2009. "Biomineralization Associated with Microbial Reduction of Fe3+ and Oxidation of Fe2+ in Solid Minerals ." American Mineralogist 94(7):1049-1058. Abstract Iron- reducing and oxidizing microorganisms gain energy through reduction or oxidation of iron, and by doing so they play an important role in geochemical cycling of iron in a wide range of environments. This study was undertaken to investigate iron redox cycling in the deep subsurface by taking an advantage of the Chinese Continental Scientific Deep Drilling project. A fluid sample from 2450 m was collected and Fe(III)-reducing microorganisms were enriched using specific media (pH 6.2). Nontronite, an Fe(III)-rich clay mineral, was used in initial enrichments with lactate and acetate as electron donors under strictly anaerobic condition at the in-situ temperature of the fluid sample (65oC). Instead of a monotonic increase in Fe(II) concentration with time as would have been expected if Fe(III) bioreduction was the sole process, Fe(II) concentration initially increased, reached a peak, but then decreased to a minimum level. Continued incubation revealed an iron cycling with a cycling period of five to ten days. These initial results suggested that there might be Fe(III) reducers and Fe(II) oxidizers in the enrichment culture. Subsequently, multiple transfers were made with an attempt to isolate individual Fe(III) reducers and Fe(II) oxidizers. However, iron cycling persisted after multiple transfers. Additional experiments were conducted to ensure that iron reduction and oxidation was indeed biological. Biological Fe(II) oxidation was further confirmed in a series of roll tubes (with a pH gradient) where FeS and siderite were used as the sole electron donor. The oxidation of FeS occurred only at pH 10, and goethite, lepidocrocite, and ferrihydrite formed as oxidation products. Although molecular evidence (16S rRNA gene analysis) collectively suggested that only a single organism (a strain of Thermoanaerobacter ethanolicus) might be responsible for both Fe(III) reduction and Fe(II) oxidation, we could not rule out the possibility that Fe(III) reduction and Fe(II) oxidation may be accomplished by a consortia of organisms. Nonetheless, our data were definitive in showing that iron redox cycling exists in the deep subsurface.
2009. "Reduction of Hg(II) to Hg(0) by Magnetite." Environmental Science & Technology 43(14):5307-5313. doi:10.1021/es9003608 Abstract Mercury (Hg) is a highly toxic element, and its contamination of groundwater presents a significant threat to terrestrial ecosystems. Understanding the geochemical processes that mediate mercury transformations in the subsurface is necessary to predict its fate and transport. In this study, we investigated the redox transformation of mercuric Hg (Hg[II]) in the presence of the Fe(II)/Fe(III) mixed valence iron oxide mineral magnetite. Kinetic and spectroscopic experiments were performed to elucidate reaction rates and mechanisms. The experimental data demonstrated that reaction of Hg(II) with magnetite results in the loss of Hg(II) and the formation of volatile elemental Hg (Hg[0]). Kinetic experiments showed that Hg(II) reduction occurred within minutes, with reaction rates increasing with increasing magnetite suspension density (0.05 to 0.2 g/L) and solution pH (4.8 to 6.7), and decreasing with increasing chloride concentration (10-6 to 10-2 mol/L). Mössbauer spectroscopic analysis of reacted magnetite samples revealed a decrease in Fe(II) content, corresponding the oxidation of Fe(II) to Fe(III) in the magnetite structure. X-ray photoelectron spectroscopy detected the presence of Hg(II) on magnetite surfaces, suggesting that adsorption is involved in the electron transfer process. These results suggest that Hg(II) reaction with solid-phase Fe(II) is a kinetically favorable pathway for Hg(II) reduction in magnetite-bearing environmental systems.
2009. "Uranium Extraction From Laboratory Synthesized, Uranium-Doped Hydrous Ferric Oxides ." Environmental Science & Technology 43:2341-2347. Abstract The extractability of uranium (U) from synthetic hydrous ferric oxides has been shown to decrease as a function of mineral ripening, consistent with the hypothesis that the ripening process decrease contaminant lability. To evaluate this process, three hydrous ferric oxide (HFO) suspensions were co-precipitated with uranyl (UO22+) and maintained at pH 7.0 ± 0.1. Uranyl was added to the HFO post-precipitation in a fourth suspension. Two suspensions also contained either co-precipitated silicate (Si-U-HFO) or phosphate (P-U-HFO). After precipitation of the HFOs, at time intervals of one week, one month, six months, one year, and 2 years, aliquots of the suspensions were contacted with five solutions for a range of time. The extracts were analyzed for U and iron (Fe). The results are consistent with the hypothesis that U and Fe extractability will decrease as the mineral phase ripens. All extracting solutions exhibited some degree of selectivity for U, as the proportional extraction of U exceeded that for congruent dissolution. Micro X-ray diffraction analysis indicates the transformation from an amorphous phase to a material containing substantial proportions of crystalline goethite and hematite, except the P-U-HFO which remained primarily amorphous. Further analysis of the co-precipitates by the Mössbauer technique and scanning electron microscopy (SEM) provides further evidence of mineralogic ripening
2008. "Reduction of Tc(VII) by Fe(II) Sorbed on Al (hydr)oxides." Environmental Science & Technology 42(15):5499-5506. doi:10.1021/es8003156 Abstract Technetium speciation, solubility and sorption behavior is strongly dependent on its valence state. Under oxic conditions, Tc exists as the soluble, weakly-sorbing pertechnetate [TcO4-] anion. The reduced form of technetium, Tc(IV), is stable in anoxic environments and is sparingly soluble as TcO2·xH2O(s). Here we investigate the heterogeneous reduction of Tc(VII) by Fe(II) sorbed on Al (hydr)oxides [diaspore (α-AlOOH) and corundum (α-Al2O3)]. Experiments were performed to study the kinetics of Tc(VII) reduction, examine changes in Fe surface speciation during Tc(VII) reduction (Mössbauer spectroscopy), and identify the nature of Tc(IV)-containing reaction products (X-ray absorption spectroscopy). We found that Tc(VII) was completely reduced by adsorbed Fe(II) within 11d (diaspore suspension) and 4d (corundum suspension). Mössbauer measurements revealed that the Fe(II) signal became less intense with Tc(VII) reduction, and was accompanied by increase in Fe(III) doublet and magnetically-ordered Fe(III) sextet signals, with latter parameters close to those for hematite. Formation of magnetically ordered Fe(III) did not depend on the oxidant nature, as both Tc(VII) or O2 lead to the formation of a virtually identical hematite-like phase. The Fe(II) doublet displayed no differences in Mössbauer parameters before and after Tc(VII) reduction, likely due to Fe(II) adsorption to similar sites and no Fe(II) sorption to or precipitation within solid phases formed. Tc-EXAFS spectroscopy revealed that the final heterogeneous redox product on corundum was similar to Tc(IV) oxyhydroxide, TcO2·xH2O. The formation of precursor polymeric TcnOy (4n-2y)+ chains prior to TcO2⋅xH2O precipitation might explain the formation of the separate TcO2-like phase on corundum without coprecipitated Fe.
