Temperature Dependence of Isotope Effects in Uranium Chemical

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Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions a

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Yasuhiko FUJII , Nobuhiko HIGUCHI , Yoshio HARUNO , Masao NOMURA & Tastuya SUZUKI

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Research Laboratory for Nuclear Reactors , Tokyo Institute of Technology , 2-12-1, Ookayama, Meguroku, Tokyo , 152-8550 , Japan Published online: 05 Jan 2012.

To cite this article: Yasuhiko FUJII , Nobuhiko HIGUCHI , Yoshio HARUNO , Masao NOMURA & Tastuya SUZUKI (2006) Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions, Journal of Nuclear Science and Technology, 43:4, 400-406 To link to this article: http://dx.doi.org/10.1080/18811248.2006.9711111

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Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 43, No. 4, p. 400–406 (2006)

ORIGINAL PAPER

Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions Yasuhiko FUJII, Nobuhiko HIGUCHI, Yoshio HARUNO, Masao NOMURA and Tastuya SUZUKI Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguroku, Tokyo 152-8550, Japan

Downloaded by [190.198.168.148] at 11:28 24 March 2014

(Received October 26, 2005 and accepted December 3, 2005) Temperature dependence of isotope effects in U(IV)–U(VI) exchange reaction was experimentally studied by means of redox chromatography using columns packed with anion exchange resin in the temperature range 87– 160 C (360–433 K). The experimentally observed isotopic separation coefficient, ", was converted to the isotopic equilibrium constant, ln K¼"o , of the U(IV)–U(VI) exchange reaction with the aid of the experimentally measured ionic mole fractions of U(IV) and U(VI) in the resin phase and the aqueous solution phase. The plots of "o vs. 1=T led to the temperature dependence of "o ¼0:69=T82=T 2 , where the first term is due to the nuclear field effect (nuclear-electron interaction effect) and the second term is due to the molecular-vibration mass effect. The discussion is extended to the temperature dependence of the previously reported isotope effects in the U(III)–U(IV) electron exchange reaction, and the U(VI)-malate-complex ligand exchange reaction. KEYWORDS: uranium isotopes, isotope effects, isotope separation, chromatography, anion exchange resin, molecular vibration, nuclear field shift, 5f electrons, electron exchange, complex formation

I. Introduction Uranium enrichment processes based on chemical exchange systems were studied by a number of investigators because of the inherent non-proliferation nature of the processes.1–3) The chemical enrichment plants can produce only low enriched uranium, which is usable as the fuels for commercial nuclear power plants but not usable as the nuclear weapon material. Technological research and developments on the chemical uranium enrichment were conducted in several countries and successful works were reported in Japan on U(IV)–U(VI) exchange process4) and in France on U(III)–U(IV) exchange process.5) In spite of the successful development in uranium enrichment process, the scientific principle was not known at that time; the orthodox theory of isotope effects based on molecular vibration could not explain the uranium isotope effects. The industrial development work was ceased in the last decade of the 20th Century in Japan under the world-wide excess production capacity over the demand of enriched uranium. Fundamental research work, however, has been continued. The chemical system of U(IV)–U(VI) electron exchange reaction involves UO2 2þ ion in which uranium atom is bonded to two oxygen atoms with double bonds in stoichiometrical estimation. The strong two double bonds were initially expected to work as the source of the isotope effects according to the quantum effects in the molecular vibration and the heavy isotope of 238 U was anticipated to be enriched in U(VI). The observed uranium isotope effect, however, was opposite to the prediction; the light isotope 235 U was enriched in U(VI) ion, strongly bound to oxygen atoms, rather

