Uranyl Phosphate Minerals Research Articles

Comparison of the kinetic rate law parameters for the dissolution of natural and synthetic autunite in the presence of aqueous bicarbonate ions

This research evaluated the effect of aqueous hydrogen carbonate solutions on the uranium rate of release from natural Ca-autunite and quantified the process kinetic rate law for a better prediction of the stability of autunite-group minerals. Testing was accomplished via a single-pass flow-through (SPFT) apparatus using buffered aqueous bicarbonate solutions (0.0005 to 0.003M) at temperatures of 23–90°C and pH values of 7–11. The release rate of uranium from Ca-autunite was directly correlated to increasing concentrations of hydrogen carbonate solutions and showed strong pH dependency. Ca-autunite kinetic rate law parameters were compared to the values obtained for synthetic Na-autunite. The power law coefficient and intrinsic rate constant were higher at pH9–11 for Ca-autunite than for Na-autunite. The lower stability of Ca-autunite was attributed to the high Ca-autunite surface cracking, fractures and basal plane cleavages as compared to Na-autunite and the combined effect of the formation of aqueous uranyl–carbonate and calcium uranyl carbonate species as a driving force for uranium(VI) detachment and the formation of secondary Ca–P hydroxyapatite and uranyl phosphate mineral phases as a driving force for phosphate and calcium detachment controlling the net release of elements.

Raman spectroscopic characterization of a synthetic, non-stoichiometric Cu–Ba uranyl phosphate

Crystals of phases belonging to the autunite group (general formula X2+(UO2)2(X5+O4)2⋅nH2O), specifically the uranyl phosphates (X5+=P) metauranocircite (X2+=Ba2+), metatorbernite (X2+=Cu2+) and a barian metatorbenite phase (X2+=Cu2+/Ba2+), have been synthesized in a silica gel medium and characterized by Raman spectroscopy. The Raman spectra showed bands in the range 750–1100cm−1, which were attributed to the ν1 and ν3(PO4)3- and (UO2)2+ stretching vibrations. By using the wavenumbers of the most intense and well defined ν1(UO2)2+ vibration, the U–O bonds lengths were calculated for the three uranyl phosphate minerals. The results are in good agreement with previous single crystal structure analysis data. Bands in the spectra from 350 to 700cm−1 were attributed to the (PO4)3- bending modes. Moreover, in the range 70–350cm−1, two groups of bands could be defined. The first group, with vibrations at lower wavenumbers, was attributed to the lattice modes and the second group, from 150 to 350cm−1, was assigned to the ν2(UO2)2+ bending mode. Finally, in the case of the barian metatorbernite, bands in the range 1500–3800cm−1 were assigned to the OH stretching and the ν2 bending vibrations of water molecules. In this phase, all the vibrations show bandshifts when compared to the vibrations in metatorbernite. These bandshifts can be related to transitional Cu–O and Ba–O bond lengths.

Mineral replacement reactions in naturally occurring hydrated uranyl phosphates from the Tarabau deposit: Examples in the Cu–Ba uranyl phosphate system

Uranyl phosphates are a mineral group which include a wide range of different species, each containing specific cations within the hydrated interlayer, and often display a geochemical/mineralogical relationship with Fe(III) oxy-hydroxides. The environmental relevance of these U-phases arises from their low solubility at most surface and groundwater conditions, where they can ultimately control aqueous U levels.In the present work, samples of naturally occurring uranyl phosphates from the Tarabau site, included in the Nisa deposit, located in central-eastern Portugal, are studied with X-ray diffraction (XRD), Electron Microprobe (EMP) and Scanning Electron Microscopy, with the purpose of i) identifying uranyl phosphate mineral paragenesis, ii) assessing chemical homogeneity and stoichiometry of the most relevant phases and iii) unraveling possible textural features of mineral reequilibration processes. XRD studies revealed that the analyzed samples comprehend metatorbernite-like structures, consistent with Cu(UO2)2(PO4)2·8H2O formula. Further EMP determinations allowed the definition of nearly stoichiometric Cu and Ba hydrated uranyl phosphates; CuxBa1−x(UO2)2(PO4)2·nH2O intermediate compositions and interlayer cation-depleted phases. The obtained results, combined with textural observations, allowed us to decipher mineral reequilibration reactions affecting the studied samples. Thus, reactive paths involving the replacement of Cu-bearing by Ba-bearing uranyl phosphates, cation-bearing uranyl phosphate by cation-depleted uranyl phosphate and cation-bearing uranyl phosphate by Fe, Al oxy-hydroxides have been defined. However, the studied textural features point toward two different mechanisms of mineral replacement, with superimposed expressions. On one hand, the replacement of Cu by Ba uranyl phosphate phases, and these last by oxy-hydroxides, takes place by coupled dissolution–precipitation reactions. On the other hand, cation depletion affecting uranyl phosphates occurs by a cation exchange process, possibly giving rise to increasing mineral porosity.

