Extended X-ray Absorption Fine Structure Investigation of Adsorption

Anal. Chem. 2007, 79, 8016-8023

Extended X-ray Absorption Fine Structure Investigation of Adsorption and Separation Phenomena of Metal Ions in Organic Resin Atsushi Ikeda,*,†,‡,§ Tsuyoshi Yaita,† Yoshihiro Okamoto,† Hideaki Shiwaku,† Shinichi Suzuki,† Tatsuya Suzuki,‡ and Yasuhiko Fujii‡

Synchrotron Radiation Research Center (SPring-8), Japan Atomic Energy Agency, Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan, and Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8550, Japan

Analytical technique using organic resins has already been well-developed, and its applications are employed in various fields; nevertheless, the chemical phenomena occurring inside the resin remain unclear for the most part. In the present study, we apply EXAFS spectroscopy to elucidate the adsorption and separation phenomena of metal ions by organic resin. That is, the chemical species of trivalent lanthanides (Ln(III)) adsorbed in a tertiary pyridine resin from hydrochloric acid and nitric acid solutions have been determined by EXAFS. The results in HCl solutions suggest that Ln(III) ions are partly dehydrated in the resin phase, enabling the pyridine groups of the resin and chloride ions to coordinate to the Ln(III) ions in their primary coordination sphere. On the other hand, Ln(III) ions are tightly coordinated by several nitrate ions in HNO3 solutions and they keep forming the nitrate complex even in the resin phase. The lighter Ln of Nd tends to form an anionic nitrate complex, [Nd(NO3)4‚nH2O]-, in the resin phase, while the middle Ln of Sm exists as a cationic nitrate complex, [Sm(NO3)2‚ nH2O]+, for the most part. On the basis of these EXAFS results, the adsorption and separation mechanisms of the pyridine resin in HCl solutions are interpreted as the direct coordination of pyridine groups to metal ions, while the mechanisms in HNO3 solutions are mainly dominated by the anion-exchange reaction between the protonated pyridine groups and the anionic nitrate complexes of Ln(III). The obtained results demonstrate that the hydration of metal ions weakens, and instead, other complexations are enhanced in the resin phase. Organic resin extractants, such as ion-exchange resins, are widely employed in basic and applied chemistry research and they are also applied to various industrial fields. The chemical phenomenon by organic resins, such as adsorption or separation of ions, occurs at the interface between the resin surface and the * To whom correspondence should be addressed. E-mail: aikeda@ lax.kuramae.ne.jp. † Japan Atomic Energy Agency. ‡ Tokyo Institute of Technology. § Present address: Institute of Radiochemistry, Forschungszentrum DresdenRossendorf, P.O. Box 510119, 01314 Dresden, Germany.

8016 Analytical Chemistry, Vol. 79, No. 21, November 1, 2007

solution phase. The chemical environment in this minimal reaction field is obviously different from that in bulk solution. For instance, it has been suggested that the hydration of ions becomes weaker and other complexation is enhanced in organic resins.1,2 However, it is experimentally difficult to investigate such kinds of chemical phenomena inside the resin. Thermodynamical approaches can provide only the information about the whole system, and thus, it is hard to discriminate the information about the resin-solution interface from that for bulk solution. Standard spectroscopic techniques, such as UV-visible absorption, IR/Raman, or NMR, are also not suitable because these techniques are strongly affected by the presence of other matrixes in the system. That is, resin-solution mixed samples have various matrixes of ions, solvent, and resin matrix and these matrixes would make the desired information about the resin-solution interface indistinguishable. Furthermore, these techniques often have a restriction of sample form. Accordingly, the chemical phenomena occurring inside organic resins remain conjectural, and they are still open questions for the most part. For investigating such a “limited” minimal field, extended X-ray absorption fine structure (EXAFS) spectroscopy is a very powerful and effective tool. Although it also provides the total information about the observed chemical system, we can distinguish different chemical species qualitatively. Additionally, this technique is highly element-selective so that we can exclude the undesirable effect of other matrixes in the system. In this study, we employ EXAFS spectroscopy to probe into the chemical phenomena inside organic resins. That is, we identify the chemical species of lanthanides (Ln(III)) that are adsorbed in a tertiary pyridine resin (Figure 1) from HCl and HNO3 solutions by EXAFS in order to elucidate the adsorption and separation mechanisms of metal ions by the pyridine resin. The obtained results give an important example for understanding the chemical phenomena occurring in organic resins and the mechanisms of their related analytical techniques, such as chromatography. (1) (a) Arisaka, M.; Kimura, T.; Suganuma, H.; Yoshida, Z. Radiochim. Acta 2001, 89, 593-598. (b) Arisaka, M.; Kimura, T.; Suganuma, H.; Yoshida, Z. Radiochim. Acta 2002, 90, 193-197. (c) Arisaka, M.; Kimura, T.; Suganuma, H.; Yoshida, Z. J. Radioanal. Nucl. Chem. 2003, 255, 385-389. (2) (a) Harada, M.; Okada, T.; Watanabe, I. J. Phys. Chem. B 2002, 106, 3440. (b) Okada, T.; Harada, M. Anal. Chem. 2004, 76, 4564-4571. (c) Harada, M.; Okada, T. J. Chromatogr., A 2005, 1085, 3-7. 10.1021/ac070700n CCC: $37.00

© 2007 American Chemical Society Published on Web 10/06/2007

Figure 1. Structure of tertiary pyridine resin and its protonation reaction.

