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• Dissolved U is quantitatively retained (> 99.99 %) by hydroxyapatite (HAP). • At acid pH, the high solubility of HAP favors the formation of a uranyl phosphate. • The phosphate phase is a solid solution rich in Na meta-autunite. • At neutral to basic pH, surface complexation prevails at low surface loadings. • Non-stoichiometric dissolution of HAP explains pH-dependent surface complexation. The immobilization of hexavalent uranium U(VI) by hydroxyapatite (HAP) was investigated under oxic conditions as a function of pH and surface loading ([U(VI)] 0 /[HAP] 0) to evaluate the applicability of HAP as migration barriers or waste forms in nuclear waste repositories. This study focused on the retention mechanisms, which were sought by integrating the solid-phase characterization (e.g., X-ray diffraction, scanning electron microscopy with energy dispersive spectroscopy, and U L III -edge X-ray absorption spectroscopy) with the solution-phase analysis of dissolved Ca, P, and U. At acidic pH, due to the increasing solubility of HAP, U(VI) retention is mainly controlled by the formation of a uranyl phosphate following HAP dissolution. Importantly, this phosphate is in the form of a solid solution with Na meta-autunite being the principal component, which considerably expands the stable domain of uranyl phosphates compared to the pure phases (e.g., chernikovite, Na meta-autunite, and meta-autunite). At neutral to basic pH, however, the uranyl phosphate formation does not prevail till [U(VI)] 0 /[HAP] 0 = 25 mg/g, below which the formation of U(VI)-phosphate ternary surface complexes mainly accounts for U(VI) retention. At basic pH, the surface substitution of PO 4 3− for CO 3 2− and the dominance of uranyl carbonate species make surface complexation less favorable under this pH condition. In this study, the initially present U(VI) is near completely immobilized (> 99.99 %) by HAP, with the greatest retention noted at circumneutral pH, thus demonstrating the effectiveness of HAP for the containment of U(VI) under ambient geochemical conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

The remediation of uranium (U) through phosphate-solubilizing bacteria (PSB) is an emerging technique as well as an interesting phenomenon for transforming mobile U into stable minerals in the environment. While studies are well needed for in-depth understanding of the mechanism of U(VI) immobilization by PSB. In this study, two PSB were isolated from a U-tailing repository site. These bacterial strains (ZJ-1 and ZJ-3) were identified as Bacillus spp. by the sequence analysis of 16S ribosomal RNA (rRNA) genes. Incubation of PSB in liquid medium showed that the isolate ZJ-3 could solubilize more than 230 mg L–1 P from glycerol-3-phosphate and simultaneously removed over 70% of 50 mg L–1 U(VI) within 1 h. During this process, the rapid appearance of yellow precipitates was observed. The microscopic and spectroscopic analysis demonstrated that the precipitates were associated with U-phosphate compound in the form of saleeite-like substances. Besides, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis of the precipitates confirmed that the extracellular polymeric substances (EPS) might also play a key role in U sequestration. Furthermore, SEM and FTIR analysis revealed that part of U(VI) was adsorbed on the bacterial surface through cellular phosphate, hydroxy, carboxyl, and amide groups. This study provides new insights into the synergistic strategies enhancing U immobilization rates by Bacillus spp. that uses glycerol-3-phosphate as the phosphorus source, the process of which contributes to harmful pollutant biodegradation.

The remediation of uranium (U) through phosphate-solubilizing bacteria (PSB) is an emerging technique as well as an interesting phenomenon for transforming mobile U into stable minerals in the environment. While studies are well needed for in-depth understanding of the mechanism of U(VI) immobilization by PSB. In this study, two PSB were isolated from a U-tailing repository site. These bacterial strains (ZJ-1 and ZJ-3) were identified as Bacillus spp. by the sequence analysis of 16S ribosomal RNA (rRNA) genes. Incubation of PSB in liquid medium showed that the isolate ZJ-3 could solubilize more than 230 mg L–1 P from glycerol-3-phosphate and simultaneously removed over 70% of 50 mg L–1 U(VI) within 1 h. During this process, the rapid appearance of yellow precipitates was observed. The microscopic and spectroscopic analysis demonstrated that the precipitates were associated with U-phosphate compound in the form of saleeite-like substances. Besides, scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR) analysis of the precipitates confirmed that the extracellular polymeric substances (EPS) might also play a key role in U sequestration. Furthermore, SEM and FTIR analysis revealed that part of U(VI) was adsorbed on the bacterial surface through cellular phosphate, hydroxy, carboxyl, and amide groups. This study provides new insights into the synergistic strategies enhancing U immobilization rates by Bacillus spp. that uses glycerol-3-phosphate as the phosphorus source, the process of which contributes to harmful pollutant biodegradation. [ABSTRACT FROM AUTHOR]

