B13E-01 INVITED
Understanding the Reduction of Tetrahedral Fe in Nontronites Using Visible and Infrared Spectroscopies
The reduction of the nontronites NAu-1 and NAu-2 were compared using visible and infrared spectroscopies to better understand the role of tetrahedral Fe, which is found in significant quantities in NAu-2. Changes in the diffuse reflectance (visible) spectra of NAu-2 were followed through a series of stepwise reductions. The later stages of reduction, which are dominated by Fe2+ /Fe3+ intervalence charge transfer (IVCT) features in the octahedral sheet, resemble those in NAu-1. However, the evolution of the electronic spectra in the very early stages of reduction suggest that tetrahedral Fe3+ is preferentially reduced before the octahedral Fe3+ is reduced to any appreciable extent. Additionally, it was noted that the spectroscopic transitions assigned to tetrahedral Fe3+ exhibit an inverse temperature dependence. Consequently, spectra of NAu-1 at liquid nitrogen temperatures allowed the identification of a small amount of tetrahedral Fe3+ in NAu-1 that has not been previously noted. Attenuated total internal reflection Fourier-transform infrared spectroscopy (ATR-FTIR) employing plane- polarized incident radiation was used to compare changes in the structures of the two nontronites resulting from reduction. In both minerals, reduction results in a shift of the in-plane Si-O stretches to lower frequencies, while the out-of-plane Si-O stretch shifts to higher frequencies. The magnitude of these shifts is greater in NAu-2 than in NAu-1, but the crystallinity of the tetrahedral silicate sheet of NAu-2 is preserved upon reduction. In both nontronites, the orientation of the out-of-plane Si-O bond changes and becomes completely perpendicular to the basal (001) surface of the clay, indicating the formation of trioctahedral domains wherein the individual tetrahedra reorient relative to the plane of the clay layer.
B13E-02
Microbe-Clay Mineral Reactions and Characterization Techniques
Clays and clay minerals are ubiquitous in soils, sediments, and sedimentary rocks. They play an important role in environmental processes such as nutrient cycling, plant growth, contaminant migration, organic matter maturation, and petroleum production. The changes in the oxidation state of the structural iron in clay minerals, in part, control their physical and chemical properties in natural environments, such as clay particle flocculation, dispersion, swelling, hydraulic conductivity, surface area, cation and anion exchange capacity, and reactivity towards organic and inorganic contaminants. The structural ferric iron [Fe(III)] in clay minerals can be reduced either chemically or biologically. Many different chemical reductants have been tried, but the most commonly used agent is dithionite. Biological reductants are bacteria, including dissimilatory iron reducing prokaryotes (DIRP) and sulfate-reducing bacteria (SRB). A wide variety of DIRP have been used to reduce ferric iron in clay minerals, including mesophilic, thermophilic, and hyperthermophilic prokaryotes. Multiple clay minerals have been used for microbial reduction studies, including smectite, nontronite (iron-rich smectite variety), illite, illite/smectite, chlorite, and their various mixtures. All these clay minerals are reducible by microorganisms under various conditions with smectite (nontronite) being the most reducible. The reduction extent and rate of ferric iron in clay minerals are measured by wet chemistry, and the reduced clay mineral products are typically characterized with chemical methods, X-ray diffraction, scanning and transmission electron microscopy, Mössbauer spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), UV-vis spectroscopy, and synchrotron-based techniques (such as EXAFS). Microbially reduced smectites (nontronites) have been found to be reactive in reducing a variety of organic and inorganic contaminants. Degradable organic contaminants include pesticides, solvents, explosives, and nitroaromatic and polychlorinated compounds. Inorganic contaminants include Cr(VI), U(VI), and Tc(VII). Despite significant efforts, our understanding of mechanisms of chemical and microbial reduction of ferric iron in clay minerals is still limited. While some studies have presented evidence for a solid-state reduction mechanism, others argue that the clay mineral structure dissolves when the extent of reduction is higher (greater than 30 percent). The electron transfer process is also dependent on the reducing agent. While chemical reduction of ferric iron appears to occur at the basal surfaces, bacteria appear to attack clay minerals at the edges.
