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1.
X-ray absorption fine structure (XAFS) measurements was used at the U L3-edge to directly determine the pH dependence of the cell wall functional groups responsible for the absorption of aqueous UO22+ to Bacillus subtilis from pH 1.67 to 4.80. Surface complexation modeling can be used to predict metal distributions in water-rock systems, and it has been used to quantify bacterial adsorption of metal cations. However, successful application of these models requires a detailed knowledge not only of the type of bacterial surface site involved in metal adsorption/desorption, but also of the binding geometry. Previous acid-base titrations of B. subtilis cells suggested that three surface functional group types are important on the cell wall; these groups have been postulated to correspond to carboxyl, phosphoryl, and hydroxyl sites. When the U(VI) adsorption to B. subtilis is measured, observed is a significant pH-independent absorption at low pH values (<3.0), ascribed to an interaction between the uranyl cation and a neutrally charged phosphoryl group on the cell wall. The present study provides independent quantitative constraints on the types of sites involved in uranyl binding to B. subtilis from pH 1.67 to 4.80. The XAFS results indicate that at extremely low pH (pH 1.67) UO22+ binds exclusively to phosphoryl functional groups on the cell wall, with an average distance between the U atom and the P atom of 3.64 ± 0.01 Å. This U-P distance indicates an inner-sphere complex with an oxygen atom shared between the UO22+ and the phosphoryl ligand. The P signal at extremely low pH value is consistent with the UO22+ binding to a protonated phosphoryl group, as previously ascribed. With increasing pH (3.22 and 4.80), UO22+ binds increasingly to bacterial surface carboxyl functional groups, with an average distance between the U atom and the C atom of 2.89 ± 0.02 Å. This U-C distance indicates an inner-sphere complex with two oxygen atoms shared between the UO22+ and the carboxyl ligand. The results of this XAFS study confirm the uranyl-bacterial surface speciation model.  相似文献   

2.
The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO22+) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10−3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)6] octahedral sites over [Al(O,OH)6] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO2. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO2.  相似文献   

3.
Remediation of uranium in the deep unsaturated zone is a challenging task, especially in the presence of oxygenated, high-carbonate alkalinity soil and pore water composition typical for arid and semi-arid environments of the western regions of the U.S. This study evaluates the effect of various pore water constituencies on changes of uranium concentrations in alkaline conditions, created in the presence of reactive gases such as NH3 to effectively mitigate uranium contamination in the vadose zone sediments. This contaminant is a potential source for groundwater pollution through slow infiltration of soluble and highly mobile uranium species towards the water table. The objective of this research was to evaluate uranium sequestration efficiencies in the alkaline synthetic pore water solutions prepared in a broad range of Si, Al, and bicarbonate concentrations typically present in field systems of the western U.S. regions and identify solid uranium-bearing phases that result from ammonia gas treatment. In previous studies (Szecsody et al. 2012; Zhong et al. 2015), although uranium mobility was greatly decreased, solid phases could not be identified at the low uranium concentrations in field-contaminated sediments. The chemical composition of the synthetic pore water used in the experiments varied for silica (5–250 mM), Al3+ (2.8 or 5 mM), HCO3− (0–100 mM) and U(VI) (0.0021–0.0084 mM) in the solution mixture. Experiment results suggested that solutions with Si concentrations higher than 50 mM exhibited greater removal efficiencies of U(VI). Solutions with higher concentrations of bicarbonate also exhibited greater removal efficiencies for Si, Al, and U(VI). Overall, the silica polymerization reaction leading to the formation of Si gel correlated with the removal of U(VI), Si, and Al from the solution. If no Si polymerization was observed, there was no U removal from the supernatant solution. Speciation modeling indicated that the dominant uranium species in the presence of bicarbonate were anionic uranyl carbonate complexes (UO2(CO3)2−2 and UO2(CO3)3−4) and in the absence of bicarbonate in the solution, U(VI) major species appeared as uranyl-hydroxide (UO2(OH)3− and UO2(OH)4−2) species. The model also predicted the formation of uranium solid phases. Uranyl carbonates as rutherfordine [UO2CO3], cejkaite [Na4(UO2)(CO3)3] and hydrated uranyl silicate phases as Na-boltwoodite [Na(UO2)(SiO4)·1.5H2O] were anticipated for most of the synthetic pore water compositions amended from medium (2.9 mM) to high (100 mM) bicarbonate concentrations.  相似文献   

