Environ. Sci. Technol. 2009, 43, 8593–8598
Uranium Uptake by Hectorite and Montmorillonite: A Solution Chemistry and Polarized EXAFS Study M I C H E L L . S C H L E G E L * ,†,‡ A N D M I C H A E L D E S C O S T E S ‡,§,| CEA, DEN, DPC/SCP/Laboratory for the Reactivity of Surfaces and Interfaces, F-91191 Gif-sur-Yvette, France, Universite´ d’Evry-Val d’Essonne, UMR 8587 “LAMBE”, F 91025 Evry, France, and CEA, DEN, DPC/SECR/Laboratory of Radionuclides Migration Measurements and Modelling, F-91191 Gif-sur-Yvette, France
Received July 20, 2009. Revised manuscript received September 25, 2009. Accepted October 8, 2009.
The mechanism of U(VI) retention on montmorillonite and hectorite at high ionic strength (0.5 M NaCl) was investigated by solution chemistry and, at near-neutral pH, polarized EXAFS spectroscopy. Uranium(VI) sorption increases from pH 3 to 7 on the two clays, but with a steeper edge for hectorite. Uranium(VI) is no longer retained at pH > 9, presumably owing to the formation of soluble anionic complexes. Polarized EXAFS showed that U(VI) retains its uranyl conformation on montmorillonite (U_mont) and hectorite (U_hect), with uranyl O at 1.79(2) Å for U_mont and 1.82(2) Å for U_hect, and split equatorial O shells at 2.29(2) and 2.47(2) Å (U_mont), or 2.35(2) and 2.53(2) Å (U_hect). An additional atomic shell of ∼0.5 Al/ Si at 3.3 Å is detected for U_mont, but neither the oxygen nor the cationic shell exhibit clear angular dependence. These results indicate the formation of mononuclear complexes at the edges of montmorillonite platelets, with the orientation of the uranyl axis equal to the magic angle, as constrained by the edges’ structural properties. In contrast to U_mont, the U-O signal varies with the polarization angle in U_hect, and the cationic Mg/ Si contribution at 3.2 Å is weak. The structure of this surface complex is not completely elucidated; it may correspond either to sorption on silanol sites, or to coprecipitation. These results lay out the fundamental molecular-scale basis to understand U retention by neoformed clay layers of nuclear glasses.
Introduction Nuclear energy relies on uranium as the prime fissile element in power plants. Thus, U dispersed in mine tailings or concentrated in nuclear spent fuel is a potent contaminant (1). Yet, migration and bioavailability of U can be mitigated by sorption or (co)precipitation on mineral surfaces. Thus, * Corresponding author e-mail:
[email protected]; phone: +33 1 69 08 93 84; fax: +33 1 69 08 54 11. † CEA, DEN, DPC/SCP/Laboratory for the Reactivity of Surfaces and Interfaces. ‡ Universite´ d’Evry-Val d’Essonne. § CEA, DEN, DPC/SECR/Laboratory of Radionuclides Migration Measurements and Modelling. | Present address: AREVA NC, Mining Business Unit, F-92084 Paris la De´fense, France. 10.1021/es902001k CCC: $40.75
Published on Web 10/20/2009
2009 American Chemical Society
U interaction with montmorillonite, a highly reactive Alsmectite which is common in soils and bentonite used for engineering barriers (2), has been much studied by solution chemistry (3–6), luminescence spectroscopy (7, 8), and extended X-ray absorption fine structure (EXAFS) spectroscopy (9–16). Uranium(VI) forms an outer-sphere surface complex on basal plane sites at low ionic strength and pH. At near-neutral pH, U(VI) was suggested to form inner-sphere surface complexes at layer edges, but the exact nature of the sorption complex remains unclear, as U(VI) was proposed to bind either Al or Fe octahedra (14, 15). Direct EXAFS insight into the binding mechanism of U(VI) on solids is hindered by interferences between single-scattering (SS) EXAFS paths from nearest cationic shells, and multiple scattering (MS) paths within the uranyl moiety (17). This difficulty can be circumvented by polarized X-ray absorption spectroscopy (P-XAS), and using this technique confirmed a direct link between U(VI) and the sorbent phase (9, 16). However, P-XAS studies were performed at low pH (eq1 ) 66(10)° and βOeq2 ) 72(10)°, confirming that Oeq shells are oriented in plane, nearly