Anal. Chem. 2000, 72, 2841-2845
Anomaly of 155Gd and 157Gd Isotope Effects in Ligand Exchange Reactions Observed by Ion Exchange Chromatography Ibrahim M. Ismail,†* Akira Fukami,‡ Masao Nomura, and Yasuhiko Fujii
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-Okayama, Meguro-Ku, Tokyo 152, Japan
The isotope effects of gadolinium in Gd-EDTA ligand exchange system were studied by means of ion exchange chromatography. The separation coefficients of gadolinium isotopes, E, and the local enrichment factors, β, were calculated from the observed isotopic ratios at the front and rear boundaries of the gadolinium adsorption band. Clear mass independent anomalies were observed in the isotope effects of 155Gd and 157Gd. The relation between the isotope effects of gadolinium isotopes, studied by the three-isotope plot and the separation coefficient methods, and the mass of gadolinium isotopes was found to be related to the change in the mean square radius of the nuclear charge distribution parameter, 〈r2〉, of these isotopes, which suggests that the nucleus shape and size highly affect the gadolinium isotope effects in chemical exchange reactions. It is believed that there is a little difference in the chemical properties of the isotopes of the same element. Isotopes of a given element may show some quantitative differences in chemical reaction equilibria and/or reaction rates; the former is the equilibrium isotope effects and the latter is the kinetic isotope effects. The chemical exchange separation of isotopes was extensively studied by H. Urey, who had successfully studied the isotope effects of nitrogen, carbon, sulfur, lithium, oxygen, and potassium by different chemical exchange methods.1,2 G. Clewett and W. Schaap3 explained that the isotope effects in a chemical exchange reaction are due to a slight difference in the affinity of the isotopes for a given molecule or complex due to minor variances in the internal energies, mainly vibrational energy, of the molecule. On the basis of the quantum molecular vibration energy, Bigeleisen formulated the method to calculate the isotope exchange equilibrium constant from spectroscopic * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +81-3-5734-2959. † Present address: Chemical Engineering Department, Cairo University, Giza, Egypt. ‡ Present Address: Nuclear Fuel Industries, LTD., Osaka, Sennon-Gun, Kumatori-Cho, Noda 950, Japan. (1) Taylor, T.; Urey, H. J. Chem. Phys. 1938, 6, 429. (2) Bigeleisen, J. Proceedings of The International Symposium on Isotope separation and Chemical Exchange Uranium Enrichment, Tokyo, Oct 29Nov 1, 1990; published as special issue 1 of the bulletin of the research lab for Nuclear Reactors; Fujii, Y., Ishida, T., Takeuchi, K., Eds.; 1992; p 3. (3) Clewett, G.; Schaap, W. Report Y-41, Y-12 Plant; Union Carbide Corp.: 1947. 10.1021/ac991187j CCC: $19.00 Published on Web 05/18/2000
© 2000 American Chemical Society
data.4 This method was used to calculate the equilibrium constant of the isotopic exchange of many elements ranging from hydrogen to uranium. Unfortunately, this method could not explain the anomalous isotope effects of the odd isotopes 233U5 and 235U6 among the other uranium even isotopes. This anomaly was found to be similar to the odd-even staggering of the isotope shift in the atomic spectra. According to the new theory derived by Bigeleisen, this anomaly is believed to be due to the field shift.7 Later on, this anomaly was reported for other elements. Nishizawa reported anomalous mass effects in the isotope separation of the odd-mass 67Zn from the even-mass 64Zn, 66Zn, and 68Zn. He attributed this anomaly to the isotope shift in the orbital energy.8 Lanthanides and actinides are known to have deformed nuclei, which cause the charge distribution effects in the isotope shifts of the atomic emission spectral lines. Although almost all of the lanthanides have more than one isotope, only a few investigations have been reported on the isotope effects of these elements. Uranium was the only exception as it was the subject of extensive research due to the importance of the 235U isotope. The high performance liquid chromatography technique was used for separating the isotopes of