Solvent Extraction Research and Development, Japan, Vol. 19, (2012)

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1 Solvent Extraction Research and Development, Japan, Vol. 19, (2012) Actinides Extractability Trends for Multidentate Diamides and osphine xides Yuji SASAKI, Yoshihiro KITATSUJI, Yumi SUG, Yasuhiro TSUBATA, Takaumi KIMURA and Yasuji MRITA uclear Science and Engineering Directorate, Japan Atomic Energy Research Institute, Tokai, Ibaraki , Japan (Received December 28, 2011; Accepted February 14, 2012) Diamides and phosphine oxides having many types of central frames were synthesized and employed in order to determine the behavior of the actinides (An)(III), (IV), (V) and (VI). Diamides synthesized here have two, three, or four donor atoms of amidic and etheric oxygen (nitrogen or sulfur, instead) atoms in their central frames, namely the extractants can work in bi-, tri-, and tetradentate modes. Di-phosphine di-oxides ((Bis(diphenylphosphoryl)methane (BDPPM) and bis(diphenylphosphoryl)ethylene (BDPPE)), carbamoylmethylphosphine oxide (octylphenyl-,-diisobutyl-carbamoylmethylphosphine oxide (MP)) are used as the representative actinide extractants. Eu(III), Th(IV), U(VI), p(v), Pu(IV) and Am(III) ions in perchloric acid were extracted into nitrobenzene or n-dodecane. Measurement of the D value at different extractant concentrations gave information on the extraction reaction and extraction ability. From the present work, the best extractant is BDPPM, due to the strong affinity provided P= group and its bidentate form resulting in six-membered ring chelation. However, this extractant has too low a solubility in n-dodecane. From the properties required for application, e.g., high solubility in n-dodecane, easy organic synthesis, gasification by combustion and high actinide extractability, into consideration, diglycolamide (DGA) is a promising extractant for radioactive waste treatment. 1. Introduction The extraction and separation of actinides by novel extractants have been studied widely in order to develop a partition method for treatment of high level radioactive waste containing actinides. For instance, many different structures of monoamides, diamides, carbamoylmethylphosphine oxides, and

2 diphosphine dioxides have been developed particularly in the last three decades [1-6]. However, there are few reports concerning extractants with different central frames [7-8], because of difficulties in their organic synthesis. Besides, the forms of the central frame play an important role in actinide extraction and thus a comparison of the several types of extractant is required to determine the best chemical structure. In this research field, the focus is on multidentate diamide extractants, due to their high extractability of actinides. Diamides with two, three, or four of carboxylic and etheric oxygen (nitrogen or sulfur) donor atoms in their central frames were synthesized. In addition, bidentate phosphine oxides, as strong actinide extractants, are also employed. All extractants obtained here are used for extraction of Eu(III), Am(III), Pu(IV), p(v) and U(VI). ne of the main purposes of this work is to determine their order of extractability, and the relation of actinide extractability with their central frame are discussed. The extractants used in this work are summarized in Figures 1 and 2. The newly synthesized,, -dimethyl-, -dioctyl-2-(3-oxapentadecane)-malonamide (DMDPDMA),,,, -tetraoctyl-3,6-dioxaoctanediamide (DDA), methoxy-,,, -tetraoctyl pentanediamide (MTPDA),,,, -tetraoctyl-oxidipropionamide (DPA),,,, -tetraoctyl-2-phenylmalonamide (PMA), trioxaheptacosane (TH),,,','-tetraoctyl-3-thiadiglycolamide (TTDGA),,,, -tetraoctyl-malonamide (TMA), methylimino-bis-(,-dioctylacetamide) (MIDA), and,,, -tetraoctyl-3-oxopentanediamide (TXPDA) are shown in Figure 1. These compounds, which have high lipophilicity and good solubility in n-dodecane, belong to division 1. xalamide (XA), malonamide (MA), succinamide(sa), maleamide(mla), glutalamide(gla), diglycolamide(dga), oxydipropionamide(dpa), thiadiglycolamide (TDGA), and thiadipropionamide(tdpa) are illustrated in Figure 2. These extractants, which have been used previously[7], belong to Division 2. Diluents are used (n-dodecane or nitrobenzene) as appropriate and the D values obtained are compared under the same experimental conditions in order to determine the extraction ability trend. 8H 17,,','-tetraoctyl -3-diglycolamide (TDGA) 8H 17 H H 2 H 3 8H 17 8H 17 methoxy-,,, -tetraoctyl pentanediamide (MTPDA) 8H 17 8H 17 H 3 8 H 17 2H H H 3 8H 17, -dimethyl-, -dioctyl-2-,,, -tetraoctyl-3,6- (3-oxapentadecane)-malonamidedioxaoctanediamide (DDA) (DMDPDMA),,, -tetraoctyloxidipropionamide (DPA) 8H 17 8H H ,,, -tetraoctyl- 2-phenylmalonamide (PMA) 101 trioxaheptacosane (TH) 10H21 8H 17 8H 17 S 8H 17 8H 17,,','-tetraoctyl-3- Thiadiglycolamide (TTDGA) 8H 17 8H 17 8H 17 8 H 17,,, -tetraoctyl malonamide (TMA) 8H 17 8 H 17 8H 17 H 2 H 3 8H 17 8H 17 methylimino-bis-(,-dioctylacetamide) MIDA Fig. 1 ewly synthesized extractants (in Division 1) 8H 17 8H 17 8H 17,,, -tetraoctyl-3- xopentanediamide (TXPDA)

