Density Functional Model Studies of Uranyl Adsorption on (001

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Langmuir 2008, 24, 9515-9524

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Density Functional Model Studies of Uranyl Adsorption on (001) Surfaces of Kaolinite Alena Kremleva, Sven Krüger, and Notker Rösch* Department Chemie, Theoretische Chemie, Technische UniVersita¨t Mu¨nchen, 85747 Garching, Germany ReceiVed April 23, 2008. ReVised Manuscript ReceiVed June 6, 2008 The adsorption of uranyl on two types of neutral (001) surfaces of kaolinite, tetrahedral Si(t) and octahedral Al(o), was studied by means of density functional periodic slab model calculations. Various types of model surface complexes, adsorbed at different sites, were optimized and adsorption energies were estimated. As expected, the Si(t) surface was found to be less reactive than the Al(o) surface. At the neutral Al(o) surface, only adsorption at protonated sites is calculated to be exothermic for inner- as well as outer-sphere adsorption complexes, with monodentate coordination being preferred. Adsorption energies as well as structural features of the adsorption complexes are mainly determined by the number of deprotonated surface hydroxyl groups involved. Outer-sphere complexes on both surfaces exhibit a shorter U-O bond to the aqua ligand of uranyl that is in direct contact with the surface than to the other aqua ligands. This splitting of the shell of equatorial U-O bonds is at variance with common expectations for outer-sphere surface complexes of uranyl.

1. Introduction Retardation of radionuclide migration in the environment is an ongoing concern in environmental research.1 Precipitation and adsorption on mineral surfaces are inherent mechanisms which affect the mobility of contaminants under natural conditions. Therefore, a comprehensive description of how actinides interact with mineral-water interfaces is crucial for understanding the distribution of these elements in the environment and also for reliable long-term risk assessment of nuclear waste repositories.2 Uranium is the central element in the nuclear fuel cycle, as fuel of nuclear reactors and as major component of the final waste. The chemically most stable form of U(VI) is the uranyl ion UO22+, which is highly mobile and readily complexates with organic and inorganic matter.3 Therefore, uranium and its decay products are hazardous pollutants of the environment. Furthermore, uranyl is the dominant uranium species in contaminated groundwater systems.4 Uranyl adsorption on various mineral surfaces has been studied by different experimental methods such as extended X-ray adsorption finestructure (EXAFS),5–13 time-resolved laser-induced fluorescence * To whom correspondence should be addressed. E-mail: [email protected]. (1) Thompson, B. M.; Longmire, P. A.; Brookings, D. G. Appl. Geochem. 1986, 1, 335–343. (2) Kristallin-I, Safety Assessment Report; Nagra Technical Reports NTB 9322E; National Cooperative for the Disposal of Radioactive Waste: Wettingen, Switzerland, 1994. (3) Grenthe, I.; Fuger, R. J. M.; Konings, R. J.; Lemire, A. B.; Muller, A. B.; Nguyen-Tsung Cregu, C.; Wanner, H. Chemical Thermodynamics of Uranium; Wanner, H., Forest, I., Eds.; Elsevier: Amsterdam, 1992. (4) Zielinsli, R. A.; Chafin, D. T.; Banta, E. R.; Szabo, B. J. EnViron. Geol. 1997, 32, 124–136. (5) Reich, T.; Moll, H.; Denecke, M. A.; Geipel, G.; Bernhard, G.; Nitsche, H.; Allen, P. G.; Bucher, J. J.; Kaltsoyannis, N.; Edelstein, N. M.; Shuh, D. K. Radiochim. Acta 1996, 74, 219–223. (6) Reich, T.; Moll, H.; Arnold, T.; Denecke, M. A.; Hennig, C.; Geipel, G.; Bernhard, G.; Nitsche, H.; Allen, P. G.; Bucher, J. J.; Edelstein, N. M.; Shuh, D. K. J. Electron. Spectrosc. 1998, 96, 237–243. (7) Hennig, C.; Reich, T.; Da¨hn, R.; Scheidegger, A. M. Radiochim. Acta 2002, 90, 653–657. (8) Sylvester, E. R.; Hudson, E. A.; Allen, P. G. Geochim. Cosmochim. Acta 2000, 64, 2431–2438. (9) Dent, A.; Ramsay, J. D. F.; Swanton, S. W. J. Colloid Interface Sci. 1992, 150, 45–60. (10) Thompson, H.; Parks, G.; Brown, J. In Adsorption of Metals by Geomedia; Jenne, E., Ed.; Academic Press: San Diego, CA, 1998; p 350-371.