The effect of pH and natural microbial phosphatase activity on the speciation of uranium in subsurface soils

The biomineralization of U(VI) phosphate as a result of microbial phosphatase activity is a promising new bioremediation approach to immobilize uranium in both aerobic and anaerobic conditions. In contrast to reduced uranium minerals such as uraninite, uranium phosphate precipitates are not susceptible to changes in oxidation conditions and may represent a long-term sink for uranium in contaminated environments. So far, the biomineralization of U(VI) phosphate has been demonstrated with pure cultures only. In this study, two uranium contaminated soils from the Department of Energy Oak Ridge Field Research Center (ORFRC) were amended with glycerol phosphate as model organophosphate source in small flow-through columns under aerobic conditions to determine whether natural phosphatase activity of indigenous soil bacteria was able to promote the precipitation of uranium(VI) at pH 5.5 and 7.0. High concentrations of phosphate (1–3 mM) were detected in the effluent of these columns at both pH compared to control columns amended with U(VI) only, suggesting that phosphatase-liberating microorganisms were readily stimulated by the organophosphate substrate. Net phosphate production rates were higher in the low pH soil (0.73 ± 0.17 mM d −1) compared to the circumneutral pH soil (0.43 ± 0.31 mM d −1), suggesting that non-specific acid phosphatase activity was expressed constitutively in these soils. A sequential solid-phase extraction scheme and X-ray absorption spectroscopy measurements were combined to demonstrate that U(VI) was primarily precipitated as uranyl phosphate minerals at low pH, whereas it was mainly adsorbed to iron oxides and partially precipitated as uranyl phosphate at circumneutral pH. These findings suggest that, in the presence of organophosphates, microbial phosphatase activity can contribute to uranium immobilization in both low and circumneutral pH soils through the formation of stable uranyl phosphate minerals.

Evidence for multiple modes of uranium immobilization by an anaerobic bacterium

Microbial reduction of hexavalent uranium has been studied widely for its potential role in bioremediation and immobilization of soluble U(VI) in contaminated groundwater. More recently, some microorganisms have been examined for their role in immobilization of U(VI) via precipitation of uranyl phosphate minerals mediated by microbial phosphate release, alleviating the requirement for long-term redox control. Here, we investigated the mechanism of U(VI) removal mediated by an environmental isolate, strain UFO1, that is indigenous to the Field Research Center (FRC) in Oak Ridge, TN and has been detected in U(VI)-contaminated sediments. Changes in U(VI) speciation were examined in the presence and absence of the electron-shuttling moiety, anthraquinone-2,6-disulfonate (AQDS). Cell suspensions were capable of nearly complete removal of 100 μM U(VI) from solution within 48 h; U(VI) removal was not dependent on the presence of an exogenous electron donor or AQDS, although AQDS increased the rate of U(VI) removal. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic measurements indicated that U(IV) was the predominant oxidation state of uranium in cell suspensions in both the absence and presence of 100 μM AQDS. Interestingly, 17% of the cell-associated precipitates in a U(VI)-treated suspension that lacked AQDS had spectral characteristics consistent with a uranyl phosphate solid phase. The potential involvement of phosphate was consistent with observed increases in soluble phosphate concentrations over time in UFO1 cell suspensions, which suggested phosphate liberation from the cells. TEM-EDS confirmed the presence of uranyl phosphate with a U:P ratio consistent with autunite (1:1). EXAFS analyses further suggested that U(IV) was bound to low-Z neighbors such as C or P, inferred to be present as functional groups on biomass. These results suggest that strain UFO1 has the ability to facilitate U(VI) removal from solution via reductive and phosphate precipitation mechanisms. Both mechanisms offer potential for the remediation of U-contaminated sediments at the FRC or elsewhere.

