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1.
The O 1s spectrum is examined for 19 uranyl minerals of different composition and structure. Spectra from single crystals were measured with a Kratos Axis Ultra X-ray Photoelectron Spectrometer with a magnetic-confinement charge-compensation system. Well-resolved spectra with distinct maxima, shoulders and inflections points, in combination with reported and measured binding energies for specific O2− species and structural data of the uranyl minerals are used to resolve the fine structure of the O 1s envelope. The resolution of the O 1s spectra includes, for the first time, different O2− bands, which are assigned to O atoms linking uranyl with uranyl polyhedra (UOU) and O atoms of uranyl groups (OUO). The resolved bands in the O 1s spectrum occur at distinct ranges in binding energy: bands for (UOU) occur at 529.6-530.4 eV, bands for (OUO) occur at 530.6-531.4 eV, bands for O2− in the equatorial plane of the uranyl polyhedra linking uranyl polyhedra with (TOn) groups (T = Si, S, C, P, Se) (TO) occur at 530.9-532.2 eV; bands for (OH) groups in the equatorial plane of the uranyl polyhedra (OH) occur at 532.0-532.5 eV, bands of (H2O) groups in the interstitial complex of the uranyl minerals (H2Ointerst) occur at 533.0-533.8 eV and bands of physisorbed (H2O) groups on the surface of uranyl minerals (H2Oadsorb) occur at 534.8-535.2 eV. Treatment of uranyl minerals with acidic solutions results in a decrease in UOU and an increase in OH. Differences in the ratio of OH : OUO between the surface and bulk structure is larger for uranyl minerals with a high number of UOU and TO species in the bulk structure which is explained by protonation of underbonded UO, UOU and TO terminations on the surface. The difference in the ratio of H2Ointerst : OUO between the bulk and surface structures is larger for uranyl minerals with higher percentages of H2Ointerst as well as, with a higher number of interstitial H2O groups that are not bonded to interstitial cations, resulting in easier dehydration of interstitial H2O groups in uranyl minerals during exposure to a vacuum.  相似文献   

2.
The current study provides an investigation of abiotic reduction of an oversaturated uranyl solution driven by iron nanoparticle oxidation. The reactivity of nano-scale zero-valent iron (ZVI) under mildly oxic conditions (1.2% O2 and 0.0017% CO2) was studied in 1000 ppm uranyl solution in the pH range 3-7, at reaction times from 10 min to 4 h. Reductive precipitation of UO2 was observed as the main process responsible for the removal of uranium from solution with the kinetics of reaction becoming increasingly favourable at higher pH. Despite working with an oversaturated uranium solution, the precipitation of UO2 occurred in preference to precipitation of UO3·2H2O (metaschoepite) at reaction times between 1 and 4 h and for uranyl solutions initially set up at pH ?5. Characterisation of both solid and solution phases was performed using X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) imaging, X-ray diffraction (XRD) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).  相似文献   