Uranyl phosphate synthesis (MUO2PO4, M = Na+, K+, NH4+) by diffusion method, direct and pH-controlled precipitation has been performed and compared. Chemical and thermogravimetric analysis revealed compound stoichiometry to be in good agreement with ideal mineral phases (sodium uranyl phosphate (NaUO2PO4⋅xH2O), meta-ankoleite (KUO2PO4⋅3H2O) and uramphite ((NH4)UO2PO4⋅3H2O)) except for sodium and ammonium cations. Layered platelet-type crystal structures associated with uranyl phosphates were confirmed. Some long-range crystal structure was observed under direct precipitation conditions. No visible evidence of long-range crystal structures for samples formed during pH-controlled precipitation was found due to dilute formation conditions and rapid nucleation kinetics. [ABSTRACT FROM AUTHOR]

The crystal structure of phurcalite, Ca2[(UO2)3O2(PO4)2]·7H2O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å3, space group Pbca, Z = 8 has been refined from single‐crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen‐bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca2+ cations and H2O groups. In contrast to previous studies the approach here reveals five transformer H2O groups (compared to three expected by a previous study) and two non‐transformer H2O groups. One of the transformer H2O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H2O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca2(H2[3]O)5(H2[4]O)2}[(UO2)3O2(PO4)2], Z = 8. [ABSTRACT FROM AUTHOR]

Uranyl phosphate minerals are widespread in uranium deposits and normally exhibit very low solubility in aqueous systems. Uranyl phosphates of the autunite group and metaautunite subgroup impact the mobility of uranium in the environment and have inspired groundwater remediation strategies that emphasize their low solubility. The importance of soluble uranium-bearing macro-anions, including nanoscale uranyl peroxide cage clusters, is largely unexplored relative to solubilization of normally low-solubility uranium minerals. Eight synthetic analogs of metaautunite subgroup minerals have been prepared and placed in various alkaline aqueous solutions containing hydrogen peroxide and tetraethylammonium hydroxide. Each uranyl phosphate studied has a topologically identical anionic sheet of uranyl square bipyramids and phosphate tetrahedra combined with various cations (Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Ba2+) and water in the interlayer. Uranyl peroxides formed under many of the experimental conditions examined, including solid studtite [(UO2)(O2)(H2O)2](H2O)2 and soluble uranyl peroxide cage clusters containing as many as 28 uranyl ions. Uranyl phosphate solids in contact with solutions in which uranyl peroxide cage clusters formed dissolved extensively or completely. The greatest dissolution of uranyl phosphates occurred in systems that contained cations with larger hydrated radii, Li+ and Na+. The details of the uranium speciation in solution depended on the pH and counter cations provided from the interlayers of the uranyl phosphate solids. [ABSTRACT FROM AUTHOR]

Kamitugaite is a rare supergene uranyl phosphate of aluminum and lead occurring at the Kobokobo pegmatite in the Sud-Kivu Province, Democratic Republic of Congo; its structure has remained unknown until now. Based on single-crystal X-ray diffraction data carried out on the type specimen of kamitugaite (no. 13986, Royal Museum for Central Africa, Tervuren), it is triclinic, space group P-1, with a = 9.0296(8), b = 10.9557(8), c = 15.8249(15) Å, α = 89.585(7)°, β = 85.349(8)°, γ = 84.251(7)°, V = 1552.5(2) Å3 and Z = 2. The structure was refined from diffraction data to R = 0.1074 for 2697 unique observed reflections. The structure of kamitugaite is based upon infinite sheets of uranyl and phosphate polyhedra, stacked perpendicular to c; these sheets result from edge-sharing of UO7 and UO8 bipyramids, forming chains approximately parallel to b, which are linked by (P,As)O4 tetrahedra. Such a sheet has not been observed in minerals or synthetic compounds and is related to the phosphuranylite topology; the ring symbol is 6¹544³34. There are two distinct interlayer complexes in kamitugaite: one involving Pb2+ and H2O groups and another involving octahedrally coordinated Al3+ and isolated H2O groups. Adjacent sheets are linked a) through the Pb2+-O and H-bonds, and b) via H-bonds only in case of the interlayer with Al, the bonding differences being largely attributable to the very different stereochemistry of Pb2+ compared to Al3+. The unique combination of these two elements is probably a key reason for the scarcity of kamitugaite. [ABSTRACT FROM AUTHOR]