3 P bis(diphenylphosphoryl) methane (BDPPM) P P P bis(diphenylphosphoryl) ethylene (BDPPE) 8 H 17 P iso 4 H 9 iso 4 H 9 octylphenyl-,-diisobutylcarbamoylmethylphosphine oxide (MP) H 3 6 H 13 xalamide (XA) H 3 6 H 13 H 3 6 H 13 H 3 H 3 6 H 13 6 H 13 H 3 6 H 13 H 3 6 H 13 H H H 3 6 H 13 H 3 6 H 13 H 3 6 H 13 malonamide (MA) succinamide (SA) maleamide (MLA) glutalamide (GLA) H H 3 2 H 3 S H 3 H 3 6 H 13 6 H 13 6 H 13 6 H 13 8 H 17 8 H 17 H 3 6H 13 8 H 17 S H 3 diglycolamide (DGA) thiadiglycolamide (TDGA) oxydipropionamide (DPA) 8 H 17 thiadipropionamide (TDPA) 6H 13 Fig. 2 Extractants produced previously (in division 2) 2. Experimental 2.1 Preparation of extractants Diamide extractants (XA, MA, SA, MLA, GLA, DGA, TDGA, DPA, and TDPA) were synthesized and purified by the same method as reported in the literatures [9-10]. XA is synthesized from oxalic chloride and,-methylhexylamine, MA is synthesized from malonic ester and a secondary amine with the addition of hydrochloric acid. SA, MLA, GLA, DGA, TDGA, DPA, and TDPA were synthesized from the corresponding initial reagents, carboxylic acids and a secondary amine by a similar method to XA. BDPPM and BDPPE were synthesized by oxidation of 2 P P 2 and 2 P(= )P 2 using hydrogen peroxide, and these extractants can be purified by recrystallization[11-12]. ther novel compounds, such as MIDA, PMA, TH, TXPDA, MTPDA, TMA, DMDPDMA and DDA, were obtained from Wako-Pure hemical Industries. PMA, TMA and DMDPDMA can be synthesized by the method for MA. MIDA, TXPDA and DDA can be synthesized by the same method as for XA. TH was synthesized from -( 2 H 4 l) 2 and Br. MTPDA was synthesized from TXPDA and H 3 I in methanol with the addition of K 2 3. All products were washed and purified by distillation and/or silica gel column chromatography. The purified samples were identified by MR (UITY plus 400, varian), IR and G. 2.2 Solvent extraction The procedure for solvent extraction was the same as in previous papers [7-8]. The extractants in Division 1 and Division 2 were dissolved in n-dodecane and nitrobenzene, respectively. The organic

