Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 34, No. 12, p (December 1997)

Transcription

1 Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 34, No. 12, p (December 1997) A Nuclear Magnetic Resonance Study on Ligand Exchange Reaction in U(VI) Nitrato Complex with n-octyl(phenyl)-n,n-diisobutylcarbamoylmethylphosphine Oxide in Acetone Containing Oxalic Acid Koh HATAKEYAMA*,t1, Kazuhiro ARAI*, Masayuki HARADA*, Yasuhisa IKEDA**,t2 and Hiroshi TOMIYASU*,t2 * Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology Kashiwa Laboratory, Institute of Research and Innovation ** (Received May 19, 1997) In order to examine the effect of oxalic acid(h2ox) on the exchange reaction of n-octyl(phenyl)-n, N- diisobutylcarbamoylmethylphosphine oxide (opcmpo) in U(VI) nitrato complex with opcmpo, structural and kinetic studies have been carried out for the solutions prepared by dissolving UO2(NO3)2(opcmpo), opcmpo, and H2ox in CD3COCD3 using 31P and 13C NMR method. The number of coordinated opcmpo was determined to be 2 by the area integration of 31P NMR signals of free and coordinated opcmpo. From the area integration of 13C signals of free H2ox and coordinated oxalate(cox), the number of Cox was estimated to be 1. Hence, the complex with composition ratio of UO2+2:opcmpo: Cox=1:2:1 was supposed to be formed in the present solution system. The exchange rate of opcmpo in this complex was found to show a first-order dependence on the free opcmpo concentrations and to decrease with increasing free H2ox concentrations. From these results, it is proposed that the exchange reaction of opcmpo proceeds through the interchange(i) mechanism and that the retardation effect of free H2ox is due to the formation of outer-sphere complex. KEYWORDS: uranyl complexes, exchange reaction, chemical reaction kinetics, concentration dependence, ligand exchange, NMR, CMPO, oxalic acid, acetone I. INTRODUCTION It has been known that carbamoylmethylphosphine oxides(cmpo, R2P(O)CH2C(O)NR'2) are useful extractants for actinide and lanthanide elements from acidic nuclear waste solutions(1)-(3). Hence, the extraction chemistry for actinide and lanthanide ions with CMPO has been extensively studied(4)-(9). In spite of many studies on the extractabilities of CMPO, little information is available concerning the kinetic data and structures of CMPO complexes in liquid phase, which are important to understand the extraction mechanism and the properties of CMPO complexes in liquid phase. On the basis of this background, we have studied the exchange reactions of n-oxtyl(phenyl)-n, N- diisobutylcarbamoylmethylphosphine oxide(opcmpo) in its La(III), Nd(III), and U(VI) nitrato complexes in CD3COCD3 by using 31P NMR. As a result, it was clarified that three opcmpo molecules coordinate to La(III), Nd(III), and U(VI) nitrato complexes and that the opcmpo exchange reactions proceed dissociatively(10)(11). * O -okayama, Meguro-ku, Tokyo 152. ** T akada, Kashiwa-shi Present address: Naka tenergy Research Center, Mitsubishi Materials Co., Naka-machi, Naka-gun, Ibaraki-ken C t orresponding author, Tel , Fax , ikeda@iri.or.jp Furthermore, the TRUEX(TRansUranium EXtraction) process, where CMPO is used as extractant, has been proposed for managing liquid wastes containing TRU elements(1)(12)-(15). In the TRUEX process for treating High Level Liquid Wastes(HLLW), dilute HNO3 solutions containing oxalic acid(h2ox) are used as scrub and strip solutions, because co-extraction of Fe, Zr, and Mo is largely suppressed by addition of H2ox. Therefore, it is important to investigate the effect of H2ox as complexing agent in the TRUEX process. From