The crystal structure of phuralumite, Al2[(UO2)3(PO4)2O(OH)](OH)3(H2O)9, from the Kobokobo pegmatite (Kivu, Democratic Republic of Congo), was refined from single crystal X-ray diffraction data. Four samples have been investigated in this study, and the crystal structure was refined to R1 = 0.0566, 0.0544, 0.0661 and 0.0353, for 5739, 5953, 6007 and 5793 unique observed reflections, for the samples VC2048, VC2048, VC2054 and VC2055, respectively. Phuralumite is monoclinic, space group P21/n, a = 9.4407(3), b = 20.8596(8), c = 13.4326(4) Å, β = 107.905(4)°, V = 2517.40(17) Å3, and Z = 4 (sample VC2048). The structure consists of [(UO2)3(PO4)2O(OH)]3- layers, parallel to (010), which are connected by Al3+ ions and H2O molecules. The uranyl phosphate sheets show the phosphuranylite anion topology, while the Al3+ ions occur in 5- and 6-fold coordination and are connected together to form Al4O4(OH)6(H2O)4 clusters. The crystallographic data obtained from the four structural models converge and confirm the previously determined structure for phuralumite. However, these new data also show some discrepancies in the bond-valence analysis, especially in the assignment of the OH- groups and water molecules. As a consequence of this study, the structural formula of phuralumite, previously reported as Al2[(UO2)3(PO4)2O(OH)](OH)3(H2O)10, must be modified to Al2[(UO2)3(PO4)2O(OH)](OH)3(H2O)9. [ABSTRACT FROM AUTHOR]

In alkaline soil, abundant carbonates will mobilize uranium (U) and increase its ecotoxicity, which is a serious threat to crop growth. However, the knowledge of U remediation in alkaline soils remains very limited. In this study, U-contaminated alkaline soil (tillage layer) was collected from the Ili mining area of Xinjiang, the soil remediation was carried out by using phosphorus (P) fertilizers of different solubility (including KH 2 PO 4 , Ca(H 2 PO 4) 2 , CaHPO 4 , and Ca 3 (PO 4) 2), and the pathways and mechanisms of U passivation in the alkaline soil were revealed. The results showed that water-soluble P fertilizers, KH 2 PO 4 and Ca(H 2 PO 4) 2 , were highly effective at immobilizing U, and significantly reduced the bioavailability of soil U. The exchangeable U was reduced by 70.5 ± 0.1% (KH 2 PO 4) and 68.2 ± 1.9% (Ca(H 2 PO 4) 2), which was converted into the Fe–Mn oxide-bound and residual phases. Pot experiments showed that soil remediation by KH 2 PO 4 significantly promoted crop growth, especially for roots, and reduced U uptake in crops by 94.5 ± 1.0%. The immobilization of U by KH 2 PO 4 could be attributed to the release of phosphate anions, which react with the uranyl ion (UO 2 2+) forming a stable mineral of meta-ankoleite and enhancing the binding of UO 2 2+ to the soil Fe–Mn oxides. In addition, KH 2 PO 4 dissolution produces acidity and P fertilizer, which can reduce soil alkalinity and improve crop growth. The findings in this work demonstrate that a rational application of P fertilizer can effectively, conveniently, and cheaply remediate U contamination and improve crop yield and safety on alkaline farmland. [Display omitted] • The solubility of phosphorus fertilizers impacts the U immobilization in alkaline soil. • Phosphates efficiently immobilized U through precipitation and adsorption reactions. • Uranium of 99.8 ± 0.1% in solution and 70.5 ± 0.1% in soil was immobilized by KH 2 PO 4. • Phosphate might act as a "bridge" to improve U adsorption on iron (hydr)oxides. [ABSTRACT FROM AUTHOR]

Uranium (U) contamination often occurs in the topsoil (arable layer), and is a serious threat to crop growth. However, conventional microbial reduction methods are sensitive to oxygen and cannot be used to treat aerobic topsoils. In this study, phosphate-solubilizing microorganisms (PSM) were isolated from U-contaminated topsoil and used for soil remediation. Microbial metabolites and products were analyzed, and the pathways and mechanisms of PSM immobilization were revealed. The results showed that strain PSM8 had the highest phosphate-solubilizing capacity (dissolved P was 208 ± 5 mg/L) and the highest U removal rate (97.3 ± 0.1%). Multi-technical analyses indicated that bacterial surface functional groups adsorbed (UO 2)2+ ions on the cell surface, glycolysis produced 3–10 mg/L of lactic acid (pH 4.7–6.0), and lactic acid solubilized Ca 3 (PO 4) 2 to form stable chernikovite (a type of uranyl phosphate) on the cell surface. The coupled application of Ca 3 (PO 4) 2 and strain PSM8 significantly reduced the bioavailability of soil U (62 ± 11%), converting U from the exchangeable to the residual phase and P from the steady to the available form. In addition, pot experiments showed that soil remediation promoted crop growth and significantly reduced U uptake and toxicity to photosynthetic systems. These findings demonstrate that PSM and Ca 3 (PO 4) 2 are good coupled fertilizers for U-contaminated agricultural soil. [Display omitted] • Ca 3 (PO 4) 2 stimulated PSM8 producing lactic acid which lowered solution pH < 5.0. • Ca 3 (PO 4) 2 was recrystallized to stable chernikovite in a "selective pH range" of 3.0–6.0. • Uranium immobilization rate reached 97.3 ± 0.1% in solution and 62 ± 11% in soil. • Soil remediation twice promoted pakchoi growth and reduced U accumulation by > 50%. [ABSTRACT FROM AUTHOR]

