Zinc Isotope Separation by Ligand Exchange Chromatography Using Cation Exchange Resin

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1 46 J. ION EXCHANGE Articles Zinc Isotope Separation by Ligand Exchange Chromatography Using Cation Exchange Resin Yasutoshi BAN*, Masao AIDA, Masao NOMURA and Yasuhiko FUJII Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology O-okayama, Meguro-ku, Tokyo, , (Manuscript submitted December 3, 2001; in final form received March 18, 2002) Abstract Zinc isotope effects in Zn-EDTA ligand exchange were studied by using a macroporous strongly acidic cation exchange resin packed in columns. Both front and rear boundaries of a zinc band were sharp after 28m migration at 60 Ž. Zinc isotope ratios of 66Zn/64Zn were measured by a mass spectrometer with surface ionization source. The enrichment of the heavier isotope, 66Zn, was found at the front band boundary region. Isotope separation coefficient per unit mass, ƒã/ M, and height equivalent to a theoretical plate were calculated as 8.0 ~10-5 and 0.9mm, respectively. Discussion is extended to the profile analysis on the chromatographic isotope separation. The results were compared with previously reported isotope effects in the ligand exchange reaction of other elements. Introduction Column chromatographic separation with ion-exchange resins is one of the promising methods not only for studying isotope effects in the laboratory scale but also for producing large amount of isotopes such as 15N, 10B and 235 U. Number of studies on isotope effects in ion-exchange reaction for various elements have been made utilizing different types of ion-exchange resins, such as strongly acidic cation exchange resins18), strongly acidic anion exchange resins9), and weakly basic anion exchange resins10-12). Many of these previous studies were made on the isotope effects caused by exchanging ligands between two different types of complexes of the isotopes; such isotope separation systems are referred to the ligand-exchange system. There are a few reports on zinc isotope effects in chemical exchange systems in published literatures, e.g. using cryptand resin chromatography13), and liquid-liquid extraction with dicyclohexano-18-crown-6 in chloroform14,15), however, the ligand-exchange system has not been used for the separation of zinc isotopes so far to the authors' knowledge. Usually the isotope effects in chemical reactions are very small. The precise analysis of isotopic ratios of experimental systems is very difficult. In particular, zinc has the highest work function among the metallic elements next to mercury, and therefore the surface ionization of zinc in mass spectrometry is very difficult. Since the small deviation in the zinc isotopic ratio after small migration of zinc adsorption band is undetectable, in the present work zinc band was migrated 28 m in the columns of a cation-exchange resin in order to observe the zinc isotope effects clearly. Zinc isotopes have an important role in the field of nuclear engineering. It was reported that trace amount of soluble zinc in the coolant water reduces 60Co build up in water piping systems16-18). Since 64Zn is converted to radioactive 65Zn in nuclear reactors, the effect of the radiation reduction is enhanced by using 64Zn depleted zinc. Therefore, the development of convenient and economical zinc isotope separation methods would be of importance. The present work aims at evaluating zinc isotope separation power in the ligand-exchange system by using a mac- * Corresponding author phuca0414@nikkeimail.ne.jp (8)

