- Email: [email protected]
Phosphine oxide and quaternary ammonium extraction of americium(III) from concentrated chloride solutions
J inorg, nuct Chem.. 1976, Vok 38, pp. 155-159. Pergamon Press. Printed in Great Britain
PHOSPHINE OXIDE AND QUATERNARY AMMONIUM EXTRACTION OF AMERICIUM(III) FROM CONCENTRATED CHLORIDE SOLUTIONS* H. D. HARMON? and J. R. PETERSON Department of Chemistry, University of Tennessee. Knoxville, TN 37916, U.S.A. and Oak Ridge National Laboratory and W. J. McDOWELL and C. F. COLEMAN Chemical TechnologyDivision,Oak Ridge National Laboratory, Oak Ridge,TN 37830. U.S.A.
(Received 14 February 1975) Abstract--The extraction of Am(Ill) and Eu(lll) by tri-n-octylphosphineoxide (TOPO) and methyl trialkyl(C~-C,,) ammonium chloride (Adogen 464) from HC1 and slightly acidic LiC1 solutions was investigated. Linear extraction isotherms showed that macro Eu(III) species exist with the same degree of association in both organic and aqueous phases suggestingthe same behavior for the actinide analog Am(Ill). Reagent dependences indicated the extraction of primarily AmCL.TOPO and AmCL.3TOPO from 1 and 5 M LiCk respectively, and the extraction of only R4NAmCL (where I~N + is the quaternary ammonium cation of Adogen 464) from both I and 5 M LiCI. However. detailed studies of the TOPO-LiCI and Adogen 464-LIC1 systems showed wide variations in lithium and water extraction as a function of aqueous LiCI molarity, so that these systems are not suitable for investigatingthe aqueous chloride complex equilibria.
INTRODUCTION
THE tri-n-octylphosphine oxide (TOPO) extraction of trivalent actinides has been studied from HCI[1,2], HNO3[I-4], NH4NO~[1] and NaNO313,4] solutions. Tertiary amine extraction of trivalent actinides has been reported from various chloride solutions. Tri-isooctylamine (TIOA)[5-8] and trilaurylamine (TLA)[9, 10] have undergone extensive study as actinide extractants, while several other tertiary amines have undergone more limited investigations[5-7, 11]. In addition, trivalent actinide extraction by quaternary ammonium salts in the thiocyanate[12-14] and nitrate[15] systems has been reported. There is, however, very limited information on the reactions and mechanisms included in the extraction of trivalent lanthanides and actinides from concentrated chloride solutions, particularly at known or reasonably constant HCI activity. Such studies are greatly hindered by the severe lack of information regarding chloride complexes of these trivalent elements in concentrated chloride solutions. Previous investigations of actinide(III) chloro complex formation in solutions with ionic strengths < 4 M were summarized in an earlier publication[16] and, in addition, some studies of actinide(III) chloro complexes in LiCI and HCI solutions have been made using anion exchange [17], tertiary amine extraction[17], spectrophotometric[18], and paper electrophoresis[19] techniques. However, none of these have resulted in understanding of the chloro complex formation of either the trivalent lanthanides or actinides *Research sponsored by the Energy Research and Development Administration under contract with the Union Carbide Corporation. +National Defense Education Act Title IV Fellow, University of Tennessee, Knoxville. Present address: Savannah River Laboratory, E. I. du Pont de Nemours & Co., Aiken, SC 29801. U.S.A.
in very concentrated chloride solutions, above ~5 M. The purpose of this study was to gain information concerning the nature of the extraction of trivalent lanthanides and actinides from concentrated chloride systems by TOPO and Adogen 464 and to assess their possible utility for studies of aqueous chloride complex equilibria, par~Iicularly at high chloride concentrations.