parallel to clay platelets. Note that self-supporting films are not perfectly textured, which causes some reduction of the angular dependence (38). Therefore, structural βOyl and βOeq are closer to 0° and 90°, respectively, than they experimentally seem to be. Finally, peak D could be fit with MS contributions plus a Si/Mg backscattering shell at 3.20(4) R values greater than uncertainties. Å (Table 2), with NU-Mg/Si EXAFS Peak A for U_mont was modeled with ∼2.5(7) Oyl at RU-O yl ) 1.79(2) Å (σ ) 0.059 Å; Table 2), consistent with the expected U-Oyl distance and Oyl coordination. Next-nearest contributions were modeled with Oeq shells at 2.29(2) and 2.47(2) Å (σ ) 0.073 Å). Finally, both MS contributions and a shell of ∼0.5 Al/Si at 3.31(3) Å (σ ) 0.059 Å) were needed to fit peak D.
Discussion Spectral analysis shows that U(VI) has distinct bonding environments in U_mont and U_hect. For U_hect, the uranyl axis is oriented mostly out of plane and thus the Oeq shell is oriented mostly in plane. This anisotropy is clear evidence for the structural association of uranyl with the clay platelet. However, the only EXAFS contribution of the neighboring cations is weak. In contrast, no angular dependence is observed for U_mont, but a significant cationic contribution close to 3.3 Å is observed. For the two phyllosilicates, invoking the presence of C atoms at ∼2.9 Å and of related MS contributions at R + ∆R ∼ 4.2 Å yields poorer fits, meaning that smectite-U(VI)-carbonate ternary complexes are marginal, if present at all. 8596
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Mechanism of U Sorption on Montmorillonite. Best fit results for U_mont yield two Oeq subshells at ∼2.29 and ∼2.47 EXAFS value Å, and one Al/Si shell at ∼3.31 Å. The average RU-O eq compares favorably with distances reported previously (e.g., 2.35-2.37 Å) (14), and N OR eq values are fairly consistent with a split equatorial shell of two Oeq1 and three Oeq2 at shorter and longer distances, respectively. Also, REXAFS U-Al/Si ) 3.31 Å equals the value reported for U(VI)-sorbed imogolite (39), but is EXAFS ≈ 3.4 Å from Hennig et al. significantly shorter than RU-Al/Si (14). Interestingly, these authors estimated from ionic radii an U-Al distance of ∼3.3 Å for U(VI) attached to the edge EXAFS suggests the formation of of an Al octahedron. Thus, RU-Al/Si an edge-sharing bidentate complexe with Al octahedra at the edges of montmorillonite layers (Figure 3a). The small NRAl values (0.6(3)-0.4(4)) and the lack of angular dependence R can be easily explained by damping interferences of NAl between the SS U(VI)-Al path and MS U-Oyl paths. Also, the proposed sorption geometry implies the presence of Si neighbors at about the same distance as Al, with U-Al and U-Si pairs being misaligned. Therefore, the resulting (Al, Si) contribution would be essentially only slightly variant with R. As U(VI) binds to the edge of Al octahedra at the montmorillonite surface, βOyl can be estimated from the shape of Al octahedra (Figure 3b). If U is in the same plane as the Al atoms, then the U-Oyl vector makes an angle βOyl of ∼55° with the direction perpendicular to the clay platelets. This βOyl value corresponds to the magic angle, for which NR is invariant with R. The exact value of βOyl depends on the flattening of phyllosilicate structural octahedra. However, this flattening is modest for Al phyllosilicates (40) and would modify the orientation of the uranyl group by only ∼5°, far smaller than experimental uncertainty. Therefore, simple geometrical considerations show that the absence of angular dependence for the Oyl shell is fully consistent with the formation of an edge surface complex. Mechanism of U Sorption on Hectorite. Although REXAFS U - Oeq values are larger by 0.06 Å for U_hect than for U_mont, NROeq are smaller for U_hect, contrary to the expected increase in coordination with increasing average U-O distance (41). The strong angular dependence of the U-Oeq pairs and the absence of U(VI) precipitation support the formation of an inner-sphere surface complex, either monomeric or oligomeric, on hectorite. Thus, the small contribution at ∼3.2 Å can result from Mg or Si atoms, or both. Because of the structural similarities between hectorite and montmorillonite edges, it would seem logical to suggest that U(VI) binds to Mg octahedra. However, Mg-O distances in Mg octahedra (2.12 Å) are larger than Al-O distances in Al octahedra (1.90 EXAFS EXAFS Å) (42), and so RU-Mg/Si is expected to be larger than RU-Al , which is not the case. Another argument against predomi-