Corresponding author, E-mail: [email protected]

than U(IV), or U4þ ion, which does not include strong chemical bondings. In addition, the isotope separation was not strictly proportional to the mass difference between the isotopes, but showed so called ‘‘odd even staggering’’ which is a kind of mass independent isotope effects.6–8) Based on the experimental observation, the uranium isotope effects in the redox exchange reaction have been explained to originate mainly from the nuclear field due to the interaction between the nucleus and electrons, in particular, s electrons. These two factors of the molecular vibration and the nuclear field are expected to show the different temperature dependence of the isotope effects, as theoretically predicted by Bigeleisen.9) From the theory derived by Bigeleisen, the isotope effect ", or the separation factor ln 1, in U(IV)– U(VI) exchange reaction is described by " ¼ A=T B=T 2 ;

ð1Þ

where T is the temperature, and A and B are constants. The first term in the right hand side of Eq. (1) is due to the nuclear field, or nuclear volume effect, the second is due to the molecular vibration, or isotopic mass effect. Since the first terms and the second term have opposite signs each other, or A and B are positive, Eq. (1) becomes a parabolic curve with a maximum at a certain temperature, when "o is plotted as a function of 1=T. In a previous paper,8) the temperature dependence was derived by using experimentally determined relativeisotope-separation-coefficients among isotopes of 232 U, 233 U, 234 U, 235 U, 236 U and 238 U, the reported relativemass-effects of uranium isotopes derived from the isotope shifts in atomic emission spectra and the reported chemical isotope effect "o 10) at a room temperature 35 C. The estimated temperature dependence of "o for the pair 235 U–238 U was expressed as,

Atomic Energy Society of Japan 400

401

Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions

"o ¼ 0:65=T 75:6=T 2 :

ð2Þ

This relation is also very similar to the temperature dependence derived by Bigeleisen for the U(IV)–U(VI) exchange system as,9) "o ¼ 0:63=T 71:6=T 2 : In the present work, chromatographic uranium isotope separation experiments are conducted at higher temperatures to experimentally study the temperature dependence of the isotope separation coefficient of U(IV)–U(VI) exchange and to confirm above-mentioned theoretical treatments.


II. Experimental 1. Chromatographic Isotope Separation To study the isotope effects in U(IV)–U(VI) electron exchange process, redox chromatographic experiments were conducted using anion exchange resin columns at different temperatures 87, 100, 120, 140 and 160 C (360, 373, 393, 413 and 433 K); the unit ‘‘ C’’ is used in the present work to clearly indicate the experimental conditions over 100 C. Commercially available anion exchange resins are not usable at these high temperatures. The porous small particle (ca. 60 mm), benzimidazole quaternary ammonium ion type resin was specially developed by Asahi Chemical Industries, and used in the present work. This resin was packed in four titanium alloy columns (2 cm I.D., 107 cm resin bed height each) with oil jackets for temperature controlling. The titanium alloy was selected as the endurable material for the columns used at high temperatures and at high HCl concentrations. The experimental apparatus is depicted in Fig. 1.

In each experiment, the columns were heated to the given temperature by circulating themostated oil through the jackets and the resin in the columns was pretreated with 4 M HCl solution. Thereafter, oxidizing reagent was fed into the four columns connected in series to make an Fe(III) adsorption band in the resin bed. Then the uranium solution was fed into the columns. Finally the uranium adsorption band was eluted by an eluent containing reducing reagent V(III) ions. A back pressure of approximately 10 kg was necessary to conduct the column operation at the temperatures higher than 100 C. The ionic constituents of solutions fed into the columns in the present work are listed in Table 1. The volumes of these solutions used are mentioned in Table 2. Other operational conditions of the chromatographic experiments are also summarized in Table 2. The effluent was collected in fractions, and the concentration of uranium in each fraction was measured by photometry. The isotopic abundance ratio of uranium was determined by a mass spectrometer Finnigan-MAT 261 with thermal ionization source. In the previous work,11) Ti(III) was used as the reducing reagent. However, Ti(III) is not stable at high temperatures, and generated Ti(IV) by the redox reaction is very easily precipitated in the form of TiO2 in the resin bed and may cause troubles in the column operation. Therefore, Ti(III) is not an appropriate reducing material at higher temperatures than 100 C. On the other hand, V(III) and V(IV) are stable at high temperatures. This is the reason why V(III) was used in the present work as the reducing reagent. 2. Two-phase Distribution of Uranium Ions To determine the isotope exchange equilibrium constant of U(IV)/U(VI) system, we have to know the distribution of these uranium ions between two phases, the resin and the solution, in addition to the isotope separation coefficient observed by the experiments above mentioned. To determine the concentration of U(IV) and U(VI) in the solution phase, Table 1 Ionic compositions of feed solutions