this point of view, we have studied the effect of H2ox on the exchange reaction of opcmpo in uranyl nitrato complex with opcmpo in CD3COCD3. II. EXPERIMENTAL Anhydrous H2ox(Wako Pure Chemical Ind. Ltd.), 13C enriched H2ox(Isotec Inc., 13C: 99 at%), and opcmpo(hokko Chemical Ind. Co. Ltd.) were used without further purification. The UO2(NO3)2(opcmpo) complex was synthesized by the same method as reported previously(11). Experimental solutions were prepared by dissolving UO2(NO3)2(opcmpo), opcmpo, and H2ox of appropriate concentrations in CD3COCD3 (Merck, 99.8%). All concentrations in this paper are described as molality, m(=mol,kg-1). Acetone-d6 used as solvent and NMR lock reagent was dried over 4A molecular sieves(wako). NMR spectra of proton 1133

2 1134 K. HATAKEYAMA et al. decoupling 31P and 13C were recorded on a JEOL JNM EX-270 and JEOL FX-100 FT-NMR spectrometers, respectively. The typical conditions of 31P and 13C(shown in parentheses) NMR measurements were as follows: resonance frequency=109.15(25.05)mhz; data points=8(8)k; spectral width=5,000(5,000)hz; pulse width=5.5(10.0)ms; pulse delay time=3.97(0.5)s; number of scan=4,000(2,000). In 13C NMR measurements, signal of acetone was used as reference of chemical shift. Kinetic analyses for the exchange reactions of opcmpo in U(VI) nitrato complexes with opcmpo in CD3COCD3 were carried out by using following equations as explained in a previous paper(11)(16). (1) (2) where G and A are the total transverse magnetization and an arbitrary scaling factor, t and P with the subscripts F and C indicate the life times and mole fractions of the free and coordinated sites, and the values of 1/T2 and w0 with subscripts F and C refer to the line widths and the chemical shifts of two sites in the absence of chemical exchange, respectively. (3) Fig. 1 31P{1H} NMR spectra of opcmpo in a solution prepared by dissolving UO2(NO3)2(opcmpo)(7.05x 10-2m), opcmpo(2.07x10-1m), and H2ox(1.38x 10-1m) in CD3COCD3 at various temperatures III. RESULTS AND DISCUSSION about 160ppm is assigned to the signal of free H2ox. Oxalic acid is known to be in rapid equilibrium with oxalate 1. Structural Studies by 31P and 13C NMR Measurements ions (Hox- and ox2-) as follows: Figure 1 shows the 31P{1H} NMR spectra of H2ox_??_H++Hox- (4) opcmpo in a solution prepared by dissolving UO2(NO3)2- Hox-_??_H++ox2-. (5) (opcmpo), opcmpo, and H2ox in CD3COCD3 at various temperatures. As reported in previous paper(11), The above equilibrium is much faster than NMR relaxation process. Hence, the 13C NMR spectrum of two peaks(low-field(c) and high-field(f)) observed in the low temperature region were assigned as signals of phosphines of coordinated and free opcmpo, respectively. The age of signals for three species, H2ox, Hox-, and ox2-. free H2ox is observed as one singlet peak at the aver- number of coordinated opcmpo molecule was determined Two singlet peaks(c) at around 173 and 177ppm can by area integration of 31P signals of C and F at 15dc. be assigned as the signals of coordinated oxalates, because these peaks are observed only in the presence of The results are listed in Table 1 and indicate that the number of coordinated opcmpo molecules is 2 without uranyl complex. From the 13C NMR area measurement, total number of coordinated oxalate(ox or Hox, depending on H2ox and opcmpo concentrations in the present experimental conditions. This is different from abbreviated as Cox) was determined to be 0.9. Considering the accuracy of area integration method, the the previous result that three opcmpo molecules coordinate to the uranyl nitrato complex in the CD3COCD3 obtained value