Hydrochemical characteristics of shallow groundwater in aquifer containing uranyl phosphate minerals, in the Köprübaşı (Manisa) area, Turkey

The concentrations of uranium, iron and the major constituents were determined in groundwater samples from aquifer containing uranyl phosphate minerals (meta-autunite, meta-torbernite and torbernite) in the Koprubasi area. Groundwater samples from wells located at shallow depths (0.5–6 m) show usually near neutral pH values (6.2–7.1) and oxidizing conditions (Eh = 119–275 mV). Electrical conductivity (EC) values of samples are between 87 and 329 μS/cm−1. They are mostly characterized by mixed cationic Ca dominating bicarbonate types. The main hydrogeochemical process is weathering of the silicates in the shallow groundwater system. All groundwater in the study area are considered undersaturated with respect to torbernite and autunite. PHREEQC predicted UO2(HPO4)22− as the unique species. The excellent positive correlation coefficient (r = 0.99) between U and PO4 indicates the dissolved uranium in groundwater would be associated with the dissolution of uranyl phosphate minerals. The groundwater show U content in the range 1.71–70.45 μg/l but they are mostly lower than US EPA (2003) maximum contaminant level of 30 μg/l. This low U concentrations in oxic groundwater samples is attributed to the low solubility of U(VI) phosphate minerals under near neutral pH and low bicarbonate conditions. Iron closely associated with studied sediments, were also detected in groundwater. The maximum concentration of Fe in groundwater samples was 2837 μg/l, while the drinking water guidelines of Turkish (TSE 1997) and US EPA (2003) were suggested 200 and 300 μg/l, respectively. Furthermore, iron and uranium showed a significant correlation to each other with a correlation coefficient (r) of 0.94. This high correlation is probably related to the iron-rich sediments which contain also significant amounts of uranium mineralization. In addition to pH and bicarbonate controlling dissolution of uranyl phosphates, association of uranyl phosphates with iron (hydr) oxides seems to play important role in the amount of dissolved U in shallow groundwater.

A cryogenic fluorescence spectroscopic study of uranyl carbonate, phosphate and oxyhydroxide minerals

Abstract In this work we applied time-resolved laser-induced fluorescence spectroscopy (TRLIF) at both room temperature (RT) and near liquid-helium temperature (6 K) to characterize a series of natural and synthetic minerals of uranium carbonate, phosphate and oxyhydroxides including rutherfordine, zellerite, liebigite, phosphuranylite, meta-autunite, meta-torbernite, uranyl phosphate, sodium-uranyl-phosphate, becquerelite, schoepite, meta-schoepite, dehydrated schoepite and compreignacite, and have compared the spectral characteristics among these minerals as well as our previously published data on uranyl silicates. For the carbonate minerals, the fluorescence spectra of rutherfordine showed significant difference from those of zellerite and liebigite. The fluorescence spectra of the phosphate minerals closely resemble each other despite the differences in their composition and structure. For all uranium oxyhydroxides, the fluorescence spectra are largely red-shifted as compared to those of the uranium carbonates and phosphates and their vibronic bands are broad and less resolved at RT. The enhanced spectra resolution at 6 K allows more accurate determination of the fluorescence band origin and offers a complemental method to measure the O=U=O symmetrical stretch frequency, ν 1, from the spacings of the vibronic bands of the fluorescence spectra. The average ν 1 values appear to be inversely correlated with the average pK a values of the anions.

Lakebogaite, CaNaFe23+H(UO2)2(PO4)4(OH)2(H2O)8, a new uranyl phosphate with a unique crystal structure from Victoria, Australia

