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THE NOVEL EXTRACTANTS, DIGLYCOLAMIDES, FORTHE EXTRACTION OF LANTHANIDES AND ACTINIDES INHNO3–n-DODECANE SYSTEMYuji Sasaki a , Yumi Sugo a , Shinichi Suzuki a & Shoichi Tachimori aa Research Group for Separation Chemistry, Department of Materials Science, JapanAtomic Energy Research Institute, Tokai, Ibaraki, 319-1195, JapanPublished online: 15 Feb 2007.

To cite this article: Yuji Sasaki , Yumi Sugo , Shinichi Suzuki & Shoichi Tachimori (2001): THE NOVEL EXTRACTANTS,DIGLYCOLAMIDES, FOR THE EXTRACTION OF LANTHANIDES AND ACTINIDES IN HNO3–n-DODECANE SYSTEM, SolventExtraction and Ion Exchange, 19:1, 91-103

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SOLVENT EXTRACTION AND ION EXCHANGE, 19(1), 91–103 (2001)

THE NOVEL EXTRACTANTS,DIGLYCOLAMIDES, FOR THE

EXTRACTION OF LANTHANIDESAND ACTINIDES IN HNO3–n-DODECANE

SYSTEM

Yuji Sasaki, Yumi Sugo, Shinichi Suzuki,and Shoichi Tachimori

Research Group for Separation Chemistry, Departmentof Materials Science, Japan Atomic Energy Research

Institute, Tokai, Ibaraki 319-1195, Japan

ABSTRACT

The novel extractants, six diglycolamides synthesized in ourlaboratory, were investigated for actinide extraction from nitricacid into n-dodecane. The dependence of the distribution coef-ficients of Eu(III) and Am(III) on the length of alkyl chain inthe extractants was examined. Among the diglycolamides stud-ied, N ,N ,N ′,N ′-tetraoctyl-3-oxapentanediamide (TOOPDA) andN ,N ,N ′,N ′-tetradecyl-3-oxapentanediamide (TDOPDA) showeda sufficient solubility in n-dodecane, due to their appropriate lipophi-licity modified by increasing the length of the alkyl chain attachedto amidic N atoms. The distribution coefficients, DM, of actinidesobtained by TOOPDA increased with an increase in HNO3 concen-tration. The number of diglycolamide molecules coordinated to the

Address correspondence to Yuji Sasaki.

91

Copyright C© 2001 by Marcel Dekker, Inc. www.dekker.com

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actinide ions was estimated to be three for Th(IV), U(VI), Pu(IV),and four for Am(III) and Cm(III) by slope analysis. The order ofDM for the actinides at high nitric acid concentrations is An(III),An(IV) > An(VI) > An(V) and that for lanthanides, heavier >

lighter. It is concluded that TOOPDA diluted in n-dodecane canextract both actinides and lanthanides completely from aqueousHNO3 solution.

INTRODUCTION

The development of the novel extractants for the trivalent actinides is in-dispensable to the chemical treatment of the high-level radioactive liquid waste(HLW). The neutral bidentate ligands, octyl(phenyl)-N ,N -diisobutylcarbamoyl-methylphosphine oxide (OφD(iB)CMPO) (1), diphenyl-N,N -di-n-butylcarbamoy-lmethylphosphine oxide (DφDBCMPO) (2), and dimethyldibutyltetradecylmalon-amide (DMDBTDMA) (3), have been proposed as extractants for the recovery ofactinides directly from the HLW and have been examined thoroughly for the ex-traction of not only actinides but also fission products. The diamide derivativespossessing a good extractability are advantageous for the treatment of the HLW,because they are completely incinerable and produce no radioactive solid wasteby combustion.

Stefan et al. (4,5) reported the extraction of various metal ions with multi-dentate amide podands; Yao et al. (6) and Sasaki and Choppin (7,8) investigated theextractions of lanthanides(III) and actinides with diglycolamides, which have anether oxygen between two amide groups. These preceding studies have focused onthe extraction of the metal ions from aqueous solutions of pH range between 1 and4. Narita et al. (9) studied the lanthanide(III) extractions from acidic solutions ofHCl and HNO3 into chloroform by a diglycolamide having two phenyl groups, andproved the tridentate coordination of the diglycolamide to the lanthanides not onlyin solid form but also in solution by XRD and EXAFS studies (10). As an alkanediluent has usually been utilized in liquid–liquid extraction processes involvingthe HLW, n-hexane and n-dodecane, together with some polar solvents, have beenused in the present investigation to assess the applicability of the extractants. In thiswork, diglycolamides having the same central frame but different alkyl chains at-tached to amidic N atoms were synthesized. The existence of the long alkyl chain inan extractant may decrease the polarity of its metal complex and increase the stabil-ity in the nonpolar solvents. After confirming the solubility of the extractants in thesolvents in question, the extraction of actinides and lanthanides from nitric acid wasinvestigated.

