Uranium Phosphate Research Articles

Comprehensive Characterization of the Electronic Structure of U(4+) in Uranium(IV) Phosphate Chloride.

Emerald-green single crystals of U(PO4)Cl were grown by chemical vapor transport in a temperature gradient (1000 → 900 °C). The crystal structure of U(PO4)Cl (Cmcm, Z = 4, a = 5.2289(7) Å, b = 11.709(2) Å, c = 6.9991(8) Å) consists of a three-dimensional network of [PO4] tetrahedra and bicapped octahedral [U(IV)O6Cl2] groups. Polarized absorption spectra measured for two perpendicular polarization directions show a large number of well-resolved electronic transitions. These transitions can be fully assigned on the basis of a detailed ligand-field treatment within the framework of the angular overlap model. The magnetic behavior predicted on the basis of the spectroscopic data is in agreement with an f (2) system and perfectly matched by the results of temperature-dependent susceptibility measurements.

Screening of bacterial strains isolated from uranium mill tailings porewaters for bioremediation purposes

The present work characterizes at different levels a number of bacterial strains isolated from porewaters sampled in the vicinity of two French uranium tailing repositories. The 16S rRNA gene from 33 bacterial isolates, corresponding to the different morphotypes recovered, was almost fully sequenced. The resulting sequences belonged to 13 bacterial genera comprised in the phyla Firmicutes, Actinobacteria and Proteobacteria. Further characterization at physiological level and metals/metalloid tolerance provided evidences for an appropriate selection of bacterial strains potentially useful for immobilization of uranium and other common contaminants. By using High Resolution Transmission Electron Microscope (HRTEM), this potential ability to immobilize uranium as U phosphate mineral phases was confirmed for the bacterial strains Br3 and Br5 corresponding to Arthrobacter sp. and Microbacterium oxydans, respectively. Scanning Transmission Electron Microscope- High-Angle Annular Dark-Field (STEM-HAADF) analysis showed U accumulates on the surface and within bacterial cytoplasm, in addition to the extracellular space. Energy Dispersive X-ray (EDX) element-distribution maps demonstrated the presence of U and P within these accumulates. These results indicate the potential of certain bacterial strains isolated from porewaters of U mill tailings for immobilizing uranium, likely as uranium phosphates. Some of these bacterial isolates might be considered as promising candidates in the design of uranium bioremediation strategies.

Uranium bioprecipitation mediated by yeasts utilizing organic phosphorus substrates.

In this research, we have demonstrated the ability of several yeast species to mediate U(VI) biomineralization through uranium phosphate biomineral formation when utilizing an organic source of phosphorus (glycerol 2-phosphate disodium salt hydrate (C3H7Na2O6P·xH2O (G2P)) or phytic acid sodium salt hydrate (C6H18O24P6·xNa(+)·yH2O (PyA))) in the presence of soluble UO2(NO3)2. The formation of meta-ankoleite (K2(UO2)2(PO4)2·6(H2O)), chernikovite ((H3O)2(UO2)2(PO4)2·6(H2O)), bassetite (Fe(++)(UO2)2(PO4)2·8(H2O)), and uramphite ((NH4)(UO2)(PO4)·3(H2O)) on cell surfaces was confirmed by X-ray diffraction in yeasts grown in a defined liquid medium amended with uranium and an organic phosphorus source, as well as in yeasts pre-grown in organic phosphorus-containing media and then subsequently exposed to UO2(NO3)2. The resulting minerals depended on the yeast species as well as physico-chemical conditions. The results obtained in this study demonstrate that phosphatase-mediated uranium biomineralization can occur in yeasts supplied with an organic phosphate substrate as sole source of phosphorus. Further understanding of yeast interactions with uranium may be relevant to development of potential treatment methods for uranium waste and utilization of organic phosphate sources and for prediction of microbial impacts on the fate of uranium in the environment.