Lakebogaite, ideally CaNaFe23+H(UO2)2(PO4)4(OH)2(H2O)8, is a new Ca-Na-Fe uranyl phosphate mineral from a quarry in Upper Devonian granite near Lake Boga, northern Victoria, Australia. It is associated with Na-analogue of meurigite (IMA 2007-024), torbernite, and saléeite on a matrix of microcline, albite, smoky quartz, and muscovite. Lakebogaite occurs as bright lemon-yellow transparent prismatic crystals up to 0.4 mm across. The crystals have a vitreous luster and a pale yellow streak. Mohs hardness is about 3. The fracture is even to conchoidal. In transmitted light, the mineral is pale yellow with very weak pleochroism: X = yellow, Y = grayish yellow, Z = grayish yellow: dispersion r > v, strong. Lakebogaite crystals are biaxial (+), with slightly variable refractive indices within the ranges: nα = 1.650(2)-1.652(2), nβ = 1.660(4)-1.664(3), nγ = 1.681(3)-1.686(2), measured using white light, and with 2Vmeas = 80-85° and 2Vcalc = 70-74°. Orientation: Y = b; crystals are elongated along [010], resulting in straight extinction. The empirical chemical formula (mean of nine electron microprobe analyses) calculated on the basis of 30 anions is (Ca1.00Na0.80Sr0.10)Σ1.90(Fe3+1.85Al0.30)Σ2.15(UO2)1.80 (PO4)4.07(OH,H2O)10.12. Lakebogaite is monoclinic, space group Cc, a = 19.6441(5), b = 7.0958(2), c = 18.7029(5) Å, β = 115.692(1)°, V = 2349.3(7) Å3, Z = 4. The seven strongest reflections in the powder X-ray diffraction pattern are [dobs in Å (I) (hkl)]: 6.60 (100) (110), 3.16 (40) (514̅, 604̅), 4.07 (20) (404̅), 3.80 (20) (314̅), 3.56 (20) (020, 312), 3.31 (20) (114, 220), 2.797 (20) (006). The crystal structure was solved from single-crystal X-ray diffraction data and refined to R1 = 0.038 on the basis of 5222 unique reflections with F > 4σF. It comprises pairs of edge-shared UO7 pentagonal bipyramids that are inter-linked via corner-sharing with PO4 tetrahedra, to form chains parallel to the c-axis. Each UO7 polyhedron also shares one of its edges with another PO4 tetrahedron. The (UO2)2(PO4)4 chains are cross-linked via corner-sharing between the PO4 tetrahedra and Fe3+O4(OH)2 octahedra. The octahedra join together by corner-sharing via OH anions to form chains parallel to b. The Na+ and Ca2+ cations, and 4 water molecules occupy eight-sided channels along [010]. The remaining water molecules occupy large ten-sided channels directed along [001] and intersecting the [010] channels. The mineral is named for the nearest township.

Raman spectroscopic study of the uranyl phosphate minerals phosphuranylite and yingjiangite

AbstractRaman spectra of phosphuranylite and yingjiangite were measured and interpreted, and compared with the published infrared spectra of both minerals. UO bond lengths were calculated using the Bartlett–Cooney empirical relations and the OHċO hydrogen bond lengths were inferred on the basis of Libowitzky's empirical relation. The presence of oxonium and (H3O)+ ions, expected from the single crystal structure analysis of phosphuranylite, was not inferred from the Raman spectra. It was assumed that phosphuranylite and yingjiangite are identical and the name yingjiangite should be discarded because the name phosphuranylite has priority. Copyright © 2007 John Wiley & Sons, Ltd.

Dissolution kinetics of synthetic and natural meta‐autunite minerals, X3−n(n)+ [(UO2)(PO4)]2 · xH2O, under acidic conditions

Mass transport within the uranium geochemical cycle is impacted by the availability of phosphorous. In oxidizing environments, in which the uranyl (UO22+) ionic species is typically mobile, formation of sparingly soluble uranyl phosphate minerals exerts a strong influence on uranium transport. Autunite group minerals, X3 − n(n)+ [(UO2)(PO4)]2 · xH2O, have been identified as the long‐term uranium‐controlling phases in many systems of geochemical interest. Anthropogenic operations related to uranium mining operations have created acidic environments exposing uranyl phosphate minerals to low‐pH groundwaters. Investigations regarding the dissolution behavior of autunite group minerals under acidic conditions have not been reported; consequently, knowledge of the longevity of uranium‐controlling solids is incomplete. The purpose of this investigation was threefold: (1) to quantify the dissolution kinetics of natural calcium meta‐autunite, Ca[(UO2)2(PO4)2] · 3H2O, and synthetic sodium meta‐autunite, Na2[(UO2)2(PO4)2] · 3H2O, under acidic conditions; (2) to measure the effect of temperature and pH on meta‐autunite mineral dissolution; and (3) to investigate the formation of secondary uranyl phosphate phases as long‐term controls on uranium migration. Single‐pass flow‐through (SPFT) dissolution tests were conducted over the pH range of 2 to 5 and from 5° to 70°C. Results presented here illustrate meta‐autunite dissolution kinetics are strongly dependent on pH but are relatively insensitive to temperature variations. In addition, the formation of secondary uranyl‐phosphate phases such as uranyl phosphate, (UO2)3(PO4)2 · x H2O, may serve as a secondary phase limiting the migration of uranium in the environment.