EXPERIMENTAL
Materials Organic extractants. Stock solutions of TOPO and Adogen 464 were prepared by dissolving the required amount of each material in p-xylene. The TOPO was purchased from Eastman Organic Chemicals, and Adogen 464 was purchased in the chloride form from the Ashland Chemical Company; both were used without further purification. Aqueous solutions. Lithium chloride (99.9% purity) was obtained from the Y-12 Plant of Union Carbide Corporation. Nuclear Division, and was recrystallized twice from water before use. A stock solution of -10M LiCI was prepared and standardized by the Mohr method[20]. Solutions of lower concentration were prepared by quantitative dilution and were adjusted to a pH of 1 by the addition of reagent-grade HCI. Solutions of HC1 from 1 to 10 M were prepared by quantitative dilution of the concentrated reagent. The EuC13 solution was prepared from Eu(NO~), which was obtained from the Michigan Chemical Corporation with 9Y.9% purity. The Eu(IIl) was precipitated with NH4OH and subsequently was converted to EuCI~ by dissolution in HC1. Tracer solutions. The 2"'Am and '~4Eu tracer solutions were obtained from lhe ORNL Isotopes Division and were diluted to the appropriate level (-10' cpm/ml). The tritiated water solntion was supplied by the New England Nuclear Corporation. Equilibration procedure. Although the equilibration procedure varied slightly in the different extraction experiments, the basic steps were as follows: volumes of 10 ml of the organic extractant solution were preequilibrated with equal volumes of the desired aqueous chloride solution. Then 5 ml of the preequilibrated organic solution and 5 ml of the aqueous solution were pipetted 155
156
W.J. McDOWELL et al.
into screwcapped vials. After 0.1 ml of the 2"~Amtracer solution had been added, the phases were equilibrated on a verticalrotating mixer for 1 hr at room temperature. Separate experiments had established that 1 hr was more than adequate to ensure that equilibrium had been attained. The vials were centrifuged to effect phase separation, providing this was necessary for clean phase separation after prolonged standing. Finally, samples (0.1 ml) of each phase were withdrawn and analyzed by liquid scintillation counting. Liquid scintillation counting. The organic- and aqueous-phase samples from the above extractions were dissolved in a scintillation liquid which contained 80g of naphthalene, 5g of 2,5-diphenyloxazole (PPO), and 50 rng of bis[2-(5phenyloxazolyl)]benzene (POPOP) dissolved in a diluent mixture consisting of 385 ml of p-xylene, 385 ml of dioxane and 230 ml of ethanol. All these chemicals were either reagent or scintillation grade. The alpha activity of Z4~Amin each sample was measured on a Packard Tri-Carb Liquid Scintillation Spectrometer (Model No. 3214) with appropriate window settings to include the alpha peak and to minimize background. Water extraction. The extraction of water by 0.5 M TOPO and 1.0 M Adogen 464 in p-xylene from aqueous LiC1 solutions was studied by employing a tritium tracer technique. Aliquots (0.1 ml) of the tritium solution (-109 cpm/ml) were added to aqueous LiCI solutions, and the tritium beta activity of each solution was carefully measured by a standard liquid scintillation procedure. Then the solutions were equilibrated with the organic phases, and samples (0.1 ml) of each organic phase were analyzed for tritium beta activity. Quenching corrections were made on both the initial aqueous- and organic-phase samples. The weights of water and LiCI in the aqueous solutions were determined by density measurements and by chloride analyses utilizing the Mohr method [20]. Lithium extraction. The extraction of lithium by 0.5 M TOPO and 1.0M Adogen 464 was examined from 0.5 and 10M LiCl solutions both in the presence and absence of 0.0125 M EuCI3. The extractions were carried out according to the procedure outlined earlier. The organic phases were analyzed for lithium via flame photometry by the staff of the Analytical Chemistry Division of ORNL. The extent of Eu(III) extraction was determined with ~"Eu tracer in extraction experiments identical to, but separate from, those carried out for lithium analysis. The organic-phase Eu(III) concentration was calculated from the results of Eu(III) extraction measurements and the initial aqueous EuCI3 concentration. Eu(III) isotherm study. Aqueous solutions of 5 M LiCI were prepared with EuCI3 concentrations varying from 2× 10 6 to 1× 10-'M and with '~4Eu tracer (~10~cpm/ml). Separate 5-ml aliquots of each aqueous solution were equilibrated, as described above, with equal volumes of 1.0 M Adogen 464 and 0.5 M TOPO, each in p-xylene. Samples (l ml) of each phase were withdrawn for gamma-activity analysis. The concentration of Eu(III) in each phase was calculated from results of the extraction measurements and from the initial aqueous EuCI3 concentration. Reagent dependence o[ Am(III) extraction. Extraction of Am(Ill) from 1 and 5 M LiC1 solutions was carried out as described previously, with TOPO solutions of 0.01-0.5 M and Adogen 464 solutions of 0.02-1.0M, all in p-xylene. Samples (0.2ml) of each phase were analyzed by liquid scintillation counting as described above. RESULTS AND DISCUSSION
Theory of neutral species extraction. The extraction coefficient of a metal ion M c÷ by a neutral species extractant is given by E -
- [MCI
~
EC~ c
c
] o r g /~__N° [MCI n c - .