Fig. 1 Experimental column system for chromatographic isotope separation of uranium

Ionic species

Uranium feed solution (M)

Oxidizing reagent (M)

Reducing eluent (M)

U(VI) V(III) V(IV) Fe(II) Fe(III) Cl

0.104 0.38 0.22 1.00 — 3.66

— — 0.600 — 1.58 5.20

— 0.582 0.018 1.00 — 3.45

Concentration unit M: mole/dm3

Table 2 Experimental conditions of chromatographic operations Temperature ( C)

87

100

120

140

160

Migration distance (cm) Eluent feed rate (cm3 /min) Band velocity (m/h) Oxidizing solution (cm3 ) Uranium feed solution (cm3 ) Reducing eluent (cm3 )

428 9.0 1.0 450 450 2,288

428 9.3 1.1 400 400 2,300

428 11 1.3 400 300 2,180

428 9.0 1.0 450 400 2,300

428 7.9 0.90 300 350 2,300

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Y. FUJII et al.

0.0069

0.06

0.02

200

400

600

0.06

800 3

Effluent volume (cm )

Fig. 2 Displacement type chromatogram of the redox chromatography at 120 C and the measured isotopic ratios of 235 U/238 U in the fraction of the uranium band

U

238 235

U/

0.0070

Isotopic ratio,

0.0072

3

Concentration of U (mol/dm )

0.0074

0.0068

0.04 0.02 0.00 200

400

600

800

3


0.0075 0.0073 3

0.0071 0.08

0.0069 0.0067

0.06

238

0.0077

U/

0.0079

160 °C

235

Migration distance : 428cm

U


238

0.04

0.00

0.0076

Isotopic ratio,

3

0.0071

140 °C


0.0073

0.0078 Migration distance : 428cm

U/

0.0075

1. U(IV)–U(VI) Electron Exchange System The experimental conditions for the chromatographic isotope separation at 87, 100, 120, 140 and 160 C are summarized in Tables 1 and 2. The chromatographic profile and the measured isotopic abundance ratios of 235 U/238 U in the uranium migration band are shown in Figs. 2, 3 and 4 for the experimental runs at 120, 140 and 160 C, respectively. In each experiment, light isotope 235 U was enriched at the rear boundary, which means 235 U is preferentially fractionated in U(VI) or UO2 2þ ion. This result is consistent with those previously reported.6–8) Based on the experimental results, the separation coefficient " was calculated in each experiment according to the established calculation method described in previous work,12,13) and presented in Table 3.

