XPS and XANES Studies of Uranium Reduction by Clostridium sp

The biochemical behavior and mechanism of uranium(Ⅵ) bioreduction induced by natural Bacillus thuringiensis

For a broader understanding of uranium migration affected by microorganisms in natural anaerobic environment, the bioreduction of uranium(Ⅵ) (U(Ⅵ)) was revealed in Bacillus thuringiensis, a dominant bacterium strain with potential of uranium-tolerant isolated from uranium contaminated soil. The reduction behavior was systematically investigated by the quantitative analysis of U(Ⅳ) in bacteria, and mechanism was inferred from the pathway of electron transmission. Under anaerobic conditions, appropriate biomass and sodium lactate as electron donor, reduction behavior of U(Ⅵ) induced by B. thuringiensis was restricted by the activity of lactate dehydrogenase, which was directly affected by the initial pH, temperature and initial U(Ⅵ) concentration of bioreduction system. Bioreduction of U(Ⅵ) was driven by the generation of nicotinamide adenine dinucleotide (NADH) from enzymatic reaction of sodium lactate with various dehydrogenase. The transmission of the electrons from bacteria to U(Ⅵ) was mainly supported by the intracellular NADH dehydrogenase-ubiquinone system, this process could maintain the biological activity of cells.

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

Reduction of U(VI) to U(IV) drastically reduces its solubility and has been proposed as a method for remediation of uranium contamination. However, much is still unknown about the kinetics, mechanisms, and products of U(VI) bioreduction in complex systems. In this study, U(VI) bioreduction experiments were conducted with Shewanella putrefaciens strain CN32 in the presence of clay minerals and two organic ligands: citrate and EDTA. In reactors with U and Fe(III)-clay minerals, the rate of U(VI) bioreduction was enhanced due to the presence of ligands, likely because soluble Fe³⁺- and Fe²⁺-ligand complexes served as electron shuttles. In the presence of citrate, bioreduced U(IV) formed a soluble U(IV)-citrate complex in experiments with either Fe-rich or Fe-poor clay mineral. In the presence of EDTA, U(IV) occurred as a soluble U(IV)-EDTA complex in Fe-poor montmorillonite experiments. However, U(IV) remained associated with the solid phase in Fe-rich nontronite experiments through the formation of a ternary U(IV)-EDTA-surface complex, as suggested by the EXAFS analysis. Our study indicates that organic ligands and Fe(III)-bearing clays can significantly affect the microbial reduction of U(VI) and the stability of the resulting U(IV) phase.

Biomineralization of Uranium-Phosphates Fueled by Microbial Degradation of Isosaccharinic Acid (ISA)

Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated "bio-barrier" extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.

Biological Reduction of a U(V)-Organic Ligand Complex

Metal-reducing microorganisms such as Shewanella oneidensis MR-1 reduce highly soluble species of hexavalent uranyl (U(VI)) to less mobile tetravalent uranium (U(IV)) compounds. The biologically mediated immobilization of U(VI) is being considered for the remediation of U contamination. However, the mechanistic underpinnings of biological U(VI) reduction remain unresolved. It has become clear that a first electron transfer occurs to form pentavalent (U(V)) intermediates, but it has not been definitively established whether a second one-electron transfer can occur or if disproportionation of U(V) is required. Here, we utilize the unusual properties of dpaea^2- ((dpaeaH₂═bis(pyridyl-6-methyl-2-carboxylate)-ethylamine)), a ligand forming a stable soluble aqueous complex with U(V), and investigate the reduction of U(VI)-dpaea and U(V)-dpaea by S. oneidensis MR-1. We establish U speciation through time by separating U(VI) from U(IV) by ion exchange chromatography and characterize the reaction end-products using U M₄-edge high resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. We document the reduction of solid phase U(VI)-dpaea to aqueous U(V)-dpaea but, most importantly, demonstrate that of U(V)-dpaea to U(IV). This work establishes the potential for biological reduction of U(V) bound to a stabilizing ligand. Thus, further work is warranted to investigate the possible persistence of U(V)-organic complexes followed by their bioreduction in environmental systems.

Kinetic modelling of the uranium biosorption by Deinococcus radiodurans biofilm

Increasing number of reports on uranium contamination in groundwater bodies is a growing concern. Deinococcus radiodurans biofilm-based U(VI) bioremediation has great potential to provide solution. This study focuses on the kinetic modelling of uranium biosorption by D. radiodurans biofilm biomass and identification of the functional groups involved in the sequestration process. The effect of temperature, pH and amount of biofilm dry mass were studied using two uranyl ion concentrations (100 and 1000 mg/L). D. radiodurans dry biomass showed good affinity for uranyl ion adsorption. The kinetic experiments revealed that the biosorption process was spontaneous and exothermic in nature. The modelling of kinetic adsorption data revealed that U(VI) sorption by D. radiodurans biofilm biomass follows a pseudo-second-order reaction. Mechanism of U(VI) sorption was suggested to follow an intra-particle diffusion model, which includes covalent bonding between U(VI) and functional groups present on the surface of biofilm biomass, and diffusional barrier acts as a rate limiting step. External mass transfer was the rate-limiting step as evident from Boyd and Elovich plot. Chemical modifications in surface functional groups of biofilm biomass, confirmed the involvement of carboxyl, phosphate, and hydroxyl groups in uranium binding as a significant loss in U(VI) sorption capacity was recorded in these chemically modified biomasses. XRD data indicated the formation of metal deposits, predominantly as uranyl phosphates.