4 phase was contacted with the appropriate concentration(0.1 or 1 M) of (H,a)l 4, which would be employed in solvent extraction (pre-equilibration). After shaking the two phases for 10 min, phase separation was carried out by centrifugation. An aliquot of the organic phase was removed from the extraction tube and mixed with the same volume of Hl 4 in a fresh extraction tube. The radioactive tracer solution was then introduced. The amounts of the metal ions used for the extraction experiments, in μg/ 10 μl, were approximately 1 for Eu Eu, 0.1 for 230 Th, 0.1 for 233 U, 1 for 237 p, 1x10-4 for 238 Pu and 1x10-4 for 241 Am. The separation of alpha 237 p activity from beta 233 Pa activity was done weekly by solvent extraction, as described in the previous paper[13]. The extraction tube was shaken mechanically at 120 rpm for 30 min at o in a thermostated bath. After extraction, the two phases were separated by centrifugation at 1000 rpm for 5 min. The radioactivities in the aqueous and organic phases were measured using an ai scintillation counter for the gamma activity of Eu, 241 Am and a liquid scintillation counter for the alpha activity of 230 Th, 233 U, 238 Pu, 237 p and 241 Am. 3. Results and Discussion 3.1 Extraction of An(III), (IV), (V) and (VI) by the extractants in Division 1 Extractants in Division 1 were used for An extraction from Hl 4 into n-dodecane. This organic diluent is used in the PUREX process, and it was already known that n-dodecane gives the satisfactory D values for An [14]. Although it is not used in the PUREX process, perchloric acid was used as the aqueous phase in order to provide the effective conditions for ion-pair extraction and to obtain detectable and comparable D values. Some of the extractants in Figure 1 are presumed to have low extractabilities. D(An) is defined as the ratio of the metal concentration in the organic phase to that in the aqueous phase. Here, the concentration unit, mol/dm 3, is expressed as M in this paper. The D values for Eu(III), Am(III), Pu(IV) and U(VI) are shown in Figure 3 against extractant concentration. Using the straight lines in Figure 3, the slopes and the intercept values on the y axis (IV,y) are obtained and summarized in Table 1. High IV,y values indicate strong extractability. As shown in Figure 3, due to its weak complexing ability, D(Eu, Am, Pu) for TH cannot be properly plotted. Figure 3 shows that the highest D(An) is obtained for TDGA and the second highest values are for DDA (for Am, Pu), MIDA (for Pu) or DMDPDMA (for U). These diamides contain one or two ether-oxygen (or nitrogen) atoms in the central frame, which may show multidentate features. For U(VI), this cation has a coordination numbers of 4-6 on the equatorial site, and the advantage of tri and tetradentate chelation, if sterically hindered complex formation occurs, might be eliminated. Assuming that the activity coefficient of the respective diamides is almost constant over their concentration range, the slope values in Table 1 are associated with the number of diamide molecules in the extraction reactions. As shown in Table 1, high slope values give relatively high IV,y values, which indicates high extractability. The results of the extraction of p(v) are shown in Table 2. As it is well known to extract poorly, most of the extractants in Division 1 have very low D(p) values(d(p) < 0.4). It could be difficult to extract p(v) completely by these diamide extractants