Fungal transformations of uranium oxides

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.

Trace element immobilization by uranyl minerals in granite-hosted uranium ores: Evidences from the Xiazhuang ore field of Guangdong province, China

The oxidative alteration of uraninite and the fate of trace elements (Y, LREE, Zr, and Th) in a granite-hosted uranium ore deposit in north Guangdong province, China, were investigated to understand the geochemical behavior of spent UO2fuel and associated fission products and transuranium elements under oxidizing conditions. In light of the paragenetic relationship of the alteration products, two alteration series of uraninite were identified: one is the silicate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → Si-rich uranyl phase → uranophane (Ca[(UO2)(SiO3OH)]2(H2O)5), and the other is the phosphate series with a mineral paragenesis of uraninite → uranyl oxide hydrates → autunite (Ca[(UO2)(PO4)]2(H2O)6) → yingjiangite ((K2,Ca)(UO2)7(PO4)4(OH)6(H2O)6). In contrast to the wide distribution and abundance of uranophane and the uranyl phosphate minerals, uranyl oxides were only occasionally found in the ore samples, suggesting that the uranyl silicates and phosphates should be the predominant alteration products of UO2under oxidizing conditions, although uranyl oxide hydrates would be the solubility-limiting phase of uranium in the very early stage of alteration. Furthermore, the interlayer cation of the uranyl phases in the Xiazhuang uranium ore field is dominated by Ca2+, indicating that the release of uranium and other radionuclides will be limited mainly by uranophane and autunite during the oxidative alteration of spent UO2fuel in underground repositories where enhanced calcium concentration is expected due to cement/water reactions.Compositionally, the cation atomic ratios in uranyl phases often deviate considerably from their respective stoichiometric values as indicated by the nominal formulae, but the compositional variation does not result in significant structural change as indicated by X-ray diffraction patterns. This observation indicates that the structure of U6+minerals may easily be adjusted to accommodate impurity elements including crystallographically compatible radionuclides. Compared with the primary uraninites, the secondary minerals are slightly enriched in light REE, but significantly depleted in Y3+probably due to its cation-radius mismatch with interlayer Ca2+. The apparent enrichment of Zr4+in uranophane and uranyl phosphates relative to uraninite may result from the coupled substitutions: Zr4+↔ U6+and REE3+↔ Ca2+(K+). Because an adequate charge-balance mechanism and significant distortion of the coordination polyhedra are required for the substitution An4+↔ U6+, this type of substitution may not be common.

The reactivity of uranyl phosphate mineral phases via microbial weathering: Short vs. long term kinetics

The reactivity of uranyl phosphate mineral phases via microbial weathering: Short vs. long term kinetics

A Raman spectroscopic study of the uranyl phosphate mineral bergenite

Raman spectroscopy at 298 and 77 K of bergenite has been used to characterise this uranyl phosphate mineral. Bands at 995, 971 and 961 cm −1 (298 K) and 1006, 996, 971, 960 and 948 cm −1 (77 K) are assigned to the ν 1(PO 4) 3− symmetric stretching vibration. Three bands at 1059, 1107 and 1152 cm −1 (298 K) and 1061, 1114 and 1164 cm −1 (77 K) are attributed to the ν 3(PO 4) 3− antisymmetric stretching vibrations. Two bands at 810 and 798 cm −1 (298 K) and 812 and 800 cm −1 (77 K) are attributed to the ν 1 symmetric stretching vibration of the (UO 2) 2+ units. Bands at 860 cm −1 (298 K) and 866 cm −1 (77 K) are assigned to the ν 3 antisymmetric stretching vibrations of the (UO 2) 2+ units. U O bond lengths in uranyls, calculated using the wavenumbers of the ν 1 and ν 3(UO 2) 2+ vibrations with empirical relations by Bartlett and Cooney, are in agreement with the X-ray single crystal structure data. Bands at (444, 432, 408 cm −1) (298 K), and (446, 434, 410 and 393 cm −1) (77 K) are assigned to the split doubly degenerate ν 2(PO 4) 3− in-plane bending vibrations. The band at 547 cm −1 (298 K) and 549 cm −1 (77 K) are attributed to the ν 4(PO 4) 3− out-of-plane bending vibrations. Raman bands at 3607, 3459, 3295 and 2944 cm −1 are attributed to water stretching vibrations and enable the calculation of hydrogen bond distances of >3.2, 2.847, 2.740 and 2.637 Å. These bands prove the presence of structurally nonequivalent hydrogen bonded water molecules in the structure of bergenite.