3.
Uranium co-precipitation with iron oxide minerals   总被引：2，自引：0，他引：2  
In oxidizing environments, the toxic and radioactive element uranium (U) is most soluble and mobile in the hexavalent oxidation state. Sorption of U(VI) on Fe-oxides minerals (such as hematite [α-Fe2O3] and goethite [α-FeOOH]) and occlusion of U(VI) by Fe-oxide coatings are processes that can retard U transport in environments. In aged U-contaminated geologic materials, the transport and the biological availability of U toward reduction may be limited by coprecipitation with Fe-oxide minerals. These processes also affect the biological availability of U(VI) species toward reduction and precipitation as the less soluble U(IV) species by metal-reducing bacteria.To examine the dynamics of interactions between U(VI) and Fe oxides during crystallization, Fe-oxide phases (containing 0.5 to 5.4 mol% U/(U + Fe)) were synthesized by means of solutions of U(VI) and Fe(III). Wet chemical (digestions and chemical extractions) and spectroscopic techniques were used to characterize the synthesized Fe oxide coprecipitates after rinsing in deionized water. Leaching the high mol% U solids with concentrated carbonate solution (for sorbed and solid-phase U(VI) species) typically removed most of the U, leaving, on average, about 0.6 mol% U. Oxalate leaching of solids with low mol% U contents (about 1 mol% U or less) indicated that almost all of the Fe in these solids was crystalline and that most of the U was associated with these crystalline Fe oxides. X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopic studies indicate that hematite formation is preferred over that of goethite when the amount of U in the Fe-oxides exceeds 1 mol% U (∼4 wt% U). FT-IR and room temperature continuous wave luminescence spectroscopic studies with unleached U/Fe solids indicate a relationship between the mol% U in the Fe oxide and the intensity or existence of the spectra features that can be assigned to UO22+ species (such as the IR asymmetric υ3 stretch for O = U = O for uranyl). These spectral features were undetectable in carbonate- or oxalate-leached solids, suggesting solid phase and sorbed U(VI)O22+ species are extracted by the leach solutions. Uranium L3-edge x-ray absorption spectroscopic (XAFS) analyses of the unleached U-Fe oxide solids with less than 1 mol% U reveal that U(VI) exists with four O atoms at radial distances of 2.19 and 2.36 Å and second shell Fe at a radial distance at 3.19 Å.Because of the large ionic radius of UO22+ (∼1.8 Å) relative to that of Fe3+ (0.65 Å), the UO22+ ion is unlikely to be incorporated in the place of Fe in Fe(III)-oxide structures. Solid-phase U(VI) can exist as the uranyl [U(VI)O22+] species with two axial U-O double bonds and four or more equatorial U-O bonds or as the uranate species (such as γ-UO3) without axial U-O bonds. Our findings indicate U6+ (with ionic radii of 0.72 to 0.8 Å, depending on the coordination environment) is incorporated in the Fe oxides as uranate (without axial O atoms) until a point of saturation is reached. Beyond this excess in U concentration, precipitating U(VI) forms discrete crystalline uranyl phases that resemble the uranyl oxide hydrate schoepite [UO2(OH)2·2H2O]. Molecular modeling studies reveal that U6+ species could bond with O atoms from distorted Fe octahedra in the hematite structure with an environment that is consistent with the results of the XAFS. The results provide compelling evidence of U incorporation within the hematite structure.  相似文献   

4.
Potential-based molecular dynamics simulations of aqueous uranyl carbonate species (MxUO2(CO3)y2+2x−2y with M = Mg, Ca, or Sr) were carried out to gain molecular-level insight into the hydration properties of these species. The simulation results were used to estimate the self-diffusion coefficients of these uranyl carbonate species, which often dominate uranyl speciation in groundwater systems. The diffusion coefficients obtained for the monoatomic alkaline-earth cations and polyatomic ions (uranyl, carbonate, and uranyl tri-carbonate) were compared with those calculated from the Stokes-Einstein (SE) equation and its variant formulation by Impey et al. (1983). Our results show that the equation of Impey et al. (1983), originally formulated for monovalent monoatomic ions, can be extended to divalent monoatomic ions, with some success in reproducing the absolute values and the overall trend determined from the molecular dynamics simulations, but not to polyatomic ions, for which the hydration shell is not spherically symmetrical. Despite the quantitative failure of both SE formulations, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed that a general linear dependence is observed for these complexes as expected from the SE equation. The nature of the alkaline-earth cation in the uranyl carbonate complexes was not found to have a significant effect on the ion’s diffusion coefficient, which suggests that the use of a single diffusion coefficient for different alkaline-earth uranyl carbonate complexes in microscopic diffusion models is appropriate.The potential model reproduced well published quantum mechanical and experimental data of and of the individual constituent ions, and therefore is expected to offer reliable predictions of the structure of magnesium and strontium uranyl carbonate aqueous species, for which there is no structural data available to date. In addition, the interatomic distances reported for could help with the refinement of the interpretation of EXAFS data of these species, which is made difficult by the similar uranium-distant carbonate oxygen and uranium-calcium distances.An analysis of the dynamics of water exchange around the alkaline-earth cations revealed that the presence of the uranyl tri-carbonate molecule has a strong influence on the geometry of the cation’s first hydration shell, which, in turn, can considerably affect the water exchange kinetics depending on whether the imposed geometry matches that around the isolated alkaline-earth cation. This result shows that the alkaline-earth uranyl carbonate complexes have distinct water exchange dynamics, which may lead to different reactivities. Finally, significant changes in water residence time were also predicted when replacing carbonate for water ligands in the uranyl coordination shell.  相似文献   