.
.
.
.n
]a q
,
(1)
where ft, = [MCI, c "]/[MC+][C1-]", E c is the extraction
coefficient of the neutral species MCIc, and N is the number of ligands in the most highly coordinated complex[21]. In systems where the ionic strength is varied considerably, the mean ionic activity of the chloride salt act may be substituted for [CI ] in eqn (1), and the 13, values become "effective stability constants" as defined by/3* = [MCl,C-"]/[MC+][ao]" [22]. It can be shown from eqn (1) that the extraction of a metal ion which forms aqueous cationic, neutral, and anionic complexes should exhibit a plot of IogE vs log [CI-] with an "inverted-U" shape. A region of positive slope results at low ligand concentrations, where the metal ion is predominantly uncomplexed. A maximum, or plateau, is seen where the neutral complex predominates, and a negative slope is found at ligand concentrations where anionic complexes exist in high concentrations [21]. Am(III) extraction results. The results of Am(III) extraction from HC1 and LiCI solutions are shown in Fig. 1 as plots of the log of the extraction coefficient, E, vs the log of the mean ionic activity of LiC1 or HCI. The necessary activity coefficients and density data required for the calculation of the activities were obtained from standard sources[23-26]. The curves for TOPO and Adogen 464 extractions of Am(III) from HC1 solutions exhibit the previously discussed "inverted-U" shape, but the maxima are not at the same HCI activity. Thus, it is probably not valid to interpret these maxima as indicating the existence of neutral and anionic complexes. As concluded in previous TOPO-HNO3 extraction studies[2], it is likely that the decrease in extraction at high HCI concentrations is due to competition between HCl and Am(III) for the organic extractant. The Am(III) extractions by TOPO and Adogen 464 from LiCI solutions were much greater than from HCI, and these extraction curves do not show any decreasing portion to suggest significant concentrations of neutral or anionic aqueousphase complexes. However, results of additional studies (described below) carried out on the TOPO and Adogen 464 systems suggest that these extraction data cannot be used to indicate the type of complex species present in the aqueous phase. Reagent dependence studies. The extraction of a trivalent actinide, M 3+, by a neutral species extractant such as TOPO can be expressed as 3+
M(~ + (n - 3)H~a)+ n Cl~a~+ yTOPO~o) = MC1, H,-3"yTOPO~o),
(2)
where n is the number of CI ions in the extracted species, y is the number of extractant molecules associated with each M 3÷ species, the subscripts a and 0 represent the aqueous and organic phases, respectively, and possible polymerization is neglected. The concentration quotient Q for this reaction can be written as [MCI, H, 3' y TOPO]o Q = [M3+], [H+]o" 3[C1-]," [TOPO],~ E = [H+]a"-3[C1-]o"[TOPO]J '
(3)
where the extraction coefficient E = [MCI, H,_3-yTOPO]o/[M3+]a. When [H÷]~ and [C1-] are held constant, it can be shown from eqn (3) that a plot of log E vs log[TOPO] should have a slope of y. For equilibrium methods, Adogen 464 chloride can be considered a neutral species extractant and, therefor-e, can also be treated according to eqn (2)[11, 27].
157
Phusphine oxide and quaternary ammonium extraction of americium(Ill) - - ] ] -T ....
I
Iil
I
7-T
.........
1o~ L
t i
I I
,
2 i
'
! i
i
I
~0°5 L-
t I !1111 ~
~
~ - ~
•
z; ADOGEN 4 6 4 - L C • TOPO - HCI • ADOGEN 464 HC 2
1:? } F
•
~0- t
A /
¢D
g x
J
i¢d i
t0 2 5
•
*0 3 i
5 •
• 5.O'M L:CI •tO-M LIC!
•
I
~d ~
2 i
i
/
"
]
•
: lii:
_~.__LIi !