This study is also significant for the development of the separation technique for trivalent actinides (An(III)) and Ln(III). The inter- and intragroup separation of An(III) and Ln(III) is known as one of the most challenging subjects in the field of separation science because the chemical properties of An(III) and Ln(III) are quite analogous to each other: they have similar ionic radii3 with the identical trivalent oxidation state (M3+), giving rise to difficulty in their separation. One of the promising methods for the An(III) and Ln(III) separation is the use of soft donortype ligands that coordinate to metal ions with soft donor atoms, such as the N or S atom. Although both An(III) and Ln(III) are classified as a typical hard cation in the HSAB theory by Pearson,4 An(III) exhibit slightly softer property than Ln(III),5,6 and in consequence, the soft donor ligands display a greater selectivity for An(III) over Ln(III) in solvent extraction processes.7 By exploiting this high selectivity of soft donor ligands, we have recently developed a novel chromatography technique using the pyridine resin (Figure 1) and reported several successful results on the inter- and intragroup separation of An(III) and Ln(III).8-16 Further improvement of this chromatographic separation tech(3) Shannon, R. D. Acta Crystallogr. 1976, A32, 751-767. (4) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533-3539. (5) (a) Musikas, C.; Cuillerdier, C.; Livet, J.; Forchioni, A.; Chachaty, C. Inorg. Chem. 1983, 22, 2513-2518. (b) Musikas, C. In Proceedings of the International Symposium on Actinide/Lanthanide Separations; Choppin, G. R., Navratil, J. D., Schulz, W. W., Eds.; World Scientific: Philadelphia, 1985; pp. 19-30. (c) Zhu, Y.; Chen, J.; Jiao, R. Solvent Extr. Ion Exch. 1996, 14, 61-68. (d) Miguirditchian, M.; Guillaneux, D.; Guillaumont, D.; Moisy, P.; Madic. C; Jensen, M. P.; Nash, K. L. Inorg. Chem. 2005, 44, 1404-1412. (6) Jensen, M. P.; Morss, L. R.; Beitz, J. V.; Ensor, D. D. J. Alloys Compd. 2000, 303-304, 137-141. (7) For instance: (a) Ensor, D. D.; Jarvinen, G. D.; Smith, B. F. Solvent Extr. Ion Exch. 1988, 6, 439-445. (b) Chen, J.; Zhu, Y.; Jiao, R. Sep. Sci. Technol. 1996, 31, 2723-2731. (c) Kolarik, Z.; Mu ¨ llich, U. Solvent Extr. Ion Exch. 1997, 15, 361-379. (d) Cordier, P. Y.; Hill, C.; Baron, P.; Madic, C.; Hudson, M. J.; Liljenzin, J. O. J. Alloys Compd. 1998, 271-273, 738-741. (e) Watanabe, M.; Mirvaliev, R.; Tachimori, S.; Takeshita, K.; Nakano, Y.; Morikawa, K.; Mori, R. Chem. Lett. 2002, 31, 1230-1231. (8) Suzuki, T.; Aida, M.; Ban, Y.; Fujii, Y.; Hara, M.; Mitsugashira, T. J. Radioanal. Nucl. Chem. 2003, 255, 581-583. (9) Ikeda, A.; Suzuki, T.; Aida, M.; Ohtake, K.; Fujii, Y.; Itoh, K.; Hara, M.; Mitsugashira, T. J. Alloys Compd. 2004, 374, 245-248. (10) Ikeda, A.; Suzuki, T.; Aida, M.; Fujii, Y.; Itoh, K.; Mitsugashira, T.; Hara, M.; Ozawa, M. J. Chromatogr., A 2004, 1041, 195-200. (11) Ikeda, A.; Suzuki, T.; Aida, M.; Otake, K.; Fujii, Y.; Itoh, K.; Mitsugashira, T.; Hara, M.; Ozawa, M. J. Nucl. Sci. Technol. 2004, 41, 915-918. (12) Suzuki, T.; Otake, K.; Sato, M.; Ikeda, A.; Aida, M.; Fujii, Y.; Hara, M.; Mitsugashira, T.; Ozawa, M. J. Radioanal. Nucl. Chem. 2007, 272, 257262. (13) Ikeda, A.; Suzuki, T.; Aida, M.; Fujii, Y.; Mitsugashira, T.; Hara, M.; Ozawa, M. J. Radioanal. Nucl. Chem. 2005, 263, 605-611. (14) Ikeda, A.; Suzuki, T.; Aida, M.; Fujii, Y.; Mitsugashira, T.; Hara, M.; Ozawa, M. Prog. Nucl. Energy 2005, 47, 454-461. (15) Ikeda, A.; Itoh, K.; Suzuki, T.; Aida, M.; Fujii, Y.; Mitsugashira, T.; Hara, M.; Ozawa, M. J. Alloys Compd. 2006, 408-412, 1052-1055. (16) Suzuki, T.; Itoh, K.; Ikeda, A.; Aida, M.; Ozawa, M.; Fujii, Y. J. Alloys Compd. 2006, 408-412, 1013-1016.