Azolla filiculoides L. as a source of metal-tolerant microorganisms

The metal hyperaccumulator Azolla filiculoides is accompanied by a microbiome potentially supporting plant during exposition to heavy metals. We hypothesized that the microbiome exposition to selected heavy metals will reveal metal tolerant strains. We used Next Generation Sequencing technique to identify possible metal tolerant strains isolated from the metal-treated plant (Pb, Cd, Cr(VI), Ni, Au, Ag). The main dominants were Cyanobacteria and Proteobacteria constituting together more than 97% of all reads. Metal treatment led to changes in the composition of the microbiome and showed significantly higher richness in the Pb-, Cd- and Cr-treated plant in comparison with other (95–105 versus 36–44). In these treatments the share of subdominant Actinobacteria (0.4–0.8%), Firmicutes (0.5–0.9%) and Bacteroidetes (0.2–0.9%) were higher than in non-treated plant (respectively: 0.02, 0.2 and 0.001%) and Ni-, Au- and Ag-treatments (respectively: <0.4%, <0.2% and up to 0.2%). The exception was Au-treatment displaying the abundance 1.86% of Bacteroidetes. In addition, possible metal tolerant genera, namely: Acinetobacter, Asticcacaulis, Anabaena, Bacillus, Brevundimonas, Burkholderia, Dyella, Methyloversatilis, Rhizobium and Staphylococcus, which form the core microbiome, were recognized by combining their abundance in all samples with literature data. Additionally, the presence of known metal tolerant genera was confirmed: Mucilaginibacter, Pseudomonas, Mycobacterium, Corynebacterium, Stenotrophomonas, Clostridium, Micrococcus, Achromobacter, Geobacter, Flavobacterium, Arthrobacter and Delftia. We have evidenced that A. filiculoides possess a microbiome whose representatives belong to metal-resistant species which makes the fern the source of biotechnologically useful microorganisms for remediation processes.

Engineering and kinetic aspects of bacterial uranium reduction for the remediation of uranium contaminated environments

Biological reduction of soluble uranium from U(VI) to insoluble U(IV) coupled to the oxidation of an electron donor (hydrogen or organic compounds) is a potentially cost-efficient way to reduce the U concentrations in contaminated waters to below regulatory limits. A variety of microorganisms originating from both U contaminated and non-contaminated environments have demonstrated U(VI) reduction capacity under anaerobic conditions. Bioreduction of U(VI) is considered especially promising for in situ remediation, where the activity of indigenous microorganisms is stimulated by supplying a suitable electron donor to the subsurface to contain U contamination to a specific location in a sparingly soluble form. Less studied microbial biofilm-based bioreactors and bioelectrochemical systems have also shown potential for efficient U(VI) reduction to remove U from contaminated water streams. This review compares the advantages and challenges of U(VI)-reducing in situ remediation processes, bioreactors and bioelectrochemical systems. In addition, the current knowledge of U(VI) bioreduction mechanisms and factors affecting U(VI) reduction kinetics (e.g. pH, temperature, and the chemical composition of the contaminated water) are discussed, as both of these aspects are important in designing efficient remediation processes.

The biogeochemical fate of nickel during microbial ISA degradation; implications for nuclear waste disposal

Intermediate level radioactive waste (ILW) generally contains a heterogeneous range of organic and inorganic materials, of which some are encapsulated in cement. Of particular concern are cellulosic waste items, which will chemically degrade under the conditions predicted during waste disposal, forming significant quantities of isosaccharinic acid (ISA), a strongly chelating ligand. ISA therefore has the potential to increase the mobility of a wide range of radionuclides via complex formation, including Ni-63 and Ni-59. Although ISA is known to be metabolized by anaerobic microorganisms, the biodegradation of metal-ISA complexes remains unexplored. This study investigates the fate of a Ni-ISA complex in Fe(III)-reducing enrichment cultures at neutral pH, representative of a microbial community in the subsurface. After initial sorption of Ni onto Fe(III)oxyhydroxides, microbial ISA biodegradation resulted in >90% removal of the remaining Ni from solution when present at 0.1 mM, whereas higher concentrations of Ni proved toxic. The microbial consortium associated with ISA degradation was dominated by close relatives to Clostridia and Geobacter species. Nickel was preferentially immobilized with trace amounts of biogenic amorphous iron sulfides. This study highlights the potential for microbial activity to help remove chelating agents and radionuclides from the groundwater in the subsurface geosphere surrounding a geodisposal facility.

Total electron acceptor loading and composition affect hexavalent uranium reduction and microbial community structure in a membrane biofilm reactor

Molecular microbiology tools (i.e., 16S rDNA gene sequencing) were employed to elucidate changes in the microbial community structure according to the total electron acceptor loading (controlled by influent flow rate and/or medium composition) in a H₂-based membrane biofilm reactor evaluated for removal of hexavalent uranium. Once nitrate, sulfate, and dissolved oxygen were replaced by U(VI) and bicarbonate and the total acceptor loading was lowered, slow-growing bacteria capable of reducing U(VI) to U(IV) dominated in the biofilm community: Replacing denitrifying bacteria Rhodocyclales and Burkholderiales were spore-producing Clostridiales and Natranaerobiales. Though potentially competing for electrons with U(VI) reducers, homo-acetogens helped attain steady U(VI) reduction, while methanogenesis inhibited U(VI) reduction. U(VI) reduction was reinstated through suppression of methanogenesis by addition of bromoethanesulfonate or by competition from SRB when sulfate was re-introduced. Predictive metagenome analysis further points out community changes in response to alterations in the electron-acceptor loading: Sporulation and homo-acetogenesis were critical factors for strengthening stable microbial U(VI) reduction. This study documents that sporulation was important to long-term U(VI) reduction, whether or not microorganisms that carry out U(VI) reduction mediated by cytochrome c₃, such as SRB and ferric-iron-reducers, were inhibited.