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EXTRACTION WITH DIGLYCOLAMIDES 93

EXPERIMENTAL

Chemicals

The reagents used in the organic synthesis were commercially available.Standard solutions of lanthanides for atomic absorption spectrometry (Wako) wereemployed. The radioactive tracers of 233U, 237Np, and 238Pu were obtained fromSceti Company, Ltd., and 230Th, 241Am, 244Cm, and 252Cf were from IsotopeProducts Laboratories. The nitric acid and the organic diluents were of analyticalgrade.

Organic Synthesis

The diglycolic anhydride and one of six kinds of amine, (1) dipropylamine,(2) dibutylamine, (3) diamylamine, (4) dihexylamine, (5) dioctylamine, or (6) dide-cylamine, were reacted with dicyclohexylcarbodiimide in ethyl acetate, followingthe procedure in Refs. (7) and (8). After conclusion of the organic reactions, thesynthesized products dissolved in ethyl acetate were separated from the precipi-tates, and the solvent was evaporated. The sample liquids were heated at 300◦Cunder vacuum, volatilizing the byproducts and leaving materials desired in theflask. The extractants were purified by the silica gel column chromatography morethan twice. After purification, the sample was identified by NMR (UNITY plus 400,Varian) and GC–MS (TurboMass, Perkin Elmer, Inc.) spectrometers. The generalstructures of the products synthesized were R,R–N-CO-CH2-O-CH2-CO-N–R,R(R = CnH2n + 1, 3 � n � 10). They are designated as N ,N ,N ′,N ′-tetrapropyl-3-oxapentanediamide (TPOPDA), N ,N ,N ′,N ′-tetrabutyl-3-oxapentanediamide(TBOPDA), N ,N ,N ′,N ′-tetraamyl-3-oxapentanediamide (TAOPDA), N ,N ,N ′,N ′-tetrahexyl-3-oxapentanediamide (THOPDA), N ,N ,N ′,N ′-tetraoctyl-3-oxapentanediamide (TOOPDA), and N ,N ,N ′,N ′-tetradecyl-3-oxapentanediamide(TDOPDA) for the products prepared by the combination of diglycolic anhydridewith amine (1), (2), (3), (4), (5), or (6).

In the H NMR spectrum of TPOPDA, the signal peaks were triplets at 0.92,1.56, 3.18, and 3.27, and singlet at 4.39 ppm; these peaks were assigned to theprotons of N-CH2-CH2-CH∗

3, N-CH2-CH∗2-CH3, N-CH∗

2-CH2-CH3 and O-CH∗2-

CO in the structure of O(-CH2-CO-N(-CH2-CH2-CH3)2)2. In the case of othermaterials, the signals appeared at the same ppm as those in TPOPDA. Com-plex peaks with two, three, four, six, and eight times the integrated intensity ofTPOPDA were found around 1.2–1.6 ppm, which corresponds to the integrals of theprotons in O(-CH2-CO-N(-CH2-C(n−2)H∗

2(n−2)-CH3)2)2 for TBOPDA, TAOPDA,THOPDA, TOOPDA, and TDOPDA, respectively. Results obtained by GC-MS

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give the amounts of impurities coexisiting in the synthesized diglycolamides: itwas obvious that these are less than 5%.

Preparation of the Oxidation State of Actinides

The neptunium and plutonium ions were prepared as follows: Np ion wasadded to less than 0.5 mL concentrated HClO4 and the Np-HClO4 solution wasevaporated to dryness. Dissolving the residue in 0.01 M HClO4, the valence ofNp was (VI). In order to reduce the oxidation state from Np(VI) to Np(V), NOx

gas, generated from the dissolution of Cu metal by HNO3, was introduced into thesample solution during 2–3 min.

The plutonium ion was fumed with HClO4. Evaporating to dryness and dis-solving Pu by the use of 1 mL of 1 M HNO3, the oxidation state of Pu was then(VI). The Pu was reduced to (III), by adding 1 mL of 1 M NH2OH·HNO3 intothe Pu solution, and the solution was kept at 60◦C for 1 h. By introduction of1 mL of 1 M NaNO2 into the Pu(III) solution, Pu(IV) was prepared and kept in7 M HNO3 to prevent disproportionation to other oxidation states. After prepara-tion of Np(V) and Pu(IV), these solutions were assayed by UV–Vis spectroscopy(V-570, JASCO Corp.). All the preparation methods followed those given inRefs. (11,12).