2008. "Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface." Geochimica et Cosmochimica Acta 72(6):1521-1539. doi:10.1016/j.gca.2008.01.004 Abstract Technetium-99 is a byproduct of uranium fission. It exists in two stable valence states (IV/VII) and exhibits a half-cell potential of intermediate value (Eo = 0.748 V). The oxidized form of 99Tc [pertechnetate, TcO4-] is an oxyanion that sorbs poorly to amphoteric surfaces and forms few solid phases with geochemically relevant cations. It is consequently highly mobile in oxic water-rock systems. The reduced valence state [Tc(IV)] is relatively insoluble (<10-8 mol L-1), and, hence immobile as an oxyhydroxide precipitate [TcO2•nH2O(s)]. In spite of favorable thermodynamics, Tc(VII) reacts slowly with Fe2+(aq) under anoxic conditions. Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) sorbed to hematite and goethite, and by structural Fe(II) in a dithionite-citrate-bicarbonate (DCB) reduced natural phyllosilicate mixture containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) sorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe2+(aq) adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by structural Fe(II) in the DCB-reduced phyllollsilicates, were not sensitive to pH or the concentration of ion-exchangeable Fe(II), and were orders of magnitude slower than observed for the Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mössbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by sorbed and structural Fe(II) in anoxic environments may be a very important geochemical process that will proceed at very different rates and that will yield different surface species depending subsurface pH and mineralogy.
2008. "Biogeochemical Processes In Ethanol Stimulated Uranium Contaminated Subsurface Sediments." Environmental Science & Technology 42(12):4384-4390. Abstract A laboratory incubation experiment was conducted with uranium contaminated subsurface sediment to assess the geochemical and microbial community response to ethanol amendment. A classical sequence of TEAPs was observed in ethanol-amended slurries, with NO3- reduction, Fe(III) reduction, SO4 2- reduction, and CH4 production proceeding in sequence until all of the added 13C-ethanol (9 mM) was consumed. Approximately 60% of the U(VI) content of the sediment was reduced during the period of Fe(III) reduction. No additional U(VI) reduction took place during the sulfate-reducing and methanogenic phases of the experiment. Only gradual reduction of NO3 -, and no reduction of U(VI), took place in ethanol-free slurries. Stimulation of additional Fe(III) or SO4 2- reduction in the ethanol-amended slurries failed to promote further U(VI) reduction. Reverse transcribed 16S rRNA clone libraries revealed major increases in the abundance of organisms related to Dechloromonas, Geobacter, and Oxalobacter in the ethanolamended slurries. PLFAs indicative of Geobacter showed a distinct increase in the amended slurries, and analysis of PLFA 13C/12C ratios confirmed the incorporation of ethanol into these PLFAs. A increase in the abundance of 13C-labeled PLFAs indicative of Desulfobacter, Desulfotomaculum, and Desulfovibrio took place during the brief period of sulfate reduction which followed the Fe(III) reduction phase. Our results show that major redox processes in ethanol-amended sediments can be reliably interpreted in terms of standard conceptual models of TEAPs in sediments. However, the redox speciation of uranium is complex and cannot be explained based on simplified thermodynamic considerations.
2008. "Long-Term Dynamics of Uranium Reduction/Reoxidation under Low Sulfate Conditions." Geochimica et Cosmochimica Acta 72(15):3603-3615. doi:10.1016/j.gca.2008.05.040 Abstract The biological reduction and precipitation of uranium has shown potential to prevent uranium migration from contaminated areas. Although previous research has shown that uranium bioremediation is maximized during iron reducing conditions, little research has been performed to understand how long iron/uranium reducing conditions can be maintained. Similarly, questions remain about the stability of the bioreduced uranium and that of the uranium-reducing microbial population after iron/uranium biostimulation conditions are terminated and an oxidant (i.e. oxygen) is introduced into the previously reduced zone. To gain further insights into these processes, columns, packed with sediment containing iron as Fe-oxides (mainly Al-goethite) and silicate Fe (Fe-containing clays), were operated in the laboratory under field-relevant flow conditions to measure the long-term (> 200 d) removal efficiency of uranium from a simulated groundwater during biostimulation with acetate under low sulfate conditions. The biostimulation experiments were then followed by reoxidation of the reduced sediments with oxygen. During biostimulation, Fe(III) reduction occurred simultaneously with U(VI) reduction. Both Fe-oxides and silicate Fe(III) were partly reduced, and silicate Fe(III) reduction was detected only during the first half of the biostimulation phase while Fe-oxide reduction occurred throughout the whole biostimulation period. Mössbauer measurements indicated that the biogenic Fe(II) precipitate resulting from Fe-oxide reduction was neither siderite nor FeS0.09 (mackinawite). U(VI) reduction efficiency increased throughout the bioreduction period, while the Fe(III) reduction gradually decreased with time. Effluent Fe(II) concentrations decreased linearly by 30% over the final 100 days of biostimulation, indicating that bioreducible Fe(III) in the sediment was not exhausted at the termination of the experiment. Even though Fe(III) reduction did not change substantially with time, microorganisms not typically associated with Fe(III) and U(VI) reduction (including methanogens) became a significant fraction of the total microbial population during long-term biostimulation, meaning that most acetate was utilized for other biological processes than Fe(III) and U(VI) reduction. Selected columns were reoxidized after 209 days by discontinuing acetate addition and purging the influent media with a gas containing 20% oxygen. Uranium reoxidation occurred rapidly with 61% of the precipitated uranium resolubilized and transported out of the column after 21 days and virtually all of the uranium being removed by day 122. During the first 21 days of reoxidation, the Fe(III) and U(VI) reducing microbial population remained at pre-oxidation levels (even though the methanogen population decreased by 99%) indicating that short-term disruptions in biostimulation (equipment failure, etc.) would not negatively affect the uranium reducing microbial population.