spectroscopy (TRLFS),14–16 as well as batch experiments.17,18 Most of the experiments were carried out on powder substrates, where the results are averaged over different surface orientations. However, in a few cases substrates with a well-defined surface orientation have been studied, making it possible to identify the atomic structure of specific sites.19–22 We explore in this work the adsorption of uranyl on kaolinite, a main mineral component of clay-rich host rock formations, considered as potentially appropriate for nuclear waste repositories.23 The adsorption of uranyl on mineral surfaces is affected by many parameters such as the pH of the solution8,9,12,14,16–18 as well as the concentration of uranyl and different counterions in solution.6,8,14,15,17 The presence of CO2 also affects the adsorption of uranyl.12,17 On kaolinite, uranyl sorption increases with pH, reaching saturation at pH ) 6.12,17 In the presence of CO2, uranyl desorbs again from kaolinite at pH > 8, whereas no desorption is observed for the system without CO2.12,17 At about neutral pH conditions, inner-sphere complexation of uranyl was determined.10,12,13 (11) Den Auwer, C.; Simoni, E.; Conradson, S.; Madic, C. Eur. J. Inorg. Chem. 2003, 21, 3843–3859. (12) Reich, T.; Reich, T. Ye.; Amayri, S.; Drebert, J.; Banik, N. L.; Buda, R. A.; Kratz, J. V.; Trautmann, N. AIP Conf. Proc. 2007, 882, 179–183. (13) Kr˘epelová, A.; Reich, T.; Sachs, S.; Drebert, J.; Bernhard, G. J. Colloid Interface Sci. 2008, 319, 40–47. (14) Baumann, N.; Brendler, V.; Arnold, T.; Geipel, G.; Bernhard, G. J. Colloid Interface Sci. 2005, 290, 318–324. (15) Wang, Z. M.; Zachara, J. M.; Gassman, P. L.; Liu, C. X.; Qafoku, O.; Yantasee, W.; Catalan, J. G. Geochim. Cosmochim. Acta 2005, 6, 1391–1403. (16) Kr˘epelová, A.; Brendler, V.; Sachs, S.; Baumann, N.; Bernhard, G. EnViron. Sci. Technol. 2007, 41, 6142–6147. (17) Kr˘epelová, A.; Sachs, S.; Bernhard, G. Radiochim. Acta 2006, 94, 825– 833. (18) Chisholm-Brause, C. J.; Berg, J. M.; Little, K. M.; Matzner, R. A.; Morris, D. E. J. Colloid Interface Sci. 2004, 277, 366–382. (19) Catalano, J. G.; Trainor, T. P.; Eng, P. J.; Waychunas, G. A.; Brown, G. E. Geochim. Cosmochim. Acta 2005, 69, 3555–3572. (20) Denecke, M. A.; Bosbach, D.; Dardenne, K.; Lindqvist-Reis, P.; Rothe, J.; Yin, R. Z. Phys. Scr. 2005, T115, 877–881. (21) Bargar, J. R.; Reitmeyer, R.; Lenhart, J. J.; Davis, J. A. Geochim. Cosmochim. Acta 2000, 64, 2737–2749. (22) Towle, S. N.; Bargar, J. R.; Brown, G. E. J. Colloid Interface Sci. 1999, 217, 312–321. (23) Hoth, P.; Wirth, H.; Reinhold, K.; Bräuer, V.; Krull, P.; Feldrappe, H. Endlagerung RadioaktiVer Abfa¨lle in Deutschland. Untersuchung und Bewertung Von Tongesteinsformationen; Bundesanstalt fu¨r Geowissenschaften und Rohstoffe: Hannover and Berlin, Germany, 2007.

