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Modeling Adsorption of the Uranyl Dication on the Hydroxylated r-Al2O3(0001) Surface in an Aqueous Medium. Density Functional Study Lyudmila V. Moskaleva,† Vladimir A. Nasluzov,‡ and Notker Ro¨sch*,† Department Chemie, Theoretische Chemie, Technische UniVersita¨t Mu¨nchen, 85747 Garching, Germany, and Institute of Chemistry and Chemical Technology, Russian Academy of Sciences, 660049 Krasnoyarsk, Russia ReceiVed NoVember 4, 2005. In Final Form: December 6, 2005 As a first step toward modeling the interaction of dissolved actinide contaminants with mineral surfaces, we studied low-coverage adsorption of aqueous uranyl, UO22+, on the hydroxylated R-Al2O3(0001) surface. We carried out density functional periodic slab model calculations and modeled solvation effects by explicit aqua ligands. We explored the formation of both inner- and outer-sphere complexes and estimated the corresponding adsorption energies. Effects of solvation were accounted for by explicit consideration of the first hydration shell of uranyl and by means of a posteriori corrections for long-range solvent effect. With energetics described at the GGA-PW91 level and under the assumption of a fully protonated ideal surface, we predict a weakly bound outer-sphere adsorption complex.
Introduction Interest in actinide chemistry has greatly increased in recent years due to an ongoing concern associated with hazardous and radioactive materials introduced in the environment as a result of human activities.1 An important factor that governs the migration of radionuclides in the environment is their interaction with mineral-water interfaces.2,3 To predict the long-term fate of radionuclides by geochemical modeling, it is necessary to understand fundamental processes such as surface precipitation and surface complexation. The number of experimental X-ray absorption fine-structure (XAFS) studies of actinide adsorption onto mineral surfaces aiming at direct structural information is growing,4-12 and substantial progress has been achieved; however, detailed atomistic understanding of these processes is still lacking. Important questions addressed by experimental studies are the explicit chemical environment around a single atom or ion, inner-sphere vs outer-sphere complexation (i.e., formation of new chemical bonds between the adsorbate and the surface or simple ion exchange), and mono- vs polydentate binding modes; for details, e.g., see ref 4 and references therein. Most experiments † ‡
Technische Universita¨t Mu¨nchen. Russian Academy of Sciences.
(1) Johnson, J. Chem. Eng. News 2002, 80, 20-25. (2) Silva, R. J.; Nitsche, H. Radiochim. Acta 1995, 70, 377-396. (3) Den Auwer, C.; Simoni, E.; Conradson, S.; Madic, C. Eur. J. Inorg. Chem. 2003, 21, 3843-3859. (4) Catalano, J. G.; Trainor, T. P.; Eng, P. J.; Waychunas, G. A.; Brown, G. E., Jr. Geochim. Cosmochim. Acta 2005, 69, 3555-3572. (5) Denecke, M. A.; Bosbach, D.; Dardenne, K.; Lindqvist-Reis, P.; Rothe, J.; Yin, R. Z. Phys. Scr., T 2005, T115, 877-881. (6) Bargar, J. R.; Reitmeyer, R.; Lenhart, J. J.; Davis, J. A. Geochim. Cosmochim. Acta 2000, 64, 2737-2749. (7) Den Auwer, C.; Drot, R.; Simoni, E.; Conradson, S. D.; Gailhanou, M.; de Leon, J. M. New J. Chem. 2003, 27, 648-655. (8) Reich, T.; Moll, H.; Denecke, M. A.; Geipel, G.; Bernard, G.; Nitsche, H.; Allen, P. G.; Bucher, J. J.; Kaltsoyannis, N.; Edelstein, N. M.; Shuh, D. K. Radiochim. Acta 1996, 74, 219-223. (9) Reich, T.; Moll, H.; Arnold, T.; Denecke, M. A.; Henning, C.; Geipel, G.; Bernard, G.; Nitsche, H.; Allen, P. G.; Bucher, J. J.; Edelstein, N. M.; Shuh, D. K. J. Electron Spectrosc. 1998, 96, 237-243. (10) Waite, T. D.; Davis, J. A.; Payne, T. E.; Waychunas, G. A.; Xu, N. Geochim. Cosmochim. Acta 1994, 58, 5465-5478. (11) Sylvester, E. R.; Hudson, E. A.; Allen, P. G. Geochim. Cosmochim. Acta 2000, 64, 2431-2438. (12) Dent, A.; Ramsay, J. D. F.; Swanton, S. W. J. Colloid Interface Sci. 1992, 150, 45-60.