Performance and Mechanism of Uranium Adsorption from Seawater to Poly(dopamine)-Inspired Sorbents

Developing facile and robust technologies for effective enrichment of uranium from seawater is of great significance for resource sustainability and environmental safety. By exploiting mussel-inspired polydopamine (PDA) chemistry, diverse types of PDA-functionalized sorbents including magnetic nanoparticle (MNP), ordered mesoporous carbon (OMC), and glass fiber carpet (GFC) were synthesized. The PDA functional layers with abundant catechol and amine/imine groups provided an excellent platform for binding to uranium. Due to the distinctive structure of PDA, the sorbents exhibited multistage kinetics which was simultaneously controlled by chemisorption and intralayer diffusion. Applying the diverse PDA-modified sorbents for enrichment of low concentration (parts per billion) uranium in laboratory-prepared solutions and unpurified seawater was fully evaluated under different scenarios: that is, by batch adsorption for MNP and OMC and by selective filtration for GFC. Moreover, high-resolution X-ray photoelectron spectroscopic and extended X-ray absorption fine structure studies were performed for probing the underlying coordination mechanism between PDA and U(VI). The catechol hydroxyls of PDA were identified as the main bidentate ligands to coordinate U(VI) at the equatorial plane. This study assessed the potential of versatile PDA chemistry for development of efficient uranium sorbents and provided new insights into the interaction mechanism between PDA and uranium.

Microbial reduction of uranium (VI) by Bacillus sp. dwc-2: A macroscopic and spectroscopic study

The microbial reduction of U(VI) by Bacillus sp. dwc-2, isolated from soil in Southwest China, was explored using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). Our studies indicated that approximately 16.0% of U(VI) at an initial concentration of 100mg/L uranium nitrate could be reduced by Bacillus sp. dwc-2 at pH8.2 under anaerobic conditions at room temperature. Additionally, natural organic matter (NOM) played an important role in enhancing the bioreduction of U(VI) by Bacillus sp. dwc-2. XPS results demonstrated that the uranium presented mixed valence states (U(VI) and U(IV)) after bioreduction, which was subsequently confirmed by XANES. Furthermore, the TEM and high resolution transmission electron microscopy (HRTEM) analysis suggested that the reduced uranium was bioaccumulated mainly within the cell and as a crystalline structure on the cell wall. These observations implied that the reduction of uranium may have a significant effect on its fate in the soil environment in which these bacterial strains occur.

Mechanical investigation of U(VI) on pyrrhotite by batch, EXAFS and modeling techniques

The interaction mechanism of U(VI) on pyrrhotite was demonstrated by batch, spectroscopic and modeling techniques. Pyrite was selected as control group in this study. The removal of U(VI) on pyrite and pyrrhotite significantly decreased with increasing ionic strength from 0.001 to 0.1mol/L at pH 2.0-6.0, whereas the no effect of ionic strength was observed at pH >6.0. The maximum removal capacity of U(VI) on pyrite and pyrrhotite calculated from Langmuir model was 10.20 and 21.34mgg^-1 at pH 4.0 and 333K, respectively. The XPS analysis indicated the U(VI) was primarily adsorbed on pyrrhotite and pyrite and then approximately 15.5 and 9.8% of U(VI) were reduced to U(IV) by pyrrhotite and pyrite after 20 days, respectively. Based on the XANES analysis, the adsorption edge of uranium-containing pyrrhotite located between U^IVO₂(s) and U^VIO₂²⁺ spectra. The EXAFS analysis demonstrated the inner-sphere surface complexation of U(VI) on pyrrhotite due to the occurrence of U-S shell, whereas the U-U shell revealed the reductive co-precipitates of U(VI) on pyrrhotite/pyrite with increasing reaction times. The surface complexation modeling showed that outer- and inner-surface complexation dominated the U(VI) removal at pH<4 and pH>5.0, respectively. The findings presented herein play a crucial role in the removal of radionuclides on iron sulfide in environmental cleanup applications.

Uranium Bioreduction and Biomineralization

Following the development of nuclear science and technology, uranium contamination has been an ever increasing concern worldwide because of its potential for migration from the waste repositories and long-term contaminated environments. Physical and chemical techniques for uranium pollution are expensive and challenging. An alternative to these technologies is microbially mediated uranium bioremediation in contaminated water and soil environments due to its reduced cost and environmental friendliness. To date, four basic mechanisms of uranium bioremediation-uranium bioreduction, biosorption, biomineralization, and bioaccumulation-have been established, of which uranium bioreduction and biomineralization have been studied extensively. The objective of this review is to provide an understanding of recent developments in these two fields in relation to relevant microorganisms, mechanisms, influential factors, and obstacles.