Recovery of uranium phosphate by a stepwise thermal treatment of uranium-bearing spent TBP

A stepwise thermal treatment process for the recovery of uranium phosphate from uranium-dissolved spent TBP was demonstrated. The pathway of the reactions involved in the thermal decomposition and oxidation processes of uranium-bearing spent TBP was established based on the results of thermogravimetric analyses. Relatively low-temperature pyrolysis is required to avoid the condensation of corrosive phosphoric acid via vaporization. Low-temperature pyrolysis residue was analyzed and found to be composed of pyrocarbon, phosphorus oxide (P2O5) and two types of uranium phosphate (UP2O7 and UP4O12). Uranium pyrophosphate (UP2O7) was recovered from the burning out of pyrocarbon in the pyrolysis residue after the dissolution removal of phosphorus oxide in water. A substantial recovery of uranium by the proposed stepwise thermal treatment method was successfully demonstrated by a treatment of pyrolysis residue from a bench-scale low-temperature pyrolysis process.

Favourable uranium–phosphate exploration trends guided by the application of statistical factor analysis technique on the aerial gamma spectrometric data in Syrian desert (Area-1), Syria

A scored lithological map including 10 radiometric units is established through applying factor analysis approach to aerial spectrometric data of Area-1, Syrian desert, which includes Ur, eU, eTh, K%, eU/eTh, eU/K%, and eTh/K%. A model of four rotated factors F1, F2, F3, and F4 is adapted for representing 234,829 data measured points in Area-1, where 86% of total data variance is interpreted. A geological scored pseudo-section derived from the lithological scored map is established and analyzed in order to show the possible stratigraphic and structural traps for uranium occurrences associated with phosphate deposits in the studied Area-1. These identified traps presented in this paper need detailed investigation and must be necessarily followed and checked by ground validations and subsurface well logging, in order to locate the anomalous uranium occurrences and explore with more confidence and certitude their characteristics as a function of depth.

Removel of Uranium from Aqueous Solutions by Spirodela Punctata as the Mechanism of Biomineralization

Click This study evaluates the feasibility of biosorption of uranium from aqueous solutions using living and dried S. punctata. The uranium maximum removal efficiency for the plant cultivar occurrs in the soluation at pH 4-5 and the uranium removal efficiency exceeds 90%. In the kinetics studies, the dried powder of duckweed can reach nearly 80% adsorption within 5min, and the batch adsorption equilibrium can be reached within 24h for the living and dried powder of duckweed. Both for the living and dried powder of duckweed, the experimental data were fitted by the pseudo-second-order rate model with the degree of fitting (R2) higher than 0.99. The uranium was deposited onto the root surface of living S. punctata as 100-500 nanometer size schistose structures that consist of primarily U (82.5%,wt%) and P(8.76%,wt %), and no carbon; while no similar uranium phosphate minerals were found on the dried powder of S. punctata, as revealed by scanning electron microscope and energy dispersive spectrometer analysis. The results suggest that there is one particularly promising biosportion processes, namely, biomineralization. And the root surface of the living S. punctata plays an important role in the uranium phosphate precipitate processes, not only play as a carrier of the functional groups but also as a substrate inducing the nucleation of uranium-phosphate nanoparticles. The dried powder of S. punctata adsorption of U is mainly through the effect of electrostatic attraction, ion exchange, and complexation coordination.

Biostimulation by Glycerol Phosphate to Precipitate Recalcitrant Uranium(IV) Phosphate.

Stimulating the microbial reduction of aqueous uranium(VI) to insoluble U(IV) via electron donor addition has been proposed as a strategy to remediate uranium-contaminated groundwater in situ. However, concerns have been raised regarding the longevity of microbially precipitated U(IV) in the subsurface, particularly given that it may become remobilized if the conditions change to become oxidizing. An alternative mechanism is to stimulate the precipitation of poorly soluble uranium phosphates via the addition of an organophosphate and promote the development of reducing conditions. Here, we selected a sediment sample from a U.K. nuclear site and stimulated the microbial community with glycerol phosphate under anaerobic conditions to assess whether uranium phosphate precipitation was a viable bioremediation strategy. Results showed that U(VI) was rapidly removed from solution and precipitated as a reduced crystalline U(IV) phosphate mineral similar to ningyoite. This mineral was considerably more recalcitrant to oxidative remobilization than the products of microbial U(VI) reduction. Bacteria closely related to Pelosinus species may have played a key role in uranium removal in these experiments. This work has implications for the stewardship of uranium-contaminated groundwater, with the formation of U(IV) phosphates potentially offering a more effective strategy for maintaining low concentrations of uranium in groundwater over long time periods.