Implication for U Fate in the Environment. This study confirms the importance of aluminol sites on montmorillonite edges for U(VI) retention at near-neutral pH (3). However, and unlike transition metals such as Zn and Ni, this “oxide”-like sorption does not promote nucleation and growth of uranyl solids at higher pH, because U(VI) desorbs at pH g 8.5, even in the absence of CO2. This results means that U(VI) fixation by montmorillonite is temporary, as a change in solution composition such as pH, concentrations, or even dissolved ligands, may easily remobilize U(VI). Hectorite retains U(VI) over a larger pH range than montmorillonite, and sorption starts at lower pH values. This increase in U(VI) uptake at lower pH may result from the formation of inner-sphere surface complexes, or of oligomers sorbing on hectorite particles. At basic pH, however, U(VI) sorbed on hectorite can be remobilized, as is the case for montmorillonite.
Acknowledgments O. Proux and J.-L. Hazemann are thanked for their assistance during EXAFS measurements on FAME. J. Catalano is gratefully acknowledged for providing EXAFS spectra of U(VI) in reference solids. This work was supported by the FrenchCRG program at ESRF and by the 7th EC-integrated project FUNMIG. We also thank three anonymous reviewers for their constructive comments and useful suggestions to improve the manuscript.
Supporting Information Available FIGURE 3. (a) Proposed structural model for U(VI) sorption on montmorillonite. Equatorial ligands (water molecules or hydroxyls) are stripped for clarity. (b) Side view of the edge complex formed on the border sites of montmorillonite. The uranyl axis (dash-dotted line) slightly deviates from the magic angle (solid line) owing to the flattening of Al octahedra. (c) Hypothetical structural model for U(VI) bound to Si sites at hectorite edges. nance of Mg-bound edge complexes is that βOyl would be not far from 55°, contrary to the experimental value. Alternatively, U(VI) may bind to silicate groups from the hectorite tetrahedral sheet (Figure 3c). For example, uranyl in solution can form bidentate complexes with sulfate groups, EXAFS ) 3.1 Å (43). Because the Si-O distance in silicate with RU-S (1.62 Å) is ∼0.13 Å longer than the S-O distance in sulfate EXAFS EXAFS is expected to be larger than RU-S by ∼0.1 (1.49 Å), RU-Si EXAFS ∼3.2 Å compares well with the Å. The predicted RU-Si experimental REXAFS U-Mg/Si. Uranyl binding to silanol sites at nearneutral pH has been proposed for Al smectite (4). Yet, that all U(VI) would bind to Si tetrahedra in our experiment would be surprising. The last possibility is the formation of disordered U(VI) oligomers at the hectorite surface. These oligomers should have a layer structure to account for the anisotropy of the EXAFS signal. Thus, soddyite and boltwoodite are candidate solids (41), inasmuch as [Si]aq is 1-2 orders of magnitude greater in hectorite suspensions than in montmorillonite ones (Figure SI-2). However, increasing [Si]aq to values typical of hectorite did not significantly enhance U(VI) sorption on montmorillonite at pH 4.5. Still another possibility is the surface precipitation of sklodowskite-like oligomers, with uranyl axes perpendicular to the sheet plane. Speciation calculations showed that [U(VI)]aq is only slightly below the solubility limit of sklodowskite (see Figure SI-1). These conditions favor the formation of positively charged oligomers, which can be stabilized by Coulombic interaction with the negative charge of the clay basal planes.