The shifts of sediment microbial community phylogenetic and functional structures during chromium (VI) reduction

The Lanzhou reach of the Yellow River, located at the upstream of Lanzhou, has been contaminated by heavy metals and polycyclic aromatic hydrocarbons over a long-time. We hypothesized that indigenous microbial communities would remediate those contaminants and some unique populations could play an important role in this process. In this study, we investigated the sediment microbial community structure and function from the Lanzhou reach. Sediment samples were collected from two nearby sites (site A and site B) in the Lanzhou reach along the Yellow River. Sediment geochemical property data showed that site A sediment samples contained significantly (p < 0.05) higher heavy metals than site B, such as chromium (Cr), manganese (Mn), and copper (Cu). Both site A and B samples were incubated with or without hexavalent chromium (Cr (VI)) for 30 days in the laboratory, and Cr (VI) reduction was only observed in site A sediment samples. After incubation, MiSeq sequencing of 16S rRNA gene amplicons revealed that the phylogenetic composition and structure of microbial communities changed in both samples, and especially Proteobacteria, as the most abundant phylum increased from 45.1 % to 68.2 % in site A, and 50.1 % to 71.3 % in site B, respectively. Some unique OTUs and populations affiliated with Geobacter, Clostridium, Desulfosporosinus and Desulfosporosinus might be involved in Cr (VI) reduction in site A. Furthermore, GeoChip 4.0 (a comprehensive functional gene array) data showed that genes involved in carbon and nitrogen cycling and metal resistance significantly (p < 0.05) increased in site A sediment samples. All the results indicated that indigenous sediment microbial communities might be able to remediate contaminants like Cr (VI), and this information provides possible strategies for future bioremediation of the Lanzhou reach.

The Role of Bacterial Spores in Metal Cycling and Their Potential Application in Metal Contaminant Bioremediation

Bacteria are one of the premier biological forces that, in combination with chemical and physical forces, drive metal availability in the environment. Bacterial spores, when found in the environment, are often considered to be dormant and metabolically inactive, in a resting state waiting for favorable conditions for them to germinate. However, this is a highly oversimplified view of spores in the environment. The surface of bacterial spores represents a potential site for chemical reactions to occur. Additionally, proteins in the outer layers (spore coats or exosporium) may also have more specific catalytic activity. As a consequence, bacterial spores can play a role in geochemical processes and may indeed find uses in various biotechnological applications. The aim of this review is to introduce the role of bacteria and bacterial spores in biogeochemical cycles and their potential use as toxic metal bioremediation agents.

Preparation and characterization of a core–shell KNO3@alginate-Ca particle with uranium-removal and slow-release of KNO3

A novel core–shell KNO₃@alginate-Ca particle was prepared by a facile method of electro-coextrusion. The core–shell KNO₃@alginate-Ca particle was a promising adsorbent for uranium removal and a slow-release material for potassium release.

Efficient removal of U(vi) from aqueous solutions by polyaniline/hydrogen-titanate nanobelt composites

The organic–inorganic hybrid material of polyaniline/hydrogen-titanate nanobelt (PANI/H-TNB) composites was fabricated as a potential adsorbent to remove U(vi) from wastewater.

The mechanism of uranium transformation from U(VI) into nano-uramphite by two indigenous Bacillus thuringiensis strains

The mechanism of uranium transformation from U(VI) into nano-uramphite by two indigenous Bacillus thuringiensis strains was investigated in the present work. Our data showed that the bacteria isolated from uranium mine possessed highly accumulation ability to U(VI), and the maximum accumulation capacity was around 400 mg U/g biomass (dry weight). X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) analyzes indicated that the U(VI) was adsorbed on the bacterial surface firstly through coordinating with phosphate, CH2 and amide groups, and then needle-like amorphous uranium compounds were formed. With the extension of time, the extracellular crystalline substances were disappeared, but some particles were appeared in the intracellular region, and these particles were characterized as tetragonal-uramphite. Moreover, the disrupted experiment indicated that the cell-free extracts had better uranium-immobilization ability than cell debris. Our findings provided the understanding of the uranium transformation process from amorphous uranium to crystalline uramphite, which would be useful in the regulation of uranium immobilization process.

Mechanism of Uranium Reduction and Immobilization in Desulfovibrio vulgaris Biofilms

The prevalent formation of noncrystalline U(IV) species in the subsurface and their enhanced susceptibility to reoxidation and remobilization, as compared to crystalline uraninite, raise concerns about the long-term sustainability of the bioremediation of U-contaminated sites. The main goal of this study was to resolve the remaining uncertainty concerning the formation mechanism of noncrystalline U(IV) in the environment. Controlled laboratory biofilm systems (biotic, abiotic, and mixed biotic-abiotic) were probed using a combination of U isotope fractionation and X-ray absorption spectroscopy (XAS). Regardless of the mechanism of U reduction, the presence of a biofilm resulted in the formation of noncrystalline U(IV). Our results also show that biotic U reduction is the most effective way to immobilize and reduce U. However, the mixed biotic-abiotic system resembled more closely an abiotic system: (i) the U(IV) solid phase lacked a typically biotic isotope signature and (ii) elemental sulfur was detected, which indicates the oxidation of sulfide coupled to U(VI) reduction. The predominance of abiotic U reduction in our systems is due to the lack of available aqueous U(VI) species for direct enzymatic reduction. In contrast, in cases where bicarbonate is present at a higher concentration, aqueous U(VI) species dominate, allowing biotic U reduction to outcompete the abiotic processes.