nique, such as optimizing operating conditions or further development of separating agents, requires a fundamental understanding of the chemical reaction between the pyridine resin and An(III)/ Ln(III). Therefore, here we investigate the adsorption and separation behavior of the pyridine resin for Ln(III) by employing EXAFS, and on the basis of the obtained results, we also deduce the mechanism of the observed intergroup separation between An(III) and Ln(III) by the pyridine resin. EXPERIMENTAL SECTION Sample Preparation. Solution samples for EXAFS measurements were prepared by dissolving a weighted amount of lanthanide trichloride hydrate (LnCl3‚nH2O, 99.9% purity) in HCl solution or lanthanide trinitrate hydrate (Ln(NO3)3‚nH2O, 99.9% purity) in HNO3 solution to give a concentration of 0.1 M Ln(III). The tertiary pyridine resin employed in this study was synthesized by the copolymerization of 4-vinylpyridine and m/p-divinylbenzene. A detailed procedure for the synthesis has been described in the previous paper.17 In the present study, the resin structure was optimized in order to multiply the information about the resinsolution interface on EXAFS measurements. That is, the crosslinking (the ratio of divinylbenzene in the monomer oil) and porosity of the resin were reduced as much as possible to increase the number of pyridine groups in the resin and to lower the volume of solution in the resin. The optimized pyridine resin had 10 wt % cross-linking with 20 vol % porosity. The monomer oil did not copolymerize successfully below these cross-linking and porosity. The synthesized resin was sifted using a 280-mesh sieve (∼50-µm diameter) to obtain a fine and uniform particle size. The adsorption capacity of tertiary pyridine resin is difficult to determine because it varies according to the pH of the solvent. However, the number of pyridine groups per unit volume can be calculated from the amount of each material in the monomer oil, and it is estimated to be 6.7 mmol of pyridine groups/cm3. The synthesized resin was washed by acetone, ethanol, and deionized water several times and dried in a vacuum prior to use. The dried resin was soaked in the Ln(III) solution, which was identical to the solution sample mentioned above, and agitated at 288 K for 1 day to achieve the adsorption equilibrium. The distribution coefficients (Kd) of Ln(III) in the studied conditions are given in Figures S1 and S2 in the Supporting Information. The resin after achieving the adsorption equilibrium was then filtered immediately before the measurement to remove an excess of solution. The wet resin (i.e., retaining solution inside the resin particles) was placed in a polystyrene cuvette or in a polyethylene bag for EXAFS measurements. The solution samples were also transferred to polystyrene cuvettes or to polyethylene bags for the measurements. Reference samples of LnCl3‚nH2O were prepared by mixing with boron nitride powder and pressing them into pellets. All the chemicals and solvents used in this study were reagent grade and supplied by Wako Pure Chemical Ind., Ltd. and Rare Metallic Co., Ltd. Solvents were used as received without further purification. Acquisition and Analysis of EXAFS Spectra. La, Ce, and Nd K-edge X-ray absorption spectra were collected at SPring-8 on the undulator beamline BL11XU under normal ring operating conditions (8 GeV, 99 mA). It has been reported that the L-edge (17) Nogami, M.; Aida, M.; Fujii, Y.; Maekawa, A.; Ohe, S.; Kawai, H.; Yoneda, M. Nucl. Technol. 1996, 115, 293-297.

Analytical Chemistry, Vol. 79, No. 21, November 1, 2007

8017

X-ray absorption spectra of lanthanides (especially the lighter lanthanides from La to Nd) always suffer from the multielectron excitation effect (MEE)18 that gives another oscillation on their EXAFS spectra and deteriorates the accuracy of structural analysis. However, no MEE appears on their K-edge absorption region, and hence, there is no disturbance of MEE on their spectra. A liquid N2-cooled Si (311) double crystal was employed to monochromatize a white X-ray from the synchrotron. The gap position of the insertion device was adjusted continuously in order to obtain a smooth curve of intense X-rays. The beam position of incident X-ray was controlled by the monochromator stabilization (MOSTAB) system.19 Sm LIII-edge X-ray absorption measurements were performed at the Photon Factory, High Energy Accelerator Research Organization (KEK) on the beamline BL-27B under normal ring operating conditions (2.5 GeV, 100∼250 mA) using a Si (111) double-crystal monochromator. The incident X-ray was detuned to 60% of its maximum intensity for reducing higher order harmonics. All the K-edge spectra were collected in transmission mode using ionization chambers filled with Ar-N2 mixture (Ar/ N2 ) 50/50 (v/v)). The LIII-edge spectra for solution samples were also collected in transmission mode using ionization chambers filled with N2 and Ar-N2 mixture for monitoring incident and transmitted X-rays, respectively. The LIII-edge measurements for resin samples were carried out in fluorescence mode with a sevenelement Ge detector. The sample thicknesses for transmission measurements were adjusted to 1 cm, 3 cm, and 1 mm for the solution and resin samples for K-edge measurements and the solution samples for LIII-edge measurements, respectively. All the measurements were performed at ambient temperature (298 K). The obtained spectra were analyzed according to a standard method20 using the program WinXAS (Ver. 3.1).21 Theoretical phase shifts and backscattering amplitude functions were calculated by the program code FEFF8.0022 on the basis of appropriate crystallographic data.23 The curve fits of the extracted EXAFS spectra took account of the contribution of possible single scattering Ln-O (H2O and NO3), Ln-Cl, Ln-N (NO3 and PyN), and Ln-C (Py-C) paths and a multiple scattering24 Ln-N-O (NO3) path, as illustrated in Figure 2. The energy of each spectrum was self-calibrated on their absorption edges. The EXAFS threshold energies, E0, were defined at 38935, 40446, 43573, and 6711 eV for La-K, Ce-K, Nd-K, and Sm-LIII edges, respectively. The (18) Solera, J. A.; Garcıá, J.; Proietti, M. G. Phys. Rev. B 1995, 51, 2678-2686. (19) (a) Alkire, R. W.; Rosenbaum, G.; Evans, G. J. Synchrotron Radiat. 2000, 7, 61-68. (b) Kudo, T. P.; Nishino, Y.; Suzuki, M.; Tanida, H.; Furukawa, Y.; Hirono, T.; Ishikawa, T. Houshakou 2003, 16, 173-177 (in Japanese). (20) Prins, R.; Koningsberger, D. E. X-ray Absorption: Principles, Applications, Techniques for EXAFS, SEXAFS, and XANES; Wiley-Interscience: New York, 1988. (21) Ressler, T. J. Synchrotron Radiat. 1998, 5, 118-122. (22) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B 1998, 58, 7565-7576. (23) [LnCl2(H2O)6]Cl (Habenschuss, A.; Spedding, F. H. Cryst. Struct. Commun. 1980, 9, 71-76) for the data of aqueous solution and HCl/MeOH solution samples, [Ln(bpy)Cl2(H2O)4]Cl (bpy: 2,2′-bipyridine, Semenova, L. I.; Skelton, B.; White, W.; Aust, A. H. J. Chem. 1999, 52, 551-569) for the resin samples prepared from the HCl/MeOH mixed solution, [Ln(H2O)5(NO)3]3 (Milinski, N.; Riba´r, B.; Sataric´, M. Cryst. Struct. Commun. 1980, 9, 473-477) for HNO3/MeOH mixed solution samples and the resin samples prepared from the HNO3/MeOH mixed solution, and [Ln(bpy)(NO3)3] (Kepert, D. L.; Semenova, L. I.; Sobolev, A. N.; White, A. L. Aust. J. Chem. 1996, 49, 1005-1008) for the resin samples prepared from the HNO3/ MeOH mixed solution, respectively. (24) Lee, P. A.; Pendry, J. B. Phys. Rev. B 1975, 11, 2795-2811.