B13E-03
Microbial Reduction of Al-Substituted Fe(III) (Hydr)oxides: Redefining the Reducing Capacity of Fe Phases in Natural Soils
Aluminum, one of the most abundant elements in soils and sediments, is also commonly found co- precipitated with Fe in natural Fe (III) (hydr)oxides. Although significant progress has been made elucidating the rates and solid-phase products of Fe(III) reduction by dissimilatory iron-reducing bacteria (DIRB) grown on pure, synthetic iron (hydr)oxides, relatively little is known about the impact of Al co-precipitation within Fe(III) (hydr)oxides on growth and bacterial Fe reduction by DIRB. Two previous studies investigating bacterial Fe reduction of Al-containing goethite minerals found contrasting results. To better understand the role of Al-substitution in controlling the rate, extent, and products of bacterial Fe(III) reduction, we have performed Fe(III) reduction experiments with the model DIRB, Shewanella putrefaciens CN32 grown on synthetic ferrihydrite, lepidocrocite, and goethite containing between 0 and 13 mole % Al. These experiments reveal that the impact of Al varies among Fe(III) (hydr)oxide minerals. Increasing Al-substitution in ferrihydrite results in a decrease of bacterial growth and Fe(III) reduction, while increasing Al content within lepidocrocite causes increased bacterial growth and Fe(III) reduction. For goethite, no change in Fe(III) reduction or growth is observed when growth on goethite containing increasing Al-substitution. Given the prevalence of Al-substitution in natural Fe(III) (hydr)oxides, our results bring into question the conventional assumptions about Fe(III) oxide bioavailability and suggest a more prominent role of natural lepidocrocite phases in impacting DIRB activity in soils and sediments.
B13E-04 INVITED
Active Early Chemistry Proposed for Mawrth Vallis, Mars, Based on Changing Redox Processes in Ancient Phyllosilicate Rocks
MRO/CRISM observations of Mawrth Vallis have revealed an expansive region with a common phyllosilicate stratigraphy, indicating multiple, large-scale aqueous events [1,2]. The lowest phyllosilicate unit is Fe/Mg- smectite that likely formed via aqueous alteration of basalt. This unit is overlain by rocks rich in hydrated silica, montmorillonite, and kaolinite that probably formed via subsequent leaching of Fe and Mg through extended aqueous events or a change in aqueous chemistry. An Fe2+ phase is present at the transition from Fe/Mg-smectite to Al/Si-rich material and implies active redox processes here during the Noachian period. The ferrous phase is identified by an increasing spectral slope from about 1 to 1.8 μm that is common to a variety of ferrous minerals including clays, carbonates, sulfates, and olivine. As we do not observe direct spectral evidence of any of these minerals, we attribute the ferrous phase to a ferrous mica, which has only weak OH bands and is the most likely material interstratified with the clay units. At the CRISM scale of 18 or 36 meters/pixel this ferrous unit is mixed with the Fe/Mg-smectite below it as well as the hydrated silica unit above it. Fe2+-bearing materials in terrestrial sediments are typically associated with microorganisms or changes in pH or cations, and could be explained here by hydrothermal activity. The stratigraphy of Fe/Mg-smectite overlain by a ferrous phase, hydrated silica and then Al-phyllosilicates implies a complex aqueous history. References: [1] Bishop J. L. et al. (2008), Phyllosilicate diversity and past aqueous activity revealed at Mawrth Vallis, Mars. Science, 321, DOI: 10.1126/science.1159699, pp. 830-833. [2] McKeown N. K. et al. (2008) Characterization of phyllosilicates observed in the central Mawrth Vallis region, Mars, their potential formational processes, and implications for past climate. JGR, to be submitted.
B13E-05
The Effect of Iron and Sulfate Levels on the Transition from Iron to Sulfate Reduction during Biostimulation
During biostimulation of microbial iron reduction for the purpose of U(VI) removal at the Rifle Integrated Field Challenge (IFC) site, the onset of sulfate reduction is usually observed within 20 to 30 days of biostimulation. A series of flow-through sediment column experiments were performed to determine if the onset of sulfate reducing conditions occurs while bioavailable Fe(III) is still present, if it the onset of sulfate reduction can be delayed by increasing the amount of bioavailable Fe(III), and to determine how the bioreduction of uranium is affected by the switch from iron-dominated to sulfate-dominated reducing conditions. The experiment also focused on the changes in the microbial population and how it is affected by varying the iron content in the sediment. For this purpose a set of column experiments was conducted using Rifle site sediments and two levels of sulfate in the inflow, while a second set of experiments was conducted with Rifle sediments augmented with small amounts of Fe-57 goethite. Fe-57 goethite was used in this experiment to track minute Fe(III) changes in augmented goethite via Mössbauer spectroscopy before and after the onset of sulfate reduction. Columns were sacrificed at regular intervals to determine extractable Fe(II) and Fe(III), precipitated U(IV), and analyze for the changes in biomass composition. The results showed that under low sulfate levels, iron reduction could be maintained for over two-hundred days, while in the presence of high sulfate levels, sulfate reduction was observed within thirty days, indicating that during biostimulation sulfate reduction can commence even though a significant pool of bioavailable Fe(III) is still present. The rate of U(VI) reduction was not negatively affected by the commencement of sulfate reducing conditions, an observation that differs from field results where U(VI) reduction has been observed to decrease after the onset of sulfate reduction. The addition of goethite to the sediments did not affect the time for the onset of sulfate reduction but appears to slightly suppress the amount of sulfate reduced. The sediment-attached microbial population is dominated by 6 TRFLP peaks. Three of these peaks have been identified, a Desulfovibrio-like clone, a Rhodoferax-like clone, and a Geobacter-like clone, totaling 14.5%, 26%, and 3.1% of the overall community profile, respectively. Addition of the goethite to the sediment had a noticeable effect on the overall composition of the microbial population.