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At present, the structure of adsorption complexes of actinyls and their mechanisms of adsorption on mineral surfaces are not well understood at the atomic level. While experimentally innerand outer-sphere complexation or mono- and polynuclear adsorption species are distinguished,8,10,12,14 several questions arise with respect to experiments on uranyl sorption at clay minerals. (i) Which adsorption sites are involved, aluminol (AlOH) or silanol (SiOH) groups? (ii) Which surface orientations are preferred for adsorption, basal or edge surfaces? (iii) Is there one preferred adsorption complex or is there an equilibrium between several of them? While different experimental techniques were applied to explore uranyl adsorption, only EXAFS provides structural parameters on molecular level. Nevertheless, an unequivocal determination of composition and structure of adsorbed complexes by means of XAFS is difficult, as these techniques average over all species present.11,24 Quantum chemistry calculations may help to interpret experimental data and to improve our understanding of structures and mechanisms of uranyl adsorption at the atomic level. In recent years, quantum chemistry studies on actinide coordination complexes in solution have become popular to complement experimental data.25–27 However, only few and mainly rather approximate theoretical models have been applied to actinide chemisorption.28–30 Mostly molecular dynamics on the basis of empirical force fields was used to explore sorption of actinides on mineral surfaces because of the inherent size and complexity of these systems.26,27 Nevertheless, the first density functional studies of uranyl adsorption on mineral surfaces, TiO2(110) and R-Al2O3(0001), have recently been published.31,32 In this work we present a density functional study of uranyl adsorption at the basal (001) surfaces of kaolinite, Al(o) and Si(t), using periodic slab models. We examined both inner- and outer-sphere complexes as well as bidentate and monodentate types of surface complexation, and we estimated the corresponding adsorption energies. We accounted for effects of solvation by explicit consideration of the first hydration shell of uranyl and by means of a posteriori corrections for long-range solvent effects. Here we are applying a slightly different model approach as used in our previous work on uranyl sorption at R-Al2O3(0001),32 but also compare uranyl sorption on kaolinite and alumina resorting to that earlier procedure.

2. Computational Details We carried out first-principles density functional (DF) calculations on supercell models of kaolinite surfaces, using the plane-wave-based Vienna ab initio simulation package (VASP).33 We optimized structures with the local density approximation (24) Denecke, M. Coord. Chem. ReV. 2006, 250, 730–754. (25) Vallet, V.; Wahlgren, U.; Szabo´, Z.; Grenthe, I. Inorg. Chem. 2002, 41, 5626–5633. (26) Kaltsoyannis, N. Chem. Soc. ReV. 2003, 32, 9–16. (27) Hay, P. J.; Martin, R. L.; Schreckenbach, G. J. Phys. Chem. A 2000, 104, 6259–6270. (28) Greathouse, J. A.; Stellalevinsohn, H. R.; Denecke, M. A.; Bauer, A.; Pabalan, R. T. Clays Clay Miner. 2005, 53, 278–286. (29) Greathouse, J. A.; Cygan, R. T. EnViron. Sci. 2006, 40, 3865–3871. (30) Wheaton, V.; Majumdar, D.; Balasubramanian, K.; Chauffe, L.; Allen, P. G. Chem. Phys. Lett. 2003, 371, 349–359. (31) Perron, H.; Domain, C.; Roques, J.; Drot, R.; Simoni, E.; Catalette, H. Inorg. Chem. 2006, 45, 6568–6570. (32) Moskaleva, L.; Nasluzov, V.; Ro¨sch, N. Langmuir 2006, 22, 2141–2145. (33) (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.

