Eu(III) Sorption to TiO2 (Anatase and Rutile): Batch, XPS, and EXAFS

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Environ. Sci. Technol. 2009, 43, 3115–3121

Eu(III) Sorption to TiO2 (Anatase and Rutile): Batch, XPS, and EXAFS Studies XIAOLI TAN,† QIAOHUI FAN,† X I A N G K E W A N G , †,* A N D BERND GRAMBOW‡ Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, 230031 Hefei, P.R. China, and Laboratory SUBATECH, Groupe de Radiochimie, UMR 6457 Ecole des Mines de Nantes/ IN2P3-CNRS/Universite´ de Nantes, 4 rue A. Kastler, BP 20722, 44307 Nantes cedex 03, France

Received December 3, 2008. Revised manuscript received February 9, 2009. Accepted March 3, 2009.

The sorption of Eu(III) on anatase and rutile was studied as a function of ionic strength, humic acid (HA, 7.5 mg/L), and electrolyte anions over a large range of pH (2-12). The presence of HA significantly affected Eu(III) sorption to anatase and rutile. The sorption of Eu(III) on anatase and rutile was independent of ionic strength. Results of an X-ray photoelectron spectroscopy (XPS) analysis showed that Eu(III) was chemically present within the near-surface of TiO2 due to the formation of tSOEu and tSOHAEu complexes. An extended X-ray absorption fine structure (EXAFS) technique was applied to characterize the local structural environment of the adsorbed Eu(III), and the results indicated that Eu(III) was bound to about seven or eight O atoms at a distance of about 2.40 Å. The functional groups of surface-bound HA were expected to be involved in the sorption process. The measured Eu-Ti distance confirmed the formation of inner-sphere sorption complexes on a TiO2 surface.

Introduction The environmental behavior of lanthanides and actinides has created intense interest in environmental impact assessment of disposed long-lived radioactive waste. To assess the potential pollution of the natural environment, we need knowledge of the sorption mechanisms and speciation of radionuclides on natural minerals; this is of paramount importance. Potentially released actinides such as Np, Pu, Am, and Cm may be sorbed on natural mineral surfaces and thereby decrease their transport in the natural environment (1). Eu(III) is a trivalent lanthanide and a chemical homologue to trivalent actinides. It has similar sorption properties to trivalent actinides (2). The sorption of Eu(III) on natural minerals and oxides has been studied extensively by using batch techniques (3-7), chelating resins (8), spectroscopic techniques (9-13), and capillary methods (14). The results indicate that sorption of Eu(III) is generally strongly influenced by pH values and independent of ionic strength (3-7). Surface complexation contributes mainly to the sorption of Eu(III) on minerals. * Corresponding author phone: +86-551-5592788; fax: +86-5515591310; e-mail: [email protected]. † Institute of Plasma Physics. ‡ Laboratory SUBATECH. 10.1021/es803431c CCC: $40.75

Published on Web 03/24/2009

 2009 American Chemical Society

Titanium oxides exist in two major polymorphic forms as anatase and rutile. TiO2 is an ideal model adsorbent for studying the relationship between surface charge and sorption. Its solubility is negligible, and its point of zero charge (pHpzc) near neutral pH makes it possible to study sorption on positively or negatively charged surfaces of TiO2 over a broad range of pH and ionic strengths (15-20). Weng et al. (17) found that specific chemical interaction was the major mechanism for the sorption of Cr(VI) on TiO2. Gao et al. (18) studied the sorption of Cd2+ on anatase and found that the intraparticle electrostatic repulsion may reduce the free energy of sorption reactions significantly for nanometersized particles. The sorption of Co2+ on the rutile surface planes (110) and (001) was studied, using extended X-ray absorption fine structure (EXAFS) spectroscopy (21). On both crystallographic orientations the same surface complexes were formed: a monodentate surface complex with a bridging oxygen surface group and a bidentate surface complex with two terminal oxygens. Quantum chemical calculations (22) indicate that Ni2+ adsorbs preferentially onto bridging oxygen sites and not onto terminal groups. The sorption of Th(IV) on TiO2 (19) indicated the reversible formation of an innersphere complex with a strong pH dependent Th(IV) sorption. However, little information is available on metal ion sorption to anatase and rutile under comparable experimental conditions. Using a multispectroscopic approach and distinguishing sorption on the different crystallographic planes, Vandenborre (23) reported similar surface complexes for uranyl aquo ions on rutile and anatase in the pH range of 2-4.5, which consist of a strong bidentate complex of uranyl to two bridging oxygens and a weaker bidentate complex between the uranyl ion and one bridging and one terminal oxygen. For anatase, surface complexation constants are about three orders of magnitude stronger for (001) than for (100) or (101) planes, while for rutile, surface complexation constants are two orders of magnitude stronger for (110) and (100) planes than for (101) planes. Humic substances (HS) have attracted great attention because of their high complexation ability with metal ions. For the assessment of radionuclide mobility, the interaction between metal ions and HS has been the subject of various studies (6-9, 24, 25). Recent X-ray photoelectron spectroscopy (XPS) (26-30) studies indicate that carboxylic groups are the primarily responsible binding sites for the complexation of metal ions with HS. In this paper, we used XPS to characterize Eu(III) adsorbed onto TiO2. The study provides compositional, oxidation state, and some structural information for all surface and near-surface elements (except for H). The in situ Eu bound onto the TiO2 surface was studied by using Eu LIII-edge EXAFS to determine the local structure around the selected element (10, 31-33). Additional information about atomic distances and coordination numbers of the incorporated Eu(III) ion can be derived by EXAFS spectroscopy. The combination of the two complementary spectroscopic methods (i.e., XPS and EXAFS) allows us to characterize and to quantify TiO2/Eu and TiO2/HA/Eu sorption species.