Dynamic Succession of Groundwater Functional Microbial Communities in Response to Emulsified Vegetable Oil Amendment during Sustained In Situ U(VI) Reduction

A pilot-scale field experiment demonstrated that a one-time amendment of emulsified vegetable oil (EVO) reduced groundwater U(VI) concentrations for 1 year in a fast-flowing aquifer. However, little is known about how EVO amendment stimulates the functional gene composition, structure, and dynamics of groundwater microbial communities toward prolonged U(VI) reduction. In this study, we hypothesized that EVO amendment would shift the functional gene composition and structure of groundwater microbial communities and stimulate key functional genes/groups involved in EVO biodegradation and reduction of electron acceptors in the aquifer. To test these hypotheses, groundwater microbial communities after EVO amendment were analyzed using a comprehensive functional gene microarray. Our results showed that EVO amendment stimulated sequential shifts in the functional composition and structure of groundwater microbial communities. Particularly, the relative abundance of key functional genes/groups involved in EVO biodegradation and the reduction of NO3 (-), Mn(IV), Fe(III), U(VI), and SO4 (2-) significantly increased, especially during the active U(VI) reduction period. The relative abundance for some of these key functional genes/groups remained elevated over 9 months. Montel tests suggested that the dynamics in the abundance, composition, and structure of these key functional genes/groups were significantly correlated with groundwater concentrations of acetate, NO3 (-), Mn(II), Fe(II), U(VI), and SO4 (2-). Our results suggest that EVO amendment stimulated dynamic succession of key functional microbial communities. This study improves our understanding of the composition, structure, and function changes needed for groundwater microbial communities to sustain a long-term U(VI) reduction.

Bioreduction of vanadium (V) in groundwater by autohydrogentrophic bacteria: Mechanisms and microorganisms

As one of the transition metals, vanadium (V) (V(V)) in trace amounts represents an essential element for normal cell growth, but becomes toxic when its concentration is above 1mg/L. V(V) can alter cellular differentiation, gene expression, and other biochemical and metabolic phenomena. A feasible method to detoxify V(V) is to reduce it to V(IV), which precipitates and can be readily removed from the water. The bioreduction of V(V) in a contaminated groundwater was investigated using autohydrogentrophic bacteria and hydrogen gas as the electron donor. Compared with the previous organic donors, H2 shows the advantages as an ideal electron donor, including nontoxicity and less production of excess biomass. V(V) was 95.5% removed by biochemical reduction when autohydrogentrophic bacteria and hydrogen were both present, and the reduced V(IV) precipitated, leading to total-V removal. Reduction kinetics could be described by a first-order model and were sensitive to pH and temperature, with the optimum ranges of pH7.5-8.0 and 35-40°C, respectively. Phylogenetic analysis by clone library showed that the dominant species in the experiments with V(V) bioreduction belonged to the β-Proteobacteria. Previously known V(V)-reducing species were absent, suggesting that V(V) reduction was carried out by novel species. Their selective enrichment during V(V) bioreduction suggests that Rhodocyclus, a denitrifying bacterium, and Clostridium, a fermenter known to carry out metal reduction, were responsible for V(V) bioreduction.

Exploring actinide materials through synchrotron radiation techniques

Synchrotron radiation (SR) based techniques have been utilized with increasing frequency in the past decade to explore the brilliant and challenging sciences of actinide-based materials. This trend is partially driven by the basic needs for multi-scale actinide speciation and bonding information and also the realistic needs for nuclear energy research. In this review, recent research progresses on actinide related materials by means of various SR techniques were selectively highlighted and summarized, with the emphasis on X-ray absorption spectroscopy, X-ray diffraction and scattering spectroscopy, which are powerful tools to characterize actinide materials. In addition, advanced SR techniques for exploring future advanced nuclear fuel cycles dealing with actinides are illustrated as well.

Phosphate-Mediated Remediation of Metals and Radionuclides

Worldwide industrialization activities create vast amounts of organic and inorganic waste streams that frequently result in significant soil and groundwater contamination. Metals and radionuclides are of particular concern due to their mobility and long-term persistence in aquatic and terrestrial environments. As the global population increases, the demand for safe, contaminant-free soil and groundwater will increase as will the need for effective and inexpensive remediation strategies. Remediation strategies that include physical and chemical methods (i.e., abiotic) or biological activities have been shown to impede the migration of radionuclide and metal contaminants within soil and groundwater. However, abiotic remediation methods are often too costly owing to the quantities and volumes of soils and/or groundwater requiring treatment. The in situ sequestration of metals and radionuclides mediated by biological activities associated with microbial phosphorus metabolism is a promising and less costly addition to our existing remediation methods. This review highlights the current strategies for abiotic and microbial phosphate-mediated techniques for uranium and metal remediation.

Uranium reduction by Shewanella oneidensis MR-1 as a function of NaHCO3 concentration: surface complexation control of reduction kinetics

It is crucial to determine the controls on the kinetics of U(VI) bioreduction in order to understand and model the fate and mobility of U in groundwater systems and also to enhance the effectiveness of U bioremediation strategies. In this study, we measured the rate of U(VI) reduction by Shewanella oneidensis strain MR-1 as function of NaHCO3 concentration. The experiments demonstrate that increasing concentrations of NaHCO3 in the system lead to slower U(VI) reduction kinetics. The NaHCO3 concentration also strongly affects the speciation of U(VI) on the bacterial cell envelope. We used a thermodynamic surface complexation modeling approach to determine the speciation and concentration of U(VI) adsorbed onto the bacteria as a function of the NaHCO3 concentration in the experimental systems. We observed a strong positive correlation between the measured U(VI) reduction rates and the calculated total concentration of U(VI) surface complexes formed on the bacterial cell envelope. This positive correlation indicates that the speciation and concentration of U(VI) adsorbed on the bacterial cell envelope control the kinetics of U(VI) bioreduction under the experimental conditions. The results of this study serve as a basis for developing speciation-based kinetic rate laws for enzymatic reduction of U(VI) by bacteria.