Figure SI-1 displays solubility diagrams for U in the investigated suspensions; Figure SI-2 shows [Si]aq, [Mg]aq and [Al]aq in the suspensions at the end of the sorption experiments; Figure SI-3 compares powder EXAFS spectra and FTs for U(VI) sorbed on montmorllonite, sorbed on hectorite, and in references; Figure SI-4 compares FTs and IFTs for UO22+(aq), U_hect, and U_mont in the [2.5, 3.5 Å] distance range; Figure SI-5 presents FTs from P-EXAFS data with simulations. The derivation of the equation between apparent and real number of neighboring atoms in a L3-edge P-EXAFS experiment also is detailed. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Burns, P. C., Finch, R., Eds. Uranium: Mineralogy, Geochemistry and the Environment; Reviews in Mineralogy, Vol. 38; Mineralogical Society of America: Washington, DC, 1999. (2) Madsen, F. T. Clay mineralogical investigations related to nuclear waste disposal. Clay Miner. 1998, 33, 109–129. (3) McKinley, J. P.; Zachara, J. M.; Smith, S. C.; Turner, G. D. The influence of uranyl hydrolysis and multiple site-binding reactions on adsorption of U(VI) to montmorillonite. Clays Clay Miner. 1995, 43, 586–598. (4) Turner, G. D.; Zachara, J. M.; McKinley, J. P.; Smith, S. C. Surfacecharge properties and UO22+ adsorption of a subsurface smectite. Geochim. Cosmochim. Acta 1996, 60, 3399–3414. (5) Akc¸ay, H. Aqueous speciation and pH effect on the sorption behavior of uranium by montmorillonite. J. Radioanal. Nucl. Chem. 1998, 237, 133–137. (6) Hyun, S. P.; Cho, Y. H.; Hahn, P. S.; Kim, S. J. Sorption mechanism of U(VI) on a reference montmorillonite: Binding to the internal and external surfaces. J. Radioanal. Nucl. Chem. 2001, 250, 55– 62. (7) Morris, D. E.; Chisholm-Brause, C. J.; Barr, M. E.; Conradson, S. D.; Eller, P. G. Optical spectroscopic studies of the adsorption of UO22+ species on a reference smectite. Geochim. Cosmochim. Acta 1994, 58, 3613–3623. (8) Chisholm-Brause, C. J.; Berg, J. M.; Matzner, R. A.; Morris, D. E. Uranium(VI) sorption complexes on montmorillonite as a function of solution chemistry. J. Colloid Interface Sci. 2001, 233, 38–49. (9) Hudson, E. A.; Terminello, L. J.; Viani, B. E.; Denecke, M.; Reich, T.; Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Edelstein, N. M. The structure of U6+ sorption complexes on vermiculite and hydrobiotite. Clays Clay Miner. 1999, 47, 439–457. VOL. 43, NO. 22, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
8597
(10) Dent, A. J.; Ramsay, J. D. F.; Swanton, S. W. An EXAFS study of uranyl ion in solution and sorbed onto silica and montmorillonite clay suspensions. J. Colloid Interface Sci. 1992, 150, 45– 60. (11) Chisholm-Brause, C. J.; Conradson, S. D.; Buscher, C. T.; Eller, P. G.; Morris, D. E. Speciation of uranyl sorbed at multiple binding sites on montmorillonite. Geochim. Cosmochim. Acta 1994, 58, 3625–2631. (12) Sylwester, E. R.; Hudson, E. A.; Allen, P. G. The structure of uranium (VI) sorption complexes on silica, alumina, and montmorillonite. Geochim. Cosmochim. Acta 2000, 64, 2431– 2438. (13) Kowal-Fouchard, A.; Drot, R.; Simoni, E.; Ehrhardt, J. J. Use of spectroscopic techniques for uranium (VI)/montmorillonite interaction modeling. Environ. Sci. Technol. 