Uranium (IV)-(VI) Electron Exchange Reactions in

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, 5[4], F (April 1968) 179 Uranium (IV)-(VI) Electron Exchange Reactions in Anion Exchange Resin, Tri-n-Octyl Amine and Tri-Butyl Phosphate Kozo GONDA*, Nobuyoshi KAWASHIMA* and Hidetake KAKIHANA** Received October 23, 1967 A study has been made of the U(IV)-U(VI) electron exchange reactions taking place between natural U(IV) and depleted U(VI) in anion exchange resin, as well as in TnOA/benzene and TBP/benzene systems. The exchange rate in the anion exchange resin was smaller than the corresponding rates in 10 M HCl, which might be explained by considering that the diffusion of the exchangeable species is much slower in the anion exchange resin than in 8~10 M HCl. The exchange rates in TnOA/benzene and in TBP/benzene were accelerated by decreasing the concentration of TnOA or TBP in benzene. The reason may be that TnOA and TBP, including the exchangeable species, is more easily diffused in dilute than in concentrated solution. Based on activation energy values, the mechanism of exchange in the anion exchange resin was estimated to be similar to that in 8~10 M HCl solution, and the corresponding mechanism for TBP/ benzene similar to that for the cation exchange resin. In the case of TnOA/benzene, the exchange process was presumed to resemble that of the anion exchange resin, judging from the similarity of ionic species extracted thereinto. I. INTRODUCTION During previous studies(1) on the isotope effect of the electron exchange reaction between U(IV) and U(IV), it was by chance observed that the reaction was catalyzed by large concentration of hydrochloric acid(2). Prior to this, a similar catalytic effect brought about by cation exchange resin on the exchange reaction had been reported by Shimokawa, et al.(3) The present paper covers a study on the rates of reaction in electron exchange with anion exchange resin, TnOA and TBP, all of which have been considered very promising media for uranium isotope separationw(4). II. EXPERIMENTAL 1. U(IV)-U(VI) Electron Exchange Reaction with Anion Exchange Resin The experimental arrangement is shown in Fig. 1(a). (1) Preparation of U(VI) Chloride Solution (depleted uranium) Uranyl nitrate (supplied by Yokosawa Chemical Co. Ltd., 235U isotope fraction 0.291%) was dissolved in water, and ammonia added. The precipitate was ignited to oxide (UO3) at 280dc. Uranyl chloride solution was prepared by dissolving the oxide with suitable amounts of hydrochloric acid. The concentration of U(VI) in the solution was estimated to be 0.11 M by colorimetry with 3% H2O2, and the concentration of free acid determined to be 8.0 M by subtracting the U(VI) concentration from the total chloride concentration. (2) Preparation of U(IV) Chloride Solution (natural uranium) Pure uranium metal (from the Power Reactor and Nuclear Fuel Developement Corporation of Japan) was washed with ethanol, dilute nitric acid and water, and then dissolved in concentrated hydrochloric acid under cooling. The U(IV) and free acid concentrations were adjusted to 0.11 M and 8.0 M, respectively, by adding suitable amounts of hydrochloric acid. * Tokai Works, Power Reactor and Nuclear Fuel Developement Corporation, Tokai-nzura, Ibaraki-ken. Research Laboratory of ** Nuclear Reactor, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo

2 180 J. Nucl. Sci. Technol., Fig. 1(a) (b) Scheme of experiment on U(IV)-U(VI) electron exchange reaction 36