2 Vol.13 No.2 (2002) 47 roporous strongly acidic cation exchange resin packed in columns. Experimental Ion-exchange resin and reagents The ion-exchange resin used in the present work was a macroporous strongly acidic cation exchange resin (Bio- Rad, AG-MP 50, mesh size). Analytical grade reagents supplied by Wako were used without further purifications. Chromatography acid were loaded on the platinum filament and dried in the same manner. Each filament loaded on a zinc mass sample was attached to a filament unit which was inserted into the ion source of the mass spectrometer then the ion source was pumped down to the pressure of Pa. The filament current was gradually increased up to A in min. The intensity of zinc ion peaks was detected by a secondary electron multiplier. The purpose of the present study was to evaluate the depletion of 64Zn in the separation system, thus the mass analysis was performed on two isotopes of 64Zn and 66Zn. The strongly acidic cation exchange resin was packed in six glass columns (100 cm long, l cm I.D.) connected in s- eries with a polytetrafluoroethylene tube (1 mm I.D.). These columns were repeatedly used in a merry-go-round way for the total migration length of 28 m. The operating temperature was kept constant at 60t by a water jacket e- quipped with the columns. Prior to the chromatographic experiment the resin packed in the columns was treated with 2 M HNO3 and the resin was converted to H+ form, then to Cue + form by feeding 0.25 M Cu(N03) M HNO3. Thereafter 0.05 M Zn (NO3) 2 solution was fed into the columns until the length of the zinc band became ca. 60 cm long. The velocity of the zinc band was 5 cm/h. Then 0.05 M EDTA (2NH4) adjusted to ph 7.5 with ammonia solution was fed into the columns as an eluent with the flow Results and discussion In the region of zinc rear band, initially adsorbed Zn2+ ions form Zn-EDTA complexes, and moved to the solution phase (eq. 1). While Zn-EDTA complexes go down in the solution phase of zinc band, the isotope exchange reaction is carried out between Zn2+ ions in the resin phase and Zn- EDTA complexes (eq. 2). When the Zn-EDTA complexes reach to the boundary region of zinc and copper band, EDTA is transferred to Cue + ions and Zn2 + ions are adsorbed in the resin phase due to the difference between the stability constant of Zn-EDTA complex (log K1 1619) ) and that of Cu-EDTA complex (log K1 1819)) (eq. 3). The chemical reactions relating to the present separation system are expressed, in the simplified form, as follows: rate of 0.74 cm3/min. When the migration length reached 28 m, the effluent was collected in 10 cm3 fractions at the bottom of the last column. Since the obtained fraction samples in the zinc front band boundary region were mixture solution of Cu and Zn, these fraction samples were further fed into a strongly basic anion exchange resin packed in a column (10 cm long, 6-8 mm I.D.) for removing Cu from the fraction samples. Analysis The concentrations and the isotopic ratios of zinc in the fraction samples were measured by flame analysis with an atomic absorption spectrometer (SAS/727, Daini-seikosha) and a mass spectrometer (Finnigan MAT261) with a surface ionization ion source, respectively. The fraction samples were burned in a crucible to decompose organic impurities, then excess amount of nitric acid was added to the sample. The nitric acid in the sample was evaporated on a plate heater under an infrared radiation lamp. Each dried mass sample was dissolved in pure water. An aliquot of the dissolved sample containing ca. 25 ƒêg zinc in the form of Zn (NO3)2 was loaded on a platinum filament and the sample was dried again by passing electric current through the filament, then 2 ml of silica gel solution and 4 ml of phosphoric (NH4)2-L+Zn2 + 2NH4++L-Zn (1) hzn2+ +L-1Zn 1Zn2++ L-hZn (2) L-Zn+Cu2+ L-Cu+Zn2+(3) where L represents a ligand; and superscripts of h and 1 denote the heavier isotope and the lighter isotope, respectively; underlines represents the species in the resin phase. The concentrations, ph values, and isotopic ratios of 66 Zn/64Zn in the eluted fraction samples after 28 m migration are shown in Fig. 1. The error bars of isotopic ratios represent standard error evaluated from 2Q. The dotted line in Fig. l indicates original isotopic ratio obtained from the sample of zinc feed solution. The zinc concentration profile indicates that both front and rear zinc band boundaries are sharp after 28 m migration. The profile of isotopic ratio clearly indicated that the heavier isotope, 66Zn, was enriched in the solution phase i.e. Zn-EDTA complex side and was accumulated in the zinc front band boundary region. On the other hand, 64Zn was enriched in zinc rear boundary region. Thus, 64Zn depleted zinc can be obtained by collecting the eluted samples in the zinc front boundary region. This enrichment and (9)