235

120 °C

Isotopic ratio,

0.0077

Migration distance : 428cm

III. Results and Discussion

U

1 m long uranium redox chromatographies were conducted at 87, 100, 120, 140 and 160 C. The effluent was collected in fractions and representative three fractions, from front, middle and rear parts of the uranium band, were subjected to ion analysis for U(IV) and U(VI). A uranium sample of 5 ml taken from the selected fraction was diluted to 100 ml with 0.1 N H2 SO4 and charged into a small column, (2 cm I.D., 12 cm long) packed with pyridine-type anion exchange resin which was initially converted to SO4 2 ion form. Both uranium ions of U(IV) and U(VI) in H2 SO4 can be adsorbed in the anion exchange resin, but Fe and V ions are eluted out by washing the resin with 0.1 N H2 SO4 . Then the column was charged with 5 M HCl solution to separate U(IV) ion of which adsorbability on anion exchange resin is much smaller than that of U(VI) in 5 M HCl. The adsorbed U(VI) ions were eluted with pure water. The amounts of eluted U(IV) and U(VI) were measured by colorimetry. From the experiences, it has been known that U(VI) is strongly adsorbed in the resin phase and the concentration of U(IV) in the resin phase is very low in the chromatographic conditions, when the benzimidasole type anion exchange rein is used even at high temperatures. To confirm this fact, breakthrough chromatographic experiments were conducted at 87 and 160 C to measure the concentration of U(IV) and U(VI) ions in the resin phase, as precisely as possible. The feed solution initially contained U(VI), V(III), Fe(II) and Cl at the concentrations of 0.12, 0.38, 1.0 and 3.66 M, respectively. Uranium(IV) ions were generated in the column by the redox reaction. The effluent solution was also collected in fractions and the concentrations of U(IV) and U(VI) in the fractions were determined in the same procedure as described above. The adsorption of U(IV) and U(VI) were determined from the breakthrough point of each uranium ion in the effluent. The mole fractions of U(IV) and U(VI) ions in the resin phase were calculated from the amounts of adsorbed U(IV) and U(VI) ions. The mole fractions of U(IV) and U(VI) in the resin phase at other temperatures were calculated by interpolating the measured values at 87 and 160 C.



402

0.04 0.02 0.00

0

200

400

600 3



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Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions Table 3 Determined separation coefficients and isotopic equilibrium constants Temperature ( C) 3

Separation coefficient "10 Mole fraction of U(IV) (%): Solution Resin Isotopic equilibrium constant "o 103

87

100

120

140

160

0.87 80 3.3 1.3

1.08 81 3.5 1.4

1.06 84 4.0 1.3

1.05 93 4.4 1.2

0.8 92 4.9 1.1

The errors are estimated for " as 15% and for "o as 20% of each value.

the same way, ½BX=½BX0 ¼rs fB . Then the equilibrium constant K is given by,

160°C

0.0078

K ¼ f B = f A ¼ 1 þ "o :

Isotopic ratio,


235

U/

238

U

Migration distance: 428cm

140°C 120°C

0.0076

100°C

0.0074 87°C

0.0072

0

100 200 3 Effluent volume (cm )

Fig. 5 Accumulation of 235 U at the band boundaries in the chromatographic elution conducted at different temperatures from 87–160 C

The measured isotopic ratios of 235 U/238 U in different chromatographic experiments in the present work are summarized in Fig. 5. It is also seen in Fig. 5 that the isotopic enrichment curves in the uranium band are steepened with increase in the operational temperature. The steepness of the curve is directly related to the productivity of the enrichment process. Apparently, the higher temperatures are efficient conditions for the production of enriched uranium by the present process. The separation coefficient " is important from the engineering point of view since " directly affects the capability of the isotope enrichment in the separation process concerned. The production capacity of the separation plant is approximately proportional to "2 . From the scientific point of view, the experimentally observed isotope separation coefficient is correlated to the equilibrium constant of the following isotope exchange reaction, AX þ BX0 ¼ AX0 þ BX;

ð3Þ

where AX and BX are different isotopomers accommodating X0 (light) and X (heavy) isotopes. The isotopic exchange equilibrium constant K of Eq. (3) is defined as, K ¼ ½AX0 ½BX=½AX½BX0 ¼ ð½BX=½BX0 Þ=ð½AX=½AX0 Þ:

ð4Þ

The isotopic abundance ratio of isotopomer AX is expressed with the standard, or feed, isotopic ratio rs multiplied by the ‘isotope effect’ fA , thus ½AX=½AX0 ¼rs fA and therefore, in VOL. 43, NO. 4, APRIL 2006