were carried out on powder substrates, where the results are averaged over different surface orientations.8-12 However, in a few cases substrates with a well-defined surface orientation have been used, making it possible to identify the atomic structure of specific sites.4-7,13,14 The adsorption properties of hexavalent uranyl species, UO22+, have been most extensively studied, as this is the typical form in which uranium is found under natural conditions.3 In adsorption systems on silica, alumina, and aluminosilicate minerals, studies based on X-ray absorption spectroscopy (XAS) invariably found the uranyl moiety to preserve its original geometry, while U-O distances in equatorial ligand shells sometimes showed a characteristic splitting by about 0.2 Å, attributed to the formation of inner-sphere complexes with the surface.3,11 Such inner-sphere complexation was observed only at near-neutral or higher pH conditions, whereas at low pH, an outer-sphere mechanism was found to be the principal method of adsorption.11 In general, experimental conditions, such as the pH, ionic strength, concentration, and chemical nature of counterions, are expected to affect the type of adsorption complexes formed. In the XAS technique, the signal unfortunately is averaged over all possible species. Also, information that can be extracted from XAS data does not allow one to directly determine whether coordinating oxygen centers are those of a solvating aqua or hydroxo ligand, or an oxygen atom from the surface. Structural uncertainty increases when several types of complexes are formed simultaneously in comparable amounts. Thus, theoretical quantum chemical tools may be able to help in the interpretation of experimental data and in understanding the sorption mechanism at a molecular level. Although quite a few quantum chemical calculations complementing experimental data on actinide coordination complexes in solution have been published in recent years,15-22 only very (13) Towle, S. N.; Brown, G. E., Jr.; Parks, G. A. J. Colloid Interface Sci. 1999, 217, 299-311. (14) Towle, S. N.; Bargar, J. R.; Brown, G. E., Jr. J. Colloid Interface Sci. 1999, 217, 312-321. (15) Vallet, V.; Wahlgren, U.; Szabo´, Z.; Grenthe, I. Inorg. Chem. 2002, 41, 5626-5633. (16) Sonnenberg, J. L.; Hay, P. J.; Martin, R. L.; Bursten, B. E. Inorg. Chem. 2005, 44, 2255-2262.
10.1021/la052973o CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006
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approximate theoretical models have been applied to actinide chemisorption.23-25 Here, we report for the first time a periodic density functional study of aqueous and hydroxoaquo uranyl adsorption onto the hydroxylated (0001) surface of R-Al2O3. Aluminum oxide is important for its role in the environment as a constituent of sedimentary rocks; as a structural fragment, it occurs in clays and micas.26 This substrate is well-characterized experimentally as well as theoretically. Particularly, the surface structure of hydroxylated R-Al2O3(0001), a common and energetically stable termination of R-Al2O3, has recently become available from accurate experimental studies.27,28 We explored the formation of both inner- and outer-sphere complexes and estimated the corresponding adsorption energies. Our adsorption models are applicable to a low pH range, at which the surface oxygens are mostly protonated. Effects of solvation were accounted for by explicit consideration of the first hydration shell of the uranyl and by means of a posteriori corrections for long-range solvent effect. Our calculations predict the formation of an inner-sphere adsorption complex with the defect-free fully hydroxylated surface to be energetically unfavorable, whereas formation of an outer-sphere