~o 4 I0 2
2
~0-t
5
2
5
~__~LAt
t00
2
5
~0~
,5,DOGEN 4.64 MOLARITY
Fig. 2. The dependence of Am(IIl) extraction on the concentration of Adogen 464 in p-xylene at 1.0 and 5.0 M LiC1.
r
I
~dsI ~Gz
t lll,ltil
lo-1
i lll;ilh Jllilll , IIII[UL~i ~oo Io 1 ~oz MEAN IONIC ACTIVITY OF LICI OR HCI
I Jill ~o3 2
Fig. l. The extraction of Am(Ill) as a function of the mean ionic activity of LiC1 or HC1.
! [111111
I
I IIIIIII
!
2_-
5 --
Plots of log E vs log [extractant] for the extraction of Am(lII) from 1.0 and 5.0M LiCI by Adogen 464 and TOPO are shown in Figs. 2 and 3, respectively. The slopes of 1.0 in Fig. 2 indicate that the extracted species is R4NAmC14, where R4N ~ represents the quaternary ammonium cation. This result is consistent with quaternary ammonium reagent dependence studies of Am(IIl) extraction in the SCN- system[13,14] and in the NO, system[15], where values of y = 1.0 were also obtained. With tertiary amines, complexes containing two molecules of alkylammonium salt per metal atom have been reported in the extraction of actinides from concentrated aqueous chloride solutions [5-7, 27]. The value of y found for the TOPO extraction was not constant over the 1-5 M LiCI concentration range, Nonintegral values of 1.2 and 2.7 were obtained with 1.0 and 5.0M LiCI, respectively. Such nonintegral values may indicate either the simultaneous existence of species with different y values or variations in the organic-phase activity coefficients, or both. However, these results do suggest that the species extracted from 1.0 and 5.0 M LiCI are predominantly AmCh.TOPO and AmC13.3TOPO, respectively. As the water activity is lowered by increasing the LiC1 concentration from 1 to 5M, it is possible that the water coordination of the extracted Am(III) complex decreases with a concomitant increase in TOPO coordination. The AmC13.3TOPO species is in agreement with y values of - 3 found for trivalent actinide extraction by TOPO in the NO3 system[l, 3]. Eu(IH) extraction isotherm. The variation of Eu(IlI)
Z 2 400
= _Z _ -
5
~, _~ 2 ~ t0 -t ~, 5 g r,='~ 2 n
~0-2 5
,~
ITI
_=
&
/
--
2 tO - 3 5 L 2
--
PE = 2 . 7
/
/
/
t0 -2
LiCI
II 4. O-M
LiCI
A ~OPE = ~ 2
/ [ I 1~411H
~0 - 4
• 5.O-M
I I IIIlill
t0 -~
i t0 0
IiL t0 ~
TOPO M O L A R I T Y
Fig. 3. The reagent dependence of Am(llI) extraction by TOPO solutions in p-xylene from 1-0 and 5.0 M LiC1.
158
W. J. McDOWELLet al.
extraction from 5 M LiCI with varying aqueous Eu(III) concentration was studied to determine whether the extracted Eu(III) species, and thus the Am(Ill) species by analogy, has the same degree of association in the aqueous and organic phases. Europium(Ill) extraction coefficients were determined over the Eu(III) concentration range of 2× 10-6 to 1 × 10-'M, and the equilibrium concentration of Eu(III) in each phase was calculated. Plots of log [Eu(III)]o vs log [Eu(III)], for the extraction by 1.0 M Adogen 464 and 0.5 M TOPO are given in Fig. 4. Each of the extraction isotherms in Fig. 4 has a slope of 1.0, which indicates that the Eu(III) species have the same degree of association (presumably monomeric) in both the aqueous and organic phases. 164 ~_ 5
I
I [ [11111
] i I IIII1~,,~1
Z~ t.0-M ADOGEN 464
I I II1~
/ 6 ¢
/a
~d 5
_
~
5
O
"
166 _z
z
-510-7
I 2
I
I IIIIII 5 tO-6
I 2
I
I [11111 5 10-5
I 2
]
I IIIJ 5 10-4
AQUEOUS Eu(III ) MOLAR ITY
Fig. 4. Extraction isotherms for Eu(III) extraction from 5 M LiCI by 1.0 M Adogen464 and 0.5 M TOPO, both in p-xylene. Water extraction. In order to utilize the TOPO and
Adogen 464 systems in studies of aqueous complex equilibria, the organic-phase composition must remain reasonably constant. Therefore, the variation in the extraction of water by 0.5 M TOPO and 1.0 M Adogen 464, both in p-xylene, was examined as a function of aqueous LiCl concentration. The results, in terms of moles of water extracted per mole of the organic reagent, are shown in Fig. 5. Water activities were obtained by graphical interpolation of published values[23]. The organic-phase water is considered to be bound largely by the extractant, since Roddy[28] has found that p-xylene extracts only 0.0183 mM of water per ml of p-xylene from pure water. These results show decreased water extrac-
7
tO
0.96
0.92
o
~
z
WATER ACTIVITY 07t
0.28
6 a:
o
t 0
~
4 5 6 7 L~ClMOLARITY
8
9
to
Fig. 5. The water content of 1.0 M Adogen 464 and 0.5 M TOPO, both in p-xylene,as a function of LiCImolarity.
