samples in their natural and often hydrated state, synchrotron radiation has been a powerful tool for environmental scientists for decades. Thus, the crucial role that a highly coherent and high-brightness hard X-ray source such as the Advance Photon Source (APS) can play in addressing many of the outstanding questions in molecular environmental science (MES) was recognized even before 'first light' at the facility. No single synchrotron-based technique or experimental approach can adequately address the tremendous temporal and spatial heterogeneities of the chemistry, physics, and biology of natural environmental samples. Thus, it is common at the APS that multiple X-ray techniques and experimental systems are employed to investigate environmental samples, often chosen for their ability to focus on solute species, plants, microbes, organics, interfacial species, or solids.