seems to be consistent with the expected one i.e., 1. Hence, the complexes with com- without containing free H2ox(11). A decrease in number of coordinated opcmpo molecules is supposed to be due position ratio of UO2+2: opcmpo: Cox=1:2:1 (abbreviated as UO2(NO3)2(opcmpo)2(Cox)) are supposed to to the replacement of one of three coordinated opcmpo molecules by one oxalate molecule. be formed in the present solution system. However, In order to examine the coordination behavior of the structure and charge of UO2(NO3)2(opcmpo)2(Cox) oxalate, the 13C{1H} NMR measurements were performed for solutions prepared by dissolving UO2(NO3)2- tion of two singlet signals due to the coordinated oxalate complex are still ambiguous as follows. The observa- (opcmpo), opcmpo, and 13C enriched H2ox, and by means that the 13C atoms of Cox exist in two different treating with anhydrous magnesium sulfate for removing trace amount of water. A typical result is shown Hox-, or ox2-) coordinates to the uranyl equatorial sites of uranyl equatorial plane. If one oxalate(h2ox, in Fig. 2. In this figure(at 20dc), a singlet peak(f) at plane as a unidentate or bidentate ligand, one singlet JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

3 Ligand Exchange Reaction in U(VI) Nitrato Complex with CMPO 1135 Table 1 Solution compositions and kinetic parameters for the exchange of opcmpo in uranyl nitrato complex with opcmpo in CD3COCD3 containing H2ox m: mol/kg, C.N.: Number of coordinated opcmpo molecules and 31P NMR spectra cannot be reasonably interpreted on the basis of configuration of coordinated opcmpo and oxalate. Hence, further structural studies on supposed complex, UO2(NO3)2(opcmpo)2(Cox), are necessary. 2. Exchange Reaction of opcmpo in UO2(NO3)2(opcmpo)2(Cox) The 31P signals of free and coordinated opcmpo become broad with increasing temperature as shown in Fig. 1. This phenomenon indicates that the exchange reaction takes place between the free opcmpo and coordinated opcmpo in UO2(NO3)2(opcmpo)2(Cox). The best-fit life times(t-values) were determined by Eq. (1) on the assumption that the number of coordinated opcmpo is 2 in the present temperature range. From the -values, the first-order rate constants (kex) were calculated by Eqs. (2) and t (6): kex=1/tc=rate/2[uo2(no3)2(opcmpo)2(cox)] =(kt/h)exp(-dh=//rt)exp(ds=//r). (6) Fig. 2 13C{1H} NMR spectra of a solution prepared by dissolving UO2(NO3)2(opcmpo)(6.97x10-2m), opcmpo(1.38x10-1m), and H2ox(1.37x10-1m) in CD3COCD3 at various temperatures peak or equivalent two peaks should be observed, respectively. Furthermore, if the uranyl complexes with unidentate and bidentate oxalates are formed, three singlet peaks due to Cox should be detected. On the other hand, the 31P signal of coordinated opcmpo is singlet in the range from 15 to 35dc. This means that the coordinated two opcmpo molecules are present in an equivalent site of uranyl equatorial plane in this temperature region. This phenomenon is in conflict with the observation of two singlet peaks due to Cox. Thus, the results of 13C Kinetic experiments were also carried out for other solutions(i-iv, vi, and vii) listed in Table 1. Figure 3 shows the semilogarithmic plots of kex vs. the reciprocal temperature. It is found from Fig. 3 and Table 1 that the exchange rate of opcmpo increases with an increase in the free opcmpo concentrations ([opcmpo]f=[opcmpo]-[uo2(no3)2(opcmpo)]) and decrease with increasing free H2ox concentrations ([H2ox]f=[H2ox]-[UO2(NO3)2(opcmpo)]). The plots of kex values against [opcmpo]f give straight lines without the intercept (see Fig. 4) and result in kex=k*[opcmpo]f. (7) The values of k* were obtained from the slopes of the plots in Fig. 4 and are listed in Table 2. Figure 5 shows the plots of kex vs. [H2ox]f (solutions v-vii in Table 1) and indicates that the exchange rates become slow and approach to constant values as [H2ox]f increases. Furthermore, the plots of 1/kex against [H2ox]f give straight lines with intercepts as shown in Fig. 6 and lead to 1/kex=k1+k2[H2ox]f. (8) VOL. 34, NO. 12, DECEMBER 1997