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(LDA);34 LDA calculations often furnish more accurate results for molecular geometries, whereas calculations with the generalized gradient approximation (GGA)35,36 yield improved adsorption energies. This also holds for actinide complexes.37,38 Thus, energies were evaluated with the gradient-corrected exchangecorrelation functional PW91.39 To represent the effect of the core levels, we used the full-potential projector augmented wave (PAW) method as implemented in VASP.40,41 Scalar relativistic effects are incorporated in the PAW potential via mass-velocity and Darwin corrections.42 We adopted an energy cutoff of 400 eV for surface models and of 520 eV for the kaolinite bulk (to reduce the effect of Pulay forces when the unit cell is optimized). This cutoff value was found sufficient in optimizations of a clean kaolinite (001) slab model; test calculations with an energy cutoff of 520 eV lead to negligible changes in structural parameters of surface models ( B ≈ D > C ≈ E (Table 2). Adsorption of a uranyl dication at a doubly deprotonated site (A) was calculated to be endothermic, by 248 kJ mol-1. Mono- or bidentate complexation at a singly deprotonated site (B, D) is still endothermic, by ∼44 kJ mol-1 (Table 2); both models B and D are energetically and structurally similar (see above). For inner- (C, E) as well as outer-sphere (F) complexation, uranyl adsorption was determined to be exothermic at the neutral surface Al(o)-KL (Table 2). The calculated adsorption energies are -155, -88, and -42 kJ mol-1 for monodentate (E), bidentate (C), and outer-sphere (F) structures, respectively. Also, adsorption energies for the same type of complexes at adsorption sites involving “lOH” groups follow the same trends; see Supporting Information (Table S1). The majority of experimental results on uranyl adsorption at mineral surfaces are interpreted as outer-sphere (at low pH)8,9 or inner-sphere bidentate complexes (at higher pH level).5–9 In contrast to these assignments, we determined a monodentate uranyl complex (E) on Al(o)-KL to be preferred at moderate pH values. As adsorption energies are always calculated with respect to the neutral surface of kaolinite, our result refers to a pH level of about pHZPC ∼ 5.5.60 Also recall that edge surfaces may play a crucial role in uranyl adsorption on kaolinite, while only basal surfaces have been investigated in the present study. EXAFS results of uranyl sorption on mineral surfaces at higher pH8,10,12,13 show two values for the equatorial U-O distances as well as a short enough U-Al/Si distance to be detectable. Therefore, these systems have been interpreted as chemisorbed inner-sphere complexes. At lower pH, physisorption seems to be the preferred mode of adsorption,8,9 resulting in (proper) outersphere complexes that are characterized by a single equatorial U-O distance, similar to that of solvated uranyl, and no detectable U-Al/Si distance. An earlier EXAFS study on uranyl adsorption on kaolinite determined two shorter equatorial U-O bonds of ∼228 pm and three longer ones of ∼248 pm, together with a U-Al/Si distance of 330 pm at a pH of 6-7.9.10 Two recent studies reported comparable results,12,13 where uranyl bond lengths of 180 pm12 and 177 pm13 and unresolved U-Oeq distances of 236-240 pm12 and 234 pm13 have been determined. In both of these studies, two U-Al/Si distances were identified, a shorter one at ∼310 pm and a longer one at ∼330 pm,12,13 which were interpreted as indicative for sorption at edge-sharing Al octahedra or Si tetrahedra.12,13 An alternative interpretation assigns these results to a mixture of two adsorption complexes.

Uranyl Adsorption on (001) Surfaces of Kaolinite

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This scenario is supported by a recent TRLFS study16 of uranyl adsorption at kaolinite, where two different fluorescence lifetimes were detected and attributed to bidentate surface complexes that differ by the number of aqua ligands of uranyl. Similar TRLFS findings for uranyl on gibbsite14 and muscovite64 have been interpreted as a consequence of coexisting mono- and polynuclear sorption complexes. The presence of contacts U-Al/Si is a clear signature of innersphere complexes. As EXAFS averages over all species present, a short U-O bond of 228 pm, associated with a coordination number of 2, may be interpreted to indicate the presence of U-O bonds of about that size.10 Our results for inner-sphere uranyl adsorption at the basal Al(o) surface can be compared to the experimental results if one takes them as model results for sorption at aluminol sites and keeps in mind that uranyl sorption at edge surfaces65 of kaolinite is very likely also present in experimental samples. The calculated results collected in Table 2 show that a short equatorial U-O distance of at most 230 pm, as seen in experiment,10 is only obtainable at the Al(o)-KL surface if a deprotonated hydroxyl group is involved in the adsorption sites. Also, with the exception of the bidentate coordination to two Ogroups (model A), only a single short U-O bond is calculated. In model A, the two bonds to surface oxygen atoms differ by 15 pm on site “uu” and by 5 pm on site “ul” (Table S1). As experimental results range from 310 to 330 pm for U-Al/Si contacts and from 234 to 240 pm for U-Oeq values,10,12,13 one concludes good agreement with the characteristics calculated for complexes B (U-Al ) 327 pm, U-Oeq ) 243 pm) and A (U-Al ) 308 pm, U-Oeq ) 237 pm). Complexes B and A involve bidentate coordination of uranyl at O-/OH or O-/Osites, respectively. Whereas the partially deprotonated site B is in line with the experimental pH condition slightly above pHZPC, a 2-fold depotonated site seems less plausible, although deprotonation of surface hydroxyl groups is facilitated in the field of the uranyl ion. When comparing with experiment, one has to keep in mind that none of the models used in the present study accounts for solvation of the mineral surface. In summary, we found qualitative agreement of geometrical parameters with experiment for bidentate surface complexes at partially deprotonated sites. Thus, although not energetically preferred in the present study, these complexes are expected as most probable in experimental samples. Inner-sphere complexes with similar geometric parameters were experimentally observed for adsorption on montmorillonite,7,8 R-Fe2O3(11j02),19 R-Al2O3(11j20),20 and hematite.21 4.4. Adsorption on Si(t)-KL. The results calculated for uranyl adsorption on Si(t)-KL are shown in the lower part of Table 2. The U-Ot distance of both inner- and outer-sphere complexes is about the same, 178-179 pm, and close to the U-Ot bond length of the solvated uranyl ion, 177 pm. In the bidentate innersphere complex G on Si(t)-KL, the distance U-Oeq, calculated at 246 pm, is longer than U-Oeq obtained for a free uranyl aqua complex, 240 pm (Table 2). Bond distances to surface oxygen centers are similar: 246 and 249 pm. Interestingly, we did not obtain any appreciable “splitting” of the first coordination shell when uranyl adsorbs as an inner-sphere complex: U-O distances vary between 241 and 249 pm, including those to surface O centers. While U-Ot, U-Oeq, and U-Si distances of the innersphere complex G are in qualitative agreement with EXAFS results for uranyl sorption on kaolinite,10,12,13 a short equatorial