Dynamic interplay between uranyl phosphate precipitation, sorption, and phase evolution

Natural examples demonstrate uranyl-phosphate minerals can maintain extremely low levels of aqueous uranium in groundwaters due to their low solubility. Therefore, greater understanding of the geochemical factors leading to uranyl phosphate precipitation may lead to successful application of phosphate-based remediation methods. However, the solubility of uranyl phosphate phases varies over >3 orders of magnitude, with the most soluble phases typically observed in lab experiments. To understand the role of common soil/sediment mineral surfaces in the nucleation and transformation of uranyl phosphate minerals under environmentally relevant conditions, batch experiments were carried out with goethite and mica at pH 6 in mixed electrolyte solutions ranging from 1–800μM U and 1–800μM P. All experiments ended with uranium concentrations below the USEPA MCL for U, but with 2–3 orders of magnitude difference in uranium concentrations. Despite the presence of many cations that are well known to incorporate into less soluble autunite-group minerals, chernikovite rapidly precipitated in all experiments containing U and P, except for solutions with 1μM U and 1μM P that were calculated to be undersaturated. Textures of uranyl phosphates observed by AFM and TEM indicate that nucleation was homogenous and independent of the initial mineral content. Comparison of time-course U and P concentrations from the experiments with thermodynamic modeling of solution equilibria demonstrated that aqueous uranium concentrations in the experimental systems evolved as increasingly sparingly soluble uranyl phosphate phases nucleated over time, with sorption accelerating the transition between phases by influencing solution chemistry. Aqueous uranium concentrations consistent with partially dehydrated (meta-) autunite were achieved only in experiments containing goethite and/or mica. These dynamic nucleation-growth-sorption-nucleation-growth-sorption cycles occur over the time scales of weeks, not hours or days at room temperature. Lab experiments and field-based investigations of uranium phosphate should consider these or longer time scales for the greatest long-term relevance.

Uranium phosphate biomineralization by fungi

Geoactive soil fungi were investigated for phosphatase-mediated uranium precipitation during growth on an organic phosphorus source. Aspergillus niger and Paecilomyces javanicus were grown on modified Czapek-Dox medium amended with glycerol 2-phosphate (G2P) as sole P source and uranium nitrate. Both organisms showed reduced growth on uranium-containing media but were able to extensively precipitate uranium and phosphorus-containing minerals on hyphal surfaces, and these were identified by X-ray powder diffraction as uranyl phosphate species, including potassium uranyl phosphate hydrate (KPUO6 .3H2 O), meta-ankoleite [(K1.7 Ba0.2 )(UO2 )2 (PO4 )2 .6H2 O], uranyl phosphate hydrate [(UO2 )3 (PO4 )2 .4H2 O], meta-ankoleite (K(UO2 )(PO4 ).3H2 O), uramphite (NH4 UO2 PO4 .3H2 O) and chernikovite [(H3 O)2 (UO2 )2 (PO4 )2 .6H2 O]. Some minerals with a morphology similar to bacterial hydrogen uranyl phosphate were detected on A. niger biomass. Geochemical modelling confirmed the complexity of uranium speciation, and the presence of meta-ankoleite, uramphite and uranyl phosphate hydrate between pH 3 and 8 closely matched the experimental data, with potassium as the dominant cation. We have therefore demonstrated that fungi can precipitate U-containing phosphate biominerals when grown with an organic source of P, with the hyphal matrix serving to localize the resultant uranium minerals. The findings throw further light on potential fungal roles in U and P biogeochemistry as well as the application of these mechanisms for element recovery or bioremediation.