Experimental Section Materials. Eu(III) stock solution was prepared from Eu2O3 after dissolution, evaporation, and redissolution in 10-3 mol/L perchloric acid. Commercially available anatase and rutile were used (Shanghai Chemical Reagent Co., Ltd., Country Medicine Group, China). The points of zero charge (PZC) were found to be 6.2 for anatase and 5.4 for rutile using potentiometric titration, which were in good agreement with VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Sorption edges of Eu(III) adsorbed onto anatase and rutile (A), effect of anions (B), effect of HA (C) and (D). In panels A, C, and D, C(TiO2) ) 0.3 g/L, C[Eu(III)]initial ) 1.0 × 10-7 mol/L, C(HA)initial ) 7.5 mg/L, T ) 25 ( 1 °C, C(NaClO4) ) 0.1 or 0.01 mol/L. the values reported in the literature (34, 35). The N2-BET surface areas of anatase and rutile were measured as 52.4 and 110.6 m2/g, respectively. The XRD patterns and SEM images of the samples are shown in Figures SI-1 and SI-2 of the Supporting Information, respectively. Soil humic acid (HA) was extracted from the soil of HuaJia County (Gansu Province, China) and was characterized in detail (36, 37). Sorption Experiments. Sorption experiments were carried out with 0.3 g/L TiO2, 1.0 × 10-7 mol/L Eu(III), and ionic strength solutions (0.1 and 0.01 mol/L NaClO4, 0.1 mol/L NaNO3, and NaCl) at T ) 25 ( 1 °C under ambient conditions in the presence and absence of 7.5 mg/L HA, using a batch technique. The detailed experimental process is shown in the Supporting Information. Sample Preparation for XPS and EXAFS Analysis. Detailed processes for the preparation are shown in Supporting Information. XPS Analysis. XPS spectra were recorded on powders with a thermo ESCALAB 250 spectrometer using an Al Ka monochromatized source and a multidetection analyzer under 10-8 Pa residual pressure. Surface charging effects were corrected with a C 1s peak at 284.6 eV as a reference. A Shirley background correction and Gaussian-Lorentzian fitting were used to transform peak areas to total intensities. EXAFS Analysis. Europium LIII-edge X-ray absorption spectra at 6976.9 eV were recorded at the National Synchrotron Radiation Laboratory (NSRL, China). Detailed descriptions of EXAFS analyses are listed in the Supporting Information.

Results and Discussion Anatase and Rutile/Eu(III) Systems. The sorption edge of Eu(III) on anatase and rutile, i.e., the effect of pH on the sorption percentage, is shown in Figure 1A. A pronounced rise of Eu(III) sorption from almost zero to about 100% within the pH range of 2-6 is observed, which is typical for many examples of metal ion sorption on oxide surfaces (2-5). For the rutile/Eu(III) system, the slope of the sorption edge curve is steeper than that of the sorption edge curve for the anatase/ 3116