Bioprecipitation of uranium (U) into uranyl phosphate (U-P) mediated by soluble ortho-phosphate is an attractive proposition for U bioremediation. As an alternative to the microbial phosphatase, we have investigated the dissolution of phosphate by the organic acids produced by bacteria to aid in U precipitation. The bacterium Acinetobacter sp. YU-SS-SB-29, isolated from monazite sand of natural background radiation site solubilized 952.0 ± 46.7 mg L −1 phosphate from tri-calcium phosphate (TCP) in the Pikovskaya's medium and showed tolerance to 120 ppm U(VI). U(VI) bioprecipitation was investigated by adding different concentrations of U(VI) to a cell-free culture supernatant containing ortho-phosphate released from TCP by the bacterium. A yellow precipitate was immediately formed following which there was a reduction in U(VI) concentration. A strong positive correlation ( R 2 = 0.98) was observed between % decrease in phosphate and U(VI) concentration (up to 750 ppm U) added. FTIR and EDX spectra of the yellow precipitate demonstrated the involvement of phosphate groups in U(VI) binding. Furthermore, the XRD pattern of the precipitate agrees well with that of chernikovite, a uranyl phosphate mineral. The results from this study demonstrate the potential of the U tolerant, phosphate solubilizing bacterium Acinetobacter sp. YU-SS-SB-29 for non-reductive in situ bioprecipitation of uranium. [ABSTRACT FROM AUTHOR]

Abstract: Single crystals of Zn4(OH)2[(UO2)(PO4)2(OH)2(H2O)] (UZnP), Cs[(UO2)(HPO4)NO3] (UCsP), and In3[(UO2)2(PO4)4OH(H2O)6].2H2O (UInP) were obtained from hydrothermal reactions and have been structurally and chemically characterized. UZnP crystallizes in space group Pbcn, a=8.8817(7), b=6.6109(5), c=19.569(1)Å; UCsP crystallizes in P−1, a=7.015(2), b=7.441(1), c=9.393(2)Å, α=72.974(2), β=74.261(2), γ=79.498(2); and UInP crystallizes in P−1, a=7.9856(5), b=9.159(1), c=9.2398(6)Å α=101.289(1), β=114.642(1), γ=99.203(2). The U6+ cations are present as (UO2)2+ uranyl ions coordinated by five O atoms to give pentagonal bipyramids. The structural unit in UZnP is a finite cluster containing a uranyl pentagonal bipyramid that shares corners with two phosphate tetrahedra. The structural unit in UCsP is composed of uranyl pentagonal bipyramids with one chelating nitrate group that are linked into chains by three bridging hydrogen phosphate tetrahedra. In UInP, the structural unit contains pairs of edge-sharing uranyl pentagonal bipyramids with two chelating phosphate tetrahedra that are linked into chains through two bridging phosphate tetrahedra. Indium octahedra link these uranyl phosphate chains into a 3-dimensional framework. All three compounds exhibit unique structural units that deviate from the typical layered structures observed in uranyl phosphate solid-state chemistry. [Copyright &y& Elsevier]

Abstract: 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; Cu x Ba1 −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. [Copyright &y& Elsevier]

Abstract: Perceived environmental problems of nuclear fuel fabrication, use and treatment limit the acceptability of nuclear power as an alternative to fossil fuels. This applies to nuclear fuel processing and reprocessing but contamination also occurs at source via run-offs from current and historic mining activities. The price of uranium (U3O8) in the1990s was US$10/lb but is currently US$58/lb, peaking in 2007 at US$135/lb. With the potential global expansion of nuclear power as an alternative to fossil fuels the market and strategic values of U will rise. A new biotechnology was demonstrated for efficient recovery of U from mine waters as pure hydrogen uranyl phosphate (HUP) but an economic assessment (discounting the value of U) showed that the limitation as a waste treatment process was the cost of the phosphate feed supplement which contained 1mol/mol phosphate. We describe the use of phytic acid (inositol phosphate; 6mol phosphate/mol), a ubiquitous plant waste, to support the removal of uranium as HUP by an immobilised cell reactor and shift the focus away from bioremediation to value product manufacturing from wastes, and resource efficiency. [ABSTRACT FROM AUTHOR]