Toxicity of ionic liquids to Clostridium sp. and effects on uranium biosorption

As green solvents ionic liquids (ILs) show high potential in nuclear industry for extraction and purification of actinides. However, to date relatively little information has been gained on ILs application in microbial processes, for example biosorption of radionuclides. We investigated the effects of three ILs, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6), N-ethylpyridinium trifluoroacetate (EtPyCF3COO) and N-ethylpyridinium tetrafluoroborate (EtPyBF4) on the growth and biosorption of uranium by Clostridium sp. The ILs affected the growth of the bacterium as evidenced by decreases in optical density, total gas production, and organic acids production from glucose metabolism. The IC50-48h of three ILs decreased in the order of BMIMPF6 (8.26mM)>EtPyBF4 (7.04mM)>EtPyCF3COO (4.05mM). Uranium biosorption by the bacterial cells decreased by 75% in the presence of 1% (v/v) BMIMPF6 and by about 90% with 1% (v/v) EtPyBF4 or EtPyCF3COO, in comparison to the control without ILs. The diminished biosorption may be attributed to the membrane damages induced by EtPyBF4 and EtPyCF3COO, which can be visualized by Transmission Electron Microscope (TEM) analysis. Energy-dispersive X-ray spectroscopy (EDS) analysis revealed the accumulation of uranium inside peripheral membrane of the cells exposed to uranium alone or with BMIMPF6, while little or no accumulation was observed in the presence of EtPyBF4 and EtPyCF3COO. These results imply that potential toxicity of ILs towards microorganisms is a particularly important issue in limiting its biotechnological applications.

Fermentation and hydrogen metabolism affect uranium reduction by clostridia

Previously, it has been shown that not only is uranium reduction under fermentation condition common among clostridia species, but also the strains differed in the extent of their capability and the pH of the culture significantly affected uranium(VI) reduction. In this study, using HPLC and GC techniques, metabolic properties of those clostridial strains active in uranium reduction under fermentation conditions have been characterized and their effects on capability variance of uranium reduction discussed. Then, the relationship between hydrogen metabolism and uranium reduction has been further explored and the important role played by hydrogenase in uranium(VI) and iron(III) reduction by clostridia demonstrated. When hydrogen was provided as the headspace gas, uranium(VI) reduction occurred in the presence of whole cells of clostridia. This is in contrast to that of nitrogen as the headspace gas. Without clostridia cells, hydrogen alone could not result in uranium(VI) reduction. In alignment with this observation, it was also found that either copper(II) addition or iron depletion in the medium could compromise uranium reduction by clostridia. In the end, a comprehensive model was proposed to explain uranium reduction by clostridia and its relationship to the overall metabolism especially hydrogen (H2) production.

Uranium interaction with two multi-resistant environmental bacteria: Cupriavidus metallidurans CH34 and Rhodopseudomonas palustris

Depending on speciation, U environmental contamination may be spread through the environment or inversely restrained to a limited area. Induction of U precipitation via biogenic or non-biogenic processes would reduce the dissemination of U contamination. To this aim U oxidation/reduction processes triggered by bacteria are presently intensively studied. Using X-ray absorption analysis, we describe in the present article the ability of Cupriavidus metallidurans CH34 and Rhodopseudomonas palustris, highly resistant to a variety of metals and metalloids or to organic pollutants, to withstand high concentrations of U and to immobilize it either through biosorption or through reduction to non-uraninite U(IV)-phosphate or U(IV)-carboxylate compounds. These bacterial strains are thus good candidates for U bioremediation strategies, particularly in the context of multi-pollutant or mixed-waste contaminations.

Linking bacterial diversity and geochemistry of uranium-contaminated groundwater

To understand the link between bacterial diversity and geochemistry in uranium-contaminated groundwater, microbial communities were assessed based on clone libraries of 16S rDNA genes from the USDOE Oak Ridge Field Research Centre (FRC) site. Four groundwater wells (GW835, GW836, FW113-47 and FW215-49) with a wide range of pH (3 to 7), nitrate (44 to 23,400 mg L(-1)), uranium (0.73 to 60.36 mg L(-1)) and other metal contamination, were investigated. Results indicated that bacterial diversity correlated with the geochemistry of the groundwater. Microbial diversity decreased in relation to the contamination levels of the wells. The highly contaminated well (FW113-47) had lower gene diversity than less contaminated wells (FW215-49, GW835 and GW836). The high concentrations of contaminants present in well FW113-47 stimulated the growth of organisms capable of reducing uranium (Shewanella and Pseudomonas), nitrate (Pseudomonas, Rhodanobacter and Xanthomonas) and iron (Stenotrophomonas), and which were unique to this well. The clone libraries consisted primarily of sequences closely related to the phylum Proteobacteria, with FW-113-47 almost exclusively containing this phylum. Metal-reducing bacteria were present in all four wells, which may suggest that there is potential for successful bioremediation of the groundwater at the Oak Ridge FRC. The microbial community information gained from this study and previous studies at the site can be used to develop predictive multivariate and geographical information system (GIS) based models for microbial populations at the Oak Ridge FRC. This will allow for a better understanding of what organisms are likely to occur where and when, based on geochemistry, and how these organisms relate to bioremediation processes at the site.