Extraction Procedure

Three milliliters of the organic solvent containing 0.001–0.2 M diglyco-lamide pre-equilibrated with the same volume of the nitric acid of desired con-centration without tracers or metal ions. After centrifugation, 2 mL of the pre-equilibrated organic phase was mixed with 2 mL of the same concentration ofnitric acid and spiked with 10 µL of radioactive tracer solution or nonradioactivelanthanide solution. The concentrations of metal ions (in ppm) used in each ex-traction experiment were 10 for lanthanides, 5 × 10−2 for Th, 0.1 for U, 1 for Np,6 × 10−5 for Pu, 3 × 10−4 for Am, 1 × 10−5 for Cm, and 2 × 10−6 for Cf. Themixture was shaken mechanically for 2 h at 25 ± 0.1◦C. After centrifuging andseparating both phases, duplicate 0.50 mL aliquots were analyzed by NaI scin-tillation counting (COBRA 5003, Packard Instrument Company) for the gammaactivities of 152Eu and 241Am, or liquid scintillation counting (Tri-Carb 1600 TR,Packard Instrument Company) for the alpha activities of 230Th, 233U, 237Np, 238Pu,244Cm, and 252Cf in 5 mL of PICO-AQUA cocktail. The amounts of lanthanides inthe sample solutions prepared from the aqueous and the organic phases were mea-sured by ICP-MS (VG-PQ �, Fisons). All extraction experiments were repeatedmore than twice for each condition.

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EXTRACTION WITH DIGLYCOLAMIDES 95

RESULTS AND DISCUSSION

Solubility of Diglycolamides in the Organic Diluents and Water

Nakamura and Miyake (13) studied the solubility of malonamide insome diluents, and they reported low solubility in the non polar solvents.In this work, the dissolution of synthesized diglycolamides was tested using fourkinds of solvents, chloroform, toluene, n-hexane, and n-dodecane. TPOPDA,TBOPDA, TAOPDA, and THOPDA are only slightly soluble in n-hexane andn-dodecane, and their dissolved solutions were not clear. The reason of the lowsolubilities in such solvents is the high polarity of these organic compounds.The carbon/oxygen (C/O) atom ratio, which related to lipophilicity, in themolecules of TPOPDA, TBOPDA, TAOPDA, and THOPDA is 5.3, 6.7, 8, and9.3, respectively. As these values are lower than those of the extractants men-tioned above, OφD(iB)CMPO: 12 and DMDBTDMA: 13.5, TPOPDA, TBOPDA,TAOPDA, and THOPDA are not applicable to HNO3–n-dodecane extractionsystem.

The solubilities in mM , of the six diglycolamides in water are 57 for TPOPDA,2.3 for TBOPDA, 0.27 for TAOPDA, 0.11 for THOPDA, 0.042 for TOOPDA, and0.042 for TDOPDA. These values also support the fact that the diglycolamideswith shorter alkyl chain attached to amidic N atoms are more hydrophilic. Onthe other hand, TOOPDA and TDOPDA, which show low solubility in water,can be dissolved in n-dodecane satisfactorily. Gasparini and Grossi (14) sug-gested that an amide with more than 12–13 carbon atoms will not form a thirdphase. TOOPDA and TDOPDA, having 36 and 44 carbon atoms respectively,did not form a third phase even when equilibrated with an aqueous solution of6 M HNO3, despite the presence of three polar oxygen atoms in each extractantmolecule.

Extraction of Eu(III) and Am(III) with Different Diglycolamides

The distribution coefficients, DM, of Eu(III) and Am(III) in the system of1 M HNO3/0.1 M diglycolamide/diluent are shown in Figure 1 for several diluents,as a function of the length of amidic alkyl chain (n). Apparently, the DM valuesgradually decreased from TPOPDA (n = 3) to TDOPDA (n = 10), showing thatthe longer alkyl chain suppresses the extraction. There are few papers concerningthe effect of the length of alkyl chain in monoamide and diamide derivativeson distribution of actinides (15–18): the correlations, however, were not definedclearly. As shown in Figure 1, our results have the correlation between the lengthof n, which corresponds to the hydrophilic property of diglycolamide, and DEu

and DAm.