5.
Pyrochlores, microlites, and U-betafites of pyrochlore group minerals were obtained from mixing experiments of the corresponding oxides and fluorides by hydrothermal synthesis at T = 800 °C and P = 200 MPa in the solution of 1.0 M NaF. The presence of U4+ in pyrochlore does not affect the cell parameter, which for the phases of pyrochlore–microlite series is 10.42 ± 0.01 Å. In a system with an excess of UO2, pyrochlores and microlites, containing uranium up to 0.2–0.3 atoms per formula unit (apfu), are formed. In the uranium-free system of betafites composition, perovskites and Ti-bearing pyrochlores are formed. U-pyrochlores of betafite series, containing 2Ti = Nb + Ta in moles, have cubic cell parameters of 10.26 ± 0.02 Å and U4+ isomorphic capacity of 0.4–0.5 apfu. In the pyrochlore structure, U4+ may substitute for Ca2+ and Na+ cations in the eightfold site. In pyrochlores of pyrochlore–microlite series, Ca2+ is replaced by U4+, while in pyrochlores of betafite series, U4+ replaces Na+. Phases with pyrochlore structure, containing U5+ and U6+ in the sixfold site, usually occupied by Nb5+, Ta5+, and Ti4+, are formed under oxidizing conditions (Cu–Cu2O buffer). They are characterized by low content of Nb5+, Ta5+ (<0.1 apfu), and anomalous behavior of the crystal lattice (compression, instead of expansion). Under natural conditions, the formation of pyrochlores containing a significant amount of U5+ and U6+ is unlikely.  相似文献   

6.
The structure of H2O-saturated silicate melts and of silicate-saturated aqueous solutions, as well as that of supercritical silicate-rich aqueous liquids, has been characterized in-situ while the sample was at high temperature (to 800 °C) and pressure (up to 796 MPa). Structural information was obtained with confocal microRaman and with FTIR spectroscopy. Two Al-bearing glasses compositionally along the join Na2O•4SiO2-Na2O•4(NaAl)O2-H2O (5 and 10 mol% Al2O3, denoted NA5 and NA10) were used as starting materials. Fluids and melts were examined along pressure-temperature trajectories of isochores of H2O at nominal densities (from PVT properties of pure H2O) of 0.85 g/cm3 (NA10 experiments) and 0.86 g/cm3 (NA5 experiments) with the aluminosilicate + H2O sample contained in an externally-heated, Ir-gasketed hydrothermal diamond anvil cell.Molecular H2O (H2O°) and OH groups that form bonds with cations exist in all three phases. The OH/H2O° ratio is positively correlated with temperature and pressure (and, therefore, fugacity of H2O, fH2O) with (OH/H2O°)melt > (OH/H2O°)fluid at all pressures and temperatures. Structural units of Q3, Q2, Q1, and Q0 type occur together in fluids, in melts, and, when outside the two-phase melt + fluid boundary, in single-phase liquids. The abundance of Q0 and Q1 increases and Q2 and Q3 decrease with fH2O. Therefore, the NBO/T (nonbridging oxygen per tetrahedrally coordination cations), of melt is a positive function of fH2O. The NBO/T of silicate in coexisting aqueous fluid, although greater than in melt, is less sensitive to fH2O.The melt structural data are used to describe relationships between activity of H2O and melting phase relations of silicate systems at high pressure and temperature. The data were also combined with available partial molar configurational heat capacity of Qn-species in melts to illustrate how these quantities can be employed to estimate relationships between heat capacity of melts and their H2O content.  相似文献   