tion, with increasing LiC1 concentration, of 33 and 34%, respectively, for the 1.0 M Adogen 464 and 0.5 M TOPO systems. Thus, it seems unlikely that the organic-phase activity coefficients remain constant. Lithium extraction. Marcus[17] has concluded that lithium is coextracted from LiCI solutions in a 1:1 ratio with macro amounts of Am(III) by tri-n-octylamine hydrochloride. However, Havelka and Duyckaerts[29] failed to confirm the coextraction of lithium with macro amounts of Am(III). In an effort to resolve this question, we investigated the extraction of lithium by 0.5 M TOPO and 1-0 M Adogen 464 in the presence as well as in the absence of macro concentrations of Eu(III), the lanthanide analogue of Am(Ill). The results of this study are listed in Table 1. First, the extremely high variation in lithium extraction from 0.5 to 10 M LiC1 by each reagent should be noted. At 10 M LiC1, the extracted lithium is a significant molar percentage of the organic reagent concentration. A similar result was obtained by Seeley and Crouse[30] for the extraction of lithium by 0.1 M Aliquat 336, a quaternary ammonium salt that has essentially the same composition as Adogen 464. Secondly, the addition of 0.0125 M EuCI3 to each aqueous solution resulted in a decrease in lithium extraction by both Adogen 464 and TOPO. These results are not consistent with the extraction of a mixed Li-Eu complex. However, this conclusion cannot be applied directly to Marcus' work [17] since the extractants are not identical. Table 1. The extraction of lithium by 1.0 M Adogen464 and 0.5 M TOPO as a function of aqueous LiC1and EuCI3concentration Aqueous Phase a
Per
Mole of Ado~en 464
Per Mole of TOPO
0.5 M LiCI, 0.0125 M EuCI 3
3.93 x 10 -3
1.07 x 10-5
0.5 M LICI
4.44 x 10-3
1.27 x 10-5
i0 M LiCI, 0.0125 M EuC13
0.654
0,206
10 M LiCl
0.746
0.354
i0 M LiCI, 0.2 M HCI[30]
0.7
aThe LICI solutions in this work were maintained at a pH of i.