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Figure 1. The log D values obtained using the following conditions: 1 M HNO3 and0.1 M diglycolamide, O(-CH2-CO-N(-CnH2n+1)2)2 (n = 3, 4, 5, 6, 8, 10) in chloroform (a),toluene (b), n-hexane (c), and n-dodecane (d). �: Eu(III), �: Am(III). The diglycolamides,O(-CH2-CO-N(-CnH2n+1)2)2, of n = 3, 4, 5, 6, 8, and 10 correspond to TPOPDA, TBOPDA,TAOPDA, THOPDA, TOOPDA, and TDOPDA, respectively.

Effect of Diluents on DM

As shown in Figure 1, n-hexane and n-dodecane provided higher DM valuesthan chloroform or toluene. To clarify the effect of diluent, the DAm and DEu

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Table 1. DM Values of Eu(III) and Am(III) in the Extraction System0.1 M TOOPDA and 1 M HNO3 for Various Organic Diluents

Organic Solvent DEu DAm

Nitrobenzene >500 2201,2-Dichloroethane 34 9.91-Octanol >500 81Ethyl acetate >500 >500Chloroform 0.097 0.12Diethylether 300 100Benzene 0.83 0.39Toluene 0.79 0.3Tetrachloromethane 0.39 0.095n-Dodecane 265 30n-Hexane >500 33

values were measured for 11 selected diluents, under the same conditions. Theresults are summarized in Table 1. The DM values are high with nitrobenzeneand 1-octanol, which have high dielectric constants, but low with the chlorinatedand aromatic solvents. Horwitz and Schulz, and Horwitz et al. (19,20) studiedthe DAm values with several diluents in the system of 0.2 M CMPO–0.25 MHNO3, and reported that DAm was higher with n-dodecane than with toluene orCCl4. Our results are in good agreement with their results for CMPO. Aromaticor halogenated solvents have high accepter numbers and they may react with thedonor oxygen of diglycolamide, which results in the decrease of the activities ofextractants.

Dependence of DM of Actinides (Ans) on Nitric Acid Concentration

The extraction behaviors of Th(IV), U(VI), Np(V), Pu(IV), Am(III), andCm(III) with 0.1 M TOOPDA/n-dodecane as a function of nitric acid concentra-tion are illustrated in Figure 2. The log DM values of all actinides increase withan increase in HNO3 concentration, which indicates that the anion, NO−

3 , plays animportant role in this extraction system. The increase of DM is steady even at theHNO3 concentration of 6 M . A similar phenomenon was reported by Narita et al.(9) in the system of N ,N ′-dimethyl-N ,N ′-diphenyl-diglycolamide/chloroform,where the DM of lanthanides(III) increased up to 8 M HNO3 or HCl. DM ofPu(IV) shows an unexpected tendency, the slope is approximately unity, anda plausible reason might be the effect of hydrolysis of Pu(IV) in a low-acidicregion.

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Figure 2. Extraction behavior of Th(IV), U(VI), Np(V), Pu(IV), Am(III), and Cm(III)with TOOPDA against HNO3 concentration.

Extraction Mechanisms of Ans with Diglycolamide

From the neutral character of the diglycolamide and the strong dependency ofDM on HNO3 concentration, the main extraction reaction of Mn+ by diglycolamideis expressed as

Mn+ + n(NO−3 ) + aDA ⇀↽ M (NO3)n(DA)a (1)

where Mn+ is an actinide or a lanthanide ion and DA is diglycolamide. The ex-traction equilibrium concentration constant, Kex, in reaction (1) is described as

Kex = [M(NO3)n(DA)a]/[Mn+][NO−3 ]n[DA]a (2)

The concentration terms, [M(NO3)n(DA)a], [Mn+], [NO−3 ], and [DA], in Equa-

tion (2) are given in molarity. The distribution coefficient, DM, is defined as

DM = [M(NO3)n(DA)a]/[Mn+] (3)

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By substituting Equation (3) into Equation (2), and converting Equation (2) intothe logarithmic form, Equations (4) and (5) are obtained:

log Kex = log DM − a log [DA] − n log [NO−3 ] (4)

log DM = log Kex + a log [DA] + n log [NO−3 ] (5)

To determine the stoichiometric number, a, in reaction (1) by slope analysis, the DM

values of actinides at the different TOOPDA concentrations and a constant aqueous1 M HNO3 were measured and are plotted in Figure 3. Here, the log Kex value inEquation (5) was assumed to be constant in these extraction conditions. The slopesof these lines of Figure 3 were determined by linear regression, and the log Kex

value was obtained from the intercept according to Equation (4). The values of aand log Kex are summarized in Table 2. The a values indicate the average numberof diglycolamide molecules coordinated to each metal ion. The principal extraction

Figure 3. Plots of log DM of Th(IV), U(VI), Np(V), Pu(IV), Am(III), and Cm(III) versuslog [TOOPDA].