Abstract: Cavernous weathering such as tafoni, alveoles and honeycomb structures have been recorded from a great variety of bedrocks and landforms. In the present study cavernous weathering from late Variscan granites was discussed as to its physical–chemical regime of formation. U/Pb dating yielded a maximum age of 1.52±0.03Ma. Supergene U mineralization is accompanied by kaolinite, nontronite and Fe(III) phosphates. Based upon Eh–pH diagrams calculated for U–Fe–P mineralization the physical–chemical conditions may be described as oxidizing with pH values fluctuating around neutral at near-ambient temperatures of 25°C. Alteration occurs in two stages: dissolution of rock-forming minerals and neoformation of hydrosilicates under mildly acidic conditions, followed by phosphate precipitation under near-neutral conditions. [Copyright &y& Elsevier]

Uranyl phosphate (UO2)3(PO4)2·8H2O was synthezied. Its dehydration was studied by X-ray diffraction, IR spectroscopy, and thermal and chemical analysis. The dehydration products were isolated and characterized by X-ray diffraction and IR spectroscopy. Their structural features were determined. [ABSTRACT FROM AUTHOR]

Abstract: Single crystals of the new compounds Li6[(UO2)12(PO4)8(P4O13)] (1), Li5[(UO2)13(AsO4)9(As2O7)] (2), Li[(UO2)4(AsO4)3] (3) and Li3[(UO2)7(AsO4)5O)] (4) have been prepared using high-temperature solid state reactions. The crystal structures have been solved by direct methods: 1—monoclinic, C2/m, a=26.963(3)Å, b=7.063(1)Å, c=19.639(1)Å, β=126.890(4)°, V=2991.2(6)Å3, Z=2, R 1=0.0357 for 3248 unique reflections with |F 0|≥4σF ; 2—triclinic, , a=7.1410(8)Å, b=13.959(1)Å, c=31.925(1)Å, α=82.850(2)°, β=88.691(2)°, γ=79.774(3)°, V=3107.4(4)Å3, Z=2, R 1=0.0722 for 9161 unique reflections with |F 0|≥4σF ; 3—tetragonal, I 41/amd, a=7.160(3)Å, c=33.775(9)Å, V=1732(1)Å3, Z=4, R 1=0.0356 for 318 unique reflections with |F 0|≥4σF ; 4—tetragonal, , a=7.2160(5)Å, c=14.6540(7)Å, V=763.04(8)Å3, Z=1, R 1=0.0423 for 1600 unique reflections with |F 0|≥4σF . Structures of all the phases under consideration are based on complex 3D frameworks consisting of different types of uranium polyhedra (UO6 and UO7) and different types of tetrahedral T O4 anions ( T =P or As): PO4 and P4O13 in 1, AsO4 and As2O7 in 2, and single AsO4 tetrahedra in 3 and 4. In the structures of 1 and 2, UO7 pentagonal bipyramids share edges to form (UO5)∞ chains extended along the b axis in 1 and along the a axis in 2. The chains are linked via single T O4 tetrahedra into tubular units with external diameters of 11Å in 1 and 11.5Å in 2, and internal diameters of 4.1Å in 1 and 4.5Å in 2. The channels accommodate Li+ cations. The tubular units are linked into 3D frameworks by intertubular complexes. Structures of 3 and 4 are based on 3D frameworks composed on layers united by (UO5)∞ infinite chains. Cation–cation interactions are observed in 2, 3, and 4. In 2, the structure contains a trimeric unit with composition [OU(1)O]–U(13)–[OU(2)O]. In the structures of 3 and 4, T-shaped dimers are observed. In all the structures, Li+ cations are located in different types of cages and channels and compensate negative charges of anionic 3D frameworks. [Copyright &y& Elsevier]

Abstract: The three new framework Rb uranyl phosphates and arsenates with anionic parts based on different type of polymeric anions have been prepared by high-temperature solid-state reactions: Rb4[(UO2)6(P2O7)4(H2O)] (1), Rb2[(UO2)3(P2O7)(P4O12)] (2), Rb[(UO2)2(As3O10)] (3). The crystal structures of the synthesized compounds have been solved by direct methods: 1—monoclinic P21/c, a=9.672(1)Å, b=12.951(1)Å, c=32.231(3)Å, β=90.116(4)°, V=4037.3(6)Å3, Z=4, R 1=0.0926 for 6351 unique reflections with ; 2—monoclinic, P21/c, a=6.791(1)Å, b=16.155(3)Å, c=19.856(4)Å, β=97.48(5)°, V=2159.8(7)Å3, Z=4, R 1=0.0286 for 3617 unique reflections with ; 3—orthorhombic, Pbcn, a=10.558(1)Å, b=11.037(1)Å, c=11.464(1)Å, V=1335.9(2)Å3, Z=4, R 1=0.0489 for 1384 unique reflections with . The structures of title are compounds based on 3D negatively charged frameworks with chemical compositions [(UO2)6(P2O7)4(H2O)]4− in 1, [(UO2)3(P2O7)(P4O12)]2− in 2 and [(UO2)2(As3O10)]− in 3. These negative charges are compensated by rubidium cations which are in the channels of 1 and closed cages in structures of 2 and 3. The channels in 1 are directed along the a direction and have minimum dimensions ∼5Å×6Å. This is the first example of porosity generation through solid state synthesis in uranyl compounds. For the first time in uranium chemistry polymeric anionic groups P4O12 and As3O10 were observed in structure of 2 and 3. [Copyright &y& Elsevier]