A Raman spectroscopic study of the uranyl phosphate mineral parsonsite

AbstractThe mineral parsonsite, with samples from The Ranger Uranium Mine, Australia, and the La Faye Mine, Grury, Saone‐et‐Loire, Burgundy, France, has been characterised by Raman spectroscopy at 298 and 77 K and complemented with infrared spectroscopy. Two Raman bands close to 807 and 796 cm−1 are attributed to the ν1(UO2)2+ symmetric stretching modes, while two bands close to 953 or 945 cm−1 and 863–873 cm−1 are assigned to the ν3(UO2)2+ anti‐symmetric stretching vibrations. Four or five bands (953, 926, 910, 883 cm−1) are observed in the infrared spectrum in this region. Bands at 965–967 and 972 cm−1 are assigned to the ν1(PO4)3− symmetric stretching modes and bands that are observed in the 987 to 1078 cm−1region to the ν3(PO4)3− anti‐symmetric stretching modes. Bands at 465, 439, 406, 394 cm−1 (298 K) and 466, 442, 405, 395 cm−1(77 K) are assigned to the split, doubly degenerate ν2(PO4)−3bending vibrations. Bands of very low intensity at 609, 595, 591, 582, 560 and 540 cm−1are attributed to the split, triply degenerate ν4(PO4)−3bending modes. Bands observed at wavenumbers lower than 300 cm−1are connected with the split ν2(δ) (UO2)2+ bending, ν(UOligand), δ(UOligand) and lattice vibrations. UO bond lengths in uranyl were calculated from the Raman and infrared spectra, which are in agreement with those from the available X‐ray single crystal structure analysis of parsonsite. A short comment is given on the water content and the possibility of a hydrogen‐bonding network in the parsonsite crystal structure. Copyright © 2006 John Wiley & Sons, Ltd.

A Raman spectroscopic study of the uranyl phosphate mineral threadgoldite

Raman spectra of threadgoldite at 298 and 77K are measured and interpreted for the first time. Bands related the (UO2)2+ and (PO4)3− stretching and bending vibrations are tenatively attributed together with the bands assigned to the stretching a and bending vibrations of water molecules and hydroxyls. Hydrogen-bonding network and H2O and (OH)−1 libration modes are mentioned. U–O bond lengths in uranyls are calculated via empirical relations RU–O=f[ν1 and ν3(UO2)2+]Å. They are comparable to the values inferred from the single crystal structure analysis of threadgoldite.

Uranium Sequestration by Aluminum Phosphate Minerals in Unsaturated Soils

AbstractA mineralogical and geochemical study of soils developed from the unmined Coles Hill uranium deposit (Virginia) was undertaken to determine how phosphorous influences the speciation of uranium in an oxidizing soil/saprolite system typical of the eastern US. This paper presents mineralogical and geochemical results that identify and quantify the processes by which uranium has been sequestered in these soils. It was found that uranium is not leached from the saturated soil zone (saprolites) overlying the deposit due to the formation of a sparingly soluble uranyl phosphate mineral of the meta-autunite group. The concentration of uranium in the saprolites is approximately 1000 mg uranium per kg of saprolite. It was also found that a significant amount of uranium was retained in the unsaturated soil zone overlying uranium-rich saprolites. The uranium concentration in the unsaturated soils is approximately 200 mg uranium per kg of soil (20 times higher than uranium concentrations in similar soils adjacent to the deposit). Mineralogical evidence indicates that uranium in this zone is sequestered by a barium-strontium-calcium aluminum phosphate mineral of the crandallite group (gorceixite). This mineral is intimately inter-grown with iron and manganese oxides that also contain uranium. The amount of uranium associated with both the aluminum phosphates (as much as 1.4 weight percent) has been measured by electron microprobe microanalyses and the geochemical conditions under which these minerals formed has been studied using thermodynamic reaction path modeling. The geochemical data and modeling results suggests the meta-autunite group minerals present in the saprolites overlying the deposit are unstable in the unsaturated zone soils overlying the deposit due to a decrease in soil pH (down to a pH of 4.5) at depths less than 5 meters below the surface. Mineralogical observations suggest that, once exposed to the unsaturated environment, the meta-autunite group minerals react to form U(VI)-bearing aluminum phosphates.