3 48 J. ION EXCHANGE x is the distance from the starting point of the migration, and the subscripts o and L indicate the original and the total migration distance, respectively20). This mathematical model is rearranged for the present system of the band chromatography which has front and rear boundaries. By replacing (L-x) of eq. 4 with XB, we obtain the isotopic enrichment profile for the front boundary region, ƒá-ƒá o= (ƒáfl-ƒáo) exp [-kxb] (5) where XB is defined as the distance from the front boundary and rfl represents the maximum isotopic ratio on the front boundary. For the rear boundary region, (L-x) is converted to ( WB-XB) and we obtain, ƒá-ƒá o= (ƒáƒál-ƒáo) exp [-k(wb-xb)](6) Fig. 1 Profiles of concentrations ( for Zn, for Cu), ph ( ), and isotopic ratios ( œ) in the eluted fraction samples. where WB is band width and rrl represents the minimum isotopic ratio on the rear boundary. In the present case of zinc, whereƒá is defined as 66Zn/64Zn, (ƒá-ƒá0) and (ƒáƒál-ƒáo) are depletion tendency is the same as that observed in the chromatographic isotope separation of many elements i.e. Li1), Mg3), Ca4), Sr5), Cu6), Eu7), and Gd8). Although zinc isotope enrichment and depletion negative at the rear boundary side. Then the slope k is given by plotting In ƒá-ƒá0 vs. XB (Fig. 2). In ƒá-ƒá0 =1n ƒáfl/ƒál-ƒáo -kxb (7) phenomena were clearly observed in both zinc front and where ƒáfl/ƒál indicates the isotopic ratio on the boundary, front or rear. The experimentally observed In ƒá-ƒá0 is plotted in Fig. 2 as a function of XB by solid circles. The linear relation between ln ƒá-ƒá0 and XB is seen at both sides of front and rear boundaries. The dotted lines are obtained by the least square method for the data of In ƒá-ƒá0 in the boundary regions. Based on the linear relation expressed by the dotted lines in Fig. 2, the slope of the fitting lines which are equivalent to the slope coefficients (k) are obtained as 6.23 ~ 10-2 and ~10-2 for the front and rear boundary region, respectively. From these k-values and eq. 7, the theoretical isotopic enrichment ratios of zinc front (ƒáfr) and rear (ƒáre) band are given as eq. 8 and eq. 9, respectively. rear band regions, the profile of the isotopic enrichment curve shows almost straight line. This means that the system is in the transient state approaching to the steady state and the 64Zn depleted and enriched parts were mixed in the central part in the band. In such a case, the isotope separation coefficient (ƒã), calculated from the experimentally observed enrichment region, is underestimated. The conventionally used equation to obtain the isotope separation coefficient from the experimental results is applied for the non-steady state system where the plateau of the original isotopic ratio is maintained in the central part of the migration band. As this type of equation is not applicable to the present work, the following calculation based on a theoretical study of isotope separation by displacement chromatography reported by two of the present authors, Y. F and M. A20) is applied to obtain more reliable ƒã. ƒá fr=ƒáo+ (ƒáfl-ƒá0) exp [-6.23 ~10-2 E XB] (8) ƒáre=ƒáo- (ƒáo-ƒáƒál) exp [6.42 ~10-2 EXB] (9) In the ideal cases of isotope enrichment by the breakthrough displacement chromatography with a sufficiently long isotopic plateau of the original value, the isotopic ratioƒá (ƒá=66zn/64zn) is expressed by Since the isotopic enrichment is small, the deviation of isotopic ratio from original, ƒá-ƒá0, is assumed to be the sum of -ƒá0= (ƒál-ƒáo) exp [-k(l-x)] (4) two isotope enrichment curves for front and rear boundaries: where k is the slope coefficient, L is the migration distance, ƒá-ƒá o= (ƒáfr-ƒá0) + (ƒáre-ƒá0), (10) (10)