2008. "Effects of gamma-sterilization on the physico-chemical properties of natural sediments." Chemical Geology 251(1-4):1-7. doi:10.1016/j.chemgeo.2008.01.003 Abstract A series of experiments were completed to determine the effects of soil sterilization on various soil chemical properties including U(VI) sorption, soil pH, natural organic matter (NOM), cation exchange capacity (CEC), and iron oxidation state. Soils under investigation were a saprolitic sequence of interbedded weathered shale and limestone collected from the Oak Ridge Reservation (ORR). Sediments were sterilized by either steam sterilization at 121oC or by γ-irradiation using a cobalt-60 source. Subsamples of sediments were pretreated with dithionate-citrate-bicarbonate (DCB) and/or H2O2 to remove reducible Fe(III) oxides and NOM. Results from aerobic U(VI) sorption experiments indicated that γ-sterilized sediments sorbed more U(VI) compared to non-sterile sediments. Results from sorption experiments completed using DCB and H2O2-treated samples indicated that the iron oxide and NOM fractions of the sediment accounted for the majority of U(VI) sorption and that γ-irradiation of these phases resulted in increased sorption of U(VI). Mössbauer spectra of γ-sterilized sedimentsdisplayed a decrease in the amount of ferric iron associated with goethite and a small increase in the amount of reduced iron in silicate minerals compared to spectra from non-sterile samples. Our results suggest that sterilization by γ-irradiation induced iron reduction that may have increased sorption of U(VI) on these sediments.
2007. "Anaerobic Microbial-Mineral Processes with Fe(III) Oxides: Experimental Considerations and Approaches." Chapter 4 in Methods for Investigating Microbial-Mineral Interactions: CMS Workshop Lectures, vol. 14, ed. Patricia A. Maurice and Leslie A. Warren, pp. 117-165. The Clay Minerals Society, West Lafayette, IN. Abstract The biogeochemical cycle of Fe is a one of the dominant redox cycles operative in surface waters and sediments, soils and vadose zones, and groundwater systems. In this cycle which is pronounced at oxic-anoxic boundaries, Fe compounds and microorganisms couple to mediate the oxidation of organic matter by molecular oxygen. The cycle includes: i.) the reductive dissolution of Fe(III) oxides by biogenic organic acids and organic matter oxidizing-metal reducing bacteria yielding Fe2+(aq) and ferrous containing minerals as products, and ii.) the oxidation of dissolved and solid-phase ferrous iron by molecular oxygen and microaerophilic Fe(II)-oxidizing bacteria with subsequent precipitation of poorly crystalline Fe(III) oxides (e.g., ferrihydrite). These Fe(III) oxides; that may recrystallize slowly with time to goethite, hematite, and lepidocrocite; represent a primary redox buffering agent (in terms of electron equivalents) in soils, sediments, and subsurface materials. Manganese (III/IV) oxides are also important in this regard. Because of the high surface area and surface chemical reactivity of Fe(III) oxides and Fe(II) containing mineral phases, the Fe biogeochemical cycle is closely linked to those of the trace metals, phosphorous, and various organic and inorganic anthropogenic contaminants.
2007. "Reduction of Pertechnetate [Tc(VII)] by Aqueous Fe(II) and the Nature of Solid Phase Redox Products." Geochimica et Cosmochimica Acta 71(9):2137-2157. doi:doi:10.1016/j.gca.2006.10.025 Abstract The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is strongly dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 10-9atm O2) in the absence of atmospheric or aqueous carbonate. The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Tc(IV)/Fe form was observed when 11 µmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. The heterogeneous reaction was more rapid, but not quantified. The kinetics of homogeneous reduction were slow at pH 7, but increased dramatically at pH 8, and correlated with the concentration of Fe(OH)+ and Fe(OH)2o(aq). Wet chemical and Fe-x-ray absorption near edge spectroscopy measurements (XANES) indicated that both Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox reaction products. 57Fe- Mössbauer, extended x-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements on the solid phase redox products that contained 13-15% Tc indicated that they were poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with a Tc2O10 octahedral dimer bound in bidentate edge-sharing mode to an octahedral FeO6 surface or vacancy site in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2•nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2•nH2O(s) as a result of its intraparticle/intragrain residence. It is suggested that precipitates of this nature may form in anoxic sediments or groundwaters, and that the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.
2007. "Biostimulation of Iron Reduction and Subsequent Oxidation of Sediment Containing Fe-silicates and Fe-oxides: Effect of Redox Cycling on Fe(III) Bioreduction." Water Research 41(13):2996-3004. doi:10.1016/j.watres.2007.03.019 Abstract Microbial reduction of iron has been shown to be important in the transformation and remediation of contaminated sediments. Re-oxidation of microbially reduced iron may occur in sediments that experience oxidation-reduction cycling and can thus impact the extent of contaminant remediation. The purpose of this research was to quantify iron oxidation in a flow-through column filled with biologically-reduced sediment and to compare the iron phases in the re-oxidized sediment to both the pristine and biologically-reduced sediment. The sediment contained both Fe(III)-oxides (primarily goethite) and silicate Fe (illite/vermiculite) and was biologically reduced in phosphate buffered (PB) medium during a 497 day column experiment with acetate supplied as the electron donor. Long-term iron reduction resulted in partial reduction of silicate Fe(III) without any goethite reduction, based on Mössbauer spectroscopy measurements. This reduced sediment was treated with an oxygenated PB solution in a flow-through column resulting in the oxidation of 38% of the biogenic Fe(II). Additional batch experiments showed that the Fe(III) in the oxidized sediment was more quickly reduced compared to the pristine sediment, indicating that oxidation of the sediment not only regenerated Fe(III) but also enhanced iron reduction compared to the pristine sediment. Oxidation-reduction cycling may be a viable method to extend iron-reducing conditions during in-situ bioremediation.