Sonochemical precipitation of amorphous uranium phosphates from trialkyl phosphate solutions and their thermal conversion to UP2O7

Insoluble amorphous precipitates containing uranyl and phosphate ions are obtained by sonication of solutions of three uranyl precursors, UO2(X)2, X=NO3, CH3COO, CH3C(O)CHC(O)CH3 (acetylacetonate, acac), in triesters of phosphoric acid, OP(OR)3, R=Me (trimethyl phosphate, TMP), Et (triethyl phosphate, TEP). TMP and TEP are used as high-boiling solvents and they serve also as a source of phosphate anions. Sonolysis experiments were carried out under flow of Ar at 40°C on a Sonics and Materials VXC 500W system (f=20kHz, Pac=0.49Wcm−3). Powder X-ray diffraction (PXRD) reveals amorphous character of all obtained precipitates. The presence of uranyl and phosphate is evidenced by IR spectroscopy and ICP-OES analysis reveals the content of both U (38.6–43.4wt%) and P (11.0–13.6wt%). The thermal behavior of the substances was studied by TG/DSC analysis, which shows weight losses in the range of 19.21–24.08%. On heating the amorphous precipitates to 1000°C, crystalline uranium diphosphate UP2O7 is obtained in all cases as the only crystalline phase. Uranyl(VI) is reduced during thermolysis to U(IV) as there is no characteristic vibration of UO22+ in the IR spectra of solid UP2O7 products. The ICP-OES analysis of U and P content in precipitates allowed us to calculate the efficiency of precipitation of uranium from mother liquor and to compare it with the efficiency calculated from the data received by the PXRD and TG/DSC analyses. The efficiency of the uranium removal attained by our sonoprecipitation procedure was typically 30–35%. These sonochemical precipitation reactions providing insoluble uranium phosphates may be potentially interesting models for the description of behavior of uranium-containing waste or reprocessing streams.

Microbial community responses to organophosphate substrate additions in contaminated subsurface sediments.

BackgroundRadionuclide- and heavy metal-contaminated subsurface sediments remain a legacy of Cold War nuclear weapons research and recent nuclear power plant failures. Within such contaminated sediments, remediation activities are necessary to mitigate groundwater contamination. A promising approach makes use of extant microbial communities capable of hydrolyzing organophosphate substrates to promote mineralization of soluble contaminants within deep subsurface environments.Methodology/Principal FindingsUranium-contaminated sediments from the U.S. Department of Energy Oak Ridge Field Research Center (ORFRC) Area 2 site were used in slurry experiments to identify microbial communities involved in hydrolysis of 10 mM organophosphate amendments [i.e., glycerol-2-phosphate (G2P) or glycerol-3-phosphate (G3P)] in synthetic groundwater at pH 5.5 and pH 6.8. Following 36 day (G2P) and 20 day (G3P) amended treatments, maximum phosphate (PO4 3−) concentrations of 4.8 mM and 8.9 mM were measured, respectively. Use of the PhyloChip 16S rRNA microarray identified 2,120 archaeal and bacterial taxa representing 46 phyla, 66 classes, 110 orders, and 186 families among all treatments. Measures of archaeal and bacterial richness were lowest under G2P (pH 5.5) treatments and greatest with G3P (pH 6.8) treatments. Members of the phyla Crenarchaeota, Euryarchaeota, Bacteroidetes, and Proteobacteria demonstrated the greatest enrichment in response to organophosphate amendments and the OTUs that increased in relative abundance by 2-fold or greater accounted for 9%–50% and 3%–17% of total detected Archaea and Bacteria, respectively.Conclusions/SignificanceThis work provided a characterization of the distinct ORFRC subsurface microbial communities that contributed to increased concentrations of extracellular phosphate via hydrolysis of organophosphate substrate amendments. Within subsurface environments that are not ideal for reductive precipitation of uranium, strategies that harness microbial phosphate metabolism to promote uranium phosphate precipitation could offer an alternative approach for in situ sequestration.