ð5Þ

Equation (5) is identical with the definition of the theoretical isotope equilibrium constant, and fA and fB correspond to the reduced partition function ratios of the isotopomer AX and BX, respectively; symmetry numbers are neglected here since we deal with the isotopomer including only one isotope concerned. The last term "o indicating the deviation from unity in Eq. (5) is the ‘‘close isotopic equilibrium constant’’ of the above mentioned reaction, Eq. (3). Hereafter, "o is called as the isotopic equilibrium constant for convenience’ sake. As described in Appendix of the present paper, the equilibrium constant "o is correlated to experimentally determined separation coefficient of the system " and these two "’s are mutually converted by using the mole fractions of isotopomers in both phases of ion exchange resin and the aqueous solution: "o ¼ "=ð Þ

ð6Þ

In the present case, above reaction (Eq. (3)) is written as, 235

U(IV) þ 238 U(VI) ¼ 238 U(IV) þ 235 U(VI):

ð7Þ

Concentrations of U(IV) and U(VI) in both resin and solution phases were measured as mentioned in the experimental section. The determined mole fractions of U(IV) in the solution phase and the resin phase are listed in Table 3, along with "o calculated by using the values of ", and in each experiment. Thus obtained isotopic equilibrium constants, "o , are plotted as a function of 1=T in Fig. 6. In a previous work,11) the temperature dependence of the isotope effects in U(IV)–U(VI) exchange was studied in the temperature range, 30–87 C, lower than the region of the present work, by using a conventional quaternary ammonium ion type strong-base anion exchange resin and a reducing eluent of Ti(III) solution. The observed separation coefficients "’s were 7:8104 , 6:8104 , 6:7104 and 7:0 104 at 30, 50, 70 and 87 C, respectively. The calculated isotopic equilibrium constants, "o , were 1:4103 , 1:1 103 , 1:0103 and 0:9103 at 30, 50, 70 and 87 C, respectively. The experimental errors are estimated to be 20% of these observed. The magnitudes of the previously reported values are approximately in the same order as ones observed in the present work. If we discuss more precisely these results, however, previous value of "o at 87 C is apparently smaller than the value observed in the present work.

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Y. FUJII et al.

2. U(III)–U(IV) Electron Exchange System In the earlier section, the temperature dependence of U(IV)–U(VI) exchange system has been discussed and it has been confirmed that the present experimental results are in good agreement with the theoretical expectation. These facts draw our attention to the U(III)–U(IV) exchange system, which was intensively studied by French CEA in order to develop the chemical uranium enrichment process.5,14) The isotopic exchange equilibrium reaction of this system is expressed by the following equation:

0.002

Separation coefficient, (

235

U/

238

U) εo

0.003

εo =0.69/T − 82/T

2

0.001 εo =0.65/T − 75.6T

2

0.000

235 −0.001 0.000

0.002

0.004 −1


Temperature 1/T (K )

Fig. 6 Temperature dependence of determined isotopic exchange equilibrium constant "o of U(IV)–U(VI) exchange system (a) Equation fitted to experimental results: "o ¼0:69=T 82=T 2 . (Solid curve) (b) Equation estimated in Ref. 8): "o ¼0:65=T75:6=T 2 . (Dashed curve)

Probably the discrepancy is due to the adsorption of U(IV) in the conventional anion exchange resin used at high temperatures in the previous work,11) where the mole fraction of U(IV) in the aqueous solution was taken into consideration, but U(IV) adsorption in the resin was neglected. In ambient temperatures, it is true that the adsorption of U(IV) is negligible, but adsorbed U(IV) mole fraction amounts about 10–13% in the conventional type strong-base anion exchange resin at high temperatures around 70 C. Therefore, "o at higher temperature cannot be compared with the present results, and only "o at 30 C in the previous work is taken as a reference value in the low temperature region for discussion; the value of "o ð1:40:3Þ103 at 30 C is in good agreement with the value of ð1:30:4Þ103 at 35 C, reported by Florence et al.10) who observed the isotope effects in U(IV)–U(VI) exchange reaction in aqueous solution. This value of "o ¼1:4103 at 30 C is also plotted in Fig. 6. From the fitting of the experimental data plotted in Fig. 6, we obtain the following equation of the temperature dependence: "o ¼ 0:69=T 82=T 2 :

ð8Þ

As mentioned in the introduction of the present paper, the temperature dependence of Eq. (2) was previously estimated without experimental data of "o at higher temperatures. Equation (2) is also drawn in Fig. 6. Apparently Eq. (8) is considered to be quite similar to the previously estimated Eq. (2) and practically both equations are regarded as being in agreement with each other when the experimental errors and the calculation processes are considered. These results demonstrate that the temperature dependence mentioned in Introduction of the present paper and the theoretical treatment proposed by Bigeleisen concerning nuclear field effects on the chemical isotope fractionation is concluded to be valid.