Abstract: Three new lithium uranyl phosphates, Li2(UO2)3(PO4)2O (1), Li(UO2)4(PO4)3 (2) and Li3(UO2)7(PO4)5O (3) were synthesized and studied. Powders of 1 and 2 were synthesized via solid state reaction, and single crystals of the three compounds were obtained by melting of 1 and 2 powders. The structures of the three compounds have been solved and refined from single crystal X-ray diffraction data. In the three compounds, the uranium atoms occupy square and pentagonal bipyramids. The uranium square bipyramids and phosphate tetrahedra are connected by vertices to form two types of layers with autunite sheet anion-topology and denoted S and D, respectively. The uranyl pentagonal bipyramids share opposite equatorial edges to form infinite chains. Mutually perpendicular chains are hung on each side of the sheets to build frameworks with non-crossing perpendicular channels that accommodate the lithium ions. Various stacking sequences of the S and D layers, S–S, D–D and S–D, generate three different frameworks in 1, 2 and 3, respectively. These compounds are similar to the recently reported vanadate analogous. However, the phosphate tetrahedra, smaller than the vanadate ones, gives distortion of the layers and a lowering of the symmetry and/or a change of periodicity. The electrical conductivity of 1 and 2 was measured using impedance spectroscopy method. The rather low conductivity of the lithium cations is explained by the crystal structure and the Li+ position within the tunnels. These results corroborate those on the analogous three-dimensional alkaline uranyl vanadates. Crystallographic data: 293K, BRUKER X8-APEX2 X-ray diffractometer, 4K CCD detector, MoKα, λ=0.71073Å, full-matrix least-squares refinement on the basis of F. 1, Tetragonal symmetry, space group I41/amd, Z=4 with a=7.1109(2)Å and c=25.0407(8)Å, R=0.034 and wR=0.047 for 38 parameters with 479 independent reflections with I⩾3σ(I). 2, monoclinic symmetry, space group P21/c, Z=4 and a=9.8829(2)Å, b=9.8909(2)Å, c=17.4871(4)Å and β=106.198(1)°, R=0.021 and wR=0.031 for 249 parameters with 4201 independent reflections with I⩾3σ(I). 3, Tetragonal symmetry, space group with a=9.9305(2)Å and c=14.5741(3)Å, R=0.035 and wR=0.038 for 137 parameters with 4527 independent reflections with i⩾3σ(I). [Copyright &y& Elsevier]

Long-term uranium mobility in tailings is an environmental management issue. The present study focuses on two U-enriched layers, surficial and buried 14.5 m, of the tailings pile of Cominak, Niger. The acidic and oxidizing conditions of the tailings pile combined with evapotranspiration cycles related to the Sahelian climate control U speciation. Uraninite, brannerite, and moluranite as well as uranophane are relict U phases. EXAFS spectroscopy, HR-XRD, and SEM/WDS highlight the major role of uranyl sulfate groups in uranium speciation. Uranyl phosphate neoformation in the buried layer (paleolayer) acts as an efficient trap for uranium. [Display omitted] • Sulfate and phosphate groups control U mobility in the tailings environment. • Neoformation of uranyl sulfates is favored by evapotranspiration phenomena. • Uranyl phosphate minerals are efficient traps for uranium long-term mobility. • Inherited U(IV) phases included in quartz are identified in tailings. [ABSTRACT FROM AUTHOR]