Figure 2. Possible scattering paths in HCl and HNO3 solution systems.

amplitude reduction factor, S02, had little influence on the curve fits in the range of 0.9-1.0 and, furthermore, the S02 value of 0.9 gave reasonable coordination numbers (CNs) for solution samples and LnCl3‚nH2O samples, whose structures had already been determined. Therefore, it was fixed at 0.9, and besides, the energy shift parameters, ∆E0, were synchronously varied for all shells on the curve fit procedure so that we could reduce the free parameters as much as possible. RESULTS In HCl Solution. The tertiary pyridine resin can separate An(III) from Ln(III) in high concentrations of HCl solution and its adsorbability (i.e., distribution coefficient, Kd) and separability (i.e., separation factor, R) are enhanced by adding alcohol in the solution.8,10,13 The determination of the chemical species adsorbed in the resin should be carried out with as high Kd condition as possible so that we can distinguish the adsorbed species from other species in the solution phase. In a concentrated HCl (i.e., 11.9 M HCl solution, written as “conc HCl” hereafter) and MeOH mixed solution system, the maximum Kd values of Ln(III) are obtained when 50-70 vol % MeOH is added to the HCl solution (see Figure S3 in the Supporting Information). Therefore, the EXAFS measurements were performed using “conc HCl/MeOH ) 5/5 (v/v)” mixed solution (i.e., 6 M HCl (solute)/50 vol %-MeOH in solvent). Figure 3 shows the k3-weighted La K-edge EXAFS spectra for La(III) in water, conc HCl/MeOH mixed solution, adsorbed in the pyridine resin from the conc HCl/MeOH mixed solution, and a crystal of hydrated trichloride compound, LaCl3‚7H2O. The corresponding Fourier transforms (FTs) are given in Figure 4. The EXAFS oscillation in the HCl/MeOH mixed solution shows a single frequency and appears to be almost identical with that in water. Their FTs show only one peak at 2.0 Å (without the correction of phase shifts), which attributes to the O shell of water molecules. On the other hand, the EXAFS spectrum for the resin sample depicts different oscillations especially in the range of k ) 9-13 Å. The corresponding FT displays a shoulder at 2.4 Å and a small but distinguishable peak at ∼3.0 Å. The FT for the hydrated trichloride compound shows two clear peaks at 2.0 and 2.4 Å, attributed to the O shell of water molecules and Cl shell, respectively. The EXAFS structural parameters obtained from the curve fits are summarized in Table 1. The Ln(III) ions in water are found to be hydrated by 9-10 water molecules with the interatomic distance of 2.55, 2.53, and 2.47 Å for La, Ce, and Nd, respectively.

Figure 3. k3-weighted La K-edge EXAFS spectra for La(III) species (1) in water, (2) in HCl/MeOH mixed solution, (3) adsorbed in the tertiary pyridine resin, and (4) in hydrated LaCl3 crystal: solid lines (-); experimental data, circles (O); theoretical fit, solvent composition of HCl/MeOH mixture; 6 M HCl (solute)/50 vol % MeOH in solvent.

Figure 4. Fourier transforms of La K-edge EXAFS spectra in Figure 4 within the range of k ) 2.5-13.0 Å-1: solid lines (-); experimental data, circles (O); theoretical fit, phase shifts are not corrected.

This is in good agreement with the previous results by XRD,25 neutron diffraction,26 and EXAFS27,28 within an error. Almost the same parameters are obtained in the HCl/MeOH mixed solution. This suggests that, whereas higher Cl- concentration ([Cl-] ) 6 M) in the solution surely reduces the activity of water (aw),29 the Ln(III) ions keep their hydration structures unchanged at least in their primary coordination spheres even in the HCl/MeOH (25) Habenschuss, A.; Spedding, F. H. J. Chem. Phys. 1979, 70, 3758-3763. (26) Narten, A. H.; Hahn, R. L. J. Phys. Chem. 1983, 87, 3193-3197. (27) Yamaguchi, T.; Nomura, M.; Wakita, H.; Ohtaki, H. J. Chem. Phys. 1988, 89, 5153-5159. (28) Allen, P. G.; Bucher, J. J.; Shuh, D. K.; Edelstein, N. M.; Craig, I. Inorg. Chem. 2000, 39, 595-601. (29) El Guendouzi, M.; Dinane, A.; Mounir, A. J. Chem. Thermodyn. 2001, 33, 1059-1072.