4.
One option for immobilizing uranium present in subsurface contaminated groundwater is in situ bioremediation, whereby dissimilatory metal-reducing bacteria and/or sulfate-reducing bacteria are stimulated to catalyze the reduction of soluble U(VI) and precipitate it as uraninite (UO2). This is typically accomplished by amending groundwater with an organic electron donor. It has been shown, however, that once the electron donor is entirely consumed, Fe(III) (hydr)oxides can reoxidize biogenically produced UO2, thus potentially impeding cleanup efforts. On the basis of published experiments showing that such reoxidation takes place even under highly reducing conditions (e.g., sulfate-reducing conditions), thermodynamic and kinetic constraints affecting this reoxidation are examined using multicomponent biogeochemical simulations, with particular focus on the role of sulfide and Fe(II) in solution. The solubility of UO2 and Fe(III) (hydr)oxides are presented, and the effect of nanoscale particle size on stability is discussed. Thermodynamically, sulfide is preferentially oxidized by Fe(III) (hydr)oxides, compared to biogenic UO2, and for this reason the relative rates of sulfide and UO2 oxidation play a key role on whether or not UO2 reoxidizes. The amount of Fe(II) in solution is another important factor, with the precipitation of Fe(II) minerals lowering the Fe+2 activity in solution and increasing the potential for both sulfide and UO2 reoxidation. The greater (and unintuitive) UO2 reoxidation by hematite compared to ferrihydrite previously reported in some experiments can be explained by the exhaustion of this mineral from reaction with sulfide. Simulations also confirm previous studies suggesting that carbonate produced by the degradation of organic electron donors used for bioreduction may significantly increase the potential for UO2 reoxidation through formation of uranyl carbonate aqueous complexes.  相似文献   

5.
Speciation of uranium (VI) in acetate solutions between 25 and 250°C, at pH values between 1.8 and 3.8 and acetate/uranium (Ac/U) ratios of 0.5 to 100 has been investigated using uranium LIII-edge X-ray absorption spectroscopy. With increasing pH the UO2(Ac)20 species becomes more important than UO2(Ac)+ species, which is predominant below pH 2. It remains the dominant species as pH is further increased to 3.8 at an Ac/U ratio of 20. Decrease in U-Oeq bond distance and coordination number with increasing solution age indicates that steric/kinetic factors are important and that equilibrium is attained slowly in this system with initial acetate coordination to the uranyl ion being monodentate or pseudo-bridging before slow conversion to bidentate chelation. Acetate coordination to the uranyl ion appears to decrease as temperature is increased from room temperature to ∼100°C before increasing in solutions of Ac/U > 2. For solutions where Ac/U ≤ 2 at pH 2.1, there is no evidence for uranyl acetate speciation at low temperatures, but at elevated temperature bidentate uranyl-acetate ion-pairing is evident. The existence of the uranyl acetate species in the temperature range 200 to 240°C demonstrates the importance of including acetate and other organic ligands in models of uranium transport at elevated temperatures.  相似文献   

6.
The adsorption of uranyl (UO22+) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO2 pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO2)(CO3)34−, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO2-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.  相似文献   

7.
Recent studies of uranium(VI) geochemistry have focused on the potentially important role of the aqueous species, CaUO2(CO3)32− and Ca2UO2(CO3)30(aq), on inhibition of microbial reduction and uranium(VI) aqueous speciation in contaminated groundwater. However, to our knowledge, there have been no direct studies of the effects of these species on U(VI) adsorption by mineral phases. The sorption of U(VI) on quartz and ferrihydrite was investigated in NaNO3 solutions equilibrated with either ambient air (430 ppm CO2) or 2% CO2 in the presence of 0, 1.8, or 8.9 mM Ca2+. Under conditions where the Ca2UO2(CO3)30(aq) species predominates U(VI) aqueous speciation, the presence of Ca in solution lowered U(VI) adsorption on quartz from 77% in the absence of Ca to 42% and 10% at Ca concentrations of 1.8 and 8.9 mM, respectively. U(VI) adsorption to ferrihydrite decreased from 83% in the absence of Ca to 57% in the presence of 1.8 mM Ca. Surface complexation model predictions that included the formation constant for aqueous Ca2UO2(CO3)30(aq) accurately simulated the effect of Ca2+ on U(VI) sorption onto quartz and ferrihydrite within the thermodynamic uncertainty of the stability constant value. This study confirms that Ca2+ can have a significant impact on the aqueous speciation of U(VI), and consequently, on the sorption and mobility of U(VI) in aquifers.  相似文献   