Sm, Ce, and Eu, while the electron exchange system was used for studying the isotope separation of Ce and Eu. The ligand exchange system was applied to Gd,9 Eu,10 and U11-13 isotope separation. Kim et al. studied the isotope effects of uranyl complexes by means of ion exchange chromatography and reported that the malic acid eluent system had the largest separation coefficient among some selected uranyl carboxylate complexes.12 While, in another study on the isotope effects of gadolinium,9 EDTA shows a larger separation coefficient than malic acid. Therefore, in the present work, EDTA is applied as the ligand for the Gadolinium(III) complex formation system. (4) Bigeleisen, J.; Mayer, M. J. Chem. Phys. 1947, 15(5), 261. (5) Nomura, M.; Higuchi, N.; Fujii, Y. J. Am. Chem. Soc. 1996, 118, 9127. (6) Fujii, Y.; Nomura, M.; Okamoto, M.; Onitsuka, H.; Kawakami, F.; Takeda, K. Z. Naturforsch., A: Phys. Sci. 1989, 44a, 395. (7) Bigeleisen, J. J. Am. Chem. Soc. 1996, 118, 3676. (8) Nishizawa, K.; Satoyama, T.; Miki, T.; Yamamoto, T.; Nomura, M. Sep. Sci. Technol. 1996, 31(20), 2831. (9) Chen, J.; Nomura, M.; Fujii, Y.; Kawakami, F.; Okamoto, M. J. Nucl. Sci. Technol. 1992, 29(11), 1086. (10) Ismail, I.; Nomura, M.; Fujii, Y. J. Chromatogr., A 1998, 808, 185. (11) Ismail, I.; Nomura, M.; Aida, M.; Fujii, Y., to be published. (12) Kim, H.; Kakihana, M.; Aida, M.; Kogure, K.; Nomura, M.; Fujii, Y.; Okamoto, M. J. Chem. Phys. 1984, 81(12), 6266. (13) Nakagawa, A.; Sakuma, Y.; Okamoto, M.; Maeda, M. J. Chromatogr. 1983, 256, 231.
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In a previous study of gadolinium isotope effects carried out by J. Chen et al.,9 the anomaly of 157Gd was suggested but not confirmed due to the large error range of the observed values. In addition, 155Gd was not analyzed due to the mass overlapping with other lanthanide isotopes. Therefore, the aim of this work is to study the isotope effects of gadolinium, especially the odd mass number isotopes in the ligand exchange system. It is hoped that more information will be gained that may help in understanding the theory of isotope effects. EXPERIMENTAL SECTION Ion-Exchange Resins and Reagents. The cation-exchange resin used in the ligand exchange chromatographic system was a macroporous strongly acidic cation-exchange resin (Bio Rad, AG-MP 50, 100-200 mesh size). Gd2O3 99.999% was supplied by Wako pure chemical industries. All other reagents used were of analytical grade and employed without further purification. Chromatographic System. Gadolinium isotope separation experiment based on the EDTA complex formation was carried out with a cyclic displacement chromatography system which is composed of three glass columns, 0.8 cm i.d. × 100 cm long, with water jackets, connected in series with Teflon tubes, 1 mm i.d., so that they were repeatedly used in a merry-go-round way for a total migration length of 10 m. These columns were packed uniformly with the above-mentioned resin. The resin was pretreated with 2 M, mol/dm3, HCl solutions to remove impurities and to convert the resin into H+ form. This was followed by passing a solution of 0.5 M CuSO4 + 0.7 M H2SO4 to convert the resin into Cu2+ form. Then a 0.02 M Gd(NO3)3 solution was fed into the first column at a constant flow rate by a peristaltic pump to form a Gd3+ adsorption band. When the Gd3+ ion adsorption band had grown to an appropriate length, the supply of the feed solution was stopped. The Gd3+ and Cu2+ adsorption bands were eluted by an eluent solution containing 0.05 M (NH4)4EDTA + 0.1 M NH4NO3 adjusted to pH 8 with NH4OH solution. After adjusting the pH to 8, it is expected that the four carboxylic groups of the EDTA will be occupied by NH4+. This was supported by a pH titration test of EDTA, of which the four carboxylic groups were in H+ form, with NH4OH. The eluent was fed at the same flow rate as that of the feed solution. The adsorption band of Gd3+ was visible, white, in contrast with the preceding blue Cu band. When the Gd3+ adsorption band migration length reached 10 m, it was eluted out from the last column. The effluent was collected in small fractions that were, thereafter, subjected to the concentration analysis and the isotopic