2007. "Phosphate Imposed Limitations on Biological Reduction and Alteration of Ferrihydrite Mineralization." Environmental Science & Technology 41(1):166-172. Abstract Biogeochemical transformation (inclusive of dissolution) of iron (hydr)oxides resulting from dissimilatory reduction has a pronounced impact on the fate and transport of nutrients and contaminants in subsurface environments. Despite the reactivity noted for pristine (unreacted) minerals, iron (hydr)oxides within native environments will likely have a different reactivity owing in part to changes in surface composition. Accordingly, here we explore the impact of surface modifications induced by phosphate adsorption on ferrihydrite reduction by Shewanella putrefaciens under static and advective flow conditions. Alterations in surface reactivity induced by phosphate adsorption change the extent, nearly linearly, and pathway of iron biomineralization. Magnetite is the most appreciable mineralization product while minor amounts of vivianite and green rust-like phases are formed in systems having high aqueous concentrations of phosphate, ferrous iron, and biogenic bicarbonate. Goethite and lepidocrocite, characteristic biomineralization products at low ferrous-iron concentrations, are inhibited in the presence of adsorbed phosphate. Considering deviations in reactivity of iron (hydr)oxides with changes in surface composition is important for deciphering mineralization pathways under native conditions and predicting reactive characteristics.
2006. "Anaerobic Redox Cycling of Iron by Freshwater Sediment Microorganisms." Environmental Microbiology 8(1):100-113. Abstract The potential for microbially-mediated anaerobic redox cycling of iron (Fe) was examined in a first-generation enrichment culture of freshwater wetland sediment microorganisms. MPN enumerations revealed the presence of significant populations of Fe(III)-reducing (ca. 108 cells mL-1) and Fe(II)-oxidizing, nitrate-reducing organisms (ca. 105 cells mL-1) in the sediment used to inoculate the enrichment cultures. Nitrate reduction commenced immediately following inoculation of acetate-containing (ca. 1 mM) medium with a small quantity (1% vol/vol) of wetland sediment, and resulted in the transient accumulation of NO2- and production of a mixture of end-products including NH4+. Fe(III) oxide (high surface area goethite) reduction took place - after NO3- was depleted and continued until all the acetate was utilized. Addition of NO3 after Fe(III) reduction ceased resulted in the immediate oxidation of Fe(II) coupled to reduction of + NO3-to NH4 . No significant NO2- accumulation was observed during nitrate-dependent Fe(II) oxidation. No Fe(II) oxidation occurred in pasteurized controls. Microbial community structure in the enrichment was monitored by DGGE analysis of PCR amplified 16s rDNA and RT-PCR amplified 16S rRNA, as well as by construction of 16S rDNA clone libraries for four different time points during the experiment. Strong similarities in dominant members of the microbial community were observed in the Fe(III) reduction and nitrate-dependent Fe(II) oxidation phases of the experiment, specifically the common presence of organisms closely related (= 95% sequence similarity) to the genera Geobacter and Dechloromonas. These results indicate that the wetland sediments contained organisms such as Geobacter sp. which are capable of both + dissimilatory Fe(III) reduction and oxidation of Fe(II) with reduction of NO3-reduction to NH4 . Our findings suggest that microbially-catalyzed nitrate-dependent Fe(II) oxidation has the potential to contribute to a dynamic anaerobic Fe redox cycle in freshwater sediments.
2006. "Microbial Reduction of Fe(III) in the Fithian and Muloorina Illites : Contrasting Extents and Rates of Bioreduction." Clays and Clay Minerals 54(1):67-79. doi:10.1346/CCMN.2006.0540109 Abstract This study was undertaken to investigate the rate and extent of reduction of Fe(III) in two different illite samples, Muloorina and Fithian, by facultative anaerobe, Shewanella putrefaciens CN32. The Fithian illite (API reference illite from Illinois) is relatively pure with only a trace amount of goethite. The starting material contained 4% of Fe, 19% of which is Fe(II) as determined by Mössbauer spectroscopy. Mössbauer spectroscopy also confirmed the presence of two different sites for Fe(III) in the illite structure. The surface area of the Fithian illite is 85.31 m2/g. The Muloorina illite, attained from Lake Eyre, S. Australia, has a surface area of 107.68 m2/g. The <0.2 µm size fraction was separated from a rock containing the Muloorina illite and used for bioreduction experiment. This size fraction is pure with no other Fe-containing minerals. The starting material contained 9.2% Fe, 93% of which is Fe(III) as determined by chemical and Mössbauer methods. A subsurface iron-reducing bacterium, Shewenella putrefactions CN32, was used in the illite reduction experiments. For both bioreduction experiments, illite suspensions of 10 mg/mL were made in bicarbonate buffer and experimental tubes were inoculated with about 4.0 x 10-6-6.0 x 10-7 CN32 cells. In selected treatments, antraquinone-2,6-disulfonate (AQDS) was included as an electron shuttle to facilitate bioreduction while lactate was used as an electron donor. Controls were identical but no cells were added. Fe(II) production at various time points was determined by Ferrozine assay. Aqueous concentrations of major elements, Si, Al, Mg, and Fe were measured by direct current plasma emission spectroscopy. Lactate and its metabolic product acetate were measured by high performance liquid chromatography. Residual and biogenic solids were characterized by transmission electron microscopy, Mössbauer spectroscopy, and electron energy loss spectroscopy. The extent of reduction was much greater in the presence of AQDS for each illite sample, and the abiotic controls did not show any Fe(III) reduction. More importantly, the two illites exhibited contrasting extent and rate of bioreduction. Approximately 50-60% of Fe(III) in the Fithian illite (as measured by Ferrozine assay) was reduced within 4 days, whereas it took about two months to reduce 6-8% of Fe(III) in the Muloorina illite. In both cases, Fe(III) reduction was coupled with oxidation of lactate. The aqueous concentration of Fe increased over time and paralleled the trend of the biogenic Fe(II) production, but it accounted for only about 10% of total biogenic Fe(II), suggesting that most of bioproduced Fe(II) was either in biogenic solids or adsorbed onto solid surfaces. Intensive characterizations of residual and biogenic solids are underway to delineate the reasons for the differences in the bioreduction behavior of the two illite samples. Surface area does not account for the differences. We speculate that these differences are caused by different structural sites of Fe(III): there exists two structural sites for Fe(III) in the Fithian illite as opposed to one site for Fe(III) in the Muloorina illite. One of the two sites is unique to the Fithian illite, and this site may be responsible for the extensive reduction for this illite sample. This site may be accessible to bacteria and thus may subject to more bioreduction.