mixed solution. The curve fit results for hydrated LnCl3 indicate that La and Ce form LnCl2‚7H2O, which is identical to the dichloroheptaaquo chloride compound of [LnCl2(H2O)7]Cl,30 while Nd forms NdCl2‚6H2O, being identical with the dichlorohexaaquo chloride compound, [NdCl2(H2O)6]Cl.31 The obtained Ln-O and Ln-Cl distances are also comparable with the values by XRD.30,31 As shown in Figure 4, the FT for the pyridine resin sample displays a shoulder at 2.4 Å and a small peak at ∼3.0 Å. The Cl peak observed in the trichloride compound appears at the same position as the shoulder structure in the resin sample. Accordingly, it is reasonable to consider the shoulder structure of the resin sample as a Cl shell. The small peak appearing at 3.0 Å is apparently small enough to be regarded as a “ghost” peak, which arises from the data treatment procedure and spectral noise. However, if the pyridine groups of the resin coordinate directly to Ln(III) ions, it could be assigned to the single scattering of C atoms of pyridine groups (i.e., scattering path 3 in Figure 2). Unfortunately, it is difficult to distinguish between O and N atoms in the present EXAFS results. However, we can estimate Ln-N distances from the obtained Ln-C distances by applying the Pythagorean theorem (see Figure S6 in the Supporting Information). The estimated Ln-N distances are 2.67, 2.61, and 2.59 Å for La, Ce, and Nd, respectively. These values are well comparable with the interatomic distances of Ln-N in the reported Ln-pyridine complexes: La-N ) 2.65-2.77 Å,32,33 Ce-N ) 2.62-2.70 Å,32,34 and Nd-N ) 2.57-2.67 Å.32,33,35,36 This agreement in the Ln-N distances might be an indirect proof of the direct interaction between the pyridine groups and Ln(III) ions in the resin phase. Although the obtained CN values for the resin samples do not represent the adsorbed species directly,37 the present EXAFS results indicate that the hydration of Ln(III) ions weakens inside the resin phase probably due to the hydrophobicity of resin matrix, and in consequence, Cl- (and possibly the pyridine groups of the resin) can coordinate to the metal ions. In fact, as already mentioned in the introduction part, several studies1,2 have also suggested that the hydration of ions becomes weaker in organic resins so that other complexation is enhanced. This is consistent with the present EXAFS results for the resin samples. In HNO3 Solution. The Kd and R of the pyridine resin increase by adding alcohol in HNO3 solution16 in the same way as observed in HCl solutions. Hence, the mixed solution “conc HNO3 ()13.5 M HNO3)/MeOH ) 5/5 (v/v)” (i.e., 6.8 M HNO3 (solute)/50 vol %-MeOH in solvent) was employed for preparing EXAFS samples in order to get sufficient Kd values. Nd K-edge (30) Habenschuss, A.; Spedding, F. H. Cryst. Struct. Commun. 1979, 8, 511516. (31) Habenschuss, A.; Spedding, F. H. Cryst. Struct. Commun. 1980, 9, 71-76. (32) Kepert, C. J.; Weimin, L; Skelton, B. W.; White, A. H. Aust. J. Chem. 1994, 47, 365-384. (33) Semenova, L. I.; White, A. H. Aust. J. Chem. 1999, 52, 571-600. (34) Denecke, M. A.; Rossberg, A.; Panak, P. J.; Weigl, M.; Schimmelpfennig, B.; Geist, A. Inorg. Chem. 2005, 44, 8418-8425. (35) Wietzke, R.; Mazzanti, M.; Latour, J.-M.; Pećaut, J.; Cordier, P.-Y.; Madic, C. Inorg. Chem. 1998, 37, 6690-6697. (36) Yaita, T.; Tachimori, S.; Edelstein, N. M.; Bucher, J. J.; Rao, L.; Shuh, D. K.; Allen, P. G. J. Synchrotron Radiat. 2001, 8, 663-665. (37) As explained in the Experimental Section, resin samples retained some amount of solution. This means that the EXAFS spectra of the resin samples include not only the information of the adsorbed species but also the information of the bulk species (i.e. the chemical species in solution). Therefore, it is difficult to determine the absolute N values for the adsorbed species from the present EXAFS results.


8019

Table 1. Summary of EXAFS Structural Parameters Obtained for HCl Solution System water

HCl/MeOH

LnCl3‚nH2O

pyridine esin

element

shell

R/Åa

Nb

R/Åa

Nb

R/Åa

Nb

R/Åa

Nb

La

O or N Cl C O or N Cl C O or N Cl C

2.55

9.5

2.55

9.6

7.0 2.0

9.6

2.52

9.6

2.53 2.90

7.0 2.0

2.47

9.5

2.48

9.6

8.3 1.0 1.8 8.5 0.6 2.0 9.0 0.5 1.5

2.55/2.56c 2.91/2.94c

2.53

2.52 2.91 3.53 2.53 2.90 3.48 2.49 2.81 3.46

2.47/2.46d 2.81/2.82d

6.0 2.0

Ce Nd

a Estimated standard deviation; R e ( 0.01 Å. b Estimated standard deviation; N e ( 20%. c XRD; ref 30. d XRD; ref 31. Squared Debye-Waller factors, σ2, were varied in the range of 0.002-0.01 Å2. Energy shift parameters, ∆E0, were varied in the range of -10 to 10 eV. Detailed parameters are given in Table S1 in the Supporting Information.

Figure 5. k3-weighted Nd K-edge and Sm LIII-edge EXAFS spectra for the Ln(III) species (1) in water, (2) in HNO3/MeOH mixed solution, and (3) adsorbed in a tertiary pyridine resin: solid lines (-); experimental data, circles (O); theoretical fit, solvent composition of HNO3/MeOH mixture; 6.8 M HNO3 (solute)/50 vol % MeOH in solvent.

and Sm LIII-edge EXAFS spectra for the solution and resin samples are shown in Figure 5. The observed EXAFS oscillation patterns in the HNO3 solution system are more complicated than those in the HCl solution system. It has been reported that Ln(III) ions form a symmetrical complex with nitrate ions, such as Ln (NO3)+2 with bidentate, in the NO3 concentration of more than 3 M38 and these bidentate-coordinate nitrate ions make the EXAFS oscillation more intricate especially at higher k range due to the strong feature of multiple scattering (i.e., the scattering path 7 in Figure 2).39,40 Therefore, the observed EXAFS spectra probably suggest that Ln(III) ions form inner-sphere complexes with nitrate ions both in the solution phase and in the resin phase. Figure 6 shows the FTs of the Nd and Sm EXAFS spectra in the HNO3/MeOH mixed solution (left) and in the pyridine resin (right). The main peaks at ∼2 Å are expected to be the mixture of two different O shells, that is, the O atoms of water molecules and those of nitrate ions. The resolution in bond distance, ∆R, (38) Yaita, T.; Ito, D.; Tachimori, S. J. Phys. Chem. B 1998, 102, 3886-3891. (39) Allen, P. G.; Veirs, D. K.; Conradson, S. D.; Smith, C. A.; Marsh, S. F. Inorg. Chem. 1996, 35, 2841-2846. (40) Yaita, T.; Narita, H.; Suzuki, S.; Tachimori, S.; Motohashi, H.; Shiwaku, H. J. Radioanal. Nucl. Chem. 1999, 239, 371-375.