4 1136 K. HATAKEYAMA et al. Fig. 3 Semilogarithmic plots of ker against the reciprocal temperature for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) complex in CD3COCD3 Fig. 5 Plots of kex vs. [H2ox]f for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) complex in CD3COCD3 (Solutions v-vii in Table 1) Fig. 4 Plots of kex vs. [oocmpo]f for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) complex in CD3COCD3 (Solutions i-iv in Table 1) Fig. 6 Plots of k-1ex vs. [H2ox]f for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) complex in CD3COCD3 (Solutions v-vii in Table 1) Table 2 The values of k* at various temperatures for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) in CD3COCD3 The values of k1 and k2 were obtained by means of leastsquares method from the intercepts and slopes in Fig. 6 and are listed in Table Reaction Mechanism As mentioned above, the exchange rate of oocmpo exhibits a first-order dependence with respect to [oocmpo]f. This result suggests that the oocmpo exchange in UO2(NO3)2(oocmpo)2(Cox) proceeds through either the interchange(i) or associative(a) mechanism(17). However, the A mechanism seems to be JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

5 Ligand Exchange Reaction in U(VI) Nitrato Complex with CMPO 1137 Table 3 Values of k1, k2, Kos, K'os, and ki at various temperatures (10) where Kos is the formation constant for the outer-sphere complex, UO2(NO3)2(oocmpo)2(Cox)..oocmpo*, and ki is the rate constant for the interchange process between oocmpo and oocmpo* in the outer-sphere complex, respectively. In this mechanism, kex is given by the following equation(21)(22): kex=ki Kos[oocmpo]f/ 1+Kos[oocmpo]f. (11) If Kos[oocmpo]f á1, Eq. (11) can be simplified as follows: k ex=kikos[oocmpo]f. (12) [oocmpo]f=1.36x10-1m, k1(1+kos[oopcmpo]f)/(kikos[oocmpo]f 2=K'os,/(kIKos[oocmpo]f) k This equation is consistent with Eq. (7). Furthermore, the conditions of Kos[oocmpo]f á1 may be fulfilled under the experimental conditions, i.e., [oocmpo]f= ruled out by the following reasons. In the A mechanism, an intermediate of increased coordination number, UO2(NO3)2(oocmpo)2(oocmpo*)(Cox), must be formed as shown in m and[h2ox]f= m, since the reactant oocmpo is uncharged and the Kos value is expected to be relatively small as evaluated by the Fuoss equation(23). Hence, the I mechanism seems to be more likely than the A mechanism. In the I mechanism, the retardation effect of free H2ox is explained by the formation of outer-sphere complex between H2ox and UO2(NO3)2(oocmpo)2(Cox) shown in Eq.(13)(24): where the rate-determining step is the association of entering oocmpo* to UO2(NO3)2(oocmpo)2(Cox) and ka represents the rate constant for the corresponding process. If in the intermediate UO2(NO3)2(oocmpo)2- (oocmpo*)(cox) the ligands coordinate as unidentate or bidentate, the coordination number in the equatorial plane should be more than 6. In general, the coordination number in the equatorial plane of uranyl ion is known to be 4, 5, or 6(18)(19). From this point of view, the formation of the intermediate of increased coordination number should be sterically restricted in the present complex with the bulky ligand such as