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Eu(III) system. It is likely that the differences in the pHpzc of anatase (pHpzc 6.2) and rutile (pHpzc 5.4) and the site concentrations of anatase (3.86 × 10-4 mol/g) and rutile (4.49 × 10-4 mol/g) are responsible for this different sorption behavior. The site concentration is calculated from acid-base titration of the samples using a FITEQL 3.2 mode. One can see that the sorption of Eu(III) is strongly dependent on pH values and independent of ionic strength, which indicates the formation of an inner-sphere complex (4-8). The sorption properties of TiO2 depend on its crystal structure and chemical properties of the surface. Hydroxylated bridging tSOHSt groups and terminal groups tSOH (uncharged surface groups), tSOH2+ (positively charged groups), and tSO- (negatively charged groups) are present at the surface of TiO2. With increasing pH, functional groups are progressively deprotonated, forming negative surface charge. The attractive force between the negative surface sites and positive metal ions results in the formation of metal ion-ligand TiO2 complexes. The knowledge of chemical speciation of Eu(III) in solution is essential to discuss the possible Eu(III) species in the system. The relative proportions of Eu(III) species calculated with thermodynamic constants (Table SI-1 of the Supporting Information) are shown in Figure SI-3 of the Supporting Information. At pH < 6.5, the predominant species is Eu3+. Thus, the fact that more Eu(III) sorbs at high pH can also be attributed to a decrease in competition between H+ and Eu3+ at the sorption sites of TiO2. The sorption of Eu(III) at pH < pHpzc is not just controlled by electrostatic forces but is a specific binding interaction, which can be deduced from spectroscopic investigations and from the weak or missing influence of ionic strength. At high pH, the surface precipitation of Eu hydroxide or Eu carbonate complexes may contribute mainly to the sorption process (38). Effect of Electrolyte Anions. Figure 1B shows the sorption of Eu(III) on bare anatase as a function of pH values in the presence of 0.1 mol/L NaNO3 and NaCl. The sorption of Eu(III) on anatase particles is independent of the anions in the suspensions. The presence of noncomplexing anions does not affect the surface properties of anatase and, therefore,

does not influence the sorption of Eu(III) (13). Cl- and NO3can form soluble complexes with Eu(III) in solution (e.g., EuCl2+, log K ) 0.34, and Eu(NO3)2+, log K ) 1.33). The complexation constants (log K) of Eu(III) with surface groups of anatase are 3.81 in 0.1 mol/L NaCl and 5.04 in 0.1 mol/L NaNO3 ( Figure SI-4 and Table SI-1 of the Supporting Information). Compared to the soluble complexes of Eu(III) with Cl- or NO3-, we find that the surface complexation of Eu(III) with surface groups of anatase is much stronger than that of Eu(III) with Cl- or NO3-. Ternary Anatase and Rutile/HA/Eu(III) Systems. The presence of HA has a significant effect on Eu(III) sorption to HA-anatase and HA-rutile hybrids (Figure 1C,D). Compared to the sorption of Eu(III) on bare anatase/rutile, we observed a sharp increase of the sorption at pH < 4 in the presence of 7.5 mg/L HA. This could be due to the following: (1) An enhancement of Eu(III) sorption is expected because of the complexation of Eu3+ with HA adsorbed onto the solid surface. (2) Sorption of HA onto the surface of a solid produces a more negative surface charge, which could enhance the electrostatically driven sorption of Eu(III). The adsorption of HA on anatase and on rutile as a function of pH is shown in Figure SI-5 of the Supporting Information. The decrease of HA sorption with increasing pH is due to electrostatic effects and surface complexation reactions (6). At intermediate pH, sorption of Eu(III) on HA-bound anatase/rutile is consistently lower than that in the absence of HA. The mechanism of Eu(III) sorption is overlaid by the aqueous Eu(III) speciation and the formation of Eu(CO3)+(HA) and Eu(OH)2+(HA) in solution (Figure SI-3D of the Supporting Information) (6, 7). The aqueous Eu(III) species in combination with the increased concentration of soluble-free HA in solution lead to a lower amount of Eu(III) sorption. At pH > 8, the sorption rises again slightly in the presence of HA. This rise could be due to several factors: (1) the low level of HA sorption, coupled with the very large surface area on the colloids, provides evidence of adsorption of an inorganic Eu species, which is similar to the adsorption of Eu(III) onto TiO2 in the absence of HA. (2) However, the sorption does not reach the levels achieved in the systems free from HA. This could be due to residual Eu(CO3)+(HA) remaining in solution or residual HA on the mineral surface enhancing electrostatic repulsion of the Eu(CO3)2- on the mineral surface (6). XPS Analysis. XPS spectra demonstrate the sensitivity for identifying elements on the surface. Figure 2 shows the XPS spectra of Eu(III) sorbed on bare and HA-rutile hybrids at pH 4.0 in 0.1 mol/L NaClO4 solutions. The adsorbed Eu(III) can be readily identified by the Eu 3d XPS lines. The rutile/Eu sample has four O 1s peaks positioned at 529.6, 530.9, 532.1, and 533.1 eV, which can be assigned to lattice oxygen O2-, bridging OH, terminal OH, and adsorbed H2O, respectively (23). The sorption of Eu(III) can greatly be attributed to the interaction of Eu(III) with OH groups on the rutile surface (23). In the rutile/HA/Eu sample, the O 1s spectrum has been deconvoluted into four peaks. The relative intensity of the peak for bridging OH decreases, while the relative intensities of the peaks for terminal OH and adsorbed H2O increase. The increased peaks can also be assigned to other oxygen species of CO (532.2 eV) and COO (533.2 eV) (28), which complex more easily with Eu(III). These carbon/ oxygen species originate from HA adsorbed onto the rutile surface. The positions of the O 1s peaks in the rutile/Eu system are very different from those of the rutile/HA/Eu system. The C 1s spectrum in the presence of HA can be wellfitted by four Gaussian-Lorentzian functions, which are assigned to C-C (charge referenced to 284.6 eV), CxHy (285.4 eV), CO (286.9 eV), and COO (289.2 eV) (29). The last three carbon groups are due to the attachment of HA groups on the surface of rutile. These carbon-containing functional