2006. "Reductive Biotransformation of Fe in Shale-Limestone Saprolite Containing Fe(III) Oxides and Fe(II)/Fe(III) Phyllosilicates." Geochimica et Cosmochimica Acta 70(14):3662–3676. doi:10.1016/j.gca.2006.05.004 Abstract A <2.0-mm fraction of a mineralogically complex subsurface sediment containing goethite and Fe(II)/Fe(III) phyllosilicates was incubated with Shewanella putrefaciens (strain CN32) and lactate at circumneutral pH under anoxic conditions to investigate electron acceptor preference and the nature of the resulting biogenic Fe(II) fraction. Anthraquinone-2,6-disulfonate (AQDS), an electron shuttle, was included in select treatments to enhance bioreduction and subsequent biomineralization. The sediment was highly aggregated and contained two distinct clast populations: i) a highly weathered one with “sponge-like” internal porosity, large mineral crystallites, and Fe-containing micas, and ii) a dense, compact one with fine-textured Fe-containing illite and nano-sized goethite, as revealed by various forms of electron microscopic analyses. Approximately 10 to 15% of the Fe(III)TOT was bioreduced by CN32 over 60 d in media without AQDS, whereas 24% and 35% of the Fe(III)TOT was bioreduced by CN32 after 40 and 95 d in media with AQDS. Little or no Fe2+, Mn, Si, Al, and Mg were evident in aqueous filtrates after reductive incubation. Mössbauer measurements on the bioreduced sediments indicated that both goethite and phyllosilicate Fe(III) were partly reduced without bacterial preference. Goethite was more extensively reduced in the presence of AQDS whereas phyllosilicate Fe(III) reduction was not influenced by AQDS. Biogenic Fe(II) resulting from phyllosilicate Fe(III) reduction remained in a layer-silicate environment that displayed enhanced solubility in weak acid. The mineralogic nature of the goethite biotransformation product was not determined. Chemical and cryogenic Mössbauer measurements, however, indicated that the transformation product was not siderite, green rust, magnetite, Fe(OH)2, or Fe(II) adsorbed on phyllosilicate or bacterial surfaces. Several lines of evidence suggested that biogenic Fe(II) existed as surface associated phase on the residual goethite, and/or as a Fe(II)-Al coprecipitate. Sediment aggregation and mineral physical and/or chemical factors were demonstrated to play a major role on the nature and location of the biotransformation reaction and its products.
2005. "Development of High-Temperature Ferromagnetism in SnO₂ and Paramagnetism in SnO by Fe Doping." Physical Review. B, Condensed Matter and Materials Physics 72(5):054402 (14 p.). Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn₁-xFexO₂ (0.005 ≤ x ≤ 0.10) and paramagnetic behavior in an identical set of Sn₁-xFexO. The ferromagnetic Sn₀․₉₉Fe₀․₀₁O₂ showed a Curie temperature Tc = 850 K, which is among the highest reported for dilute magnetic semiconductors. No evidence of dopant segregation was detected in Sn₁-xFexO₂ or Sn1-xFexO, suggesting that the emerging magnetic interactions in these systems are strongly related to the properties of the host systems SnO and SnO₂.
2005. "Development of high-temperature ferromagnetism in SnO2 and paramagnetism in SnO by Fe doping." Virtual Journal of Nanoscale Science & Technology 12(7):, Abstract We report the development of room-temperature ferromagnetism in chemically synthesized powder samples of Sn1−xFexO2 *0.005*x*0.05* and paramagnetic behavior in an identically synthesized set of Sn1−xFexO. The ferromagnetic Sn0.99Fe0.01O2 showed a Curie temperature TC=850 K, which is among the highest reported for transition-metal-doped semiconductor oxides. With increasing Fe doping, the lattice parameters of SnO2 decreased and the saturation magnetization increased, suggesting a strong structure-magnetic property relationship. When the Sn0.95Fe0.05O2 was prepared at different temperatures between 200 and 900 °C, systematic changes in the magnetic properties were observed. Combined Mössbauer spectroscopy and magnetometry measurements showed a ferromagnetic behavior in Sn0.95Fe0.05O2 samples prepared at and above 350 °C, but the ferromagnetic component decreased gradually as preparation temperature approached 600 °C. All Sn0.95Fe0.05O2 samples prepared above 600 °C were paramagnetic. X-ray photoelectron spectroscopy, magnetometry, and particle induced x-ray emission studies showed that the Fe dopants diffuse towards the surface of the particles in samples prepared at higher temperatures, gradually destroying the ferromagnetism. Mössbauer studies showed that the magnetically ordered Fe3+ spins observed in the Sn0.95Fe0.05O2 sample prepared at 350 °C is only *24% of the uniformly incorporated Fe3+. No evidence of any iron oxide impurity phases were detected in Sn1−xFexO2 or Sn1−xFexO, suggesting that the emerging magnetic interactions in these systems are most likely related to the properties of the host systems SnO2 and SnO, and their oxygen stoichiometry.