7.
Computational and experimental studies were performed to explore heterogeneous reduction of U6+ by structural Fe2+ at magnetite (Fe3O4) surfaces. Molecular Fe-Fe-U models representing a uranyl species adsorbed in a biatomic bidentate fashion to an iron surface group were constructed. Various possible charge distributions in this model surface complex were evaluated in terms of their relative stabilities and electron exchange rates using ab initio molecular orbital methods. Freshly-cleaved, single crystals of magnetite with different initial Fe2+/Fe3+ ratios were exposed to uranyl-nitrate solution (pH ∼ 4) for 90 h. X-ray photoelectron spectroscopy and electron microscopy indicated the presence of a mixed U6+/U5+ precipitate heterogeneously nucleated and grown on stoichiometric magnetite surfaces, but only the presence of sorbed U6+ and no precipitate on sub-stoichiometric magnetite surfaces. Calculated electron transfer rates indicate that sequential multi-electron uranium reduction is not kinetically limited by conductive electron resupply to the adsorption site. Both theory and experiment point to structural Fe2+ density, taken as a measure of thermodynamic reducing potential, and sterically accessible uranium coordination environments as key controls on uranium reduction extent and rate. Uranium incorporation in solid phases where its coordination is constrained to the uranate type should widen the stability field of U5+ relative to U6+. If uranium cannot acquire 8-fold coordination then reduction may proceed to U5+ but not necessarily U4+.  相似文献   

8.
Alteration of uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the UO2 in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform uraninite and (2) fine-grained uraninite. The colloform uraninite contains 1.04-1.75 wt% of WO3, 0.16-1.70 wt% of As2O3, 0.06-0.88 wt% of MoO3; whereas, the fine-grained uraninite retains 2.25-4.93 wt% of WO3, up to 5.76 wt% of MoO3, and 0.26-0.60 wt% of As2O3. The near constant concentration of incompatible W in the colloform uraninite suggests W-incorporation into the uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO2+x and U4O9; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37-2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite. Four stages of colloform uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U6+-phases with well-defined cleavage, and (iv) fracture of the uraninite surface along the cleavage planes of the U6+-phases.  相似文献   

9.
X-ray diffraction (XRD) studies on the radioactive ore samples from various parts of Rajasthan and Haryana have revealed the presence of several uranium and other atomic mineral occurrences in the albitite belt of western India. The primary uranium minerals (PUMs) are uraninite and brannerite, whereas, the secondary uranium minerals (SUMs) show considerable speciations: phosphate, silicate, hydrous oxide hydrate, and vanadate. Multiple oxides (MOs) are davidite, fergusonite, aeschynite-(Y), microlite, samarskite, euxenite, betafite, and columbite-tantalite. The thorium minerals are huttonite, thorite, uranoan-thorite, thorianite, thorutite, and brabantite. The yttrium and REE-bearing minerals are xenotime, britholite, allanite, chevkinite, tritomite, and monazite. It is noted that the measured unit cell dimension (a0) of the investigated uraninites ranges from 5.4110 Å to 5.4646 Å. The highest unit cell dimension (5.4646 Å) represents a composition (or oxidation grade) of UO2.05, whereas, the lowest one (5.4110 Å) corresponds to a composition of UO2.54. Furthermore, it is also apparent that, with increase in oxidation grade, there is a concomitant decrease in unit cell dimension. As most of the values of ao of uraninites from the albitite belt are high (> 5.45 Å), it may be inferred that the overall temperature of formation of uraninites of the albitite belt was higher (ca. 400°C). However, the low values of a0 in certain localities could be due to the prevalence of relatively low and fluctuating temperature regimes locally (ca. 400°–100° C). Numerous occurrences of refractory, multiple oxides, and REE minerals, in association with uranium mineralisation, also support a high-temperature origin for the investigated uraninites. Binary data plots of unit cell dimension (a0) versus oxidation grade/composition (UO2+x) of uraninites (n = 36) suggest that the gross uranium mineralisation in the albitite belt of western India is mainly linked to regional metamorphism, anatexis, granitic intrusion, metasomatism, and contact metamorphosed granite-pegmatite aureoles and granite-related vein type with hydrothermal overprints, including redistribution of intrinsic sedimentary uranium and its concentration along suitable structural locales. These interpretations are consistent with the known gross geologic features of the albitite belt. Furthermore, the presence of marialite (calcian) in many places in the albitite belt also supports such a contention, as this mineral is known to be restricted to metamorphic and metasomatic environments. The speciation of secondary uranium minerals could be due to the higher oxidation of U4+ to U6+ in surface to near-surface conditions and its (U6+) remobilisation as uranyl ions. The combination of moving uranyl ions with available cations and anions en route caused re-precipitation of U as diversified assemblages of low-temperature uranyl minerals under suitable physicochemical conditions.  相似文献   