groups are abundant on the surface of rutile, which can provide numerous sorption sites and thus increase the sorption of Eu(III). The XPS spectrum reveals the characteristic doublet of Eu 3d levels. The Eu 3d5/2 peak is found at 1135.1 ( 0.2 eV (with a satellite at 1127.0 ( 0.2 eV, with respect to the main signal). This is in agreement with the value for the Eu(III) adsorbed species (30). The data leave no doubt that the sorbed Eu(III) is chemically present within the near-surface region of rutile. This XPS feature is perhaps associated with tSOEu complexes. The rutile/HA/Eu spectra are well-fitted with two components located at 1135.0 and 1137.3 eV. These different binding energies correspond to different chemical environments, which are associated with the formation of tSOEu and tSOHAEu complexes on a rutile surface. The relative intensity of the component located at 1137.3 eV is much smaller than the one at 1135.0 eV, suggesting that even a low HA concentration may influence mineral surface properties significantly and thus have some impact on the Eu(III) sorption behavior. However, differences of the satellite spacing relative to the Eu 3d main lines are expected because of the higher chemical sensitivity of the satellites. The different satellite spacing observed for the two samples confirms that the type of bonding has been altered by the presence of HA. In contrast, the Eu(III) adsorbed at pH 9.0 is expected to signify surface coprecipitation, which is discussed in Figure SI-6 of the Supporting Information. EXAFS Analysis. For all sorption samples, an intense adsorption line at 6985 eV (XANES data in Figure SI-7 of the Supporting Information) dominates the X-ray adsorption edge. The position of this line shows that Eu is trivalent in all of our sorption samples (31-33), suggesting the enrichment of Eu(III) on the surface of anatase/rutile, which is in agreement with the XPS results. The EXAFS spectra of reference samples, i.e., Eu2O3, Eu(OH)3(s) and Eu(aq), are shown in Figure 3A. The results are in agreement with the results reported by Schlegel et al. (10). Detailed interpretations are listed in the Supporting Information. The k2-weighted fluorescence Eu LIII-edge EXAFS spectra and the corresponding Fourier transforms (FT) for the samples are shown in Figure 3. EXAFS spectra obtained for Eu sorption samples look similar to each other, but different somewhat from those of the reference samples. The oscillations at 8 Å-1 for the Eu sorption samples are broadened with respect to those for Euaq. Spectral data for sorption samples and Eu(OH)3(s) are obviously different, and attempts to include the Eu-Eu shell contribution to the EXAFS data suggest the formation of surface complexes, including a polymeric form of Eu at low pH, while compelling formation of coprecipitation on a TiO2 surface at high pH. The fit to the EXAFS data is simultaneously done for all samples. The coordination number (N), interatomic distance (R), and EXAFS Debye-Waller factor (σ2) obtained from the fits are listed in Table 2. The first FT peaks at a R value around 2 Å arise from the single scattering (SS) of the photoelectron on oxygen atoms in the first coordination sphere. It is not possible to distinguish between the Eu-O distances from the hydration sphere and the Eu-O originating from interactions of Eu and oxygen atoms at the surface. The distance difference between Eu and O that belongs to the carboxylate groups [O(Ac), mean value of 2.49 Å] is not large enough to allow an unambiguous distinction in peak analysis. The found Eu-O distance of 2.40 Å of the TiO2/Eu samples is an average distance composed of the surface Eu-O contribution and the hydration sphere Eu-OH2 and the excessive carboxylate groups Eu-O(Ac) contribution in the TiO2/HA/Eu samples. It corresponds to the Eu-O distance from the Eu3+ aquo ion with a hydration sphere of 9 H2O molecules (10). Using the same fit conditions, we find that VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. XPS spectra of the survey, O 1s, C 1s, and Eu 3d; (A) rutile/Eu(III) hybrids and (B) rutile/HA/Eu(III) hybrids. C1s and O1s spectrum of the rutile/HA/Eu(III) system is the result of a fit curve. C(TiO2) ) 0.3 g/L, C[Eu(III)]initial ) 3.0 × 10-5 mol/L, C(NaClO4) ) 0.01 mol/L, C(HA)initial ) 7.5 mg/L, pH 4.0 ( 0.1, T ) 25 ( 1 °C. the fit to the Eu3+ aquo species data yields 8.8 oxygen atoms (Table 2). The error in determining the coordination numbers using EXAFS spectroscopy is in general about 10-20%. The REu-O distances are longer than REu-O ) 2.35 Å for hexacoordinated Eu in Eu2O3 and marginally shorter than REu-O ) 2.43 Å for octo-coodinated Eu in water. This suggests that adsorbed Eu is 8-fold to 7-fold coordinated. A contribution from the Eu/HA/TiO2 system is observed around 2.7 Å, which is caused by Eu · · · C single scattering and Eu-O-C scattering (32). This is attributed to the expected Eu/HA complexes on TiO2 surfaces. The number of carboxylate and phenolate groups bound to Eu could not be determined accurately. However, the spectra show a relatively 3118