Evaluation of positron emission tomography as a method to visualize subsurface microbial processes

Positron emission tomography (PET) provides spatiotemporal monitoring in a nondestructive manner and has higher sensitivity and resolution relative to other tomographic methods. Therefore, this technology was evaluated for its application to monitor in situ subsurface bacterial activity. To date, however, it has not been used to monitor or image soil microbial processes. In this study, PET imaging was applied as a "proof-of-principle" method to assess the feasibility of visualizing a radiotracer labeled subsurface bacterial strain (Rahnella sp. Y9602), previously isolated from uranium contaminated soils and shown to promote uranium phosphate precipitation. Soil columns packed with acid-purified simulated mineral soils were seeded with 2-deoxy-2-[(18)F]fluoro-D-glucose ((18)FDG) labeled Rahnella sp. Y9602. The applicability of [(18)F]fluoride ion as a tracer for measuring hydraulic conductivity and (18)FDG as a tracer to identify subsurface metabolically active bacteria was successful in our soil column studies. Our findings indicate that positron-emitting isotopes can be utilized for studies aimed at elucidating subsurface microbiology and geochemical processes important in contaminant remediation.

Microbial community succession during lactate amendment and electron acceptor limitation reveals a predominance of metal-reducing Pelosinus spp

The determination of the success of in situ bioremediation strategies is complex. By using controlled laboratory conditions, the influence of individual variables, such as U(VI), Cr(VI), and electron donors and acceptors on community structure, dynamics, and the metal-reducing potential can be studied. Triplicate anaerobic, continuous-flow reactors were inoculated with Cr(VI)-contaminated groundwater from the Hanford, WA, 100-H area, amended with lactate, and incubated for 95 days to obtain stable, enriched communities. The reactors were kept anaerobic with N(2) gas (9 ml/min) flushing the headspace and were fed a defined medium amended with 30 mM lactate and 0.05 mM sulfate with a 48-h generation time. The resultant diversity decreased from 63 genera within 12 phyla to 11 bacterial genera (from 3 phyla) and 2 archaeal genera (from 1 phylum). Final communities were dominated by Pelosinus spp. and to a lesser degree, Acetobacterium spp., with low levels of other organisms, including methanogens. Four new strains of Pelosinus were isolated, with 3 strains being capable of Cr(VI) reduction while one also reduced U(VI). Under limited sulfate, it appeared that the sulfate reducers, including Desulfovibrio spp., were outcompeted. These results suggest that during times of electron acceptor limitation in situ, organisms such as Pelosinus spp. may outcompete the more-well-studied organisms while maintaining overall metal reduction rates and extents. Finally, lab-scale simulations can test new strategies on a smaller scale while facilitating community member isolation, so that a deeper understanding of community metabolism can be revealed.

Influence of uranium on bacterial communities: a comparison of natural uranium-rich soils with controls

This study investigated the influence of uranium on the indigenous bacterial community structure in natural soils with high uranium content. Radioactive soil samples exhibiting 0.26% - 25.5% U in mass were analyzed and compared with nearby control soils containing trace uranium. EXAFS and XRD analyses of soils revealed the presence of U(VI) and uranium-phosphate mineral phases, identified as sabugalite and meta-autunite. A comparative analysis of bacterial community fingerprints using denaturing gradient gel electrophoresis (DGGE) revealed the presence of a complex population in both control and uranium-rich samples. However, bacterial communities inhabiting uraniferous soils exhibited specific fingerprints that were remarkably stable over time, in contrast to populations from nearby control samples. Representatives of Acidobacteria, Proteobacteria, and seven others phyla were detected in DGGE bands specific to uraniferous samples. In particular, sequences related to iron-reducing bacteria such as Geobacter and Geothrix were identified concomitantly with iron-oxidizing species such as Gallionella and Sideroxydans. All together, our results demonstrate that uranium exerts a permanent high pressure on soil bacterial communities and suggest the existence of a uranium redox cycle mediated by bacteria in the soil.

Palladium nanoparticles produced by fermentatively cultivated bacteria as catalyst for diatrizoate removal with biogenic hydrogen

A new biological inspired method to produce nanopalladium is the precipitation of Pd on a bacterium, i.e., bio-Pd. This bio-Pd can be applied as catalyst in dehalogenation reactions. However, large amounts of hydrogen are required as electron donor in these reactions resulting in considerable costs. This study demonstrates how bacteria, cultivated under fermentative conditions, can be used to reductively precipitate bio-Pd catalysts and generate the electron donor hydrogen. In this way, one could avoid the costs coupled to hydrogen supply. The catalytic activities of Pd(0) nanoparticles produced by different strains of bacteria (bio-Pd) cultivated under fermentative conditions were compared in terms of their ability to dehalogenate the recalcitrant aqueous pollutants diatrizoate and trichloroethylene. While all of the fermentative bio-Pd preparations followed first order kinetics in the dehalogenation of diatrizoate, the catalytic activity differed systematically according to hydrogen production and starting Pd(II) concentration in solution. Batch reactors with nanoparticles formed by Citrobacter braakii showed the highest diatrizoate dehalogenation activity with first order constants of 0.45 ± 0.02 h⁻¹ and 5.58 ± 0.6 h⁻¹ in batches with initial concentrations of 10 and 50 mg L⁻¹ Pd, respectively. Nanoparticles on C. braakii, used in a membrane bioreactor treating influent containing 20 mg L⁻¹ diatrizoate, were capable of dehalogenating 22 mg diatrizoate mg⁻¹ Pd over a period of 19 days before bio-Pd catalytic activity was exhausted. This study demonstrates the possibility to use the combination of Pd(II), a carbon source and bacteria under fermentative conditions for the abatement of environmental halogenated contaminants.