8.
Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10?5 M, (dissolved uranyl 10?5 to 10?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO3 solutions containing up to 0.01 M NaHCO3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO2OH+ and (UO2)3(OH)+5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO2OH+ surface complex, and a monodentate, bidentate or tridentate (UO2)3(OH)+5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10?2 and 10?3 M.  相似文献   

9.
Summary The crystal structure of meta-uranocircite II, Ba(UO2)2(PO4)2·6H2O, has been determined with a synthetic crystal using three-dimensional X-ray techniques.R=0.071 andR w =0.064 were obtained for 1743 observed reflections. Ba(UO2)2(PO4)2·6H2O is monoclinic, space groupP1121/a, a=9.789,b=9.822,c=16.868 Å, =89.95° andZ=4. The structure consists of slightly corrugated UO2PO4 layers parallel (001). The layers are connected by Ba atoms and H2O molecules. Uranium exhibits a (2+4)-coordination with mean U-O bond lengths of 1.78 Å for the uranyl oxygens and 2.28 Å for the phosphate oxygens. The average P-O bond length is 1.52 Å. Barium is coordinated by two uranyl oxygens. two phosphate oxygens and five water molecules. The Ba–O bond lengths vary from 2.74 to 3.11 Å. Two of the six water molecules of the formula are not bonded to barium.
Die Kristallstruktur des Meta-Uranocircits II, Ba(UO2)2(PO4)2·6H2O
Zusammenfassung Die Kristallstruktur des Meta-Uranocircits II, Ba(UO2)2(PO4)2·6H2O, wurde anhand eines künstlichen Kristalls mit dreidimensionalen Röntgendaten bearbeitet und für 1743 Reflexe aufR=0,071 undR w =0,064 verfeinert. Ba(UO2)2(PO4)2·6H2O kristallisiert monoklin in der RaumgruppeP1121/a, a=9,789,b=9,882,c=16,868 Å, =89,95° und einem Zellinhalt von vier Formeleinheiten. Die Struktur besteht aus schwach gewellten UO2PO4-Schichten parallel (001), die durch Ba-Atome und H2O-Moleküle miteinander verknüpft sind. Uran besitzt oktaedrische (2+4)-Koordination mit mittleren U-O-Abständen von 1,78 Å für die Uranylsauerstoffatome und 2,28 Å für die Phosphatsauerstoffatome. Die P-O-Abstände der Phosphattetraeder messen im Mittel 1.52 Å. Barium ist von je zwei Uranyl- und Phosphatsauerstoffatomen sowie von fünf Wassermolekülen koordiniert. Die Ba-O-Abstände betragen 2,74–3,11 Å. Von den sechs H2O-Molekülen der Formel sind zwei nicht an Barium gebunden.

With 3 Figures  相似文献   

10.
We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-LIII edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (RU-Oax ≈ 1.80 Å, RU-Oeq ≈ 2.40 Å), and towards the gibbsite surface (RU-O ≈ 2.87 Å, RU-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ∼1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO2(OH)2, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to RU-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ∼2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities.  相似文献   

11.
We measured the kinetics of U(VI) reduction by Shewanella oneidensis MR-1 under anaerobic conditions in the presence of variable concentrations of either EDTA or dissolved Ca. We measured both total dissolved U and U(VI) concentrations in solution as a function of time. In separate experiments, we also measured the extent of U(VI) adsorption onto S. oneidensis in order to quantify the thermodynamic stabilities of the important U(VI)-bacterial surface complexes. In the EDTA experiments, the rate of U(IV) production increased with increasing EDTA concentration. However, the total dissolved U concentrations remained constant and identical to the initial U concentrations during the course of the experiments for all EDTA-bearing systems. Additionally, the U(VI) reduction rate in the EDTA experiments exhibited a strong correlation to the concentration of the aqueous U4+-EDTA complex. We conclude that the U(VI) reduction rate increases with increasing EDTA concentration, likely due to U4+-EDTA aqueous complexation which removes U(IV) from the cell surface and prevents UO2 precipitation.In the Ca experiments, the U(VI) reduction rate decreased as Ca concentration increased. Our thermodynamic modeling results based on the U(VI) adsorption data demonstrate that U(VI) was adsorbed onto the bacterial surface in the form of a Ca-uranyl-carbonate complex in addition to a number of other Ca-free uranyl complexes. The observed U(VI) reduction rates in the presence of Ca exhibit a strong negative correlation to the concentration of the Ca-uranyl-carbonate bacterial surface complex, but a strong positive correlation to the total concentration of all the other Ca-free uranyl surface complexes. Thus, the concentration of these Ca-free uranyl surface complexes appears to control the rate of U(VI) reduction by S. oneidensis in the presence of dissolved Ca. Our results demonstrate that U speciation, both of U(VI) before reduction and of U(IV) after reduction, affects the reduction kinetics, and that thermodynamic modeling of the U speciation may be useful in the prediction of reduction kinetics in realistic geologic settings.  相似文献   