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large contribution from the Eu · · · C and Eu-O-C scattering paths around 2.7 Å. For the sample obtained from the sorption experiment at pH 9.0, subtle differences indicate that the data cannot be solely described by detectable surface complexes, suggesting the occurrence of a coprecipitated component in the sample. The FT shows shifts to higher distances and increased amplitudes of first-shell O peaks as the fraction of coprecipitated Eu is increased as well as an amplified Eu-Eu shell at higher R values. Both raw values and FT indicate the formation of a greater fraction of coprecipitated Eu at pH 9.0 relative to that at pH 5.0.

FIGURE 3. k2-weighted EXAFS spectra (A) and the corresponding Fourier transforms (B) of the following reference and sorption samples: Eu(III) adsorbed onto rutile at pH 5.0 (rutile/Eu5.0), at pH 4.0 (rutile/Eu4.0), and at pH 9.0 (rutile/Eu9.0); the presence of HA at pH 5.0 (rutile/HA/Eu5.0 and anatase/HA/Eu5.0); and Eu(III) adsorbed onto anatase at pH 5.0 (anatase/Eu5.0). C(TiO2) ) 0.3 g/L, C[Eu(III)]initial ) 3.0 × 10-5 mol/L, C(NaClO4) ) 0.01 mol/L, C(HA)initial ) 7.5 mg/L, pH 4.0 ( 0.1 or 5.0 ( 1 or 9.0 ( 0.2, T ) 25 ( 1 °C.

TABLE 1. XPS Results for Measured Binding Energies (BE) and Full Widths at Half-Maximum (FWHM) sample rutile/Eu

signal O 1s

C1s Eu 3d5/2

sample rutile/HA/Eu

signal O 1s

C1s

Eu 3d5/2

peak

BE (eV)

fwhm (eV)

O bridging OH terminal OH H2O C-C tSOEu satellite

529.6 530.9 532.1 533.1 284.6 1135.1 1127.0

1.71 1.79 1.17 1.64 2.58 5.34 7.37

2-

peak O2bridging OH terminal OH/C-O H2O/O-CdO C-C CxHy (in HA) C-O O-CdO tSOEu tSOHAEu satellite