CONCLUSIONS In the extraction of americium from both 1 M and 5 M LiCI by Adogen 464 in p-xylene, the organic-phase species is ILNAmCI4 (where R4N+ is the quaternary ammonium ion). Extraction of americium by TOPO from the same lithium chloride concentrations, however, appeared to be primarily AmC13.TOPO from 1 M LiC1 and AmCI3.3TOPO from 5 M LiCI, with nonintegral slopes suggesting that mixtures of species were present or that the nature of the organic phase was changing with lithium chloride concentration, or both. Europium has the same degree of association (probably monomeric) in both aqueous and organic phases, suggesting the same for its actinide analogue, americium. The large changes in extracted water and lithium chloride observed in both extractant systems as the aqueous lithium chloride concentration increased preclude conventional use of these systems for investigating the aqueous-phase chloride complex equilibria at high aqueous chloride concentrations. Acknowledgements--One of the authors (H.D.H.) would like to
acknowledge financial support from a National Defense Education Act Title IV Fellowship from the University of Tennessee
Phosphine oxide and quaternary ammonium extraction of americium(Ill) (Knoxville) and support from the Oak Ridge National Laboratory during the summer of 1970. REFERENCES
1. K. Watanabe, J. Null. S¢i. Tech. I, 39 (1964). 2. B. Martin, D. W. Ockenden and J. K. Foreman, J. Inorg. Nucl. Chem. 21, 96 (1961). 3. J.GoffartandG.Duyckaerts,Anal.Chem.Acta46,91(l%9). 4. J. Goffart, European Atomic Energy Community Report EUR-2297.e, pp. 15-26. Li6ge, Belgium (1%5). ~,. F. L. Moore. Anal. Chem. 33, 748 (1961). 6. R. D. Baybarz and B Weaver, U.S. Atomic Energy Commission Report ORNL-3185. Oak Ridge, Tennessee (1961). -7. R D. Baybarz, B. Weaver and H. B. Kinser, Nucl. Sci. Engr. 17, 457 (1963). g. A. van Dalen and W. Kraak, European Atomic Energy Community Report EUR-2640.e, pp. 1-8. Petten, Netherlands (1966). 9. W. Miiller, European Atomic Energy Community Report EUR-2297.3, pp. 3-14. Liege, Belgium (1%5). Ill. H. C. Jain and G. Duyckaerts, lnorg. Nucl. Chem. Lett. 5,553 11969). 1 I. W. Mtiller, Actinides Rev. 1, 71 (1%7). 12, F L. Moore, Anal. Chem. 36, 2158 (1964), 13. P Th. Gerontopulos, L. Rigali and P. G. Barband. Radiochimica Acta 4, 75 (1965). 14. E A. Huff..L Chromatog 27, 229 (1967).
JINC Vol 38, No. I ~ K
159
15. E. P. Horwitz, C. A. A. Bloomquist, L. J. Sauro and D. J. Henderson, J. lnorg. Nucl. Chem. 28. 2313 (1966). 16. H. D. Harmon, J. R. Peterson and W. J. McDowell. hTorg. Nucl. Chem Lett. 8, 57 (1972). 17. Y. Marcus, J. lnorg. Nucl. Chem. 28, 209 (1966) 18. M. Shiloh and Y. Marcus, J. lno~e. Nucl. Chem. 28, 7!725 (1966). 19. Bernard Marin, CEA-R-3803 (Thesis). b\S. Atomic Energy Commission Report ORNL-tr-2342 (June 1%9), 20. R. B. Fischer, Quantitative Chemical Analysis. 2rid Edn. pp. 281-284. Saunders, London (1962). 21. F. J. C. Rossotti and H. Rossotti, The Determination o/ Stability Cunstants. pp. 203-205. McGraw-Hill, New York {1961). 22. Y. Marcus, gec. Chem. Progr. 27, 105 11966). 23. R. A. Robinson, Trans. Faraday Soc. 41,756 11945). 24. H. S. Harned and R. W. Ehlers. J. Am. Chem. Soc. 55. 2179 t1963t. 25. G. Akerl6f and J. W. Teare, J. Am. Chem. Soc. 59. 1855 (1937). 26. E. W. Washburg (Editor), International Critical Tables. Vo[. lIl, pp. 54, 77. McGraw-Hill, New York (19261. 27. W. Mfiller, Chem. Ztg. 95(11), 490 (1971). 28. J. W. Roddy, Oak Ridge National Laboratory, Oak Ridge. Tennessee, personal communication (Aug. 1969). 29. S. Havelka and G, Duyckaerts, personal commtmication. 1%6; W. Miiller, Actinides Rev. 1, 71 (1967). 30. F. G. Seeley and D. J. Crouse. J. Chem. Engng Data 1 I, 424 (1966).
PHOSPHINE OXIDE AND QUATERNARY AMMONIUM EXTRACTION OF AMERICIUM(III) FROM CONCENTRATED CHLORIDE SOLUTIONS* H. D. HARMON? and J. R. PETERSON Department of Chemistry, University of Tennessee. Knoxville, TN 37916, U.S.A. and Oak Ridge National Laboratory and W. J. McDOWELL and C. F. COLEMAN Chemical TechnologyDivision,Oak Ridge National Laboratory, Oak Ridge,TN 37830. U.S.A.