2005. "Influence of Sediment Bioreduction and Reoxidation on Uranium Sorption." Environmental Science and Technology 39(11):4125-4133. Abstract The influence of sediment bioreduction and reoxidation on U(VI) sorption was studied using Fe(III) oxide-containing saprolite from the U.S. Department of Energy (DOE) Oak Ridge site. Bioreduced sediments were generated by anoxic incubation with a metal reducing bacterium, Shewanella putrefaciens strain CN32, supplied with an electron donor. The reduced sediments were subsequently reoxidized by air contact. U(VI) sorption was studied in Na-NO3-HCO3 electrolytes that were both closed and open to atmosphere, and where pH, U(VI) and carbonate concentration was varied. Mössbauer spectroscopy and chemical analyses showed that 50% of the Fe(III)-oxides were reduced to Fe(II) that was sorbed to the sediment during incubation with CN32. However, this reduction and subsequent reoxidation of the sorbed Fe(II) had negligible influence on the rate and extent of U sorption, or the extractability of sorbed U by 0.2 mol/L NaHCO3. Various results indicated that U(VI) surface complexation was the primary process responsible for uranyl sorption by the bioreduced and reoxidized sediments. A two-site, non-electrostatic surface complexation model best described U(VI) adsorption under variable pH, carbonate and U(VI) conditions. A ferrihydrite-based diffuse double layer model provided a better estimation of U(VI) adsorption without parameter adjustment than did a goethite-based model, even though a majority of the Fe(III)-oxides in the sediments were goethite.
2005. "Ferrous Hydroxy Carbonate is a Stable Transformation Product of Biogenic Magnetite." American Mineralogist 90(2-3):510-515. Abstract A ~1:1 mixture of ferrihydrite and nanocrystalline akaganeite (β-FeOOH; 10-15 nm) was incubated with Shewanella putrefaciens (strain CN32) under anoxic conditions with lactate as an electron donor and anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle. The incubation was carried out in a 1,4-piperazinediethanesulfonic acid (PIPES)-buffered medium, without PO₄³⁻ at circumneutral pH. Iron reduction was measured as a function of time (as determined by 0.5 N HCl extraction), and solids were characterized by X-ray diffraction (XRD), electron microscopy, and Mössbauer spectroscopy. The biogenic reduction of Fe(III) was rapid; with 60% of the total Fe (Feтот) reduced in one day. Only an additional 10% of Feтот was reduced over the next three years. A fine-grained (10 nm), cation-excess (CE) magnetite with a Fe(II)/Feтот ratio of 0.5-0.6 was the sole biogenic product after one day of incubation. The CE magnetite was unstable and partially transformed to micron-sized ferrous hydroxy carbonate [FHC; Fe2 (OH)2CO3⒮], a rosasite-type mineral, with time. Ferrous hydroxy carbonate dominated the mineral composition of the three year incubated sample. The Fe(II)/Feтот ratio of the residual CE magnetite after three years of incubation was lower than the day 1 sample and was close to that of stochiometric magnetite (0.33). To best of our knowledge, this is the first report of biogenic FHC, and only the third observation of this material in nature. Ferrous hydroxy carbonate appeared to form by slow reaction of microbially produced carbonate with Fe(II)-excess magnetite. The FHC may be an overlooked mineral phase that explains the infrequent occurrence of fine-grained, biogenic magnetite in anoxic sediments.
2005. "CONTROL OF FE(III) SITE OCCUPANCY ON THE RATE AND EXTENT OF MICROBIAL REDUCTION OF FE(III) IN NONTRONITE." Geochimica et Cosmochimica Acta 69(23):5429-5440. doi:10.1016/j.gca.2005.07.008 Abstract A quantitative study was performed to understand how Fe(III) site occupancy controls Fe(III) bioreduction in nontronite by Shewanella putrefaciens CN32. NAu-1 and NAu-2 were nontronites and contained Fe(III) in different structure sites with 16% and 23% total iron (w/w), respectively, with almost all iron as Fe(III). Mössbauer spectroscopy showed that Fe(III) was present in the octahedral site in NAu-1 (with a small amount of goethite), but in both the tetrahedral and the octahedral sites in NAu-2. Mössbauer data further showed that the octahedral Fe(III) in NAu-2 existed in at least two environments- trans (M1) and cis (M2) sites. The microbial Fe(III) reduction in NAu-1 and NAu-2 was studied in batch cultures at a nontronite concentration of 5mg/mL in bicarbonate buffer with lactate as the electron donor. Fe(II) production in inoculated treatments was determined by extraction with 0.5 N HCl and compared to uninoculated controls to establish the extent of biological reduction. The resulting solids were characterized by X-ray diffraction (XRD), Mössbauer spectroscopy, and transmission electron microscopy (TEM). In the presence of an electron shuttle, anthraquinone-2,6-disulfonate (AQDS), the extent of bioreduction was 11-16% for NAu-1 but 28-32% for NAu-2. The extent of reduction in the absence of AQDS was only 5-7% in NAu-1 but 14-18% in NAu-2. The reduction rate was also faster in NAu-2 than that in NAu-1. Mössbauer data of the bioreduced nontronite materials indicated that the Fe(III) reduction in NAu-1 was mostly from the presence of goethite, whereas the reduction in NAu-2 was due to the presence of the tetrahedral and trans-octahedral Fe(III) in the structure. The measured aqueous Fe(II) was negligible [< 2.5% of the total biogenic Fe(II)]. As a result of bioreduction, the average nontronite particle thickness remained nearly the same (from 2.1 to 2.5 nm) for NAu-1, but decreased significantly from 6 to 3.5 nm for NAu-2 with a concomitant change in crystal size distribution. The decrease in crystal size suggests reductive dissolution of nontronite NAu-2, which was supported by aqueous solution chemistry (i.e., aqueous Si). These data suggest that the more extensive Fe(III) bioreduction in NAu-2 was due to the presence of the tetrahedral and the trans-octahedral Fe(III), which was presumed to be more reducible. The biogenic Fe(II) was not associated with biogenic solids such as siderite or green rust or in the aqueous solution. We infer that it may be either adsorbed onto surfaces of nontronite particles/bacteria and in the structure of nontronite. Furthermore, we have demonstrated that natural nontronite clays were capable of supporting cell growth even in non-growth medium, possibly due to presence of naturally existing nutrients in the nontronite clays. These results suggest that crystal chemical environment of Fe(III) is an important determinant in controlling the rate and extent of microbial reduction of Fe(III) in nontronite.