2004, 38, 1399– 1407. (14) Hennig, C.; Reich, T.; Da¨hn, R.; Scheidegger, A. M. Structure of uranium sorption complexes at montmorillonite edge sites. Radiochim. Acta 2002, 90, 653–657. (15) Catalano, J. G.; Brown, G. E. Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation. Geochim. Cosmochim. Acta 2005, 69, 2995–3005. (16) Greathouse, J. A.; Stellalevinsohn, H. R.; Denecke, M. A.; Bauer, A.; Pabalan, R. T. Uranyl surface complexes in a mixed-charge montmorillonite: Monte Carlo computer simulation and polarized XAFS results. Clays Clay Miner. 2005, 53, 278–286. (17) Den Auwer, C.; Drot, R.; Simoni, E.; Conradson, S. D.; Gailhanou, M.; de Leon, J. M. Grazing incidence XAFS spectroscopy of uranyl sorbed onto TiO2 rutile surfaces. New J. Chem. 2003, 27, 648– 655. (18) Abdelouas, A.; Crovisier, J. L.; Lutze, W.; Grambow, B.; Dran, J. C.; Muller, R. Surface layers on a borosilicate nuclear waste glass corroded in MgCl2 solution. J. Nucl. Mater. 1997, 240, 100– 111. (19) Schlegel, M. L.; Manceau, A.; Chateigner, D.; Charlet, L. Sorption of metal ions on clay minerals. I. Polarized EXAFS evidence for the adsorption of cobalt on the edges of hectorite particles. J. Colloid Interface Sci. 1999, 215, 140–158. (20) Schlegel, M. L. Polarized EXAFS characterization of the sorption mechanism of yttrium on hectorite. Radiochim. Acta 2008, 96, 667–672. (21) Schlegel, M. L.; Manceau, A. Evidence for the nucleation and epitaxial growth of Zn phyllosilicate on montmorillonite. Geochim. Cosmochim. Acta 2006, 70, 901–917. (22) Guillaumont, R.; Fangha¨nel, T.; Fuger, J.; Grenthe, I.; Neck, V.; Palmer, D. A.; Rand, M. H. Update on the Chemical Thermodynamics of Uranium, Plutonium, Americium and Technetium; OECD, NEA: Amsterdam 2003. (23) Nguyen, S. N.; Silva, R. J.; Weed, H. C.; Andrews, J. E., Jr. Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite. J. Chem. Thermodyn. 1992, 24, 359–376. (24) Descostes, M.; Schlegel, M. L.; Eglizaud, N.; Descamps, F.; Miserque, F.; Simoni, E. Uptake of uranium and trace elements in Pyrite (FeS2) suspensions. Geochim. Cosmochim. Acta, accepted with revisions. (25) Proux, O.; Biquard, X.; Lahera, E.; Menthonnex, J.-J.; Prat, A.; Ulrich, O.; Soldo, Y.; Tre´visson, P.; Kapoujvan, G.; Perroux, G.; Taumer, P.; Grand, O.; Jeantet, P.; Deleglise, M.; Roux, J.-P.;
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9
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(26) (27) (28) (29) (30) (31)
(32) (33)
(34) (35) (36)
(37) (38) (39)
(40) (41)
(42) (43)