B13E-06
Uranium(IV) oxidation during anoxic chemical extractions of natural sediment: Importance of Fe(III)
In situ reduction of soluble U(VI) to insoluble U(IV) precipitates is one promising solution for the remediation of U-contaminated aquifers. U(VI) reduction can occur upon stimulation of the native microbial community by injection of an electron donor or by the presence of natural organic matter. Contamination from a former U mill tailings repository (Rifle, CO) provides a research site to study the effects of in situ and natural bioreduction. An accurate method for determining solid-phase U oxidation state in sediments with elevated amounts of Fe and organic matter is necessary to evaluate the extent of bioreduction. The oxidation state of U in anaerobic sediment is often measured by a two-step bicarbonate/carbonate chemical extraction when spectroscopic methods are infeasible. In this study, anaerobic sediment samples from Rifle were analyzed for labile U(VI) content by extraction in anoxic conditions (pH 9.4, 14mM NaHCO3, 2.8 mM Na2CO3). A subset of each sediment sample was oxidized by exposure to air for 2 weeks. The extraction was repeated in air, and the amount of U(IV) present in the anaerobic sample was calculated by difference between the anoxic and oxidized extractions. For comparison, the U oxidation state was measured in several preserved samples by collecting X-ray absorption spectra (XANES). The XANES measurement indicated that approximately 90% was present as U(IV) prior to the extraction. In contrast, the extractions suggested evidence of substantial oxidation (<5% as U(IV)) even in an anoxic extraction. This discrepancy was eliminated when the anoxic extractions were repeated at pH 12, suggesting that Fe(III) may be an important oxidant for reduced U species during an anoxic extraction at pH 9.4, since the thermodynamic driving force for this reaction decreases at high pH. The results of an investigation of biogenic uraninite (UO2) oxidation by ferrihydrite in the pH range 7-12 under bicarbonate/carbonate extraction concentrations will be presented. In addition, the effect of organic matter on biogenic uraninite oxidation in the presence and absence of Fe(III) will be discussed. The results of this study demonstrate the importance of reoxidation of U(IV) by Fe(III) under anoxic extraction conditions, and proposes an alternate protocol for accurate solid-phase U oxidation state determination in sediment samples containing Fe(III) and organic matter.
B13E-07 INVITED
Thiobarbituric-acid reactive substances (TBARS) response curves in the presence of 1:1 and 2:1 phyllosilicates
Clays catalyze chemical reactions including acid hydrolysis, condensations, oxidative polymerizations, etc. Here the authors propose that properties such as the content and structural distribution of structural Fe in expandable (e.g., hectorite, nontronite) or non-expandable (e.g., kaolinite) clay minerals influence the mechanism(s) and production rate of radical species in suspension, which can alter the chemical composition of biological material. The measurement of Thiobarbituric Acid Reactive Substances (TBARS) has become the method of choice for screening and monitoring lipid peroxidation, a major indicator of oxidative stress. The assay provides important information regarding free radical activity in disease states and has been used for measurement of anti-oxidant activity of several compounds and to determine lipid peroxidation. TBARS analyses for kaolinite, hectorite, and nontronites NAu-1 and NAu-2 showed variations in amounts of lipid peroxidation. The response followed the order kaolinite (0.42 nmol/mg protein), NAu-1 (1.15), hectorite (3.35), and NAu-2 (11.1). As determined by TBARS assays, clay properties including expandability, structural iron content and distribution, were found to influence the production rate. The effect of UV light incidence (Ć = 540 nm) was found to be of little influence.