complex is expected. Computational Methods We combined two modeling strategies: one based on slab models with periodic boundary conditions and one employing (finite) molecular models. In both cases, we carried out first-principles density functional calculations. The slab model calculations were carried out with the planewave-based Vienna ab initio simulation package (VASP)29 using the local density approximation (LDA) in the parametrization of Perdew and Zunger30 for structural optimizations, because LDA often yields more accurate results for molecular geometries than the generalized gradient approximation (GGA).31,32 This holds in particular for actinide complexes where this combination strategy has been proven successful in previous studies.17,18,33 Energetic properties were calculated with the gradient-corrected GGA-PW91 (17) Fuchs, M. S. K.; Shor, A. M.; Ro¨sch, N. Int. J. Quantum Chem. 2002, 86, 487-501. (18) Moskaleva, L. V.; Kru¨ger, S.; Spo¨rl, A.; Ro¨sch, N. Inorg. Chem. 2004, 43, 4080-4090. (19) Hay, P. J.; Martin, R. L.; Schreckenbach, G. J. Phys. Chem. A 2001, 104, 6259-6270. (20) Tsushima, S.; Yang, T.; Suzuki, A. Chem. Phys. Lett. 2001, 334, 365373. (21) Bolvin, H.; Wahlgren, U.; Moll, H.; Reich, T.; Geipel, G.; Fangha¨nel, T.; Grenthe, I. J Phys. Chem. A 2001, 105, 11441-11445. (22) Vazquez, J.; Bo, C.; Poblet, J. M.; de Pablo, J.; Bruno, J. Inorg. Chem. 2003, 42, 6136-6141. (23) Greathouse, J. A.; O’Brien, R. J.; Bemis, G.; Pabalan, R. T. J. Phys. Chem. B 2002, 106, 1646-1655. (24) Greathouse, J. A.; Stellalevinsohn, H. R.; Denecke, M. A.; Bauer, A.; Pabalan, R. T. Clays Clay Miner. 2005, 53, 278-286. (25) Wheaton, V.; Majumdar, D.; Balasurbramanian, K.; Chauffe, L.; Allen, P. G. Chem. Phys. Lett. 2003, 371, 349-359. (26) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford University Press: Oxford, 1984. (27) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Newville, M.; Sutton S. R.; Rivers, M. L. Science 2000, 288, 1029-1033. (28) Coustet, V.; Jupille, J. Surf. Interface Anal. 1994, 22, 280-283. (29) (a) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169-11186. (b) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251-14269. (c) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558-561. (d) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 48, 13115-13118. (e) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15-50. (30) Perdew, J. P.; Zunger, A. Phys. ReV. B 1981, 23, 5048-5079. (31) Go¨rling, A.; Trickey, S. B.; Gisdakis, P.; Ro¨sch, N. In Topics in Organometallic Chemistry; Brown, J., Hofmann, P., Eds.; Springer: Heidelberg, 1999; Vol. 4, pp 109-165. (32) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional Theory, 2nd ed.; Wiley-VCH: Weinheim, 2000. (33) Garcı´a-Herna´ndez, M.; Lauterbach, C.; Kru¨ger, S.; Matveev, A.; Ro¨sch, N. J. Comput. Chem. 2002, 23, 834-846.
MoskaleVa et al. exchange-correlation density functional.34 The interaction between atomic cores and electrons was described by the projector augmented wave (PAW) method.35,36 Scalar relativistic effects were incorporated into the PAW potential by adding the mass-velocity and Darwin correction terms. An energy cutoff of 450 eV was adopted throughout. This cutoff had been found sufficient in optimizations of a parent clean Al2O3(0001) slab model.37 Brillouin zone integrations were carried out with the help of a single k-vector (Γ point) using a generalized Gaussian smearing method38 with the standard smearing width of 0.01 eV. The Al2O3 substrate was modeled using a slab of eight layers, O-Al-Al-O-Al-Al-O-Al, exposing the (0001) surface. The resulting surface was assumed to be fully hydroxylated; i.e., an H atom was assumed to be adsorbed on top of each surface O atom. The (3 × 3) unit