10.
The Nopal I uranium deposit of the Sierra Peña Blanca, Mexico, has been the focus of numerous studies because of its economic importance and its use as a natural analog for nuclear-waste disposal in volcanic tuff. Secondary uranyl minerals such as uranophane, Ca[(UO2)(SiO3OH)]2(H2O)5, and weeksite, (K,Na)2[(UO2)2(Si5O13)](H2O)3, occur in the vadose zone of the deposit and are overgrown by silica glaze. These glazes consist mainly of opal A, which contains small particles of uraninite, UO2, and weeksite. Close to a fault between brecciated volcanic rocks and welded tuff, a greenish silica glaze coats the altered breccia. Yellow silica glazes from the center of the breccia pipe and from the high-grade pile coat uranyl-silicates, predominantly uranophane and weeksite. All silica glazes are strongly zoned with respect to U and Ca, and the distribution of these elements indicates curved features and spherical particles inside the coatings. The concentrations of U and Ca correlate in the different zones and both elements inversely correlate with the concentration of Si. Zones within the silica glazes contain U and Ca in a 1:1 ratio with maximum concentrations of 0.08 and 0.15 at.% for the greenish and yellow glazes, respectively, suggesting trapping of either Ca1U1-aqueous species or -particles in the colloidal silica. X-ray photoelectron spectroscopy (XPS), Fourier-transform infra-red spectroscopy (FTIR), and oxygen-isotope ratios measured by secondary-ion mass spectrometry (SIMS) indicate higher U6+/U4+ ratios, higher proportions of Si-OH groups and lower δ18O values for the greenish silica glaze than for the yellow silica glaze. These differences in composition reflect increasing brecciation, porosity, and permeability from the center of the breccia pipe (yellow silica glaze) toward the fault (green silica glaze), where the seepage of meteoric water and Eh are higher.  相似文献   

11.
Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO2) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO2 between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO2 in silicic, but less so in mafic bulk systems.  相似文献   

12.
《Applied Geochemistry》1997,12(5):549-565
The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository. The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite.UPb chemical ages of the 3 stages of U mineralization from Cigar Lake coincide with the 3 major fluid events that precipitated clay and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout the entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende have the lowest δ18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ18O values. Based on UPb chemical ages,δ18O values, and petrographic relationships of U alteration minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in the Athabasca Basin. Therefore, the Cigar Lake ore deposit, although surrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin.The 3 stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary context for the evaluation of the Cigar Lake deposit as a “natural analogue” for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield.  相似文献   

13.
High levels of U (up to 5570 μg/L) have been discovered in well waters near Simpsonville, South Carolina, USA. In order to characterize the mineralogical source of the U and possible structural controls on its presence, a deep (214 m) well was cored adjacent to one of the enriched wells. The highest gamma-ray emissions in the recovered core occur in coarse biotite granite at a depth just below 52 m. A slickenlined fault plane at 48.6 m and narrow pegmatite layers at depths of 113, 203 and 207 m also yield high gamma-ray counts. Thin sections were made from the above materials and along several subvertical healed fractures. Uraninite and coffinite are the principal U-rich minerals in the core. Other U-bearing minerals include thorite and thorogummite, monazite, zircon and allanite. Primary uraninite occurs in the biotite granite and in pegmatite layers. Secondary coffinite is present as tiny (<5 μm) crystals dispersed along fractures in the granite and pegmatites. Coffinite also occurs along the slickenlined fault plane, where it is associated with calcite and calcic zeolite and also replaces allanite. Coffinite lacks radiogenic Pb, hence is considerably younger than the uraninite.Dissolution of partially oxidized Ca-rich uraninite occurring in the surficial biotite granite (or secondary coffinite in fracture zones) is likely the main source for the current high levels of U in nearby area wells. The high-U well waters have a carbonate signature, consistent with pervasive calcite vein mineralization in the core. Aqueous speciation calculations suggest U transport as an uranyl (U6+) hydroxyl-carbonate complex. Later reduction resulted in secondary precipitation along fractures as a U4+ mineral (i.e., coffinite).  相似文献   