analysis. The temperatures of the columns were kept constant at 80 ( 0.2 °C by circulating the thermostated water through the water jackets surrounding the columns. The set of apparatuses for the chromatographic experiment is illustrated in Figure 1, and the experimental conditions are summarized in Table 1. Analysis. The concentration of gadolinium in each fraction of the effluents was determined by an ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometer) model SPS 1500VR, Seiko Instruments Inc. The gadolinium isotopic ratios of some selected samples were measured by using a MAT 261 mass spectrometer equipped with a thermal ionization ion source. The samples in the form of Gd(NO3)3 were loaded on the evaporation filament and thermally ionized by the ionization filament. 2842
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Figure 1. Schematic of separation system used. Table 1. Experimental Conditions in the Ligand Exchange System resin pretreatment feed solution eluent column size migration length temperature flow rate band velocity
strongly acidic cation-exchange resin (AG MP 50, 100-200 mesh size) 2 M HCl followed by 0.5 M CuSO4 + 0.7 M H2SO4 to convert resin into Cu2+ form 0.02 M Gd(NO3)3 0.05 M EDTA‚4NH4 + 0.1 M NH4NO3 at pH ) 8 0.8-cm i.d. and 100-cm length 10 m 80 °C 0.42 cm3/min 0.08 cm/min
RESULTS AND DISCUSSION Chromatographic System. The chemical reactions involved in the present system first take place at the interface between NH4+ and Gd3+ adsorption bands. When (NH4)4EDTA reaches the front boundary of the Gd3+ adsorption band, the EDTA ligands are transferred to Gd3+ because of the large stability constant of the Gd-EDTA complex compared with that of the ammonium ion-EDTA complex, eq 1. During the moving down of the solution phase, which contains Gd-EDTA complex species through the Gd3+ adsorption band in the column, the isotopic exchange reaction takes place between Gd3+ ions in the resin phase and Gd-EDTA complex species in the solution phase, eq 2. After that, the Gd-Ligand complex reaches the Cu2+ ion band, where ligands are transferred to Cu2+ ions and Gd3+ ions are adsorbed in the resin phase, eq 3. The related chemical reactions can be expressed, in the most simple form, as
(NH4)4-L + Gd3+ + H+ f 4 NH4+ + Gd-L-H
(1)
Gd3+ + LGd-L-H T LGd3+ + HGd-L-H
(2)
Gd-L-H + Cu2+ +H+ f Gd3+ + Cu-L-2H
(3)
H
where the italics represent the species in the resin phase, L represents a ligand, and HGd and LGd represent the heavy and the light gadolinium isotopes, respectively. The profile of Gd concentration in the effluent fractions, which corresponds to the Gd band profile in the column, after a 10-m migration is shown in Figure 2 along with the measured pH values of the effluent. The sharp boundaries of the band shown in this figure, which is also supported by the pH change, indicate that the chromatographic displacement was almost ideal at both boundaries. The concentration of Gd in the plateau region of the chromatogram and the concentration of EDTA in the eluent feed were used to perform a charge balance analysis for the Gd-EDTA complex. The analysis shows that about 60% of the carboxylic groups in EDTA are coordinating with Gd. The rest is assumed to be coordinating with H+ initially sorbed through mixing with Cu and Gd ions in the band.9,10 This is supported by the low pH value of the plateau region of the chromatogram as shown in Figure 2. In fact, the chemistry of the system may be more complicated than that represented by eqs 1, 2, and 3. The exact complex structure and the different possibilities of Gd and/or H2O hydrolysis are out of the scope of the present work. Gd has seven isotopes but it is not possible to precisely measure the isotope ratio of all of them. The isotope ratio measurement had been focused on the major five isotopes, 155Gd, 156Gd, 157Gd, 158Gd, and 160Gd. The measured isotopic abundance ratios of 155Gd, 156Gd, 157Gd, and 158Gd commonly against 160Gd in the effluent fractions and their standards deviation errors are plotted in Figure 3. The dotted lines show the isotopic abundance ratio in the feed solution. It is usually believed that the isotope ratio measurements of the isotope pair 155Gd/160Gd show large values