cell of this slab model contains 153 atoms: 27 H, 45 Al, and 81 O centers. The slabs were repeated periodically, with an initial vacuum spacing of 12 Å. The slab surfaces were first relaxed without adsorbates. The two “bottom” layers (O, Al) of the substrate were kept fixed at the bulk-terminated geometry obtained from a periodic LDA optimization, whereas all other layers were allowed to relax fully in all geometry optimizations. Pertinent parameters of the optimized rhombohedral primitive unit cell of the bulk structure (space group R3hc) were a ) b ) 4.78 Å and c ) 13.05 Å; the spacing between Al and O layers was 0.84 Å and that between adjacent Al layers was 0.49 Å. In the geometry optimizations, the total energy was converged to 10-3 eV, and forces acting on ions were required to be less than 10-2 eV/Å. For free species [UO2(H2O)5]2+, [UO2(H2O)3(OH)2], H2O, and H3O+, molecular calculations were performed with the linear combination of Gaussian type orbitals fitting-functions density functional method (LCGTO-FF DF)39 as implemented in the code ParaGauss.40,41 These calculations were used to calculate energetics of reactions involving charged species to avoid charged unit cells in periodic calculations. Scalar-relativistic calculations were carried out using the local density approximation (LDA) in the parametrization of Vosko, Wilk, and Nusair (VWN)42 for geometry optimizations and the gradient-corrected PW91 functional for energies. In the geometry optimizations, the total energy and elements of the density matrix were required to converge to 10-8 au; for the largest component of the forces and the geometry update step length, the convergence criteria were set to 10-5 au. The Kohn-Sham orbitals were represented by flexible Gaussian type basis sets, contracted in a generalized fashion using atomic eigenvectors of scalar relativistic LDA calculations. For U, we used a basis set of the type (24s,19p,16d,11f), contracted to [10s,7p,7d,4f];43 O and H atoms were described by standard basis sets,44 (9s,5p,1d) f [5s,4p,1d] and (6s,1p) f [4s,1p], respectively. Long-range solvation effects were taken into account with a polarizable continuum model (PCM), using the COSMO method as implemented in the code ParaGauss.17
Results and Discussion Hydroxylated r-Al2O3(0001) Surface. Theoretical37,45,46 and experimental47 studies have shown that water readily dissociates (34) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244-13249. (35) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953-17979. (36) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758-1775. (37) Moskaleva, L. V.; Nasluzov, V. A.; Chen, Z.; Ro¨sch, N. Phys. Chem. Chem. Phys. 2004, 6, 4505-4513. (38) Methfessel, M.; Paxton, A. T. Phys. ReV. B 1989, 40, 3616-3621. (39) Dunlap, B. I.; Ro¨sch, N. AdV. Quantum Chem. 1990, 21, 317-339. (40) Belling, T.; Grauschopf, T.; Kru¨ger, S.; No¨rtemann, F.; Staufer, M.; Mayer, M.; Nasluzov, V. A.; Birkenheuer, U.; Hu, A.; Matveev, A. V.; Shor, A. M.; Fuchs-Rohr, M. S. K.; Neyman, K. M.; Ganyushin, D. I.; Kerdcharoen, T.; Woiterski, A.; Gordienko, A. B.; Majumder S.; Ro¨sch, N.; ParaGauss, version 3.0; Technische Universita¨t Mu¨nchen: Mu¨nchen, 2004. (41) Belling, T.; Grauschopf, T.; Kru¨ger, S.; Mayer, M.; No¨rtemann, F.; Staufer, M.; Zenger, C.; Ro¨sch, N. In High Performance Scientific and Engineering Computing; Bungartz, H.-J., Durst, F., Zenger, C., Eds.; Lecture Notes in Computational Science and Engineering 8; Springer: Heidelberg, 1999; pp 439453. (42) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Chem. 1980, 58, 1200-1211. (43) Minami, T.; Matsuoka, O. Theor. Chim. Acta 1995, 90, 27-39. (44) Poirier, R.; Kari, R.; Csizimadia, I. G. Handbook of Gaussian Basis Sets; Elsevier: Amsterdam, 1985.