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Table 2. log Kex and a Values of Actinide Ions in the ExtractionSystem 0.1 M TOOPDA/n-Dodecane/1 M HNO3

Element log Kex a

Th(IV) 5.55 ± 0.04 3.39 ± 0.02U(VI) 2.9 ± 0.2 3.1 ± 0.2Np(V) 0.81 –Pu(IV) 4.0 ± 0.4 2.8 ± 0.3Am(III) 5.3 ± 0.1 3.75 ± 0.07Cm(III) 6.0 ± 0.2 4.2 ± 0.1

Figure 4. log DM versus atomic number of Ln and An under 0.1 M TOOPDA/n-dodecane–HNO3 system. 0.5 M HNO3: open symbols, 1.0 M HNO3: closed symbols.

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Table 3. DM Values of Ln(III) and An(III), and SFConcerning Am (DM/DAm) in the Extraction System0.1 M TOOPDA/n-dodecane–1 M HNO3

Element DM DM/DAm

La(III) 5.3 0.18Eu(III) 265 8.8Lu(III) 631 21Th(IV) 147 4.9U(VI) 0.8 0.027Np(V) 0.0056 0.00020Am(III) 30 1.0Cm(III) 78 2.6Cf(III) 156 5.2

species are estimated to be Th(DA)3(NO3)4, UO2(DA)3(NO3)2, Pu(DA)3(NO3)4,Am(DA)4(NO3)3, and Cm(DA)4(NO3)3. From the log Kex values in Table 2, theorder of extractability from 1 M HNO3 is An(III), An(IV) > An(VI) > An(V). Theoverwhelming extraction of An(III) is peculiar to the diglycolamide. Other extrac-tants such as 2-thenoyltrifluoroacetone, di-2-ethylhexylphosphoric acid, tributylphosphate, OφD(iB)CMPO, DMDBTDMA, and amidic and maronamidic deriva-tives show order of extractability of An(IV) > An(VI) > An(III) > An(V) orAn(VI) > An(IV) > An(III) > An(V) (1,3,14,17,19,21,22).

The log DM of trivalent lanthanides (Lns) and Ans are plotted against theatomic number of these elements in Figure 4. The values of log DM increasesgradually with an increase in atomic number of Lns and Ans. Turanov et al.also found an ascending pattern with phosphoryl-containing podands (23). Thisascending pattern is just opposite to or different from those obtained by otherbidentate ligands, for example, tetra(p-)tolylmethylene diphosphine dioxide (24),diamide (25), or CMPO (26–28).

The DM values of Lns and Ans and separation factor (defined as DM/DAm)with respect to Am(III) in the extraction system of 1 M HNO3 and 0.1 M TOOPDA/n-dodecane are shown in Table 3. The separation factors are not high enough toapply to either the mutual separation of Ans(III) or Lns(III) or the group separationof Ans(III) from Lns(III).

CONCLUSIONS

Six diglycolamides having different amidic alkyl chain lengths were syn-thesized and examined for the extraction of Ans and Lns from HNO3. The effectof the chain length showed that shorter chain increases the extractability. However,

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the extractants with longer chain, N ,N ,N ′,N ′-tetraoctyl-3-oxapentanediamide(TOOPDA) and N ,N ,N ′,N ′-tetradecyl-3-oxapentanediamide (TDOPDA), showedbetter solubility in n-dodecane and satisfactory extractabilities toward An(III),(IV), (VI), and Ln(III) from highly acidic solutions. The change of bidentate di-amide to the tridentate diglycolamide causes drastic variation in the log DM versusatomic number pattern of lanthanides. More detailed work to elucidate these pat-terns is underway. The diglycolamide developed seems to be a candidate extractantfor the partitioning of HLW.

ACKNOWLEDGMENTS

The authors gratefully acknowledge to Drs. H. Naganawa, T. Yaita, andH. Narita of Japan Atomic Energy Research Institute for helpful discussions, andMr. H. Takeishi of Japan Atomic Energy Research Institute for the preparation ofNp and Pu solutions.

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Received February 25, 2000

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