12.
Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)2UO2(H2O)n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO2-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)2UO2(H2O)3 which can form on the dominant {101} surface. However, in the presence of CO2, we find an enhancement of UO2 sorption at low pH and attribute this to a (>FeO)CO2UO2 ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)2UO2CO3. The three proposed surface complexes, (>FeOH)2UO2(H2O)3, >FeOCO2UO2, and (>FeOH)2UO2CO3 are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO2-OH-CO3 aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO2 environments at surface loadings of 0.02-2.0 wt% U.  相似文献   

13.
We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium(VI) was supplied as sole terminal electron acceptor to Shewanella putrefaciens (strain 200R) in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U(VI) and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U(VI), resulting in removal of insoluble amorphous UO2 from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U(VI) reduction to U(IV), to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U(VI):ligand and U(IV):ligand complexes. In the presence of strongly complexing ligands (e.g., NTA, Tiron, EDTA), UO2 precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO2, which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U4+ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U4+, which is a necessary step in UO2 precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U(VI)-organic complexes may also influence overall UO2 production rates, although the magnitude of this effect is unclear relative to the effects of U(IV)-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO2.  相似文献   

14.
Depending upon oxygen fugacity, uranium exists in three different oxidation states in magmatic silicate liquids. The hexavalent state, present as the uranyl group, UO2+2, is stable under highly oxidizing conditions, but can still be detected in the presence of the NiNiO buffer. Under the same conditions the pentavalent state forms about 30–40% of total uranium and is also characteristic of relatively high oxygen fugacities. Optical absorption spectra obtained on granitic and basaltic glasses synthesized in the presence of the NiNiO buffer are very different: this is interpreted as being due to the presence of UO+2 complexes in the former and 6-coordinated U(V) in the latter. The tetravalent state is the most stable under reducing conditions: at the FeFeO buffer, it is the only one present. An 8-coordinated U(IV) species seems the most probable, by comparison of the spectra with those of crystallized U(IV) compounds. The trivalent state was not detected, even under the most reducing conditions. Interpretation of the spectra obtained in the glasses in terms of coordination and bonding is however difficult, due to the lack of knowledge of 5f-systems in iono-covalent systems such as oxide glasses. The presence of the pentavalent state must be taken into account in discussing partition coefficients of uranium and trans-uranium compounds in natural and synthetic systems (because of the effect of oxygen fugacity and oxide ion activity on the U(IV) U(V) system). During postmagmatic hydrothermal processes U(V) is destroyed, resulting in the early precipitation of U(IV) containing minerals and possible migration of uranyl ions.  相似文献   

15.
Uraninite solubility in HF solutions (0.0001–0.5 m) was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the predominant U(IV) species in aqueous solution are U(OH)40, U(OH)3F0, and U(OH)2 F20. Using the results of uraninite solubility measurement, the Gibbs free energies of the uranium (IV) species were calculated at 500°C and 1000 bar (kJ/mol): −986.55 for UO2(aq), −1712.42 for U(OH)3F0, −1755.53 for U(OH)2F20, and the equilibrium constants of the uraninite solubility in water and HF solutions were estimated: UO2(κ) = UO2(aq), which is similar to UO2(cr) + 2H2O = U(OH)40, pK0 = 6.64; UO2(cr) + HF0 + H2O = U(OH)3F0, K1 = 0.0513; UO2(cr) + 2HF0 = U(OH)2F20K2 = 7.00 × 10−4. Approximate values K3 = 5.75 × 10−3 and K4 = 6.7 × 10−2 were obtained for equilibria UO2(cr) + 4HF0 =UF40 + 2H2O and UO2(cr) + 4HF = UF40 + 2H2O. Maximum observed in the uranium concentration curve as a function of HF concentration can be explained by the decrease (to < 1) of activity coefficient ratio of HF0 to U(OH)3F0 with increasing HF concentrations.  相似文献   