The shell close to 4.41 Å in the sorption samples corresponds mainly to Eu-Ti atomic pairs (Table 2). The Eu-Ti distance determined from EXAFS fitting supports the formation of monodentate complexes on a TiO2 surface (i.e.,

BE (eV) 530.6 531.0 532.2 533.2 284.6 285.4 286.9 289.2 1135.0 1137.3 1126.5

fwhm (eV) 1.26 1.80 1.20 2.02 2.14 1.89 2.00 1.58 4.21 3.36 6.59

Eu-Ti distances comply with the Eu-O distance of 2.4 Å and the Ti-O distance in rutile of e1.98 Å, and Eu cations are in extreme apical positions relative to the Ti octahedra). This is at variance with results reported for other cations VOL. 43, NO. 9, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. EXAFS Results of Reference Samples and Sorption Samples at Eu LIII-Edge samplea Eu2O3 Eu(OH)3 Eu(aq) rutile/Eu5.0 rutile/HA/Eu5.0

rutile/Eu4.0 anatase/Eu5.0 anatase/HA/Eu5.0

rutile/Eu9.0

shell

Rb (Å)

Nc

σ2d (Å2)

Rf e

Eu-O Eu-Eu Eu-O Eu-Eu Eu-O Eu-O Eu-Ti Eu-O Eu-C Eu-Ti Eu-O Eu-Ti Eu-O Eu-Ti Eu-O Eu-C Eu-Ti Eu-O Eu-Eu Eu-Ti

2.345(6) 3.59(2) 2.428(7) 3.61(6) 2.427(5) 2.39(3) 4.40(2) 2.40(3) 2.67(2) 4.42(3) 2.41(2) 4.41(3) 2.40(2) 4.40(1) 2.39(2) 2.7(2) 4.40(2) 2.415(7) 3.65(2) 4. 51

5.7(8) 6.4(2) 8.0(4) 2.2(3) 8.8(5) 8.3(2.2) 1.1(4) 8.5(1.6) 1.4(6) 1.3(4) 9.1(1.1) 1.2(3) 6.7(4) 1.0(6) 7.1(1.5) 2.0(6) 1.1(2) 8.0(5) 2.35(9) 1.3(6)

0.0081(13) 0.0077(2) 0.0095(1) 0.001(9) 0.0075(4) 0.0103(8) 0.0064(1) 0.0106(4) 0.0083(3) 0.0064(4) 0.0099(2) 0.0064(1) 0.0064(11) 0.0086(4) 0.0065(8) 0.0064(21) 0.0066(1) 0.0080(4) 0.0026(16) 0.0025(9)

0.040 0.030 0.020 0.065 0.023 0.075 0.048 0.034 0.134 0.138 0.041 0.245 0.020 0.102 0.033 0.124 0.114 0.021 0.164 0.137

a Eu2O3, Eu(OH)3, and Eu(aq) are named as reference samples, whereas the other samples of anatase or rutile with adsorbed Eu(III) in the presence or absence of HA are named as sorption samples. b R is the interatomic distance. c N is the number of neighbor oxygens. d σ2 is the Debye-Waller factor. e Rf is the residual factor. Rf ) ∑k(k3xexp - k3xcalc)/∑k(k3xcalc), which measures the quality of the model Fourier-filtered contribution (xcalc) with respect to the experimental contribution (xexp).

(39, 40). The evidence from the EXAFS analysis that suggests Eu(III) is adsorbed onto TiO2 as an inner-sphere complex is consistent with macroscopic studies, showing little change in the pH-dependent sorption of Eu(III) as a function of ionic strength.

Acknowledgments Financial support from National Natural Science Foundation of China (20677058, 20501019) and the 973 Project (2007CB936602) from MOST of China are acknowledged. The authors gratefully acknowledge Dr. Bo He and Dr. Zhi Xie (NSRL, USTC, China) for helpful technical assistance of EXAFS experiments. We also express our thanks to Professor H. Geckeis and Dr. Th. Rabung (INE, FZK, Germany) and Dr. G. Montavon (SUBATECH, France) for favorable discussions to improve the quality of the paper.

Supporting Information Available SEM images and XRD patterns of anatase and rutile, sorption of HA on anatase and rutile, relative species distribution of Eu(III) in solution, a surface complex speciation repartition diagram of Eu(III) sorption on anatase, XPS spectra of Eu 3d for sorption samples at pH 9.0, XANES spectra and first-shell fit of the EXAFS function of reference, and sorption samples. This information is available free of charge via the Internet at http://pubs.acs.org.

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