Figure 6. Fourier transforms of Nd K-edge and Sm LIII-edge EXAFS spectra for the sample (2) and (3) in Figure 5: solid lines (s); experimental data, circles (O); theoretical fit, k range taken over for Fourier transformation; 2.5-15.0 Å-1 for Nd and 2.0-12.0 Å-1 for Sm; phase shifts are not corrected for the total FT magnitudes (black lines) and their corresponding curve fits (circles).

for the present EXAFS results was 0.12 and 0.16 Å for Nd and Sm, respectively.41 In principle, these two O shells are indistinguishable if their difference is below the resolution limit. However, clear peaks attributed to the distal O atoms (Odist) and the multiple scattering (MS) of nitrate ions were observed at ∼3.8 Å, and in (41) The resolution in bond distance is defined as ∆R ) π/2∆k, where ∆k is the k range for Fourier transformation.

Table 2. Summary of EXAFS Structural Parameters Obtained for HNO3 Solution System water

HNO3/MeOH

pyridine resin

in crystal

element

shell

R/Åa

Nb

R/Åa

Nb

R/Åa

Nb

R/Å

Nd

Ohyd Oco N Odist MS (N-Odist-N) Ohyd Oco N Odist MS (N-Odist-N)

2.47

9.5

8.6

5.5 3.9 1.9 1.9 1.9 4.7 3.8 1.9 1.9 1.9

2.49 2.61 3.05 4.26 4.31 2.42 2.53 2.96 4.20 4.20

2.1 7.9 3.9 3.9 3.9 4.1 3.9 1.9 1.9 1.9

2.45-2.54c 2.58-2.60c

2.44

2.50 2.63 3.07 4.26 4.30 2.42 2.54 2.97 4.20 4.20

Sm

2.49d 2.55d

a Estimated standard deviation; R e ( 0.02 Å. b Estimated standard deviation; N e ( 15%. c Reference 42. d Reference 43. Squared DebyeWaller factors, σ2, were varied in the range of 0.001-0.012 Å2. Energy shift parameters, ∆E0 were varied in the range of -10 to 10 eV. Detailed parameters are given in Table S2 in the Supporting Information.

consequence, we can estimate the structural parameters for the coordinated O atoms (Oco) and N atoms of nitrate ions by correlating these parameters with those of Odist and MS paths in the fitting procedure. The EXAFS structural parameters obtained from the curve fits are given in Table 2. Both Nd(III) and Sm(III) ions are coordinated by two nitrate ions with a bidentate mode in the solution phase. The Ln-Oco distances are ∼0.1 Å longer than the interatomic distances for water molecules, Ln-Ohyd, being consistent with the previous EXAFS results for Ln(III) ions in 13 M HNO3 solution.40 On the other hand, notable results were obtained for the resin samples. That is, the NH2O of Nd decreases from 5.5 to 3.1 in the resin phase, and instead, NNO3 increases from 1.9 to 3.9, while Sm only shows a slight decrease in NH2O in the resin phase without an increase in NNO3. This suggests that the adsorption of Nd(III) ions in the pyridine resin is accompanied by dehydration followed by additional nitrate complexation, forming an anionic complex of [Nd(NO3)4‚nH2O]- in the resin phase. On the other hand, the adsorption of Sm(III) ions is also accompanied by slight dehydration, but it involves no further nitrate complexation. In consequence, Sm(III) ions keep the same cationic complexes, i.e., [Sm(NO3)2‚nH2O]+, even in the resin phase; nonetheless, the atomic number (i.e., ionic radius) of Sm is close to that of Nd. The comparison of FTs in Figure 6 clearly demonstrates the difference in the structural change of Nd and Sm between the solution phase and the resin phase. The contributions of Oco, N, Odist, and MS shells increase proportionally on the FT of Nd in the resin phase, while the contributions of these shells are almost constant in the FTs of Sm. Besides, no distinguishable contribution of Py coordination was observed in the measured spectra. DISCUSSION Here we discuss the adsorption and separation mechanisms of tertiary pyridine resin on the basis of the EXAFS results. The adsorption and separation phenomena by the pyridine resin can be interpreted in two different manners: that is, the coordinative mechanism and the ion-exchange mechanism. The tertiary pyridine resin possesses pyridine groups in its matrix and this pyridine ligand can directly coordinate to An(III) and Ln(III) ions even in