3 Vol. 5, No. 4 (Apr. 1968) 181 ( 3 ) The Starting Point of the Electron Exchange Reaction An ample amount of anion exchange resin of Cl form, Dowex 2 x 10 (200~400 mesh) was thoroughly dried at 110dc. One gram portions of dried anion exchange resin were placed one each in hario-red glass reaction vessels (50 ml) and deposited in a thermostat (30-, 40- or 50dc). To these portions were added 8 and of U(IV) solution and 4 ml of U(VI) solution. Shaking for 1 hr allowed nearly all of the U(VI) and a part of the U(IV) to be absorbed by the anion exchange resin, and ion exchange equilibrium to be attained by both U(IV) and U(VI) between the anion exchange resin and solution phases. This point was taken as the starting point of the electron exchange reaction in the anion exchange resin phase. The content of one of the vessels was used to determine at this point the isotope ratios of U(IV) in the anion exchange resin and in the solution, as well as the equilibrium distribution of U(IV) and U(Vl) between the anion exchange resin and the solution. After separating the anion exchange resin from the solution by drawing through a glass filter, the U(IV) and U(VI) in the anion exchange resin were eluted by 1 M HCI. This 1 M HCl is roughly adequate to inhibit further electron exchange reaction. The equilibrium concentration of U(IV) and U(VI) in the anion exchange resin were determined in the eluate by titration with potassium bichromate and by colorimetry with 3 % H2O2, respectively. The results are shown in Table 1(a) (c). The isotope ratios of U(IV) in the anion exchange resin and in the solution were determined by the procedure described below. Table 1(a)~(c) Experimental conditions of U(IV) -U(VI) electron exchange reaction 37

4 182 J. Nucl. Sci. Technol., ( 4 ) Preparation of Sample for Massspectrometry At regular intervals, the anion exchange resin in one of the reaction vessels was separated from the external solution by suction. The U(IV) and U(VI) in the anion exchange resin were eluted by 1 M HCl, and the concentration of hydrochloric acid in the eluate was adjusted to 4 M HCl to completely inhibit further electron exchange reaction. The U(IV) in the eluate was purified by removing U(VI) with 0.2 M TnOA, 5 % octylalcohol-kerosine. The purified U(IV) was ignited to UO3. The U(IV) in the external solution separated from the anion exchange resin was similarly purified and ignited to oxide. The oxide from both solutions was dissolved in concentrated hydrochloric acid and diluted with water, and the uranyl chloride solution thus obtained was used as the sample for mass spectrometry. The isotope ratios were determined by thermoionization, using an Atlas mass spectrometer model CH4 equipped with double collectors. 2. U( IV )-U(VI) Electron Exchange Reaction in Tn0A/henzene and in TBP/ benzene The experimental arrangement is shown in Fig. 1(b). Sample solutions of various concentrations of natural U(IV) in TnOA/benzene or in TBP/ benzene were prepared by extracting natural U(IV) from 8 M free hydrochloric acid solution containing 1 M U(IV) by means of benzene solutions of TnOA or TBP of various concentrations. Sample solutions of various concentrations of depleted U(VI) were prepared by a similar extraction procedure. The amounts of U(IV) or U(VI) and Cl were determined by colorimetry with 3 % H2O2 and by titration with Hg (I) nitrate, after stripping with 1.8 N perchloric acid the U(IV), U(VI) and Cl from organic solvent. The results for TnOA/benzene system are shown in Table 1(b), and those for TBP/benzene system in Table 1(c). Ten milliliters of TnOA/benzene solution containing natural U(IV) and 10 ml of TnOA/ benzene solution containing depleted U(VI) were mixed in a hario-red glass reaction vessel (50 ml) in a thermostat (30-, 40- or 50dc). The TBP/benzene solutions containing U(IV) and U(Vl) were similarly mixed. ( 1) Preparation Sample for Massspectrometry After timed intervals, portions of mixture of U(IV) and U(VI) in TnOA/benzene or in TBP/benzene were stripped with 1 M HCl or with water; the stripped U(IV) and U(VI) were delivered directly into suitable amounts of dilute HCl solution; and the acid concentration of the mixture was adjusted to 4 M to inhibit further electron exchange. Samples for mass spectrometry were prepared according to the procedure described in Fig. 1(b). III. CALCULATION 1. U(IV)-U(VI) Electron Exchange Reaction in Anion Exchange Resin Since nearly all U(VI) is absorbed in the anion exchange resin, there would be three possible paths for the isotope exchange reaction to follow : (1) between U(IV) in the anion exchange resin and U(VI) in the anion exchange resin, (2) between U(IV) in the anion exchange resin and U(IV) in the solution, and (3) between U(VI) in the anion exchange resin and U(IV) in the solution. The first and third paths would involve redox in the electron exchange reaction between U(IV) and U(VI) the former in the anion exchange resin phase and the latter between the anion exchange resin and solution phases. The second path is not a redox reaction, but an isotope exchange reaction between U(IV) in the anion exchange resin and U(IV) in the solution. These three isotope exchange paths are represented by the following equations(5): and (1) (2) (3) (4) 38