High-Temperature, High-Pressure Hydrothermal Synthesis, Crystal Structures, and Spectroscopic Studies of a Uranium(IV) Phosphate (Na10U2P6O24) and the Isotypic Cerium(IV) Phosphate (Na10Ce2P6O24)

A uranium(IV) phosphate, Na10U2P6O24, was synthesized under hydrothermal conditions at 570 °C and 160 MPa and structurally characterized by single-crystal X-ray diffraction. The valence state of uranium was established by UV-vis and U 4f X-ray photoelectron spectroscopy. The powder sample has a second-harmonic-generation signal, confirming the absence of a center of symmetry in the structure. The structure contains UO8 snub-disphenoidal polyhedra that are linked to monophosphate tetrahedra by vertex and edge sharing such that a three-dimensional framework with intersecting 12-sided circular and rectangular channels is formed. All 10 sodium sites are situated inside the channels and are fully occupied. This is the first uranium(IV) phosphate synthesized under high-temperature, high-pressure hydrothermal conditions. The isotypic cerium(IV) phosphate, Na10Ce2P6O24, was also synthesized under the same hydrothermal conditions. It is the first structurally characterized Ce(IV) phosphate with a P/Ce ratio of 3. Crystal data of Na10U2P6O24: orthorhombic, P212121 (No. 19), a = 6.9289(3) Å, b = 16.1850(7) Å, c = 18.7285(7) Å, V = 2100.3(2) Å(3), Z = 4, R1 = 0.0304, and wR2 = 0.0522. Crystal data of Na10Ce2P6O24: orthorhombic, P2(1)2(1)2(1) (No. 19), a = 6.9375(14) Å, b = 16.215(3) Å, c = 18.765(4) Å, V = 2111.0(7) Å(3), Z = 4, R1 = 0.0202, and wR2 = 0.0529.

Structural changes within the alkaline earth uranyl phosphites

Three new alkaline earth (AE) uranyl phosphites and one barium uranium(IV) phosphate were synthesized under hydrothermal conditions. The carbonate salts of the AE's were employed both as cation sources and as pH regulators. Despite having very similar formulas and uranyl building units, the Ca(2+), Sr(2+) and Ba(2+) uranyl phosphites have three different extended networks. The calcium compound also contains a Ca(2+)/UO2(2+) mixed cation position. The four structure types will be presented herein with the AE cations generating distinct structural transformations.

Aerobic uranium immobilization by Rhodanobacter A2-61 through formation of intracellular uranium–phosphate complexes

Severe environmental problems arise from old uranium mines, which continue to discharge uranium (U) via acid mine drainage water, resulting in soil, subsoil and groundwater contamination. Bioremediation of U contaminated environments has been attempted, but most of the conceptual models propose U removal by cell suspensions of anaerobic bacteria. In this study, strain Rhodanobacter A2-61, isolated from Urgeiriça Mine, Portugal, was shown to resist up to 2 mM of U(vi). The conditions used (low nutrient content and pH 5) potentiated the interaction of the toxic uranyl ion with the tested strain. The strain was able to remove approximately 120 μM of U(vi) when grown aerobically in the presence of 500 μM U. Under these conditions, this strain was also able to lower the phosphate concentration in the medium and increased its capacity to take up inorganic phosphate, accumulating up to 0.52 μmol phosphate per optical density unit of the medium at 600 nm, after 24 hours, corresponding approximately to the late log phase of the bacterial culture. Microscopically dense intracellular structures with nanometer size were visible. The extent of U inside the cells was quantified by LS counting. EDS analysis of heated cells showed the presence of complexes composed of phosphate and uranium, suggesting the simultaneous precipitation of U and phosphate within the cells. XRD analysis of the cells containing the U-phosphate complexes suggested the presence of a meta-autunite-like mineral structure. SEM identified, in pyrolyzed cells, crystalline nanoparticles with shape in the tetragonal system characteristic of the meta-autunite-like mineral structures. U removal has been reported previously but mainly by cell suspensions and through release of phosphate. The innovative Rhodanobacter A2-61 can actively grow aerobically, in the presence of U, and can efficiently remove U(vi) from the environment, accumulating it in a structural form consistent with that of the mineral meta-autunite inside the cell, corresponding to effective metal immobilization. This work supports previous findings that U bioremediation could be achieved via the biomineralization of U(vi) in phosphate minerals.