Dehydration processes in the meta-autunite group minerals meta-autunite, metasaleeite, and metatorbernite

We investigated dehydration processes in uranyl phosphate minerals of the meta-autunite group that consist of uranyl phosphate sheets and interlayer cations, and water molecules. Meta-autunite [Ca(UO 2 PO 4 ) 2 ⋅6H 2 O], metasaléeite [Mg(UO 2 PO 4 ) 2 ⋅8H 2 O], and metatorbernite [Cu(UO 2 PO 4 )2⋅8H 2 O] were selected for our study. The change in basal spacing between two adjacent uranyl phosphate sheets with temperature was examined by temperature-controlled X-ray diffraction (XRD) analysis from room temperature (RT) to 300 °C to determine structurally distinct, dehydrated phases. Thermogravimetric and differential thermal analyses (TG/DTA) were also performed under conditions similar to those used for the XRD analysis to clarify the hydration states of the dehydrated phases. Retention of the structure of the uranyl phosphate sheets under a high vacuum, equivalent to 300 °C, was confirmed by transmission electron microscopy. Meta-autunite, metasaléeite, and metatorbernite decreased their basal spacings by losing water molecules. Comparison of the TG/DTA and XRD results indicates that the changes in basal spacings of the dehydrated phases with temperature are as follows: 8.32 Å (6 H2O per unit formula) at RT, 7.31 (? H 2 O) and 6.68 Å (? H 2 O) at 75 °C, 6.34 Å (2 H 2 O) at 120°C, and 5.81 Å (1 H 2 O) at 300°C for meta-autunite; 8.29 (8 H 2 O) and 7.73 Å (? H 2 O) at RT, 6.62 Å (probably, 2 H 2 O) at 40 °C, 6.54 Å (2 H 2 O) at 160 °C, and 5.52 Å (1 H 2 O) at 300 °C for metasaléeite; and 8.61 Å (8 H 2 O) at RT, 8.07 Å (4 H 2 O) at 100 °C, 6.58 Å (2 H 2 O) at 200 °C, and 5.60 Å (1 H 2 O) at 300 °C for metatorbernite. The dehydration processes revealed by XRD and TG/DTA under similar experimental conditions are slightly different from those obtained by previous studies. Our results clearly demonstrate the presence of previously unknown dehydrated phases of the meta-autunite group minerals with basal spacings less than 6Å that may have distinct thermodynamic properties

An Unprecedented Uranyl Phosphate Framework in the Structure of [(UO2)3(PO4)O(OH) (H2O)2](H2O).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Resistance to, and Accumulation of, Uranium by Bacteria from a Uranium-Contaminated Site

Heterotrophic bacteria were isolated from an acidic uranium-contaminated site. Five isolates and one reference strain, Deinococcus radiodurans, were studied for uranium accumulation and resistance to uranium. Cells were incubated in a pH 4 solution containing 80 ppm of U(VI) in the form of (UO2)2+ for 1 hr. Three isolates closely related to high G+C gram-positive bacteria were resistant to (UO2)2 +. One of the high G+C Gram-positive isolates, closely related to Arthrobacter ilicis, accumulated uranium intracellularly as precipitates closely associated with polyphosphate granules. It is interpreted that sequestration of uranium into polyphosphate is a detoxification mechanism. Deinococcus radiodurans, the most radiation resistant organism known, was vulnerable to chemical toxicity of (UO2)2+. D. radiodurans precipitated nanocrystals of a uranyl phosphate mineral extracellularly, probably as the result of phosphate release during cell lysis. It is possible that Arthrobacter spp. play important roles in natural attenuation and stimulated bioremediation of sites contaminated with radionuclides and toxic organic compounds.