solution.6,35,36,44 Therefore, the observed clear separation between An(III) and Ln(III) in HCl solution8-10,13 could be explained as a result of the soft donor property of pyridine groups. On the other hand, the pyridine resin can also function as a weakly basic anion exchanger. That is, pyridine N atoms are easily protonated in acidic solutions as shown in Figure 1, behaving as an anion exchanger. In fact, the chromatographic behavior of An(III) and Ln(III) by the pyridine resin in HNO3 solution11,12,14-16 is analogous to that by anion-exchange resins,45 and no intergroup separation between An(III) and Ln(III) is observed.15 This difference in chromatographic behavior of An(III) and Ln(III) between HCl and HNO3 solution systems brings perplexity on the interpretation of adsorption and separation mechanisms by the pyridine resin. The EXAFS results for the resin samples in HCl solution can be interpreted in two different ways. Assuming that the observed adsorption and separation phenomena are caused by the ionexchange interaction, Ln(III) ions must be adsorbed in the resin as an anionic chloro complex, such as [LnCl4]-, and hence, the obtained EXAFS spectrum can be interpreted as a mixture of the anionic chloro complex (i.e., adsorbed species) and the hydrated species of Ln(H2O)n (i.e., bulk species), which is dissolved in the solution phase that remained within the pore structure of the resin. This means that Ln(III) ions can form the anionic chloro complex in the resin phase to interact with the protonated (i.e., positively charged) pyridine groups. If this is the case, Ln(III) (and An(III)) are expected to also be adsorbed by typical anion-exchange resins in HCl solution. However, our previous experiment9 has suggested that a quaternary ammonium type, strongly basic anion-exchange resin shows almost no adsorption and separation for An(III) and Ln(III) in the same HCl/MeOH mixed solution. Besides, it has also confirmed in our ongoing experiments that tertiary am(42) (a) Bu ¨ nzli, J.-C. G.; Klein, B.; Wessner, D. Inorg. Chim. Acta 1981, 54, L43L46. (b) Rogers, D. J.; Taylor, N. J.; Toogood, G. E. Acta Crystallogr. 1983, C39, 939-941. (c) Benetolio, F.; Bombieri, G.; Cassol, A.; De Paoli, G.; Legendziewicz, J. Inorg. Chim. Acta 1985, 110, 7-13. (43) Burns, J. H. Inorg. Chem. 1979, 18, 3044-3047. (44) (a) Yashiro, M.; Ishikubo, A.; Takarada, T.; Komiyama, M. Chem. Lett. 1995, 8, 665-666. (b) Assefa, Z.; Yaita, T.; Haire, R. G.; Tashimori, S. Inorg. Chem. 2003, 42, 7375-7377. (45) (a) Usuda, S. J. Radioanal. Nucl. Chem., Art. 1987, 111, 399-410. (b) Usuda, S. J. Radioanal. Nucl. Chem. Art. 1987, 111, 477-485. (c) Usuda, S. J. Radioanal. Nucl. Chem., Art. 1988, 123, 619-631. (d) Usuda, S.; Kohno, N. Sep. Sci. Technol. 1988, 23, 1119-1131.


8021

monium type (weakly basic) and quaternary pyridine type (strongly basic) anion-exchange resins give no adsorption and separation for An(III) and Ln(III) in the same solution. Considering these facts, the adsorption of Ln(III) ions by the tertiary pyridine resin in the HCl system is not likely to have originated in the ion-exchange interaction. On the other hand, we can give another explanation for the EXAFS results of the resin samples. As already discussed in the Results section, the small peak at 3.0 Å for FT-(3) in Figure 4 can be assigned as the C atoms of the pyridine groups. Although the magnitude of this peak is close to the noise level, the obtained Ln-N and Ln-C distances are reasonable for the directly coordinated pyridine group. Taking these facts and our previous chromatographic experiments8-10,13-15 into account, the coordinative interaction is more probable for the adsorption mechanism of Ln(III) by the pyridine resin in HCl solution rather than the ion-exchange interaction. Naturally, the protonation of pyridine groups is expected to be very strong46 and the pyridine groups of the resin should be fully protonated in a high acidic solution like the present HCl/ MeOH mixture. Therefore, the Py-metal interaction must be competitive with the protonation reaction. That is, the protonation of pyridine groups is a reversible reaction, and hence, the functional sites of pyridine groups are not always occupied by protons (H+). In this case, metal ions still have a possibility to interact with the pyridine groups directly. However, the coordination of metal ions is also a reversible reaction, and the coordinated metal ions are easily substituted by H+ or other metal ions. This is probably the reason why the Kd values of Ln(III) are low in the HCl solution system. In fact, the adsorbability of the pyridine resin increases drastically in a nonacidic chloride solution of LiCl solution,9 in which the protonation of pyridine groups hardly occurs. In this sense, H+ is just a negative factor that lowers the adsorbability of the pyridine resin. However, to the contrary, An(III) and Ln(III) ions (especially An(III)) are adsorbed in the resin so strongly in the absence of H+ that it takes a long time to perform the practical separation procedure. Therefore, from a practical point of view, a moderate concentration of H+ is required for the efficient separation in the chloride solution system. Additionally, adding alcohol in solution also restrains the protonation reaction of pyridine ligand.46 This gives an appropriate explanation for the observed increasing effect on Kd by adding alcohol in solvent. On the other hand, the EXAFS results have also suggested that the Cl- coordination is enhanced in the resin phase and Ln(III) ions are probably adsorbed in the resin as a cationic or neutral chloro complex, [LnClm(H2O)n]3-m (m ) 1-3). The 139La NMR study by Yaita et al.38 has indicated that the chloride ions coordinating to La(III) ions break the hydrogen bond network around the metal ions. The hydrogen bond network around metal ions surely hinders the Py-metal interaction, and hence, this structure-breaking effect by chloride ions may encourage the Py-metal interaction. Chloride ions also possess a soft donor property as compared with nitrate ions, showing a slightly larger formation constant for An(III) than for the homologous Ln(III).47 Accordingly, the observed intergroup separation between An(III) and Ln(III) by the pyridine resin in HCl solution could be (46) Kılıç , E.; Ko ¨seolu, F.; Bas¸ gut, O ¨ . Anal. Chim. Acta 1994, 294, 215-220. (47) (a) Bansal, B. M. L.; Patil, S. K.; Sharma, H. D. J. Inorg. Nucl. Chem. 1964, 26, 993-1000. (b) Sekine, T. J. Inorg. Nucl. Chem. 1964, 26, 1463-1465.