4 Vol.13 No.2 (2002) 49 Fig. 2 The slope analysis of isotope enrichment in a zinc band. Solid circles and dotted lines represent experimental data and fitting lines of experimental data in the zinc band boundary region, respectively. Solid line indicate the theoretical isotopic enrichment curve obtained from slope coefficients (k) and eq. 7. Fig.3 Estimated isotopic enrichment profiles in the zinc band for five different migration length. eq. 13 is applicable to the present system. The calculated values of the separation coefficient for front boundary (ƒãf) and rear boundary (er) were 1.6 ~10-4 and 1.1 ~10-4, therefore: respectively. Since the good correlation of (r-r0) and XB at the front boundary is seen, the slope of the front boundary ƒá=ƒáfy+ƒáre-r0 (11) is considered as more reliable than the slope of the rear boundary. Thus ƒãf is taken as E for this system. Calculated In ƒá-ƒá0 on the basis of eq. 10 is shown as the solid line in Fig. 2. When eq. 8 and 9 are substituted in eq. 11, we obtain the isotopic enrichment profile, which is In a previous paper, a convenient equation was introduced for estimating the isotope enrichment profile in the migration band at the non-steady state. presented as the dashed line in Fig. 1. Good agreement is seen between the calculated curve and the experimentally obserbed data in Fig. 1. When the theoretical isotopic enrichment curve is given by eq. 7, the separation coefficient (ƒã) is calculated as reported in a previous work10): (14) In the case of the low enrichment stage, R is replaced by r/ (r+1): (15) (12) where R, and R0 are the atomic fraction of an isotope. This equation was originally derived for the two-stable-isotopes system where the relation, R=r/(r+1), holds. In the case of the low enrichment systems, eq.12 can be approximated by replacing R with r/(r+1)as: (13) The present system involves four stable isotopes. But major two isotopes were analyzed. As long as the attained enrichment is low, the system is regarded as the two isotope system consisting of the measured two isotopes. Then The estimated isotope enrichment profiles in the zinc front band for five different migration length, i.e. 30 m, 60 m, 120 m, 240 m, and 480 m, were summarized in Fig. 3. In this figure, enrichment factor, i.e. the isotopic ratios normalized by original one, is shown on the lift hand side. The ratio of calculated 64Zn is shown on the right hand side of the figure. After 480 m migration 35% 64Zn is expected to be obtained at the extremely front band. The height equivalent to a theoretical plate, HETP, was calculated for evaluating isotope separation performance by the following equation: (16) (11)

5 50 J. ION EXCHANGE Fig. 4 Effect of band velocity and resin diameter on HETP. The HETPs for Cu and Gd are calculated based on the reported data. The slope of the solid straight line is 0.5. where H, ƒã, k, and L represents HETP, separation Fig. 5 Mass dependencies of isotope separation coefficients per unit mass (ƒã/ M) obtained from the ligand-exchange system in ion-exchange chromatography. The open circles and solid square represent previous works and present work, respectively. The solid lines have the slope of -2. coefficient, slope coefficient, and migration distance, respectively. In the present work, HETP was obtained as 0.9 mm. To compare HETPs obtained in the present and suggests that the HETP for the present system would be previous works, i.e. Fe9), Cu6), Eu7), and Gd8), HETPs are plotted against the term of the band velocity, UB, multiplied by the square of the resin diameter, dp, (Fig. 4). The HETPs of Cu and Gd migrations were also calculated based on the reported data. The experimental conditions, H, ub, and d2p values for each studies are summarized in Table 1. The isotope exchange reactions between solution phase and resin phase proceed effectively and reach equilibrium in a short migration distance when the band velocity is slow. In general, HETP is proportional to the root of d2pub. Fig. 4 agrees with this prediction, moreover this figure reduced by decreasing the band velocity, although a minimum value of HETP would exist at the very low velocity of the band migration. Both large ƒã and small HETP are needed for a practical application of the isotope separation. The value of HETP for the present separation system is not large compared to other isotope separation systems utilizing ion-exchange chromatography. On the other hand, the value of ƒã seems relatively small compared with those obtained by the separation systems utilizing crown ether (ƒã/ M= ), and )). The value of ƒã should be increased at least one order to apply the present separation system for practical use. Table 1 Experimental conditions, band velocities, and HETPs * Calculated based on the reported data. (12)