16.
Uraninite solubility in 0.001–2.0 m HCl solutions was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the following U(IV) species dominate in the aqueous solution: U(OH)40, U(OH)2Cl20, and UOH Cl30 Using the results of uraninite solubility measurement, the Gibbs free energies of U(IV) species at 500°C and 1000 bar were calculated (kJ/mol): −9865.55 for UO2(aq), −1374.57 for U(OH)2 Cl20, and −1265.49 for UOH Cl30, and the equilibrium constants of uraninite dissolution in water and aqueous HCl solutions were estimated: UO2(cr) = UO2(aq), pK 0 = 6.64; UO2(cr) + 2HCl0 = U(OH)2 Cl20, pK 2 = 3.56; and UO2(cr) + 3HCl0 = UOHcl30 + H2O, pK 3 = 3.05. The value pK 1 ≈ 5.0 was obtained as a first approximation for the equilibrium UO2(cr) + H2O + HCl0 = U(OH)3Cl0. The constant of the reaction UO2(cr) + 4HCl0 = UCl40 + 2H2O (pK 4 = 7.02) was calculated taking into account the ionization constants of U Cl40 and U(OH)40, obtained by extrapolation from 25 to 500°C at 1000 bar using the BR model. Intense dissolution and redeposition of gold (material of experimental capsules) was observed in our experiments. The analysis and modeling of this phenomenon suggested that the UO2 + x /UO2 redox pair oxidized Au(cr) to Au+(aq), which was then reduced under the influence of stronger reducers.  相似文献   

17.
The surface reactivity of biogenic, nanoparticulate UO2 with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO2. Synthesis of biogenic UO2 was accomplished by reduction of aqueous uranyl ions, by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO2 as a function of particle size and surface-associated organic matter. The N2-BET surface areas of the two biogenic UO2 samples following the washing procedures are 128.63 m2 g−1 (NAUO2) and 92.56 m2 g−1 (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO2 (both NAUO2 and BIUO2) over the pH range 3-9 is the same as that of bulk. The U LIII-edge EXAFS spectra for biogenic UO2 (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO2. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m−2 UO2 (bulk uraninite), 2.34 ± 0.12 μmol m−2 UO2 (NAUO2), and 2.57 ± 0.10 μmol m−2 UO2 (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH)6 sorbs as bidentate, edge-sharing complexes to UO8 polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO2 surface. Surface-associated organic matter on the biogenic UO2 particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO2 surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.  相似文献   

18.
The Tulul Al Hammam area in central Jordan is an advantageous natural analogue site to study long-term U(VI) retention in ~?1 Ma old U-bearing combustion metamorphic marbles with clinker-like mineralogy exposed to prolonged supergene alteration for at least ~?100 kyr. The marbles contain abundant grains of high-temperature (ca. 800–850 °C) primary double Ca-U(VI) oxides (mainly Ca3UO6 and CaUO4), which are commonly replaced by hydrated calcium uranates with various impurities (Si, Fe, Al and F). A more hydrous natural analogue of X-phase (Ca2UO5·2-3H2O) occurs as a predominant secondary U compound after primary Ca-U(VI) oxides. The phase was studied by single-crystal XRD, SEM/EDX and electron microprobe (EPMA) analyses and Raman spectroscopy. It is a non-crystalline phase with a specific finger-like microtexture consisting of thin (no wider than 1–2 μm) lamellar particles. Its Raman spectrum shows a single strong band at 706–713 cm?1, sometimes coexisting with up to three weak diffuse bands (ν ~?390, ~?540 and 1355–1400 cm?1). The find of the natural X-phase (Ca2UO5·2-3H2O) is evidence of its long-term stability in a natural environment. It proves explicitly that the compound Ca2UO5·nH2O is a solubility-limiting phase in aged cements. The results have implications for geological disposal of radioactive wastes.  相似文献   

19.
《Geochimica et cosmochimica acta》1999,63(19-20):2939-2955
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20.
《Applied Geochemistry》2002,17(4):399-408
The sorption of U(VI) onto the surface of olivine has been experimentally investigated at 25 °C under an air atmosphere as a function of pH, solid surface to volume ratio and total U concentration. Sorption has been observed to decrease as the extent of carbonate complexation of U(VI) in solution increases, which is attributed to the competition between aqueous and solid ligands for the coordination of U. The experimental results have been interpreted by means of two different approaches: (1)a semi-empirical model, exemplified by the application of a Langmuir isotherm and (2) a non-electrostatic thermodynamic surface complexation model which includes the formation of the surface species: >SO–UO2+ and >SO–UO2(OH). The following stability constants for these species have been determined from the thermodynamic analysis: K(>SO–UO2+)=289±71 and K(>SO–UO2(OH))=(3.4±0.4)×10−6. The comparison of the sorption of U onto olivine with granites of different origin indicate that the use of this mineral as additive to the backfill of deep high level nuclear waste repositories could retard the migration of U from the repository to the geosphere.  相似文献   

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