Abstract: Four new Li uranyl phosphates and arsenates have been prepared by high-temperature solid-state reactions: α-Li[(UO2)(PO4)] (1), α-Li[(UO2)(AsO4)] (2), β-Li[(UO2)(AsO4)] (3) and Li2[(UO2)3(P2O7)2] (4). The structures of the compounds have been solved by direct methods: 1—triclinic, , a=5.0271(1)Å, b=9.8799(2)Å, c=10.8920(2)Å, α=108.282(9)°, β=102.993(8)°, γ=104.13(1)°, V=470.69(2)Å3, Z=4, R 1=0.0415 for 2786 unique reflections with |F 0|⩾4σF ; 2—triclinic, , a=5.129(2)Å, b=10.105(3)Å, c=11.080(3)Å, α=107.70(2)°, β=102.53(3)°, γ=104.74(3)°, V=501.4(3)Å3, Z=4, R 1=0.055 for 1431 unique reflections with |F 0|⩾4σ F; 3—triclinic, , a=5.051(1)Å, b=5.303(1)Å, c=10.101(1)Å, α=90.31(1)°, β=97.49(1)°, γ=105.08(1)°, V=258.80(8)Å3, Z=2, R 1=0.0339 for 2055 unique reflections with |F 0|⩾4σ F; 4—triclinic, , a=5.312(1)Å, b=6.696(1)Å, c=12.542(1)Å, α=94.532(9)°, β=99.059(8)°, γ=110.189(7)°, V=409.17(10)Å3, Z=2, R 1=0.0565 for 1355 unique reflections with |F 0|⩾4σ F. The structures of all four compounds are based upon 3-D frameworks of U and T polyhedra (T=P, As). Phases 1 and 2 are isostructural and consist of U2O12 dimers and UO6 square bipyramids linked by single TO4 tetrahedra. The structure of 3 consists of 3-D framework of corner-sharing UO6 bipyramids and AsO4 tetrahedra. In the structure of 4, the framework is composed of U2O12 dimers, UO6 bipyramids and P2O7 dimers. In all the compounds, Li+ cations reside in framework cavities. The topologies of the 3-D frameworks can be described as derivatives of the PtS (cooperite) network. [Copyright &y& Elsevier]

Abstract: A new open-framework compound, [C6H14N2][(UO2)4(HPO4)2(PO4)2(H2O)]·H2O, (DUP-1) has been synthesized under mild hydrothermal conditions. The resulting structure consists of diprotonated DABCOH2 2+ (C6H14N2 2+) cations and occluded water molecules occupying the channels of a complex uranyl phosphate three-dimensional framework. The anionic lattice contains uranophane-like sheets connected by hydrated pentagonal bipyramidal UO7 units. [C6H14N2][(UO2)4(HPO4)2(PO4)2(H2O)]·H2O possesses five crystallographically unique U centers. U(VI) is present here in both six- and seven-coordinate environments. The DABCOH2 2+ cations are held within the channels by hydrogen bonds to both two uranyl oxygen atoms and a μ 2-O atom. Crystallographic data (193K, Mo Kα, λ=0.71073Å): DUP-1, monoclinic, P21/n, a=7.017(1)Å, b=21.966(4)Å, c=17.619(3)Å, β=90.198(3)°, Z=4, R(F)=4.76% for 382 parameters with 6615 reflections with I>2σ(I). [Copyright &y& Elsevier]

Two hydrated uranyl arsenates and a uranyl phosphate were synthesized by hydrothermal methods in the presence of amine structure-directing agents and their structures determined: (N2C6H14)[(UO2)(AsO4)]2(H2O)3, DabcoUAs, {NH(C2H5)3}[(UO2)2(AsO4)(AsO3OH)], TriethUAs, and (N2C4H12)(UO2)[(UO2)(PO4)]4(H2O)2, PiperUP. Intensity data were collected at room temperature using MoKα X-radiation and a CCD-based area detector. The crystal structures were refined by full-matrix least-squares techniques on the basis of F2 to agreement indices (DabcoUAs, TriethUAs, PiperUP) wR2=5.6%, 8.3%, 7.2% for all data, and R1=2.9%, 3.3%, 4.0%, calculated for 1777, 5822, 9119 unique observed reflections (|Fo|⩾4σF), respectively. DabcoUAs is monoclinic, space group C2/m, Z=2, a=18.581(1), b=7.1897(4), c=7.1909(4) Å, β=102.886(1)°, V=936.43(9) Å3, Dcalc=3.50 g/cm3. TriethUAs is monoclinic, space group P21/n, Z=4, a=9.6359(4), b=18.4678(7), c=10.0708(4) Å, β=92.282(1)°, V=1790.7(1) Å3, Dcalc=3.41 g/cm3. PiperUP is monoclinic, space group Pn, Z=2, a=9.3278(4), b=15.5529(7), c=9.6474(5) Å, β=93.266(1)°, V=1397.3(1) Å3, Dcalc=4.41 g/cm3. The structure of DabcoUAs contains the autunite-type sheet formed by the sharing of vertices between uranyl square bipyramids and arsenate tetrahedra. The triethylenediammonium cations are located in the interlayer along with two H2O groups and are disordered. Both TriethUAs and PiperUP contain sheets formed of uranyl pentagonal bipyramids and tetrahedra (arsenate and phosphate, respectively) with the uranophane sheet-anion topology. In TriethUAs, triethlyammonium cations are located in the interlayer. In PiperUP, the sheets are connected by a uranyl pentagonal bipyramid that shares corners with phosphate tetrahedra of adjacent sheets, resulting in a framework with piperazinium cations and H2O groups in the cavities of the structure. [Copyright &y& Elsevier]