5 Vol. 5, No.4 (Apr. 1968) 183 (5) (6) L where [ ]C and [ ]C represent the concentrations of uranium in the solution and in the anion exchange resin phases respectively. For the sake of simplicity, we introduce the following expressions to represent uranium concentrations and isotopic fractions, [235 U(IV)]+[238U(IV)]=a, [235U(IV)]=af(IV), 235U(IV)]+[238U(IV)]=a [, [235U(IV)])=af(IV), 235U(VI)]+[238U(VI)]=b, [ [235U(VI)]=bf(VI). During the exchange reaction, a, a and b remain constant, while f(iv), f(iv) and f(vi) are variable. Equations (4), ( 5 ) and ( 6) are reduced to (6) (4) L (5) L The apparent rate constants k,, k2 and k3 can be calculated from Eqs.( 4 ) L, ( 5 ) L and ( 6 ) L by tracing the isotopic fractions f(iv), f(iv) and f(vi), and tangents thereof against reaction time. 2. U(IV)-U(VI ) Electron Exchange Reaction in TnOA/benzene and TBP/benzene For the experiments in TnOA/benzene and TBP/benzene, -similarly to the case of aqueous solution- values for the exchange fraction were calculated from the equation(2) F={f(IV)- f0(iv)}{foo(iv)- f0(iv)}-1, ( 7 ) in which F is the fraction of exchange at time i,f(iv) the 235U fraction in the U(IV) fraction in the sample removed at time t,f0(iv) the initial 235U fraction in U(IV), and foo(iv) the final 235U fraction of the infinite time sample. The relationships of F to the exchange rate R and to the apparent rate constant k are given by ln(1 F)=ln(f(IV)-foo(IV))(f0(IV) foo(iv))-1 = R([U(IV)]+[U(VI)])([U(IV)] [U(VI)])-1t = k([u(iv)]+[u(vi)]t. ( 8) Plots of ln(1 F) vs. t represent a straight Table 2 Experimental results of U(IV)-U(VI) electron exchange reaction 39

6 184 J. Nucl. Sci. Technol., (b) In TnOA/benzene system (c) In TBP/benzene system line from whose slope R and K can be calculated. IV. RESULTS AND DISCUSSION 1. U(IV)-U(VI) Electron Exchange Reaction in Anion Exchange Resin Exchange reaction measurements in anion exchange resin were carried out at three different temperatures. In order to obtain the exchange rate R and the apparent rate constant k1, the isotopic fractions of the U(IV) in the anion exchange resin and in the solution were measured at various reaction times, and the isotopic fractions of U(VI) in the anion exchange resin were calculated. These isotopic fractions, together with the exchange rate R and the apparent rate constant k1 calculated with the use of Eq.( 4 ) L, are shown in Table 2(a). The value of 7.5 x 10-4l/mol/min for the apparent rate constant k1 at 30-C in the anion exchange resin placed in contact with 8 M HCl solution containing U(IV) was considered small compared with the value of 3.5 x 10-3l/mol/min for the apparent rate constant in 8 M HCl solution at 30dc. This fact can be explained by presuming that the hydrochloric acid, the catalyst of this electron exchange reaction, is contained in smaller amount in the anion exchange resin phase than in 8 M HCl solution, and that the exchangeable species in the anion exchange resin are not as easily diffused as in hydrochloric acid solution because of the influence of the fixed functional radicals of the anion exchange resin. The logarithms of the apparent rate con- 40