Dynamics of microbial community composition and function during in situ bioremediation of a uranium-contaminated aquifer

A pilot-scale system was established to examine the feasibility of in situ U(VI) immobilization at a highly contaminated aquifer (U.S. DOE Integrated Field Research Challenge site, Oak Ridge, TN). Ethanol was injected intermittently as an electron donor to stimulate microbial U(VI) reduction, and U(VI) concentrations fell to below the Environmental Protection Agency drinking water standard (0.03 mg liter(-1)). Microbial communities from three monitoring wells were examined during active U(VI) reduction and maintenance phases with GeoChip, a high-density, comprehensive functional gene array. The overall microbial community structure exhibited a considerable shift over the remediation phases examined. GeoChip-based analysis revealed that Fe(III)-reducing bacterial (FeRB), nitrate-reducing bacterial (NRB), and sulfate-reducing bacterial (SRB) functional populations reached their highest levels during the active U(VI) reduction phase (days 137 to 370), in which denitrification and Fe(III) and sulfate reduction occurred sequentially. A gradual decrease in these functional populations occurred when reduction reactions stabilized, suggesting that these functional populations could play an important role in both active U(VI) reduction and maintenance of the stability of reduced U(IV). These results suggest that addition of electron donors stimulated the microbial community to create biogeochemical conditions favorable to U(VI) reduction and prevent the reduced U(IV) from reoxidation and that functional FeRB, SRB, and NRB populations within this system played key roles in this process.

Fate of Neptunium in an Anaerobic, Ethanogenic Microcosm

Neptunium is found predominantly as Np(IV) in reducing environments, but as Np(V) in aerobic environments. Currently, it is not known how the interplay between biotic and abiotic processes affects Np redox speciation in the environment. To evaluate the effect of anaerobic microbial activity on the fate of Np in natural systems, Np(V) was added to a microcosm inoculated with anaerobic sediments from a metal-contaminated freshwater lake. The consortium included metal-reducing, sulfate-reducing, and methanogenic microorganisms, and acetate was supplied as the only exogenous substrate. Addition of more than 10−5M Np did not inhibit methane production. Total Np solubility in the active microcosm, as well as in sterilized control samples, decreased by nearly two orders of magnitude. A combination of analytical techniques, including VIS-NIR absorption spectroscopy and XANES, identified Np(IV) as the oxidation state associated with the sediments. The similar results from the active microcosm and the abiotic controls suggest that microbially produced Mn(II/III) and Fe(II) may serve as electron donors for Np reduction.

Non-uraninite products of microbial U(VI) reduction

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

Mercury and other heavy metals influence bacterial community structure in contaminated Tennessee streams

High concentrations of uranium, inorganic mercury [Hg(II)], and methylmercury (MeHg) have been detected in streams located in the Department of Energy reservation in Oak Ridge, TN. To determine the potential effects of the surface water contamination on the microbial community composition, surface stream sediments were collected 7 times during the year, from 5 contaminated locations and 1 control stream. Fifty-nine samples were analyzed for bacterial community composition and geochemistry. Community characterization was based on GS 454 FLX pyrosequencing with 235 Mb of 16S rRNA gene sequence targeting the V4 region. Sorting and filtering of the raw reads resulted in 588,699 high-quality sequences with lengths of >200 bp. The bacterial community consisted of 23 phyla, including Proteobacteria (ranging from 22.9 to 58.5% per sample), Cyanobacteria (0.2 to 32.0%), Acidobacteria (1.6 to 30.6%), Verrucomicrobia (3.4 to 31.0%), and unclassified bacteria. Redundancy analysis indicated no significant differences in the bacterial community structure between midchannel and near-bank samples. Significant correlations were found between the bacterial community and seasonal as well as geochemical factors. Furthermore, several community members within the Proteobacteria group that includes sulfate-reducing bacteria and within the Verrucomicrobia group appeared to be associated positively with Hg and MeHg. This study is the first to indicate an influence of MeHg on the in situ microbial community and suggests possible roles of these bacteria in the Hg/MeHg cycle.

Concomitant microbial generation of palladium nanoparticles and hydrogen to immobilize chromate

The catalytic properties of various metal nanoparticles have led to their use in environmental remediation. Our aim is to develop and apply an efficient bioremediation method based on in situ biosynthesis of bio-Pd nanoparticles and hydrogen. C. pasteurianum BC1 was used to reduce Pd(II) ions to form Pd nanoparticles (bio-Pd) that primarily precipitated on the cell wall and in the cytoplasm. C. pasteurianum BC1 cells, loaded with bio-Pd nanoparticle in the presence of glucose, were subsequently used to fermentatively produce hydrogen and to effectively catalyze the removal of soluble Cr(VI) via reductive transformation to insoluble Cr(III) species. Batch and aquifer microcosm experiments using C. pasteurianum BC1 cells loaded with bio-Pd showed efficient reductive Cr(VI) removal, while in control experiments with killed or viable but Pd-free bacterial cultures no reductive Cr(VI) removal was observed. Our results suggest a novel process where the in situ microbial production of hydrogen is directly coupled to the catalytic bio-Pd mediated reduction of chromate. This process offers significant advantages over the current groundwater treatment technologies that rely on introducing preformed catalytic nanoparticles into groundwater treatment zones and the costly addition of molecular hydrogen to above ground pump and treat systems.