5 D(Eu) D(Am) D(Pu) TDGA MIDA TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA Extractant concentration/m TDGA MIDA TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA (a) (c) D(U) TDGA MIDA TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA Extractant concentration/ M TDGA MIDA TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA (b) (d) 10-3 Extractant concentration/ M Extractant concentration/m Figure 3. Relationship between D(An) and the extractant concentration for extractants in Division 1. onditions: Diluent; n-dodecane, aqueous solution; 1 M Hl 4, and 25 o. (a): Eu(III), (b): Am(III), (c): Pu(IV), (d): U(VI). Table 1 Analysis of the straight lines in Fig. 3 Eu Am Pu U slope IV,y* slope lv,y slope IV,y slope lv,y TDGA MIDA TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA *: IV,y; intercept values of y axis for the straight lines

6 In order to determine the trend of extractability for all diamides, we calculated the extractant concentration to obtain D(M)= 1 using the straight lines in Figure 3. Both IV,y and the extractant concentration in Tables 1 and 3 are employed to determine the trend of An extractability. The results are summarized in Table 3. Lower extractant concentration to obtain D(M)= 1 means higher extractability. Thus the order is TDGA > DDA > DMDPDMA > TTDGA > TMA > DPA > MIDA PMA TH TXPDA MTPDA for Eu and Am, TDGA > DDA > MIDA > DMDPDMA > TMA > TTDGA > TXPDA > DPA > PMA TH MTPDA for Pu, and TDGA > DMDPDMA > TMA > MIDA > PMA > TTDGA > DPA > DDA > TXPDA > TH MTPDA for U. oxidation states of An is An(IV) > An(VI) > An(III) > An(V). An(III) and (V). As described elsewhere [15-16], the reactivity trend for different Few extractant have high D values for As mentioned above, U, having equatorial coordination sites, is well-extracted by bidentate malonamide-type extractants (DMDPDMA, TMA, PMA). open-ring crown ether structure, has never shown detectable D values. oxygen atoms have a very low complexing ability for An. TH, which has an This suggests that the ether Extractants with two carbonyl and one ketone oxygen atoms situated in the central frame (TXPDA and MTPDA) show low D(An) values, which suggests that there is a significant steric hindrance of the ketone group. Table 2 D values for p(iv) for the extractants D(p) TDGA MIDA TTDGA PMA < 1x10-4 TH 1.4x10-4 TXPDA MTPDA DPA TMA DMDPDMA DDA Aqueous phase: 1M Hl 4, rganic phase: 0.1M extractants/ n-dodecane Table 3 Extractant concentration to obtain D(M)=1 from the results in Fig. 3 Em(III) Am(III) Pu(IV) U(VI) TDGA 8.53x x x MIDA x TTDGA PMA TH TXPDA MTPDA DPA TMA DMDPDMA DDA 6.19x x x Aqueous phase: 1M Hl 4, diluent: n-dodecane