Geochemical and mineralogical studies of the 15- to 20-m-thick soil and saprolite profile developed over the Coles Hill uranium deposit demonstrate that uranium transport may be inhibited or naturally attenuated by the precipitation of U(VI) phosphate minerals in oxidizing and saturated soil environments. This study examines geochemical conditions in which these secondary uranium phosphates are formed and remain stable for hundreds of thousands of years. We also describe geochemical conditions that may lead to their dissolution.The lower part of the weathering profile, which consists of groundwater saturated saprolite, contains up to 1300 mg/kg uranium (within the solid). This concentration is approximately 1.5 times greater than the average ore grade of the deposit, indicating that the saturated saprolites are enriched relative to the underlying primary ore. Uranium within this zone is dominantly associated with U(VI) phosphates of the meta-autunite mineral group. Groundwaters from this zone contain less than 14 μg/l uranium suggesting that the U(VI) phosphate minerals present within the Coles Hill saprolites are capable of buffering dissolved uranium concentrations to values significantly lower (by a factor of 2 or more) than the US-EPA maximum contaminant level of 30 μg/l.Above the water table, U(VI) phosphates of the type found in the saturated saprolite zone are not present, and the soil uranium concentration drops to an average value of 250 mg/kg. Thus, the zone of water table fluctuation represents a major transition zone in uranium mineralogy. Mineralogical characterization of the upper part of the profile suggests that geochemical conditions in the unsaturated zone are not conducive to uranium stabilization by meta-autunite group minerals. Uranium within the vadose zone, however, is associated with phosphate (e.g. Al phosphate minerals and sorption with phosphate to ferric oxyhydroxides).As a whole, the Coles Hill soil profile is interpreted to have reached a steady state with respect to uranium transport and immobilization. Results of this study suggest that uranium is leached from the unsaturated part of the profile (meta-autunite unstable) and reprecipitated below the water table where the measured activity ratio of dissolved phosphate to carbonate is higher. Based on estimates of weathering rates for this region, the processes responsible for the dispersal of uranium within the Coles Hill system are estimated to have been active on a time scale of hundreds of thousands of years. This system thus represents an excellent natural laboratory for understanding the long-term mineralogical and geochemical processes involved in the coupling of uranium and phosphorus in natural, fluid-rich systems. [Copyright &y& Elsevier]

Three homeotypic hydrated alkali metal uranyl phosphates, A2(UO2)[(UO2)(PO4)]4(H2O)2, A=Cs (CsUP), Rb (RbUP), K (KUP), were synthesized by hydrothermal methods. Intensity data were collected at room temperature using MoKα radiation and a CCD-based area detector. Their crystal structures were solved by Patterson (CsUP) and direct (RbUP, KUP) methods and refined by full-matrix least-squares techniques to agreement indices (CsUP, RbUP, KUP) wR2=0.048, 0.230, 0.072 for all data, and R1=0.023, 0.078, 0.038 calculated for 5338, 4738, 4514 unique observed reflections (∣Fo∣≥4σF), respectively. The compound CsUP is orthorhombic, space group Cmc21, Z=4, a=14.854(1), b=13.879(1), c=12.987(1) A˚, V=2677.5(3) A˚3. Both RbUP and KUP are monoclinic, space group Cm, but are presented in the unconventional pseudo-orthorhombic space group Fm11 to facilitate comparison with CsUP and to allow a model for RbUP that includes the effects of pseudo-merohedral twinning. RbUP is monoclinic, space group Fm11, Z=4, a=15.72(2), b=13.84(1), c=13.05(1) A˚, α=90.39°(2), V=2839(5) A˚3; KUP is monoclinic, space group Fm11, Z=4, a=15.257(1), b=13.831(1), c=13.007(1) A˚, α=91.760°(1), V=2743.4(3) A˚3. The structures consist of sheets of phosphate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO2)(PO4)]−, that are topologically identical to the uranyl silicate sheets in uranophane-beta. These sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with two phosphate tetrahedra on each of two adjacent sheets and whose fifth equatorial vertex is an H2O group, resulting in an open framework with alkali metal cations in the larger cavities of the structures. Where CsUP and RbUP have two alkali metal positions and a H2O group in these cavities, KUP has four K atoms and two H2O groups, all of which are partially occupied, in the interstitial sites. [Copyright &y& Elsevier]