U(III) þ 238 U(IV) ¼ 238 U(III) þ 235 U(IV):

ð9Þ

The multi-stage counter current contactor was developed by French CEA with the aid of two phases of an organic solvent and an aqueous solution. In this system, U(IV) is in the form of TBP complex and extracted in the solvent phase. Uranium(III) remains in an aqueous phase of the HCl solution. French development work found that the equilibrium constant of above exchange reaction is larger than unity. This means that 235 U is enriched in the solvent phase, namely in the U(IV) TBP complex state. Anomalous isotope effects of odd mass number 235 U has been already reported for the U(III)–U(IV) exchange system, as well. The enrichment of 236 U by the chemical process is much smaller than the expected value on the basis of the enrichment of 235 U. This is an advantage for the purpose of depression of 236 U concentration increase in uranium enrichment plant. An application of this system may be the enrichment of recycled uranium from spent fuels which contain 236 5) U. The observed phenomenon suggests that the major source of the isotope effects in U(III)–U(IV) exchange process is due to the nuclear field effect rather than the molecular vibration. The temperature dependence analysis would also give a clue for the search of the origin of isotope effects. The reported values of the separation coefficients of this system at different temperatures are 2:69103 at 20 C, 2:51 103 at 40 C and 2:29103 at 70 C.14) In the case that the relative adsorptions or distributions of isotopomers in each phases are not so largely changed within the experimental temperature range, the temperature dependence of the separation coefficient " is expected to show the same pattern with that of ln K, or "o . On the assumption that ionic distributions of U(III) and U(IV) are not so largely changed in the experimental temperature range, " vs. 1=T are plotted in Fig. 7. Temperature dependence of the separation coefficients " is found to be expressed by the following relation: " ¼ 0:77=T:

ð10Þ

This result is quite interesting. The isotope separation coefficients of U(III)–U(IV) exchange system is quite large compared with those of other uranium chemical systems, in spite of the fact that there is no strong chemical bonding in both U(III) and U(IV). The observed isotopic fractionation between U(III) and U(IV) is not explained by the orthodox molecular vibration effect. The only possibility to explain the experimental results, to our knowledge, is the nuclear field effect, which appears as the first term of the temperaJOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

Temperature Dependence of Isotope Effects in Uranium Chemical Exchange Reactions

asymmetric vibrational mode of U(VI) complexes. The isotope separation coefficients theoretically calculated from the frequency of 3 , however, give much smaller values than the experimentally observed ones.15) Recently temperature dependence of isotope effects in U(VI)-malate complex ligand exchange has been studied in the temperature range from 15–70 C.16) The results show that the separation coefficient is increased with increase in temperature in the tested temperature range. The pattern of temperature dependence is found to be given by Eq. (1), as

4

2

ε

103

3

1

" ¼ 0:45=T 118=T 2 : 0 0

0.002 0.004 Temperature 1/T (K -1 )