14.
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   

15.
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   

16.
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   

17.
Mössbauer spectroscopy has been used widely to characterize the ferric (Fe3+) and ferrous (Fe2+) proportions and coordination of solid materials. To obtain these accurately, the recoilless fraction is indispensible. The recoilless fractions (f) of iron-bearing minerals, including oxides, oxyhydroxides, silicates, carbonates, phosphates and dichalcogenides, and silicate glasses were evaluated from the temperature dependence of their center shifts or absorption area with the Debye model approximation. Generally, the resolved Debye temperature (θD) of ferric iron in minerals, except dichalcogenides, through their center shifts ranging from 400 to 550 K, is significantly larger than ferrous iron ranging from 300 to 400 K, which is consistent with the conclusion from previous work. The resolved f (Fe3+)RT with the center shift model (CSM) ranges from 0.825 to 0.925, which is larger than that obtained for f(Fe2+)RT, which ranges from 0.675 to 0.750. Meanwhile, the θD and f resolved from temperature-dependence of absorption are generally lower than from center shifts, especially for ferric iron. The significant difference between f(Fe3+) and f(Fe2+) indicates the necessity of recoilless fraction correction on the Fe3+/(Fe3++Fe2+) resolved from Mössbauer spectra.  相似文献   

18.
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   

19.
Detrital zircons from the Ob, Yenisey, Lena, Amur, Volga, Dnieper, Don and Pechora rivers have been analyzed for U-Th-Pb, O and Lu-Hf isotopes to constrain the growth rate of the preserved continental crust in Greater Russia. Four major periods of zircon crystallization, 0.1-0.55, 0.95-1.3, 1.45-2.0 and 2.5-2.9 Ga, were resolved from a compilation of 1366 zircon U/Pb ages. The Archean zircons have δ18O values lying between 4.53‰ and 7.33‰, whereas Proterozoic and Phanerozoic zircons have a larger range of δ18O values in each of the recognized U/Pb time intervals with maximum δ18O values up to 12‰. We interpret the zircons with δ18O between 4.5‰ and 6.5‰ to have been derived from a magmatic precursor that contains little or no sedimentary component. The variable δ18O values of the zircons were used to constrain the 176Lu/177Hf ratios of the crustal source region of the zircons, which, in turn, were used to calculate Hf model ages (TDMV). The crustal incubation time, the time difference between primitive crust formation (dated by TDMV) and crustal melting (dated by zircon U/Pb age), varies between 300 to 1000 Myr for the majority of analyzed zircon grains, but can be up to 2500 Myr. The average TDMV Hf model age weighted by the fraction of zircons in the river load is 2.12 Ga, which is in reasonable agreement with the area-weighted average of 2.25 Ga. The TDMV Hf model age crustal growth curve for zircons with mantle-like δ18O values (4.5-6.5‰), weighted by area, shows that growth of the Great Russian continental crust started at 4.2 Ga, and that there are two principal periods of crustal growth, 3.6-3.3 Ga and 0.8-0.6 Ga, which are separated by an interval of low but more or less continuous growth. An alternative interpretation, in which the average 176Lu/177Hf ratio (0.0115) of the continental crust is used for the Paleoproterozoic zircons from the Lena River, lowers the average TDMV age of these grains by about 500 Myr and delays the onset of significant crustal growth to 3.5 Ga.The two principal growth periods recognized in Greater Russia differ from those identified from the Gondwana and the Mississippi river basin, which show peaks at 1.7-1.9 and 2.9-3.1 Ga (Hawkesworth and Kemp, 2006a) and 1.6-2.2 and 2.9-3.4 Ga (Wang et al., 2009), respectively. The older 3.6-3.3 Ga or 3.5-3.3 Ga peak for Greater Russia is slightly older than the older Gondwana-Mississippi peaks, whereas the younger 0.8-0.6 Ga peak is distinctly younger than the youngest peak in either Gondwana or the Mississippi river basin. This suggests that the two major peaks of crustal growth identified in Gondwana and the Mississippi river basin may not be global periods of enhanced continental growth and that the major periods of crustal growth may differ from continent to continent.  相似文献   

20.
Pyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]aq/[Fe]aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm−2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10−6 mol g−1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10−8 mol g−1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s).Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U(VI).  相似文献   

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