Bioleaching of apatite rich low grade Indian uranium ore

Uranium ore from Narwapahar Mines, UCIL contains 0·047% U3O8 with some refractory minerals and high apatite (5%) results in a maximum 78% recovery through conventional processing at UCIL, with a fairly high consumption of sulphuric acid and pyrolusite, and loss of uranium as uranium phosphate. To avoid usage of non-ecofriendly oxidants, obviate the influence of phosphate and improve the overall process output of uranium, an alternate extraction technology using microbial isolate(s) is elucidated in this study. A. ferrooxidans isolated from Narwapahar mine water was used in bioleaching of uranium from this apatite rich low grade uraninite ore. Optimum uranium biorecovery of 96% is achieved at 10% pulp density (w/v), pH 1·7 and 35°C in 40 days with the fine particles of <45 μm size. Under the optimum condition at pH 1·7, rise in redox potential is recorded to be 594–708 mV in 40 days. Bioleaching of uranium seems to follow the indirect mechanism of leaching with the involvement of Fe(III) biogenically generated by Acidithiobacillus ferrooxidans (A. ferrooxidans). Uranium recovery was also examined using another mesophilic isolate of Leptospirillum ferrooxidans (L. ferrooxidans) which showed 98% uranium leaching at 40°C, which shows the possibility of improving the kinetics of the process. The high R2 values in the temperature range (298–308 K) indicated uranium dissolution by the chemical reaction occurring at the ore surface with Fe(III) generated biogenically, with Ea value of 28·3 kJ mol−1. The mechanism of uranium bioleaching is also elucidated with X-ray diffraction phase identification of the leach residues with time, followed by observing the surface morphology through SEM at varying temperatures.Le minerai d’uranium des Mines de Narwapahar, d’UCIL, contient 0·047% d’U3O8 avec quelques minéraux réfractaires et une teneur élevée en apatite (5%). On note une récupération maximale de 78% par traitement conventionnel à UCIL, avec une consommation relativement élevée d’acide sulfurique et de pyrolusite, ainsi qu’une perte d’uranium sous forme de phosphate d’uranium. Afin d’éviter l’utilisation d’agents oxydants non écologiques, de prévenir l’influence du phosphate, et d’améliorer la production globale d’uranium du procédé, dans cette étude on examine une autre technologie d’extraction utilisant un (des) isolat(s) microbien(s). On a utilisé A. ferrooxidans, isolée de l’eau de mine de Narwapahar, pour la biolixiviation de l’uranium de ce minerai pauvre en uraninite et riche en apatite. La bio récupération optimale de l’uranium de 96% est obtenue à 10% PD (poids/volume) au pH de 1·7 et à 35°C en 40 jours avec les particules fines d’une taille <45 μm. Sous la condition optimale d’un pH de 1·7, on a enregistré l’augmentation du potentiel rédox à 594–708 mV en 40 jours. La biolixiviation de l’uranium semble suivre le mécanisme indirect de lixiviation avec l’implication de Fe(III) engendré bio génétiquement par A. ferrooxidans. On a également examiné la récupération de l’uranium en utilisant un autre isolat mésophile de L. ferrooxidans, qui a montré une lixiviation de 98% de l’uranium à 40°C, ce qui montre la possibilité d’améliorer la cinétique du procédé. Les valeurs élevées de R2 dans la gamme de température (298–308 K) indiquaient que la dissolution de l’uranium par réaction chimique se produisait à la surface du minerai avec Fe(III) engendré bio génétiquement, à une valeur de Ea de 28·3 kJ mol−1. Le mécanisme de biolixiviation de l’uranium est également examiné avec l’identification de phase par XRD des résidus de lixiviat en fonction du temps, suivi par l’observation de la morphologie de la surface au moyen du SEM, à des températures variées.