U-O bond as determined in ref 10 is missing in model complex G (Table 2). The outer-sphere complex H exhibits an average value U-Oeq ) 243 pm, which is only 3 pm longer than that calculated for a free uranyl aqua complex. On the other hand, the uranyloxygen distances U-Ow of different aqua ligands vary considerably, from 224 to 252 pm; curiously, the shortest distance was obtained for the ligand between uranyl and the support, just as for outer-sphere complexes F and F′ at Al(o)-KL (Section 4.3). This aqua ligand of complex H is in direct contact with both the uranyl and the Si(t) surface. Because of moderately strong hydrogen bonds between this water molecule and surface oxygen atoms (O · · · H ) 157 and 148 pm), O-H bond distances are elongated from 98 pm in [UO2(H2O)5]2+ up to 102 and 104 pm. As a rationalization, charge transfer from the surface to this water ligand may be invoked, which would strengthen the interaction with uranyl and lead to a shortening of the U-Ow bond. The other four aqua ligands exhibit common bond lengths U-Ow varying from 243 to 252 pm (Table 2). A splitting of equatorial bond lengths of uranyl to oxygen atoms appears for this model outer-sphere complex, in contradiction to common assumptions,58 as it was calculated for the outer-sphere species on Al(o) as well (Section 4.3). At the Si(t)-KL surface, adsorption energies of both innerand outer-sphere uranyl complexes are calculated positive (Table 2). Adsorption as an inner-sphere complex is more endothermic than as an outer-sphere complex, with adsorption energies of 239 and 206 kJ mol-1, respectively. These large values agree with the expectation that the Si(t) surface of kaolinite exhibits a low reactivity as long as charged defects are absent;65–67 hence, adsorption of uranyl at this surface is unlikely.

(64) Arnold, T.; Utsunomiya, S.; Geipel, G.; Ewing, R. C.; Baumann, N.; Brendler, V. EnViron. Sci. Technol. 2006, 40, 4646–4652. (65) Brady, P. V.; Cygan, R. T.; Nagy, K. L. In Adsorption of Metals by Geomedia; Jenne, E., Ed.; Academic Press: San Diego, 1998; p 371-383.

(66) Sposito, G.; Skipper, N. T.; Sutton, R.; Park, S.; Soper, A. K.; Greathouse, J. A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3358–3364. (67) Tunega, D.; Gerzabek, M. H.; Lischka, H. J. Phys. Chem. B 2004, 108, 5930–5936.

5. Summary and Conclusions We carried out a “first-principles” density-functional model study of uranyl adsorption on the basal (001) Al(o) and Si(t) surfaces of ideal kaolinite, using the plane-wave based PAW approach with periodic boundary conditions as implemented in the software VASP. The kaolinite surfaces were modeled using a single layer of kaolinite, separated from its periodic images by more than 1 nm. Geometry optimizations were performed at the LDA level, while adsorption energies were derived from “singlepoint” energies calculated at the GGA level. Care was taken to construct models with neutral unit cells, even for charged adsorption complexes. Adsorption energies of uranyl were estimated with the help of suitably chosen elementary reaction steps. A representative set comprising mono- as well as bidentate coordinated inner-sphere complexes and also models of outersphere complexes with 5-fold coordination of uranyl was considered. In the outer-sphere complexes, surface contact via the first solvation shell of uranyl was assumed. For the Al(o) surface, we studied uranyl adsorption at protonated as well as deprotonated sites. As expected, adsorption energies on the neutral Al(o) surface decrease with increasing number of deprotonated OH groups at the adsorption site. Adsorption as a bidentate inner-sphere complex of uranyl at two O- groups was calculated to be strongly endothermic, by ∼248 kJ mol-1. Slightly endothermic adsorption, with energies of ∼44 kJ mol-1, was calculated for bi- and monodentate uranyl surface complexes, which involve a single O- site. Adsorption at sites with OH groups was modeled to be