Adsorption of UO22+ on Hydroxylated R-Al2O3(0001)
to give surface hydroxyl groups on clean Al-terminated R-Al2O3(0001) surface. Recently, the structure of a hydrated R-Al2O3(0001) surface has been determined using crystal truncation rod (CTR) diffraction.27 The surface was shown to be O-terminated with a double Al layer directly underneath the surface O layer.27 This experimental structure has been corroborated by theoretical atomistic48,49 and density functional (DF) studies,46,50 which in addition give information about the equilibrium positions of hydrogen atoms not directly seen in a CTR experiment. All theoretical studies predict one of three hydrogen atoms per surface unit cell to lie approximately parallel to the surface, while the other two are directed roughly normal to the surface plane. In the present work we employed a (3 × 3) surface unit cell, which is of the same size as employed in the CPMD-BLYP study of ref 46 and in lateral dimensions larger than the (1 × 1) cell used in the PW91 calculations of ref 50. Despite somewhat different computational details, we found a qualitatively very similar equilibrium surface structure as compared to earlier periodic DF studies. The “flat-lying” OH bonds create a network of hydrogen bonds ∼2.11-2.13 Å long. This compares well to 2.08 Å determined earlier.50 In agreement with experiment, our calculation predicts a notable contraction of the spacing between the two subsurface Al layers Al1 and Al2, by 45% (cf. 53% in ref 27), and an expansion of the spacing between the layer Al1 and the “top” oxygen layer O1 by 18% compared to the bulk value (21% in ref 27). The distance between the topmost O and the next Al layer, 0.99 Å, is very close to the experimentally determined 1.01 Å.27 Inner-Sphere Complex. The two principal mechanisms postulated for metal sorption on minerals are inner-sphere and outer-sphere complexes with the surface.51 In the former case, chemical bonds to the surface O2- or OH groups are formed, whereas in the latter case, the interaction with the surface is mainly electrostatic and possibly involves hydrogen bonding to the surface, while the first coordination shell of an adsorbed ion remains intact. The characteristic experimental features of innersphere complexes are a split first coordination shell and a short distance from the metal center to the surface, whereas in the case of outer-sphere adsorption the distance to the surface is longer and the spectral characteristics are similar to those of a free hydrated ion. Intuitively, inner-sphere complexation, which can be classified as chemisorption, is expected to lead to stronger bound complexes and thus to be the predominant adsorption mode. In practice, however, for some of the adsorption systems investigated by XAS, an outer-sphere mechanism was found to be the preferred method of adsorption.11,52,53 As will be illustrated by a specific example below, inner-sphere complexation may not necessarily be energetically favorable if the stability gain due to formation of new bonds is less than the stability loss caused by breaking bonds existing at the surface. Thus, the (45) Shapovalov, V.; Truong, T. N. J. Phys. Chem. B 2000, 104, 9859-9863. (46) Hass, K. C.; Schneider, W. F.; Curioni, A.; Andreoni, W. J. Phys. Chem. B 2000, 104, 5527-5540. (47) Nelson, C. E.; Elam, J. W.; Cameron, M. A.; Tolbert, M. A.; George, S. M. Surf. Sci. 1998, 416, 341-353. (48) de Leeuw, N. H.; Parker, S. C. J. Am. Ceram. Soc. 1999, 82, 3209-1216. (49) Di Felice, R.; Northrup, J. E. Phys. ReV. B 1999, 60, R16287-R16290. (50) Yong, C. W.; Warren, M. C.; Hillier, I. H.; Vaughan, D. J. Phys. Chem. Miner. 2003, 30, 76-87. (51) Stumm, W. Chemistry of the Solid-water Interface. Processes at the mineral-water and particle-water interface in natural systems; Wiley: New York, 1992. (52) Bargar, J. R.; Towle, S. N.; Brown, G. E., Jr.; Parks, G. A. J. Colloid Interface Sci. 1997, 185, 473-492. (53) Bargar, J. R.; Towle, S. N.; Brown, G. E., Jr.; Parks, G. A. Geochim. Cosmochim. Acta 1996, 60, 3541-3547.