Copper uranyl phosphate and arsenate incorporating an organic ligand with a pillared layer structure: [Cu(4,4′-bpy)(UO2)0.5(HPO4)(H2PO4)]·H2O and [Cu(4,4′-bpy)(UO2)0.5(HAsO4)(H2AsO4)]·1.5H2O

Two mixed-metal uranium compounds, [Cu(4,4′-bpy)(UO2)0.5(HPO4)(H2PO4)]·H2O (1) and [Cu(4,4′-bpy)(UO2)0.5(HAsO4)(H2AsO4)]·1.5H2O (2) have been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction, fluorescence spectroscopy, and magnetic susceptibility. They are the first examples of mixed-metal uranium phosphate and arsenate incorporating an organic ligand. Their structures contain copper uranyl phosphate/arsenate layers which are covalently linked by 4,4′-bpy pillars to form a 3-D framework structure. The fluorescence spectrum of 1 shows the characteristic vibronic structure of the UO22+ moiety despite the presence of copper(II) ions in its structure. The two compounds are isostructural and crystallize in the monoclinic space group C2/c with a=20.184(4)Å, b=8.921(2)Å, c=19.095(3)Å, β=115.15(1)°, and R1=0.0244 for 1, and a=20.184(1)Å, b=9.0210(5)Å, c=19.714(1)Å, β=114.879(1)°, and R1=0.0399 for 2.

Systematic Evolution from Uranyl(VI) Phosphites to Uranium(IV) Phosphates

Six new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds can contain U(VI) or U(IV), and two are mixed-valent U(VI)/U(IV) compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from U(VI) to U(IV) happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.

Complete Genome Sequence of Rahnella sp. Strain Y9602, a Gammaproteobacterium Isolate from Metal- and Radionuclide-Contaminated Soil

Rahnella sp. strain Y9602 is a gammaproteobacterium isolated from contaminated subsurface soils that is capable of promoting uranium phosphate mineralization as a result of constitutive phosphatase activity. Here we report the first complete genome sequence of an isolate belonging to the genus Rahnella.

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-[18F]fluoro-d-glucose (18FDG) labeled Rahnella sp. Y9602. The applicability of [18F]fluoride ion as a tracer for measuring hydraulic conductivity and 18FDG 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.

Bio-precipitation of uranium by two bacterial isolates recovered from extreme environments as estimated by potentiometric titration, TEM and X-ray absorption spectroscopic analyses

This work describes the mechanisms of uranium biomineralization at acidic conditions by Bacillus sphaericus JG-7B and Sphingomonas sp. S15-S1 both recovered from extreme environments. The U-bacterial interaction experiments were performed at low pH values (2.0-4.5) where the uranium aqueous speciation is dominated by highly mobile uranyl ions. X-ray absorption spectroscopy (XAS) showed that the cells of the studied strains precipitated uranium at pH 3.0 and 4.5 as a uranium phosphate mineral phase belonging to the meta-autunite group. Transmission electron microscopic (TEM) analyses showed strain-specific localization of the uranium precipitates. In the case of B. sphaericus JG-7B, the U(VI) precipitate was bound to the cell wall. Whereas for Sphingomonas sp. S15-S1, the U(VI) precipitates were observed both on the cell surface and intracellularly. The observed U(VI) biomineralization was associated with the activity of indigenous acid phosphatase detected at these pH values in the absence of an organic phosphate substrate. The biomineralization of uranium was not observed at pH 2.0, and U(VI) formed complexes with organophosphate ligands from the cells. This study increases the number of bacterial strains that have been demonstrated to precipitate uranium phosphates at acidic conditions via the activity of acid phosphatase.