7 From these results, podand-type diamides (TDGA and DDA) have high D(Am, Th), on the other hand bidentate malonamide has a relatively high D(U) values. As a whole, TDGA is the best diamide extractant used in this work. 3.2 Extraction of An(III), (IV), (V), and (VI) by the extractants in Division 2 In the case of extractants in Division 2, nitrobenzene was used as the diluent instead of n-dodecane, due to their low solubility in non-polar organic diluents. The relationships between the D(An) values for phosphine oxides and diamides and the extractant concentration (Figure 2) are shown in Figure 4. The D values for Am(III), Th(IV), p(vi) and U(VI) are plotted in Figure 4(a), (b), (c) and (d), respectively. The values of the slopes of the straight lines in Figure 4 and the intercept values on the y axis (IV,y) are summarized in Table 4. The extractant concentrations to obtain D(M)= 1 were calculated using the straight lines in Figure 4, and the results are summarized in Table 5. The D values of p by MA, SA, MLA, TDGA and DPA are too small to plot in Figure 4(c). Moreover, most of the D(An) values for XA, GLA and TDPA are lower than the detection limit. The slope values in Table 4 are associated with the number of diamide molecules in the extraction reactions. Except for DPA, 2-3 molecules for Am, 1-2 for Th, 1-2 for p and 1-2 for U are associated with their extractions. omparing these values to those in Table 1, a somewhat low molar ratio of An : extractant can be seen in nitrobenzene, which may correlate with the polarity of diluent. It is well-known that nitrobenzene has one of the highest polarities of all organic solvents, whereas n-dodecane is a non-polar hydrocarbon. It could be considered that one : one or one : two complexes (= metal : ligand) might be stable in nitrobenzene, while higher metal : ligand ratios are required for stability in n-dodecane. osphine oxide-type extractants show high D values, due to the strong affinity for P=. Among extractants showing the same six-membered ring (BDPPM, MP and MA) in Figure 2, the order of the D values is BDPPM > MP > MA. This result indicates that the P=/P= (BDPPM) combination in the central frame has high extractability, next is P=/= (MP), then =/= (MA), which suggests that the P= group has a higher complexing ability than =. Using Table 4 and 5, we determined the trend of An extractability for the extractants in Division 2. The trends of An extractability are as follows, BDPPM > DGA > BDPPE > MP > TDGA > MA > DPA > MLA > SA for Am, BDPPM > BDPPE > DGA > MP > TDGA > MA > DPA > MLA > SA for Th, BDPPM > BDPPE > DGA > MP for p and BDPPM > BDPPE > MP > DGA > DPA > MA > TDGA > SA > MLA for U. A similar trend for all extractants between Am and Th can be seen, which is due to their high coordination number (8-12) in similar cubic crystal systems. It is clear that BDPPM, DGA, MP, and BDPPE are strong extractants for all An elements. ompared to all the bidentate diamides, MA, which forms a six-membered ring after complexation, shows higher D values than the diamides having other membered ring formation

8 MA SA MLA DGA TDGA DPA BDPPM BDPPE MP D(Am) D(Th) MA SA MLA DGA TDGA DPA BDPPM BDPPE MP extractant Extractant concentration/ concentration/ M (a) (b) Extractant concentration/ M 10 3 DGA BDPPM BDPPE MP MA D(p) D(U) (d) (c) MA SA MLA DGA TDGA DPA BDPPM BDPPE MP Extractant concentration/ M Extractant concentration/ M Fig. 4 Results of actinide extraction by extractants in division 2 Figure 4. Relationship between D(An) and extractant concentration for extractants in Division 2. onditions: Diluent; nitrobenzene, aqueous solution; 0.1 M al 4 at ph 3 and 25 o. (a): Am(III), (b): Th(IV), (c): p(v), (d): U(VI) Table 4 Analysis of straight lines in Fig. 4 Am Th p U slope IV,y* slope IV,y slope IV,y slope IV,y MA SA MLA DGA TDGA DPA BDPPM BDPPE MP *: IV,y; intercept values of y axis for the straight lines

9 Table 5 Extractant concentration to obtain D(M)=1 from the results in Fig. 4 Am(III) Th(IV) p(v) U(VI) MA x x10-3 SA x10-3 MLA x10-3 DGA 1.5x x x10-5 TDGA 1.2x x x10-3 DPA x x10-4 BDPPM 3.3x10-6 1x x x10-6 BDPPE 1.4x x x x10-6 MP 2.2x x x10-5 Aqueous phase: 0.1M al 4, ph 3, diluent: nitrobenzene 3.3 Effect of the central frame of the extractant on D(An) The effect of the different donor atoms (, S and ) are studied using the D values for TDGA, TTDGA and MIDA. As shown in Table 1 and 3, the order of extraction ability for each donor atom depends on the actinide ions, namely, Am(III): > S >, Pu(IV) and U(VI): > > S. Hard oxygen donor atoms show strong extractability for An(III),(IV),(VI), and soft nitrogen donors have somewhat high D values for Pu(IV) and U(VI). Here, MIDA, having a nitrogen donor atom, gives very high D values of Tc(VII) and Pd(II) [17-18]. Among the bidentate diamides, it appears that MA, with six-membered ring chelation, is stronger than the extractants having other-membered rings. Three malonamide-type extractants(dmdpdma, TMA and PMA) are used in this work (Figure 1), the order of actinide extraction is DMDPDMA > TMA > PMA. MA, having oxapentadecane bonded at the center atom, shows the highest D values, due to the positive effect of the ether oxygen atom in its oxyalkyl group. PMA having a phenyl group, shows the lowest D values. Steric hindrance by the phenyl group in PMA is likely to occur in this structure, which would result in instability in the organic phase and low D values. It is clear from Figure 4 that the D values for BDPPM and BDPPE are higher than MP and MA among the same bidentate ligands with six-membered rings. In addition, BDPPM, BDPPE and MP show a strong effect from aryl substitution [19]. The difference between BDPPM and BDPPE is the presence or the absence of the double bond in the center of the backbones. The double bond leads to a planer structure and a short distance between the two carbon atoms, which show more restriction than an extractant with single bonds. Due to these restrictions, the extractability of BDPPE is lower than BDPPM. Two diphosphine dioxides show higher D(An) values than the podand-type diamide DGA, however the main problem with these extractants is their solubility in n-dodecane. It is necessary to use a modifier in order to get high solubility. In addition, after disposal by combustion, secondary radioactive waste,