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preferred mechanism depends greatly on the chemical composition of a particular surface and also on the nature of the adsorbed ion.52,53 Uranyl adsorption on the hydrated (0001) surface of R-alumina has not been studied experimentally to date. Adsorption on a different crystal plane, (11h02),4,5 as well as on powder samples of γ-alumina11 was shown to undergo inner-sphere complexation. In our computational model we used a completely hydroxylated perfectly terminated surface. This is well justified, taking into account much experimental evidence of surface defect self-healing due to high diffusion rates.54,55 Thus a high homogeneity of the surface can typically be achieved on oriented single crystals, as studied in grazing incidence XAFS (GI-XAFS) experiments.4,5,52,53 Also, under close to neutral pH condition, the (0001) surface is expected to be uncharged and fully protonated, based on spectroscopic studies of water adsorption on R-Al2O3(0001)28 and acidity constants.56 Quite a few theoretical studies57 addressed the preferred number of water ligands of UO22+ considering four, five, and six aqua ligands in the first coordination sphere. Based on solvation energetics, several authors identified the five-coordinated complex as energetically preferred.18,19,57 Numerous experimental evidences58-62 strongly support this conclusion, although a dynamic equilibrium between four- and five-coordinated forms dominated by the latter may not be excluded.63 In the following model study, we assumed a 5-fold coordination for uranyl in solution. To simplify the models, we kept this coordination number also for the adsorption complexes. Typically, experimentally determined coordination numbers for actinyl adsorption complexes with oxide surfaces ranged from 5 to 6.3 First, we considered the inner-sphere complexation model shown in Figure 1a. In this model, the pentaaqua uranyl3 loses two of its aqua ligands and instead forms two strong bonds to two oxygen atoms of the surface, replacing two protons on these oxygens:
S(OH)2 + [UO2(H2O)5]2+ f S(O)2-UO2(H2O)3 + 2H3O+ (1) Here, the symbol “S” stands for the infinite rest of the surface, while the reacting functional groups are given explicitly. The resulting distances to the surface, U-Os, are 2.11 and 2.32 Å. There is a third close contact at 2.80 Å to a nearestneighbor OH group on the same face of an AlO6 octahedron; thus, one may consider the resulting inner-sphere complex as effectively 6-fold coordinated. The uranyl moiety bends quite notably in the direction opposite to the surface plane, resulting in an OdUdO angle of 149°. Such a distortion is not surprising, taking into account a low bending frequency of UO22+, 208 cm-1.33 Relatively long axial UdO distances, 1.84 and 1.85 Å, compared to the distances of 1.76-1.80 Å, computed for free (54) Chiarello, R. P.; Sturchio, N. C. Geochim. Cosmochim. Acta 1995, 59, 4557-4561. (55) Coustet, V.; Jupille, J. Surf. Sci. 1994, 307-309, 1161-1165. (56) Hiemstra, T.; van Riemsdijk, W. H.; Bruggenwert, M. G. M. Neth. J. Agric. Sci. 1987, 35, 281-293. (57) For an up-to-date list of references, see: Bu¨hl, M.; Diss, R.; Wipff, G. J. Am. Chem. Soc. 2005, 127, 13506-13507. (58) Alcock, N. W.; Esperås, E. J. Chem. Soc., Dalton Trans. 1977, 893-896. (59) A° berg, M.; Ferri, D.; Glaser, J.; Grenthe, I. Inorg. Chem. 1983, 22, 39863989. (60) Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Edelstein, N. M.; Reich, T. Inorg. Chem. 1997, 36, 4676-4683. (61) Wahlgren, U.; Moll, H.; Grenthe, I.; Schimmelpfennig, B.; Maron, L.; Vallet, V.; Gropen, O. J. Phys. Chem. A 1999, 103, 8257-8264. (62) Se´mon, L.; Boehme, C.; Billard, I.; Hennig, C.; Lu¨tzenkirchen, K.; Reich, T.; Rossberg, A.; Rossini, I.; Wipff, G. Chem. Phys. Chem. 2002, 2, 591-598. (63) Neuefeind, J.; Soderholm, L.; Skanhtakumar, S. J. Phys. Chem. A 2004, 108, 2733-2739.
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Table 1. Pertinent Parameters of Optimized Structures of Uranyl Adsorption Complexes on Hydroxylated r-Al2O3(0001) Surface and of Free Adsorbatesa S(O)2-UO2(H2O)3 S′‚‚‚UO2(H2O)3(OH)2 UO2(OH)2(H2O)3 UO2(H2O)52+
calculation
UOax
UOw
periodic, LDA periodic, LDA periodic, LDA finite, LDA finite, LDA
1.84; 1.85 1.80; 1.81 1.79; 1.80 1.78; 1.80 1.76; 1.77
2.45; 2.60; 2.63 2.30; 2.59; 2.68 2.50; 2.57; 2.64 2.46; 2.54; 2.69 2 × 2.39; 2 × 2.41; 2.46
UOH
UOs
OdUdO
2.11; 2.32; 2.80
149 176 173 177 171
2.21; 2.25 2.19; 2.20 2.18; 2.25
a Distances in angstroms; OdUdO angle in degrees. UOax, distance to axial O; UOw, distance to O of H2O; UOH, distance to O of OH; UOs, distance to O of the surface.