oocmpo. Furthermore, even in the uranyl nitrato complex with tributyl phosphate (TBP), which is smaller than oocmpo, the exchange reaction proceeds through the D mechanism(20). Hence, it seems unlikely that the bulky oocmpo attacks to UO2(NO3)2(oocmpo)2(Cox) associatively. On the other hand, the I mechanism is expressed as follows(17): (9) (13) where K'os is the formation constant for the outer-sphere complex, UO2(NO3)2(oocmpo)2(Cox)..H2ox*. This complex formation lowers the concentration of outersphere complex UO2(NO3)2(oocmpo)2(Cox)..oocmpo* and results in a decrease in the exchange rate. From the mechanism expressed by Eqs. (10) and (13), the following equation is derived(21)(22)(24) When Kos [oocmpo]f á(1+k'os[h20x]f) at [H2ox]f= m (ref. Solutions in Table 1), Eq. (14) can be simplified as (14) (15) Equation (15) is consistent with the experimental results expressed by Eq. (7), and k* is related to the coefficient of Eq. (15) as follows: (16) In this mechanism, however, the condition of Kos [oocmpo]f á(1+k'os[h2ox]f) must be satisfied. This condition seems to be fulfilled under the present conditions [oocmpo]f= m and [H2ox]f =0.0278m) from the same reasons as mentioned above. In fact, the Kos values for the DMF(N, N- dimethylformamide) exchange in UO2(dmf)25+ in CD2Cl2 and for the oocmpo exchange in La(NO3)3(oocmpo)3 in CD3COCD3 have been reported to be 1.59m-1 at 5dc(25) and 0.63m-1 at 25dc(11), respectively. 4 Furthermore, when [oocmpo]f =constant, the follow- VOL. 34, NO. 12, DECEMBER 1997

6 1138 K. HATAKEYAMA et al. Table 4 Kinetic parameters for the exchange reactions of oocmpo in UO2(NO3)2(oocmpo)3 and UO2(NO3)2(oocmpo)2(Cox) complexes in CD3COCD3 * : The exchanging species ing equation is derived from Eq. (14). 1/kex=1+Kos[oocmpo]f/kIKos[oocmpo]f+K'os[H2ox]f/kIKos[oocmpo]f. (17) Equation (17) is found to correspond to Eq. (8), where k1 and k2 are correlated to the coefficients of Eq. (17) as follows: k1=1+kos[oocmpo]f/kikos[oocmpo]f ( 18a) k2=k'os/kikos[oocmpo]f. (18b) k2=k'os/kikos[oocmpo]f. (18b) Therefore, the values of ki, Kos, and K'os were calculated from Eqs. (16), (18a), and (18b) by using the k* values in Table 2, the k1 and k2 values in Table 3, [oocmpo]f= 0.136m, and [H2ox]f=0.0278m. The results are listed in Table 3. The Kos values are consistent with those for the oocmpo exchange reaction in La(NO3)3(oocmpo)3 in CD3COCD3(4.6m-1 at 0dc) and the DMSO(dimethyl sulfoxide) exchange -3 reaction in UO2(acac)2dmso (acac=acetylacetonate) in CD2Cl2(4.9dm3/mol at 25dc)(23). The K'os values are larger than Kos ones. This suggests that the Cox molecules interact with uranyl ions more strongly than the oocmpo molecules. The plot of k1 vs. 1/T is shown in Fig. 7. From this plot, the kinetic parameters for ki path were obtained as listed in Table 4 with data for the oocmpo exchange in UO2(NO3)2(oocmpo)3. The DH=/ value for the present system is found to be nearly equal to that for the oocmpo exchange in UO2(NO3)2(oocmpo)3, which proceeds through the D mechanism. This indicates that the formation of activated complex in the present exchange reaction is also governed by the dissociation of oocmpo and that the exchange of oocmpo in the present system proceeds through the dissociative interchange (Id) mechanism(17). Furthermore, the DS=/ value for the present system is more largely negative than that for the oocmpo exchange in UO2(NO3)2(oocmpo)3. This may be due to the effect of solvent rearrangement around the complex as proposed by Bennetto and Caldin(26)(27) and explained in the previous paper(20), and due to the formation of outer-sphere complex, UO2(NO3)2- (oocmpo)2(cox)..oocmpo*. IV. CONCLUSIONS