10 like phosphate, would be produced. DGA shows the very strong extractability because of the effect of the ether oxygen atoms of the diamides employed here. oncerning this, Turanov et. al. studied novel podand-type diphosphine dioxides and tested them for the extraction of Eu and Am [20-21] It is clear that the extractants having longer carbon chains or oxyalkyl chains connected with two P= groups show relatively low D values. This result is not consistent with those for diamide extractants. 4. onclusion The extractability for diamides and phosphine oxides has been investigated. BDPPM is the best extractant of all examined here, its central frame has two P= groups and it forms six-membered ring with metal ions. The podand-type diamides (TDGA and DDA), which form five-membered rings after multidentate chelation, show relatively high D values for trivalent actinides, and the extractability by TDGA is next to the di-phosphine di-oxides. The phosphine oxides used in this work have poor solubility in n-dodecane. From their chemical structures, increasing their lipophilicity property by attachment of long alkyl groups requires additional organic synthesis. Moreover, phosphine oxide extractants have the disadvantage of producing secondary radioactive waste after disposal by combustion. Among the extractants used in this work, TDGA shows a satisfactory D(An), high solubility in n-dodecane, and an easy synthesis. This extractant has high applicability for trivalent An extraction in HLW. Acknowledgments We acknowledge greatly the staff of Wako-Pure hemical Industries for the organic synthesis and Shigeo Umetani for the preparation of BDPPM and BDPPE, Mrs. T. Masaki, M. Ito and Y. akahara and Dr. H. Suzuki and. Shirasu for the An extraction and analysis. The present study includes the results of Development of mutual separation technology of minor actinides by the novel hydrophilic and lipophilic diamide compounds entrusted to Japan Atomic Energy Agency by the Ministry of Education, ulture, Sports, Science and Technology of Japan (MEXT). References 1) G.M. Gasparini, G. Grossi, Solvent Extr. Ion Exch., 4, 1233 (1986). 2). Musikas, H. Hubert, Solvent Extr. Ion Exch., 5, 877 (1987). 3) L. Spjutth, J.. Lilzenjin, M.J. Hudson, M.G.B. Drew, P.B. Iveson,. Madic, Solvent Extr. Ion Exch., 18, 1 (2000). 4) E.P. Horwitz, D.G. Kalina, L. Kaplan, G.W. Mason, H. Diamond, Sep. Sci. Tech., 17, 1261 (1982). 5) R. hiarizia, E.P. Horwitz, Solvent Extr. Ion Exch., 10, 101 (1992). 6) T.M. Siddall III, J. Inorg. ucl. hem., 25, 883 (1963). 7) Y. Sasaki and S. Tachimori. Solvent Extr. Ion Exch., 20, 21 (2002)

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