6 Vol.13 No.2 (2002) 51 The value of ƒã/ M observed in the present work is plotted in Fig. 5 as a function of the atomic weight. The values of ƒã/ M obtained from the ligand-exchange system for alkali metals (K21), and Rb22)), for alkaline earth metals (Mg3), Ca9), and Sr4)), and transition metals (Cu6), Eu7), and Gd8)) are also plotted in Fig. 5. The plots are separated to two groups: one is the group of alkali and alkaline earth metals and the other group is transition metals. Both groups show the straight line between ƒã/ M and atomic weight with a slope of - 2. These experimental results agree with the general rule of isotope effect, i.e. ƒã/ M is proportional to the reciprocal square of atomic weight, that is theoretically predicted by the theory of Bigeleisen and Mayer23). Recent discussion has indicated that the isotope effects in chemical exchange reaction depend on not only the nuclear mass, but also the nuclear volume, in the case of heavy elements. The present work suggests that the isotope effects of zinc in the ligand exchange reaction is largely governed by mass effect, and therefore Fig. 5 can be used to estimate approximate values of ƒã/ M for middle heavy elements in the ligand-exchange chromatography. Conclusions Chromatographic separation of zinc isotopes has been studied by a macroporous strongly acidic cation exchange resin in ligand-exchange system. The heavier isotope (66Zn) and the lighter isotope (64Zn) are enriched in the front and the rear boundary regions, respectively. The values of HETP is small, however, more larger isotope separation coefficient is needed to apply the present isotope separation system for practical use. Isotope effects of various elements with different atomic weight, such as Li, Ca, Zn, Eu, Gd etc., in the ligand-exchange system with ion-exchange chromatography are found to approximately follow the mass dependence rule predicted from the conventional theory of isotope effects. References 1) T. Oi, K. Kawada, M. Hosoe and H. Kakihana, Sep. Sci. and Technol., 26,1353 (1991). 2) M. Ohwaki, Y. Fujii and M. Hasegawa, J. Chromatogr., 793, 223 (1998). 3) T. Oi, S. Hosoe and H. Kakihana, Sep. Sci. and Technol., 22, 2203 (1987). 4) T. Oi, N. Morioka, H. Ogino, H. Kakihana and M. Hosoe, Sep. Sci. and Technol., 11 & 12, 1971 (1993). 5) T. Oi, H. Ogino, M. Hosoe and H. Kakihana, Sep. Sci. and Technol., 27,631 (1992). 6) MD. A. Matin, M. Nomura, Y. Fujii and J. Chen, Sep. Sci. and Technol., 33,1075 (1998). 7) I. M. Ismail, M. Nomura and Y. Fujii, J. Chromatogr., 808, 185 (1998). 8) I. M. Ismail, A. Fukami, M. Nomura and Y. Fujii, Anal. Chem., 72,2841 (2000). 9) S. H. Kim, M. Aida, M. Nomura, Rifaid M. Nur and Y. Fujii, J. Ion Exchange, 11,26 (2000). 10) M. Aida, Y. Fujii and M. Okamoto, Sep. Sci. and Technol., 6 & 7,643 (1986). 11) Y. Fujii, J. Fukuda and H. Kakihana, J. Nucl. Sci. and Technol.,15,745 (1978). 12) Y. Fujii, M. Nomura, M. Okamoto, H. Onitsuka, F. Kawakami and K. Takeda, Z. Naturforsch., 44a,395 (1989). 13) K. Nishizawa, Y. Maeda, F. Kawashiro, T. Fujii, T. Yamamoto and T. Hirata, Sep. Sci. Technol., 33,2101 (1998). 14) K. Nishizawa, T. Satoyama, T. Miki, T. Yamamoto and M. Nomura, Sep. Sci. Technol., 31,2831 (1996). 15) K. Nishizawa, T. Miki, T. Satoyama, T. Fujii, T. Yamamoto and M. Nomura, Sep. Sci. Technol., 33,991(1998). 16) C. J. Wood, EPRI report, (1986). 17) W. J. Marble, C. J. Wood, C. E. Leighty and T. A. Green, Proc. the 1986 Joint ASME/ANS Nucl. Power Conf., 144 (1986). 18) C. J. Wood, EPRI report, (1989). 19) G. Schwarzenbach and L. G. Sillen, Stability constants of metal-ion complexes, with solubility products of inorganic substances. Part I, 76 (1957). 20) Y. Fujii, M. Aida, M. Okamoto and T. Oi, Sep. Sci. and Technol., 20,377 (1985). 21) K. Kawada, T. Oi, M. Hosoe and H. Kakihana, J. Chromatogr., 538,355 (1991). 22) M. Hosoe, T. Oi, K. Kawada and H. Kakihana, J. Chromatogr., 438,225 (1988). 23) J. Bigeleisen and M. G. Mayer, J. Chem. Phys., 15,261 (1947). (13)

7 52 J. ION EXCHANGE

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography

The Effect of Temperature on Uranium Isotope Effects Studied by Cation Exchange Displacement Chromatography Ibrahim M. Ismail, Masao Nomura, Masao Aida, and Yasuhiko Fujii Research Laboratory for Nuclear