with a conventional molecular method as described in the section Computational Methods. At the PW91 level, employed throughout for estimating the energetics, the calculated energy changes of reactions 2 and 3 are 29 and 38 kcal/mol, respectively. Thus, the energy change of reaction 1 is estimated at 67 kcal/mol, indicating an endothermic reaction. This hypothetical “gas-phase” or “in vacuo” calculation could be corrected for solvation effects if we subtract the solvation energies of the reactants from those of the products of reaction 1. We have
S(OH)2,aq + [UO2(H2O)5]2+aq f S(O)2-UO2(H2O)3,aq + 2H3O+aq (4) H3O+ f H3O+aq
(5)
[UO2(H2O)5]2+ f [UO2(H2O)5]2+aq
(6)
S(OH)2 f S(OH)2,aq
(7)
S(O)2-UO2(H2O)3 f S(O)2-UO2(H2O)3,aq
(8)
Under low-coverage conditions, for neutral systems S(OH)2 and S(O)2-UO2(H2O)3, we can approximately set ∆E(7) ) ∆E(8). The hydration energies of [UO2(H2O)5]2+ and H3O+, corresponding to ∆E(5) and ∆E(6), were calculated using a PCM model. Thus, one obtains for the energy change of reaction 4: Figure 1. (a) Optimized structure of inner-sphere adsorption complex of uranyl, S(O)2-UO2(H2O)3. O atoms are shown as red spheres, H atoms are shown as white spheres, and U atom is shown as a blue sphere. Inner layers of the slab are depicted schematically for clarity. (B) Optimized structure of an outer-sphere adsorption complex of uranyl, S‚‚‚UO2(H2O)3(OH)2. Same color coding as for (a); hydrogen bonds indicated as dotted lines.
species [UO2(H2O)5]2+ and [UO2(H2O)3(OH)2] (see Table 1), are clearly a consequence of this bent geometry, which makes orbital overlap of U and axial O atoms less optimal, but enhances the competing bonding to surface Os. To assess the plausibility of forming such an inner-sphere complex, one has to calculate the energetics of reaction 1. However, to avoid problems caused by charged unit cells, we calculated the energy of reaction 1 indirectly as the sum of reactions 2 and 3:
S(OH)2 + [UO2(H2O)3(OH)2]0 f S(O)2-UO2(H2O)3 + 2H2O (2) 2H2O + [UO2(H2O)5]2+ f [UO2(H2O)3(OH)2]0 + 2H3O+ (3) The energy change of eq 2, which involves only neutral species, was calculated with the periodic supercell approach. Reaction 3 does not involve surfaces and thus was conveniently treated
∆E(4) ) -∆E(6) - ∆E(7) + ∆E(1) + ∆E(8) + 2[∆E(5)] ) -∆E(6) + ∆E(1) + 2[∆E(5)] The hydration energy of two H3O+, 2[∆E(5)] ) -182 kcal/ mol, turns out to be on the same order as the hydration energy of [UO2(H2O)5]2+, ∆E(6) ) -193 kcal/mol. Therefore, the net effect of hydration is only 11 kcal/mol and renders reaction 4 to be 78 kcal/mol endothermic, even slightly more endothermic than reaction 1. At first glance, this result might seem surprising, taking into account that inner-sphere adsorption of uranyl was experimentally found on the (11h02) surface of R-Al2O34,5 and on γ-Al2O3.11 However, different surface orientations vary quite significantly in their chemical composition and adsorption properties. For example, a grazing incidence XAFS study52,53 of Pb(II) adsorption on the (0001) and (11h02) surfaces of R-Al2O3 found that Pb(II) adsorbed in an inner-sphere mode on (11h02), but as outer-sphere complexes on (0001). This difference was attributed to rather chemically inert [Al Al > OH] sites dominating the (0001) surface, -1/2] sites whereas the (11h02) surface contains charged [AlAl Al > O with unsaturated oxygen atoms.52 Based on these arguments, the formation of an inner-sphere complex without surface deprotonation, corresponding to a surface species S(OH)2-UO2(H2O)3, is not likely. In such a hypothetical complex, the surface oxygen atoms would be coordinatively oversaturated, [Al Al > (OH)-U], as manifested by the sum of the
Adsorption of UO22+ on Hydroxylated R-Al2O3(0001)
Langmuir, Vol. 22, No. 5, 2006 2145
bond valences, 2.28-2.79 valence units;4 this value is significantly above 2. Thus, this bonding arrangement is predicted to be unstable by Pauling’s electrostatic valence principle, which in turn is validated by numerous experimental observations on stable solids and aqueous species.4,52 Outer-Sphere Complex. We examined also a possible scenario for outer-sphere complexation on the R-Al2O3(0001):
S′ + [UO2(H2O)5]2+ f S′‚‚‚[UO2(H2O)5]2+
(9)