8022


interpreted as a result of the synergic soft donor effect of pyridine groups and chloride ions. On the other hand, the situation seems to be different in the HNO3 solution system. As previously mentioned, the adsorption and separation properties of tertiary pyridine resin in HNO3 solution are similar to those of typical anion-exchange resins but quite different from those in HCl solution. That is, no intergroup separation between An(III) and Ln(III) is achieved in the HNO3 solution system, while the intragroup separation between several elements, such as Am-Cm or Nd-Sm, is possible. The EXAFS results have suggested that Nd(III) ions form an anionic nitrate complex, [Nd(NO3)4(H2O)n]-, in the resin phase, while the dominant species of Sm(III) ions in the resin phase is a cationic complex, [Sm(NO3)2(H2O)n]+. In addition, no significant contribution of Py coordination has been detected. The remarkable thing is that the Kd values of Ln(III) in HNO3 solution drastically change between Nd and Sm in the practical chromatographic process:14 Nd(III) ions, which mainly exist as the anionic complex in the resin phase, are strongly adsorbed by the pyridine resin, while Sm(III) ions, which predominantly form the cationic complex in the resin phase, are weakly adsorbed. In consequence, a clear separation is observed between Nd and Sm. Considering these facts, the adsorption mechanism of the pyridine resin in the HNO3 solution system is likely to be dominated by the “anion-exchange” interaction, not the “coordinative” one. That is, the metal ions are adsorbed in the resin as an anionic complex like [M(III)(NO3)4(H2O)n]- by interacting with the protonated pyridine groups. On the other hand, the metal ions forming a cationic complex like [M(III)(NO3)2(H2O)n]+ are not attracted by the protonated pyridine groups, showing smaller Kd values. This hypothesis is consistent with the observed similarity of separation behavior between the pyridine resin and other anion-exchange resins in this solution system. Furthermore, this “anion-exchange” hypothesis also gives a good explanation for the clear individual separation between Am and Cm in HNO3/MeOH mixed solutions.11,12 The reported EXAFS studies27,28 have indicated that the An(III)-OH2 distances (An(III) ) Am and Cm) in aqueous solution are intermediate between Nd and Sm. This implies a possibility that there is also a difference in the complexation with nitrate ions between Am(III) and Cm(III) ions in the resin phase. That is, Am(III) ions, whose ionic radius is closer to Nd(III) ions,3 probably form an anionic complex with nitrate ions (i.e., [Am(NO3)4(H2O)n]-) in the resin phase, and in consequence, they strongly interact with the protonated pyridine groups in the resin like Nd ions. To the contrary, Cm(III) ions, which have an ionic radius similar to that of the Sm(III) ions,3 mainly exist as a cationic complex ([Cm(NO3)2(H2O)n]+) even in the resin phase, with the result that their adsorption in the resin becomes weaker than that of Am(III) ions. Figure 7 represents the Kd variation of An(III) and Ln(III) in a HNO3/MeOH mixed solution as a function of their ionic radii. Several studies (EXAFS,27 NMR,48 apparent molar volume measurement,49 and Raman50) have demonstrated that the (48) Lewis, W. B.; Jackson, J. A.; Lemons, J. F.; Taube, H. J. Chem. Phys. 1962, 36, 694-701. (49) (a) Spedding, F. H.; Pikal, M. J.; Ayers, B. O. J. Phys. Chem. 1966, 70, 2440-2449. (b) Spedding, F. H.; Saeger, V. W.; Gray, K. A.; Boneau, P. K.; Brown, M. A.; DeKock, C. W.; Baker, J. L.; Shiers, L. E.; Weber, H. O.; Habenschuss, A. J. Chem. Eng. Data 1975, 20, 72-81. (c) Spedding, F. H.; Shiers, L. E.; Brown, M. A.; Derer, J. L.; Swanson, D. L.; Habenschuss, A. J. Chem. Eng. Data 1975, 20, 81-88.

Assuming that the tertiary pyridine resin works as an anion exchanger, the intragroup separation between Am and Cm or Nd and Sm should be also achieved by using other anion-exchange resins in the same condition. In fact, our recent experiments using a strongly basic anion-exchange resin have brought the same separation results for Am and Cm in conc HNO3/MeOH mixed solutions. This also supports the “anion-exchange” hypothesis in the HNO3 solution system.

Figure 7. Distribution coefficients of An(III) and Ln(III) for tertiary pyridine resin in HNO3/MeOH mixed solution at 298 K: solvent composition, 8.1 M HNO3 (solute)/40 vol %-MeOH in solvent; the values of effective ionic radii were referred to ref 3; the error bars are smaller than the plotted points.

inner-sphere hydration structure of Ln series changes at around Sm-Eu, for instance, from [Ln(H2O)9]3+ to [Ln(H2O)8]3+. A similar thing could be occurring also in their nitrate complexation, causing the different complexation manner with nitrate ions between Nd and Sm. We can also explain the observed intragroup separation of An(III) in the HNO3 solution in the same way. However, the difference in the nitrate complexation between Am and Cm is expected to be very sensitive to the composition of solvent since their ionic sizes seem to be at the boundary, which decides the complexation manner with nitrate ions in the resin phase. (50) Kanno, H.; Hiraishi, J. J. Phys. Chem. 1982, 86, 1488-1490.

ACKNOWLEDGMENT The authors acknowledge M. Ozawa and Y. Ikeda (Tokyo Tech.) and T. Kimura (JAEA) for thier fruitful discussions and also thank K. Otake and K. Itoh (Tokyo Tech.) and T. Kobayashi and M. Numakura (JAEA) for their experimental support. The EXAFS measurements at SPring-8 have been carried out as the cooperative research with JAEA for synchrotron science (Proposal H17-3-n). The EXAFS measurements at Photon Factory have been performed under the approval of the Photon Factory Program Advisory Committee (Proposals 2002P015, 2004G250, 2004G279, and 2004G313). SUPPORTING INFORMATION AVAILABLE Graphs of distribution coefficients (Figures S1-S3), the EXAFS spectra not presented in the text (Figures S4 and S5), description about the estimation of Ln-N distance (Figure S6), and detailed EXAFS structural parameters (Table S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org. Received for review April 10, 2007. Accepted August 17, 2007. AC070700N


8023

Extended X-ray Absorption Fine Structure Investigation of Adsorption

Recommend Documents