Fig. 7 Semilogarithmic plot of ki against the reciprocal temperature for the exchange of oocmpo in UO2(NO3)2(oocmpo)2(Cox) complex in CD3COCD3 In order to examine the effect of oxalic acid on the extraction of actinide ions using oocmpo, the structures and ligand exchange reactions of uranyl complexes in solutions prepared by dissolving UO2(NO3)2(oocmpo), oocmpo, and H2ox in CD3COCD3 were studied by 31P and 13C NMR. As a result, two oocmpo and one oxalate molecules were found to coordinate to uranyl ions in such a solution system. Hence, the complex with composition ratio of UO22+: oocmpo: Cox=1:2:1 was supposed to be formed in the present solution system. The exchange rate of oocmpo in this complex was found to depend on the first-order with respect to [oocmpo]f and to decrease with increasing [H2ox]f. From these results, it is proposed that the exchange reaction of oocmpo proceeds through the interchange(i) mechanism. The retardation effect of free H2ox on the exchange rate of oocmpo is considered to be due to the formation of outer-sphere complex. -R EFERENCES- (1) Vandegrift, G. F., Leonard, R. A., Steindler, M. J., Horwitz, E. P., Basile, L. J., Diamond, H., Kaline, D. G., Kaplan, L.: ANL-84-45, (1984). (2) Horwitz, E. P., Kalina, D. G., Diamond, H., Vandegrift, G. F., Schulz, W. W.: Solvent Extr. Ion Exch., 3, 75 JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY

7 Ligand Exchange Reaction in U(VI) Nitrato Complex with CMPO 1139 (1985). (3) Horwitz, E. P., Kalina, D. G., Diamond, H., Kaplan, L., Vandegrift, G. F., Leonard, R. A., Steindler, M. J., Schulz, W. W.: DE , (1985). (4) Kalina, D. G., Horwitz, E. P., Kaplan, L., Muscatello, A. C.: Sep. Sci. Technol., 16, 1127 (1981). (5) Horwitz, E. P., Kalina, D. G., Kaplan, L., Mason, G. W., Diamond, H.: Sep. Sci. Technol., 17, 1261 (1982). (6) Horwitz, E. P., Martin, K. A., Diamond, H., Kaplan, L.: Solvent Extr. Ion Exch., 4, 449 (1986). (7) Marsh, S. F., Yarbro, S. L.: DE , (1988). (8) Nagasaki, S., Kinoshita, K., Enokida, Y., Suzuki, A.: J. Nucl. Sci. Technol., 29, 56 (1992). (9) Wisnubroto, D. S., Nagasaki, S., Enokida, Y., Suzuki, A.: J. Nucl. Sci. Technol., 29, 263 (1992). (10) Ikeda, Y., Miyata, A., Park, Y.-Y., Hatakeyama, K., Tomiyasu, H.: J. Nucl. Sci. Technol., 30, 720 (1993). (11) Hatakeyama, K., Park, Y.-Y., Tomiyasu, H., Ikeda, Y.: J. Nucl. Sci. Technol., 32, 1146 (1995). (12) Carnell, W. T., Choppin, G. R.: "Plutonium Chemistry", American Chem. Soc., Washington, D.C., ACS Symp. Ser., 216, 433, (1983). (13) Kolarik, Z. J., Horwitz, E. P.: Solvent Extr. Ion Exch., 6, 247 (1988). (14) Schulz, W. W., Horwitz, E. P.: Sep. Sci. Technol., 23, 1191 (1988). (15) Mincher, B. J.: Solvent Extr. Ion Exch., 7, 645 (1989). (16) Ikeda, Y., Tomiyasu, H., Fukutomi, H.: Inorg. Chem., 23, 1356 (1984). (17) Langford, C. H., Gray, H. B.: "Ligand Substitution Processes", Benjamin, London, (1974). (18) Evans, H. T.: Science, 141, 154 (1963). (19) Lincoln, S. F.: Pure Appl. Chem., 51, 2059 (1979). (20) Hatakeyama, K., Park, Y.-Y., Tomiyasu, H., Ikeda, Y.: J. Nucl. Sci. Technol., 34, 298 (1997). (21) Lo, S. T. D., Swaddle, T. W.: Inorg. Chem., 15, 1881 (1976). (22) Ikeda, Y., Tomiyasu, H., Fukutomi, H.: Bull. Chem. Soc. Jpn., 56, 1060 (1983). (23) Fuoss, R. M.: J. Am. Chem. Soc., 89, 5059 (1958). (24) Ikeda, Y., Tomiyasu, H., Fukutomi, H.: Inorg. Chem., 23, 3197 (1984). (25) Doine, H., Ikeda, Y., Tomiyasu, H., Fukutomi, H.: Bull. Chem. Soc. Jpn., 56, 1989 (1983). (26) Bennetto, H. P., Caldin, E. F.: J. Chem. Soc., A, 2198 (1973). (27) Caldin, E. F., Bennetto, H. P.: J. Solution Chem., 2, 217 (1973). VOL. 34, NO. 12, DECEMBER 1997