2005. "Effects of Sediment Iron Mineral Composition on Microbially Mediated Changes in Divalent Metal Speciation: Importance of ferrihydrite." Geochimica et Cosmochimica Acta 69(7):1739-1759. Abstract Abstract 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 utilise 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 previouslysorbed 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. Mössbauer 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.
2004. "Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer." Geochimica et Cosmochimica Acta 68(8):1791-1805. Abstract Biogeochemical Transformation of Fe Minerals in a Petroleum-Contaminated Aquifer
2004. "Copper Sorption Mechanisms on Smectites." Clays and Clay Minerals 52(3):321-333. Abstract Abstract– Due to the importance of clay minerals in metal sorption many studies have attempted to derive mechanistic models that describe adsorption processes. These models often include several different types of adsorption sites, including permanent charge sites and silanol and aluminol functional groups on the edges of clay minerals. The edge sites have similar pH-dependent adsorption properties as many oxide minerals. To provide a basis for development of adsorption models it is critical that molecular level studies be done to characterize sorption processes. In this study we conducted XAFS and ESR spectroscopic experiments on copper (II) sorbed on smectite clays using suspension pH and ionic strength as variables. At low ionic strength, results suggest that Cu is sorbing in the interlayers and maintains its hydration sphere. At high ionic strength Cu atoms are excluded from the interlayer and sorb primarily on the silanol and aluminol functional groups of the montmorillonite or beidellite structures. Interpretation of the XAFS and ESR spectroscopy results provides evidence that multinuclear complexes are forming on the edge sites. Fitting of EXAFS spectra revealed that the Cu-Cu atoms in the multinuclear complexes are 2.65 apart, and have coordination numbers near one. This structural information suggests that small Cu dimers are sorbing on the surface. These complexes are consistent with observed sorption on mica and amorphous silicon dioxide, as well as the Cu-bearing silicate minerals plancheite and shattuckite, yet are inconsistent with previous spectroscopic results for Cu sorption on montmorillonite. We hypothesize that the differences in sorption mechanisms on the edges of the montmorillonite are due to loading level. The results reported in this paper provide mechanistic data that will be valuable for modeling surface interactions of Cu with clay minerals, and predicting the geochemical cycling of Cu in the environment.
2004. "Synthesis of Colloidal Mn2+:ZnO Quantum Dots and High-TC Ferromagnetic Nanocrystalline Thin Films." Journal of the American Chemical Society 126(30):9387-9398. Abstract Abstract:We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn2+:ZnO colloids of nearly spherical shape and uniform diameter (6.1 ( 0.7 nm). Simulations of the highly resolved X-and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn2+ with very homogeneous speciation, differing from bulk Mn2+:ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 íB/Mn2+ and TC > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn2+:ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.
2004. "Biotransformation of Two-Line Silica-Ferrihydrite by a Dissimilatory Fe(III)-Reducing Bacterium: Formation of Carbonate Green Rust in the Presence of Phosphate." Geochimica et Cosmochimica Acta 68(13):2799-2814. Abstract The reductive biotransformation of two Si-ferrihydrite (0.01 and 0.05 mole% Si) coprecipiates by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ~7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate (electron shuttle) that stimulates respiration was present in the media. Experiments were performed without and with PO43- (ranging from 1 to 20 mmol/L in media containing 50 mmol/L Fe). Our objectives were to define the combined effects of SiO44- and PO43- on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction (XRD) and Mossbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl- and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of PO43- whereas carbonate green rust GR(CO32-) ([FeII(6-x)FeIIIx(OH)12]x+(CO32-)0.5x.yH2O) and vivianite [Fe3(PO4)2.8H2O], were formed when PO43- was present. GR(CO32-) dominated as a mineral product in samples with < 4 mmol/L PO43-. The Fe(II)/Fe(III) ratio of GR(CO32-) varied with PO43- concentration; it was 2 in the 1 mmol/L PO43- and approached 1 in the 4- and 10-mmol/L PO43- samples. GR appeared to form by solid-state transformation of ferrihydrite. Medium PO43- concentration dictated the mechanism of transformation. In 1 mmol/L PO43- media, an intermediate Fe(II)/Fe(III) phase with structural Fe(II), which we tentatively assigned to a protomagnetite phase, slowly transformed to GR with time. In contrast, in medium with >4 mmol/L PO43-, a residual ferrihydrite with sorbed Fe2+ phase transformed to GR. Despite similar chemistries, PO43- was shown to have a profound effect on ferrihydrite biotransformations while that of SiO44- was minimal.
2004. "Reduction of TcO4- by sediment-associated biogenic Fe(II)." Geochimica et Cosmochimica Acta 68(15):3171-3187. Abstract Abstract-The potential for reduction of 9~CO- (aq) to poorly soluble 9~COZ .nHzO(s) by biogenic sediment-associated Fe(ll) was investigated with three Fe(IlI)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy's Hanford and Oak Ridge sites contained significant amounts of Mn(IlI,IV) oxides and net bioreduction of Fe(llI) to Fe(ll) was not observed until essentially all of the 1 hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcOz .nHzO(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO-: (aq) at rates that generally increased with increasing concentrations of 0.5 N HC1F~(Il~o of the sediments showed a common relationship between th~~8I~e~oncentra-= e ~/.!I80 20 'rY'\rI) --' t~?_~ (~~1 and the first-order reduction rate (in h-1, whereas the third demonstrated a markedly different trend. A combination of chemical extractions and s7Fe Mossbauer spectroscopy were used to characterize the Fe(llI) and Fe(ll) phases. There was little evidence of the formation of secondary Fe(ll) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(ll) were predominantly surface complexes of different forms. The reduction rates of Tc(VIl)O-; were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(ll) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(IlI), either naturally or via redox manipulation, may be effective at immobiJizing TcO-: (aq) associated with groundwater contaminant plumes.