Fig. 7 Plotting of isotope separation coefficients for U(III)–U(IV) exchange system reported in Ref. 14) ("¼0:77=T)

ture dependence of Eq. (1). It is confirmed that Eq. (10) does not involve the second term, B/T 2 , which appears in Eq. (8) in the preceding discussion. By combining the results of the U(IV)–U(VI) exchange system and the U(III)–U(IV) exchange system, the enrichment tendency among U(III), U(IV) and U(VI) is generalized as in the way that the lighter isotope of 235 U is enriched in the ionic species in higher valence state. The reason for this phenomenon may be explained by the role of 5 f electrons, which can act as screening of the nuclear charge between the nucleus and electrons in the outer shell. 3. U(VI)-complex Ligand Exchange System Now it is clear that the isotope effects in uranium in electron exchange reactions are mainly attributed to the nuclear field or the nucleus-electron interaction. The present results draw our further attention to the uranium complex ligand exchange reaction. The valence of uranium ion is unchanged in the complex ligand exchange reactions so that the electronic states of uranium in the ligand exchange reactions are considered to be not so much changed as in the cases of electron exchange reactions. It is an interesting topic whether the nucleus-electron interaction affects the isotope effects of complex formation or not. In our early work, U(VI) complex-ligand (L) exchange reactions were studied for various ligands.15) The U(VI) complex ligand exchange reaction is expressed by the following equation: 235

405

U(VI)aq þ 238 U(VI)L ¼ 238 U(VI)aq þ 235 U(VI)L: ð11Þ

Among the tested ligands, malate complex has shown the largest value of the isotope separation coefficient. The isotopic fractionation of this system was somewhat strange. The light isotope 235 U is enriched in the complex form of U(VI)L instead of the free aqua ion U(VI) in an aqueous solution. The explanation was made by the loosening of U=O double bond in U(VI), which takes place when the complex formation is made with ligand L. The extent of loosening of the double bond is detected by IR measurement of 3 , the VOL. 43, NO. 4, APRIL 2006

ð12Þ

Since the uranium adsorption capacities of the experimental systems were not so much changed among the experimental conditions of the tested temperatures in the reported work,16) the distributions of the uranium chemical species in two phase are also estimated to be not so largely changed. In such cases, as previously mentioned, the temperature dependence of " shows the same pattern as that of "o . This fact suggests that the nucleus-electron interaction may affects the isotope fractionation of uranium complex formation. If this is true, this type of isotope effects are expected to be observed, in general, in the complex formation of heavy elements and lanthanide elements of which nuclear volumes are relatively large, or of which nuclear shapes are deformed. Recently anomaly in mass dependence has been found in gadolinium–EDTA complex ligand exchange system.17) Enrichment behavior of Gd isotopes was studied by means of cation-exchange chromatography. Among the isotopes, 155 Gd, 156 Gd, 157 Gd, 158 Gd and 160 Gd, odd mass number isotopes of 155 Gd and 157 Gd showed deviations from the linear mass dependence observed for even mass number isotopes of 156 Gd, 158 Gd and 160 Gd. The deviation is analogical to the pattern of nuclear size deviation hr 2 i. This suggests the isotope effects in lanthanide complex is also originated from the nuclear-electron interaction. The inverse temperature effect, namely isotope separation coefficient is increased with increase in temperature, has been observed in Eu(II)– Eu(III) electron exchange reaction.18) This phenomenon can be also explained by the isotope effects in the nucleuselectron interaction. The isotope effects in chemical binding may be interesting topics in other scientific fields, such as geo- and cosmochemistry. So far information on the isotope effects of heavy elements is limited, further study is necessary to elucidate the isotope effects.

IV. Conclusions Chromatographic uranium isotope separation experiments were conducted at 87, 100, 120, 140 and 160 C to study the temperature dependence of the isotope effects in U(IV)– U(VI) exchange reaction. The temperature dependence of observed ‘close isotope equilibrium constants’ "o in the present work and "o at 30 C reported in the previous work is expressed by, "o ¼ 0:69=T 82=T 2 ; which is very similar to previously estimated "o ¼0:65=T

406

Y. FUJII et al.

75:6=T 2 without using the experimental values of "o at high temperatures. The results confirmed that "o consists of two factors of the nuclear field effect and the vibrational mass effect. The reported separation coefficients of U(III)–U(IV) exchange system were analyzed from the view point of temperature dependence. The derived equation is given by,

10)

11)

" ¼ 0:77=T; 12)

which suggests that the vibrational mass effect is negligible. The discussion was extended to the isotope effects in U(VI)-complex ligand exchange. The pattern of temperature dependence in U(IV)–U(VI) exchange system is also seen in U(VI)-malate ligand exchange reaction reported as,


"o ¼ 0:45=T 118=T 2 : It is also suggested that the isotope effects of U(VI) complex are originated from two factors of the nuclear field effect and the molecular vibrational effect. Vibrational effect is very popular but the nuclear field effect also may generally appear in the complex-formation isotope effects of heavy elements.