Hazemann, J.-L. FAME: a new beamline for X-ray absorption investigation of very-diluted systems of environmental materials and biological interest. Phys. Scr. 2005, T115, 970–973. Teo, B. K. EXAFS: Basic Principles and Data Analysis; SpringerVerlag: Berlin, 1986. Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537–541. Manceau, A.; Combes, J.-M. Structure of Mn and Fe oxides and oxyhydroxides: a topological approach by EXAFS. Phys. Chem. Miner. 1988, 15, 283–295. Ankudinov, A. L.; Rehr, J. J. Relativistic calculations of spindependent x-ray absorption spectra. Phys. Rev. B 1997, 56, 1712– 1715. Demartin, F.; Gramaccioli, C. M.; Pilati, T. The importance of accurate crystal structure determination of uranium minerals. 2. Soddyite (UO2)2(SiO4).2H2O. Acta Crystallogr. 1992, C48, 1–4. Hudson, E. A.; Allen, P. G.; Terminello, L. J.; Denecke, M. A.; Reich, T. Polarized x-ray-absorption spectroscopy of the uranyl ion: Comparison of experiment and theory. Phys. Rev. B 1996, 54, 156–165. Schlegel, M. L. De l’adsorption du cobalt et du zinc sur l’hectorite et le quartz, a` la ne´oformation de phyllosilicates. PhD Thesis, Universite´ Joseph Fourier: Grenoble, France, 2000. Petiau, J.; Calas, G.; Petitmaire, D.; Bianconi, A.; Benfatto, M.; Marcelli, A. Delocalized versus localized 5f states and the uranium site structure in uranium oxides and glasses probed by X-ray absorption near-edge structure. Phys. Rev. B 1986, 34, 7350–7361. Manceau, A.; Bonnin, D.; Kaiser, P.; Fretigny, P. Polarized EXAFS of biotite and chlorite. Phys. Chem. Miner. 1988, 16, 180–185. Citrin, P. H. Bond lengths and coordination numbers from L2,3edge versus K-edge surface etxended x-ray absorption fine structure. Phys. Rev. B 1985, 31, 700–721. Ulrich, K. U.; Rossberg, A.; Foerstendorf, H.; Zanker, H.; Scheinost, A. C. Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids. Geochim. Cosmochim. Acta 2006, 70, 5469–5487. Barnes, C. E.; Shin, Y.; Saengkerdsub, S.; Dai, S. EXAFS study of uranyl nitrate dimer at high and low temperature. Inorg. Chem. 2000, 39, 862–864. Manceau, A.; Schlegel, M. L. Texture effect on polarized EXAFS amplitude. Phys. Chem. Miner. 2000, 28, 52–56. Arai, Y.; McBeath, M.; Bargar, J. R.; Joye, J.; Davis, J. A. Uranyl adsorption and surface speciation at the imogolite-water interface: Self-consistent spectroscopic and surface complexation models. Geochim. Cosmochim. Acta 2006, 70, 2492–2509. Gu ¨ ven, N. Smectites. In Hydrous Phyllosilicates; Bailey, S. W., Ed.; Mineralogical Society of America: Washington, DC, 1988; Vol. 19, pp 497-559. Burns, P. C.; Ewing, R. C.; Hawthorne, F. C. The crystal chemistry of hexavalent uranium: Polyhedron geometries, bond-valence parameters, and polymerization of polyhedra. Can. Mineral. 1997, 35, 1551–1570. Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. 1976, A32, 751–767. Hennig, C.; Schmeide, K.; Brendler, V.; Moll, H.; Tsushima, S.; Scheinost, A. C. EXAFS investigation of U(VI), U(IV), and Th(IV) sulfato complexes in aqueous solution. Inorg. Chem. 2007, 46, 5882–5892.
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