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exothermic. With an adsorption energy of -155 kJ mol-1, a monodentate structure is preferred compared to bidentate coordination, -88 kJ mol-1, and outer-sphere model complexes, about -42 kJ mol-1. At the Si(t) (001) surface, adsorption of bidentate and outer-sphere surface complexes of uranyl was determined to be endothermic, where the latter complex is predicted to be preferred with an adsorption energy of 206 kJ mol-1. Comparison of our results to those of an earlier model study of uranyl adsorption at R-Al2O3(0001)32 shows that uranyl adsorption at kaolinite is about 75 kJ mol-1 energetically preferred. The geometry of the inner-sphere uranyl surface complexes at Al(o)-KL is mainly determined by the number of surface Ogroups involved for which short equatorial U-O distances, 208-223 pm, were calculated. Longer bonds exist to surface OH groups, 245-260 pm; this range is comparable to the variation of U-Ow distances of aqua ligands of the first solvation shell, 236-260 pm. The stronger interaction of uranyl with deprotonated surface OH groups, as indicated by rather short bond lengths, is also reflected in an increase of the U-Ot bonds of uranyl. Values range from 177 pm for a “free” aqua complex of uranyl to ∼182 pm for outer sphere complexes, up to 185 pm for a bidentate inner-sphere complex at two surface O- sites. Smaller variations of geometry parameters of only a few picometers were calculated when one compares complexes at different surface OH groups of Al(o), oriented parallel or perpendicular to the surface. The shorter uranyl bond U-Ot of 178 pm, calculated for a bidentate inner-sphere complex at Si(t)-KL, illustrates the lower reactivity of that surface compared to that of Al(o)-KL. For both surfaces, U-O distances to aqua ligands within each given outer-sphere model complex examined vary considerably. These computational results are at variance with the common expectation that outer-sphere complexes essentially preserve the structure of a solvated uranyl. A short U-Ow bond, 225-230 pm, was calculated to the aqua ligands of uranyl that are in direct contact with the surface, while the other U-Ow bonds were determined in the usual range of about 240-250 pm. Taking these result together, one is lead to conclude that experimentally

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detected adsorbed species with similar geometric features as solvated uranyl are associated with more than a single coordination shell of aqua ligands between uranyl and the mineral surface. A tentative comparison to available EXAFS results for uranyl adsorption at kaolinite shows qualitative agreement for complexes at partially deprotonated sites and supports the hypothesis that bidentate inner-sphere complexes exist at pH values of 6 to 8.10,12,13 To relate more thoroughly experimental and computational evidence, models of “edge” surfaces of kaolinite also have to be considered. In addition, for a more quantitative description a more realistic modeling of the solvation of the uranyl and of the kaolinite surfaces is very desirable. Nevertheless, while experimental characterization of bidentate uranyl adsorption complexes at mineral surfaces commonly yields two short surface bonds of similar length,5,8,10 all models of that type of complexes examined in the present work, except model A, featured two rather different bonds to the surface. This first systematic “first-principles” model study of actinide adsorption complexes at a mineral surfaces provided valuable insight in geometric and energetic aspects. Yet, to achieve a more complete atomistic description of uranyl sorption on kaolinite, it is necessary to also explore adsorption at the more complex “edge” surfaces as well as include effects of surface solvation. Work in this direction is in progress. Acknowledgment. We thank Prof. T. Reich, Universita¨t Mainz, for stimulating discussions. This work was supported by the German Bundesministerium fu¨r Wirtschaft und Technologie (Grant No. 02E10186) and Fonds der Chemischen Industrie. Supporting Information Available: Table S1 compares pertinent geometry parameters and adsorption energies of inner-sphere uranyl complexes at the Al(o) (001) kaolinite surface at sites containing lOH and uOH surface groups. In addition, atomic coordinates of all optimized structures are provided. This material is available free of charge via the Internet at http://pubs.acs.org. LA801278J