13)

14)

15)

16)

Acknowledgment The present study was financially supported by the Grantin-Aid Program of Ministry of Education, Culture, Sports, Science and Technology (Project No. 16206095). References 1) Y. Fujii, M. Okamoto, H. Kadotani, H. Kakihana, ‘‘Equilibrium time and criticality considerations in uranium enrichment by the chemical-exchange processes,’’ Nucl. Technol., 86, 282 (1989). 2) J. Shimokawa, F. Kobayashi, ‘‘Separation of uranium isotopes by chemical exchange,’’ Isotopenpraxis, 6, 121 (1970). 3) M. Seko, T. Miyake, K. Inada, K. Takeda, ‘‘Uranium isotope enrichment by chemical method,’’ Nucl. Technol., 50, 178 (1980). 4) M. Seko, ‘‘Basic uranium-235 enrichment by Asahi Chemical Enrichment Process (ACEP),’’ Proc. Int. Symp. Chem. Exchange Uranium Enrich. & Isotope Sep., Bull. Res. Lab. Nucl. React. Special Issue, 1, 176 (1992). 5) T. Dujardin, G. Lonchampt, ‘‘Review of the French Chemicx Process,’’ Proc. Int. Symp. Chem. Exchange Uranium Enrich. & Isotope Sep., Bull. Res. Lab. Nucl. React. Special Issue, 1, 161 (1992). 6) Y. Fujii, M. Nomura, H. Onitsuka, K. Takeda, ‘‘Anomalous isotope fractionation in uranium enrichment process,’’ J. Nucl. Sci. Technol., 26, 1061 (1989). 7) Y. Fujii, M. Nomura, M. Okamoto, H. Onitsuka, F. Kawakami, K. Takeda, ‘‘An anomalous isotope effect of 235 U in U(IV)– U(VI) chemical exchange,’’ Z. Natureforsch., 44a, 359 (1989). 8) M. Nomura, N. Higuchi, Y, Fujii, ‘‘Mass dependence of uranium isotope effects in U(IV)–U(VI) exchange reaction,’’ J. Am. Chem. Soc., 118, 9127 (1996). 9) J. Bigeleisen, ‘‘Nuclear size and shape effects in chemical re-

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Appendix The experimentally observed separation coefficient " (separation factor S¼1þ") is converted to the theoretical isotope exchange equilibrium constant ("o ¼K1) of Eq. (3), by using the isotopic distribution between two phases, or mole fractions of isotopomers in each phase: S ¼ ðX/X0 Þsol. =ðX/X0 Þres. ¼ ð½AX þ ½BXÞ=ð½AX0 þ ½BX0 Þ=ð½AX þ ½BXÞ =ð½AX0 þ ½BX0 Þ ¼ ð½AX0 fA þ ½BX0 fB Þ=ð½AX0 þ ½BX0 Þ =ð½AX0 fA þ ½BX0 fB Þ=ð½AX0 þ ½BX0 Þ ¼ fð1 Þ þ ð1 þ "o Þg=fð1 Þ þ ð1 þ "o Þg ¼ ð1 þ "o Þ=ð1 þ "o Þ ¼ 1 þ ð Þ"o : Then we obtain, "o ¼"=ðÞ. In above equations, is the mole fraction of species B, the subscript ‘‘res.’’ and the underline indicate the resin phase and the subscript ‘‘sol.’’ indicates the solution phase.

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