2003. "Transformation of 2-line Ferrihydrite to 6-line Ferrihydrite under Oxic and Anoxic Conditions." American Mineralogist 88(11-12):1903-1914 pt. 2. Abstract New Insights into the Transformation of 2-line Ferrihydrite to Crystalline Iron Oxides: The First Observation of 6-line Ferrihydrite during Solid-state Transformation
2003. "Mossbauer and Optical Spectroscopic Study of Temperature and Redox Effects on Iron Local Environments in a Fe-Doped (0.5 mol% Fe2O3) 18Na(2)O-72SiO(2) Glass ." Journal of Non-crystalline Solids 317(3):301-318. Abstract Mossbauer and Optical Spectroscopic Study of Temperature and Redox Effects on Iron Local Environments in a Fe-Doped (0.5 mol% Fe2O3) 18Na(2)O-72SiO(2) Glass
2003. "Secondary Mineralization Pathways Induced by Dissimilatory Iron Reduction of Ferrihydrite Under Advective Flow." Geochimica et Cosmochimica Acta 67(16):2977-2992. Abstract Secondary Mineralization Pathways Induced by Dissimilatory Iron Reduction of Ferrihydrite Under Advective Flow
2003. "Influence of Electron Donor/Acceptor Concentrations on Hydrous Ferric Oxide (HFO) Bioreduction." Biodegradation 14(2):91-103. Abstract Dissimilatory metal-reducing bacteria (DMRB) facilitate the reduction of Fe and Mn oxides in anoxic soils and sediments and play an important role in the cycling of these metals and other elements such as carbon in aqueous environments.
2003. "Microbial Reduction of Structural Fe(III) in Illite and Goethite." Environmental Science and Technology Vol. 37(7):1268-1276 . Abstract Microbial Reduction of Structural Fe(III) in Illite and Goethite
2003. "Heterogeneous Electron-Transfer Kinetics with Synchrotron 57Fe Mossbauer Spectroscopy." Geochimica et Cosmochimica Acta 67(12):2109-2116. Abstract In the first known kinetic application of the technique, synchrotron 57Fe-Mossbauer spectroscopy was used to follow the rate of heterogeneous electron transfer between aqueous reagents and a solid phase containing Fe. The solid, a synthetic 57Fe-enriched Fe(III)-bearing pyroaurite-like phase having terephthalate (TA) in the interlayer [Mg3Fe(OH)8(TA)0.5 2H2O], was reduced by Na2S2O4 and then re-oxidized by K2Cr2O7 using a novel flow-through cell. Synchrotron Mossbauer spectra were collected in the time domain at 30-s intervals. Integration of the intensity obtained during a selected time interval in the spectra allowed sensitive determination of Fe(II) content as a function of reaction time. Analysis of reaction end member specimens by both the synchrotron technique and conventional Mossbauer spectroscopy yielded comparable values for Mossbauer parameters such as center shift and Fe(II)/Fe(III) area ratios. Slight differences in quadrupole splitting values were observed, however. A reactive diffusion model was developed that fit the experimental Fe(II) kinetic data well and allowed the extraction of second-order rate constants for each reaction. Thus, in addition to rapidly collecting high quality Mossbauer data, the synchrotron technique seems well-suited for aqueous rate experiments due to the penetrating power of 14.4 keV X-rays and high sensitivity to Fe valence state.
2002. "Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)." Geomicrobiology Journal 19:179-207. Abstract Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)
2001. "A Study of the Corrosion Products of Mild Steel in High Ionic Strength Brines." Waste Management 21:335-341. Abstract A Study of the Corrosion Products of Mild Steel in High Ionic Strength Brines
2001. "Kinetics and Mechanism of Birnessite Reduction by Catechol." Soil Science Society of America Journal 65:58-66. Abstract Kinetics and Mechanism of Birnessite Reduction by Catechol
2001. "Dissimilatory Bacterial Reduction of Al-Substituted Goethite in Subsurface Sediments." Geochimica et Cosmochimica Acta 65(17):2913-2924. Abstract Microbiologic reduction of the 0.2-2.0 mm size fraction of an Atlantic coastal plain sediment (Eatontown) was investigated using a dissimilatory Fe(III)-reducing bacterium (Shewanella putrefaciens, strain CN32) to evaluate mineralogic controls on the rate and extent of Fe(III) reduction and resulting distribution of biogenic Fe(II). Mössbauer spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) was used to show that the sedimentary Fe(III) oxide was Al-substituted goethite (11-17% Al) that existed as 1-5 mm aggregates of indistinct morphology. Bioreduction experiments were performed in two buffers [HCO3-, 1,4-piperazinediethansulfonic acid (PIPES)] both without and with 2,6-anthraquinone disulfonate (AQDS), an electron shuttle. The production of biogenic Fe(II) and the distribution of Al (aqueous and sorbed) were followed over time, as was formation of Fe(II) biominerals and physical/chemical changes to the goethite.
2001. "Biotransformation of Ni-Substituted Hydrous Ferric Oxide by an Fe(III)-Reducing Bacterium." Environmental Science and Technology 35(4):703-712. Abstract Biotransformation of Ni-Substituted Hydrous Ferric Oxide by an Fe(III)-Reducing Bacterium
2000. "Mineral Transformations Associated with the Microbial Reduction of Magnetite." Chemical Geology 169:299-318. Abstract N/A
1999. "2H Solid-State NMR Investigation of Terephthalate Dynamics and Orientation in Mixed-Anion Hydrotalcite-like Compounds." Journal of Physical Chemistry B 103:5197-5203.