Here, “S′” indicates a surface site to which a hydrated uranyl binds weakly by means of hydrogen bonds. The first coordination shell of an adsorbed ion in the resulting complex remains the same as that of a free aqueous ion. Again, to avoid charged supercells, we calculated the energy change of reaction 9 indirectly, employing reaction 10 instead:
S′ + [UO2(H2O)3(OH)2] f S′‚‚‚[UO2(H2O)3(OH)2]
(10)
We assumed the deprotonation energy of two water molecules from the first coordination sphere of a free aqueous uranyl, eq 11, to be approximately the same as the deprotonation energy of two aqua ligands in a weakly adsorbed complex (Figure 1b), eq 12, i.e., ∆E(11) ≈ ∆E(12), provided that these ligands are those most distant from the surface:
[UO2(H2O)5]2+ f [UO2(H2O)3(OH)2] + 2H+
(11)
S′‚‚‚[UO2(H2O)5]2+ f S′‚‚‚[UO2(H2O)3(OH)2] + 2H+ (12) Then one has ∆E(9) ≈ ∆E(10). The interaction energy, i.e., the reaction energy ∆E(9), estimated in this way is -6 kcal/mol; thus, reaction 9 is weakly exothermic. This result is consistent with the experimental data on Pb(II) sorption at R-Al2O3(0001),53 where an outer-sphere complex and a small uptake of ∼0.1 µmol/ m2 was reported. The effect of solvation on the energy change of reactions 9 and 10 is expected to be minimal, because the hydration energies of reactants and products should be about the same due to weak complexation. Finally, we would like to comment on the optimized adsorption geometry shown in Figure 1b. The adsorption complex is characterized by two short hydrogen bonds of 1.55 and 1.56 Å between the two H atoms of a surface-touching aqua ligand and O atoms of the surface. The uranyl moiety is practically linear, and the geometry is only slightly distorted with respect to that of the free adsorbate; see Table 1. Thus, the calculated energy
gain of 6 kcal/mol should be entirely attributed to two new strong hydrogen bonds formed. The hydrogen-bonding network of the free surface is somewhat perturbed at the adsorption site (one hydrogen bond on the surface is broken), but the net effect is still favorable. The hydrogen-bonding network is probably dynamic, fluctuating with time, as was predicted by a recent Car-Parinello molecular dynamics study for the fully hydroxylated R-Al2O3(0001) surface.46 Also, one should not exclude the possibility of a slightly lower energy minimum with two water molecules forming hydrogen bonds to the surface, but in this prescreening study we limited ourselves to one plausible complex structure.
Conclusions This work demonstrated that theoretical predictions can be useful for understanding actinide adsorption phenomena at the molecular level, modeling specific surface sites. Adsorption of aqueous uranyl on the hydroxylated R-alumina (0001) surface was studied by means of periodic slab calculations with large unit cells treating the first solvation shell explicitly. We predict weak adsorption with nearly zero enthalpy of adsorption on the regular sites of this surface via an outer-sphere complexation. We would like to emphasize the idealized nature of this study, which was carried out assuming a perfectly terminated, completely protonated surface near pH 7 or below. However, as already stated, these assumptions are expected to be rather appropriate for the experimental conditions of grazing incidence GI-XAFS measurements on oriented single crystals. Although adsorption of uranyl on hydrated R-Al2O3(0001) has not yet been experimentally addressed, our results are indirectly supported by an experimental structural study of Pb(II) adsorption on the (0001) and (11h02) surfaces of alumina.52,53 As we considered only representative adsorption sites, it is possible that inner- or outer-sphere adsorption via other complexes could render adsorption slightly more exothermic, but qualitatively we do not expect a big change to the assessments of this work. In particular, the estimated energetic preference of outer-sphere complexes, more than 80 kcal/mol, is large enough to justify the main result of our model study. Certainly, for a deeper understanding of uranyl adsorption on oxide and mineral surfaces, a more comprehensive study of further surface complexes, in particular at surface defects, is highly desirable. Acknowledgment. We thank Dr. S. Kru¨ger for his valuable comments. L.V.M. thanks the Alexander von Humboldt Foundation for a research fellowship. This work was supported by the Bundesministerium fu¨r Wirtschaft und Arbeit (Grant 02E9450), Volkswagen Foundation, and Fonds der Chemischen Industrie. LA052973O