of error due to the interference of LaO, which has a predominant mass of 155. The ionization of LaO is much easier than the ionization of 155Gd, and the mass peak at 155 due to LaO appears earlier than that of 155Gd. Actually it was found that the peak height recorded by the mass spectrometer at 155 mass number decreased with time until all LaO was ionized, and then it became constant. By using ultrapure Gd2O3 to prepare the Gd solutions and waiting for a suitable time before recording the peak height ratios, the error was reduced to a minimum value. Using the above-mentioned procedure of the isotope analysis, the recorded peak height ratios of 155Gd/160Gd were found to be almost constant within the normal experimental error like other isotopes. It is clear that the heavier isotope 160Gd was selectively fractionated in the front boundary, the complex side. This tendency is the same as that observed in the chromatographic isotope separation of calcium,14 magnesium,15 strontium,16 gadolinium,9 and copper.17,18 Since the heavier isotope is enriched in (14) Kobahashi, N.; Fujii, Y.; Okamoto, M.; Kakihana, H. Bull. Res. Lab. Nucl. React. (Tokyo Inst. Technol.) 1980, 5, 19. (15) Oi T.; Yanase, S.; Kakihana, H. Sep. Sci. Technol. 1987, 22, 2203. (16) Oi, T.; Ogino, H.; Hosoe, M.; Kakihana, H. Sep. Sci. Technol. 1992, 27(5), 631. (17) Abdul Matin, M.D.; Nomura, M.; Fujii, Y.; Chen, J. Sep. Sci. Technol. 1998, 33(8), 1075. (18) Ismail, I.; Abdul Matin, M.D.; Nomura, M.; Aida; M.; Fujii, Y., to be published
Figure 2. Concentration and pH profile in Gd band displaced by EDTA eluent.
the complex species, the observed isotopic enrichment tendency accords with the theoretically expected direction of the isotopic effects in chemical exchange. The three isotope plots can be used also to ensure the anomalous isotope effects of 155Gd and 157Gd as was explained in a previous paper.5 The isotopic local enrichment factor β is defined as
β(j/k) ) r/ro
(5)
where r is the isotopic ratio of the isotopes j and k and the subscript o denotes the feed. The slope of plotting the logarithm of the local enrichment factor of an isotope pair against that of another isotope pair is directly correlated to their isotope effects. It is equal to the ratio of their separation coefficients. This method is known as the three-isotope plots and is mainly used in geochemistry and cosmochemistry to study isotopic fractionation in minerals. The three-isotope plots of gadolinium isotopes are shown in Figure 4a,b,c. The slope of the isotope pair 156Gd/160Gd was taken as a reference in order to compare the slopes of all isotopic pairs. The slopes calculated for each pair of isotopes are listed in Table 2 and plotted against the mass difference in Figure 5. It is easy to notice that the slopes of 155Gd and 157Gd deviate from the straight line of the other even isotopes. The single-stage separation factor, R ) (1 + ) for the 160Gd/ nGd isotopic pair, is defined here as
Rn ) ([nGd]/[160Gd])/([nGd]/[160Gd])
(4)
where the italics represent the species in the resin phase and n can take the values 155, 156, 157, and 158. The separation coefficients, , were calculated using the isotopic enrichment curves of the front and rear boundaries according to the equations developed by Spedding19 and Kakihana.20 The separation coef(19) Spedding, F. H.; Powell, J. E.; Svec, H. J. J. Am. Chem. Soc. 1955, 77, 6125.
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Figure 4. Three-isotope plots of local enrichment factors: (a) between isotopes 155Gd, 156Gd, and 160Gd; (b) between isotopes 157Gd, 156Gd, and 160Gd; (c) between isotopes 158Gd, 156Gd, and 160Gd. Table 2. Slopes of Three-Isotope Plots, TIP, and the Separation Coefficients, E, of the 155Gd, 156Gd, 157Gd, and 158Gd Isotopes against 160Gd separation coefficients ( × 105)
Figure 3. Isotopic distribution of (a) 155Gd and 156Gd and (b) 157Gd and 158Gd against 160Gd in a Gd band displaced by EDTA.
isotope
slopes of TIP
rear boundary
front boundary
average
155 156 157 158
1.2 ( 0.05 1b 0.8 ( 0.03 0.5 ( 0.05
6.1 5.4 4.7 2.7
6.1 5.0 4.1 2.7
6.1 ( 0.3a 5.2 ( 0.3 4.4 ( 0.3 2.7 ( 0.3
a The errors were estimated from the errors in each isotope ratio measurements. b Control.