7 Vol. 5, No. 4 (Apr. 1968) 185 stants are plotted against the reciprocal of temperature in Fig. 2. The activation energy of the U(IV)-U(VI) electron exchange reaction in the anion exchange resin was estimated from the slope to be 22.9 kcal/mol. This value is close to the value of 24~25 kcal/mol for 6~ 10 M HCl. This similarity could be taken to indicate that the activated complex of the electron exchange reaction between U(IV) and U(VI) in the anion exchange resin could be similar to that of the activated complex in a solution of 6~10M HCl, and that the action in the anion exchange resin is governed by a rate determining ). step similar to that in 6~10 M HCl(2 Fig. 2 Logarithms k1 and k against reciprocal of temperature The electron exchange between U(VI) in the anion exchange resin, and U(IV) in the solution is too slow to permit precise estimation of the value k3 from this experiment. 2. U(IV) -U(VI) Electron Exchange Reaction in TnOA/benzene and TBP/benzene Exchange reaction measurements in TnOA/ benzene and TBP/benzene were carried out at various concentrations of TnOA and TBP in benzene at 30dc; and in order to obtain the activation energy of the exchange reaction in TBP, the exchange reaction measurements in 0.05 N TBP were performed at 40- and 50dc as well as at 30dc. The isotopic fraction of U(IV) was measured by mass-spectrometry. In Table 2(b) are given the results for TnOA/ benzene system together with exchange rate R and the apparent rate constant k calculated by using Eq.( 8 ). Corresponding data for the TBP/benzene system are presented in Table 2(c). These tables show that the exchange rates increase with decreasing TnOA or TBP concentration. The reason for this may be that TnOA and TBP -including the exchangeable species- are more easily diffused in dilute than in concentrated solution. The true rate constants in TnOA/benzene and TBP/benzene could be obtained by measuring the exchange in a dilute solution of TnOA or TBP, as the case may be. Because of the unstability of TnOA at 40- and 50dc, the activation energy of the TnOA/ benzene system could not be determined. In so far as corcerns the ionic species extracted in TnOA/benzene, the mechanism of the exchange in TnOA/benzene appears to be very similar to that of the anion exchange resin. The activation energy of the U(IV)-U(VI) electron exchange reaction in TBP was estimated from Fig. 2 to be 14.2 kcal/mol. This value is close to the 13.9 kcal/mol measured in the cation exchange resin by Shimokawa, et al.(3), which might indicate that the exchange in TBP/benzene is governed by a rate determining step similar to that of the cation exchange resin(6). Assuming that the extracted species of U(IV) and U(VI) in TnOA are (TnOA HCl)2UCl4 and (TnOA HCl)2UO2Cl2, and in the case of TBP, (TBP)2UCl4 and (TBP)2UO2Cl2(7)(8), respectively, the amounts of Cl dissolved into the organic phase are found to be as shown in Tables 1(b) and (c). From our present work, it has not been possible to ascertain whether the U(IV)- U(VI) electron exchange reaction in TnOA/ benzene or TBP/benzene is influenced by the Cl concentration. ACKNOWLEDGMENT The authors express their thanks to Mr. H. Satoh, Mr. K. Kagami and Mr. S. Suzuki of the Analytical Section in the Power Reactor and Nuclear Fuel Developement Corporation

8 186 J. Nucl. Sci. Technol. for their help in mass spectrometry. They are also indebted to Mr. M. Itani, who assisted the author in part of the experimental work. -- REFERENCES-- (1) KAKIHANA, H., GONDA, K., SATOH, H., et al.: J. At. Energy Soc. Japan, (in Japaneses), 5 [12], 990 (1963). (2) KAKIHANA,H., TOMIYASU,H., GONDA,K.: Symp. on Exchange Reaction, Brookhaven, Exchange Reaction, 121 (1965). (3) SHIMOKAWA, J., NISHIO, G., KOMORI, T. : J. Nucl. Sci. Technol., 1 [2], 51 (1964). (4) GONDA, K., KAWASHIMA, N., KAKIHANA, H.: J. At. Energy Soc. Japan, (in Japanese), 9 [7], 376 (1967). (5) KAKIHANA, H., et al.: Proc. Int. Conf. Peaceful Uses At. Energy, 12, 342 (1964). (6) SHIMOKAWA, J., NISHIO, G.: J. Nucl. Sci. Technol., 1 [7], 221 (1964). (7) NAITO, K., SUZUKI, T.: J. Phys. Chem., 66, 989 (1962). (8) WATANABE, K.: J. Nucl. Sci. Technol., 1, [5], 155 (1964)