ficients of gadolinium isotopes were calculated from the data of the rear and front boundaries, and then the average was taken. The results are shown in Table 2. If the gadolinium used had a considerable amount of La, this element would be concentrated in the front boundary after the chromatographic process. In such case, the separation coefficient of 155Gd calculated from the front boundary data would be higher than that calculated from the rear boundary data. As it can be seen from Table 2, for the case of 155Gd, the value of the separation coefficient calculated from the front boundary data was almost equal to that calculated from the rear boundary data. Such results confirm the credibility of the (20) Kakihana, H.; Kanzaki, T. Bull. Tokyo Inst. Technol. 1969, 90, 77.
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obtained results. The values of the separation coefficients are plotted as a function of the mass difference from isotope 160Gd as shown in Figure 5. It is clear that the separation coefficient of the odd 157Gd isotope is larger than the value expected from other even isotopes of 156Gd and 158Gd, while the separation coefficient of the odd 155Gd isotope is smaller. The anomalous mass dependence of 157Gd isotope effects is similar to the case of U(IV)-U(VI) system where the odd isotopes 235U and 233U show larger separation coefficients than the other even isotopes of 234U and 236U against 238U. In the case of uranium,
manner as their isotopic effects had shown in Figure 5. Therefore, it is reasonable to suggest that the mass anomaly of the odd isotopes 157Gd and 155Gd are due to the field shift, which agrees with the case of the U(IV)-U(VI) exchange system and the Eu(II)-Eu(III) exchange systems where the field shift was found to be a predominant effect. By comparing the data plotted in part a of Figure 5 with that in parts b and c, it is found that the deviations of the isotope effects of the odd-numbered isotopes from the even-numbered isotopes, Figure 5b,c, are smaller than those of δ〈r2〉, Figure 5a. This fact suggests that the chemical isotope effects of Gd-complex formation involves two factors, normal mass dependent isotope effects due to the molecular vibrations and the nuclear charge distribution effects called field shift.
Figure 5. The relation between the mass number of Gd isotopes and (a) the mean square radius of the nuclear charge distribution parameter, 〈r2〉; (b) the slope of three-isotope plots; (c) the separation coefficients.
this mass anomaly was suggested to be due to the field shift. Therefore, it is of great interest to study the field shift of the Gd isotopes. The field shift is related to the mean square radius of the nuclear charge distribution parameter, 〈r2〉. Figure 5 shows the reported values of the change in 〈r2〉 plotted for Gd isotopes.21 The 157Gd isotope shows a larger value, while the 155Gd isotope shows a lower value than the even isotopes in exactly the same (21) King, W. H. Isotope Shifts in Atomic Spectra; Plenum: New York, 1984.
CONCLUSIONS The ideal displacement chromatogram in the Gd-EDTA ligand exchange system was obtained using EDTA as a complex reagent for a relatively long migration. The heavier isotope 160Gd was clearly found to be enriched at the front boundary of the Gd adsorption band, the Gd-EDTA complex species side, and the lighter isotopes, 155Gd, 156Gd, 157Gd, and 158Gd, were enriched at the rear boundary of the gadolinium band, the Gd(III) side. The separation coefficients of 155Gd,156Gd, 157Gd, and 158Gd against 160Gd were calculated to be (6.1 ( 0.3) × 10-5, (5.2 ( 0.3) × 10-5, (4.4 + 0.3) × 10-5, and (2.7 ( 0.3) × 10-5, respectively. The gadolinium isotope effects were also studied by means of the threeisotopes plot. Both the separation coefficients method and the three-isotopes plot method had shown larger isotope effects for the 157Gd isotope and smaller isotope effects for the 155Gd isotope compared with the other even isotopes. The anomalous massindependent isotope effects of the odd isotopes 155Gd and 157Gd were found to be related to the change in the mean square radius of the nuclear charge distribution parameter, 〈r2〉, which suggests that the nucleus volume and shape highly affect the gadolinium isotope effects.
Received for review October 14, 1999. Accepted March 8, 2000. AC991187J
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