(Received 14 February 1975) Abstract--The extraction of Am(Ill) and Eu(lll) by tri-n-octylphosphineoxide (TOPO) and methyl trialkyl(C~-C,,) ammonium chloride (Adogen 464) from HC1 and slightly acidic LiC1 solutions was investigated. Linear extraction isotherms showed that macro Eu(III) species exist with the same degree of association in both organic and aqueous phases suggestingthe same behavior for the actinide analog Am(Ill). Reagent dependences indicated the extraction of primarily AmCL.TOPO and AmCL.3TOPO from 1 and 5 M LiCk respectively, and the extraction of only R4NAmCL (where I~N + is the quaternary ammonium cation of Adogen 464) from both I and 5 M LiCI. However. detailed studies of the TOPO-LiCI and Adogen 464-LIC1 systems showed wide variations in lithium and water extraction as a function of aqueous LiCI molarity, so that these systems are not suitable for investigatingthe aqueous chloride complex equilibria.
INTRODUCTION
THE tri-n-octylphosphine oxide (TOPO) extraction of trivalent actinides has been studied from HCI[1,2], HNO3[I-4], NH4NO~[1] and NaNO313,4] solutions. Tertiary amine extraction of trivalent actinides has been reported from various chloride solutions. Tri-isooctylamine (TIOA)[5-8] and trilaurylamine (TLA)[9, 10] have undergone extensive study as actinide extractants, while several other tertiary amines have undergone more limited investigations[5-7, 11]. In addition, trivalent actinide extraction by quaternary ammonium salts in the thiocyanate[12-14] and nitrate[15] systems has been reported. There is, however, very limited information on the reactions and mechanisms included in the extraction of trivalent lanthanides and actinides from concentrated chloride solutions, particularly at known or reasonably constant HCI activity. Such studies are greatly hindered by the severe lack of information regarding chloride complexes of these trivalent elements in concentrated chloride solutions. Previous investigations of actinide(III) chloro complex formation in solutions with ionic strengths < 4 M were summarized in an earlier publication[16] and, in addition, some studies of actinide(III) chloro complexes in LiCI and HCI solutions have been made using anion exchange [17], tertiary amine extraction[17], spectrophotometric[18], and paper electrophoresis[19] techniques. However, none of these have resulted in understanding of the chloro complex formation of either the trivalent lanthanides or actinides *Research sponsored by the Energy Research and Development Administration under contract with the Union Carbide Corporation. +National Defense Education Act Title IV Fellow, University of Tennessee, Knoxville. Present address: Savannah River Laboratory, E. I. du Pont de Nemours & Co., Aiken, SC 29801. U.S.A.
in very concentrated chloride solutions, above ~5 M. The purpose of this study was to gain information concerning the nature of the extraction of trivalent lanthanides and actinides from concentrated chloride systems by TOPO and Adogen 464 and to assess their possible utility for studies of aqueous chloride complex equilibria, par~Iicularly at high chloride concentrations.
EXPERIMENTAL
Materials Organic extractants. Stock solutions of TOPO and Adogen 464 were prepared by dissolving the required amount of each material in p-xylene. The TOPO was purchased from Eastman Organic Chemicals, and Adogen 464 was purchased in the chloride form from the Ashland Chemical Company; both were used without further purification. Aqueous solutions. Lithium chloride (99.9% purity) was obtained from the Y-12 Plant of Union Carbide Corporation. Nuclear Division, and was recrystallized twice from water before use. A stock solution of -10M LiCI was prepared and standardized by the Mohr method[20]. Solutions of lower concentration were prepared by quantitative dilution and were adjusted to a pH of 1 by the addition of reagent-grade HCI. Solutions of HC1 from 1 to 10 M were prepared by quantitative dilution of the concentrated reagent. The EuC13 solution was prepared from Eu(NO~), which was obtained from the Michigan Chemical Corporation with 9Y.9% purity. The Eu(IIl) was precipitated with NH4OH and subsequently was converted to EuCI~ by dissolution in HC1. Tracer solutions. The 2"'Am and '~4Eu tracer solutions were obtained from lhe ORNL Isotopes Division and were diluted to the appropriate level (-10' cpm/ml). The tritiated water solntion was supplied by the New England Nuclear Corporation. Equilibration procedure. Although the equilibration procedure varied slightly in the different extraction experiments, the basic steps were as follows: volumes of 10 ml of the organic extractant solution were preequilibrated with equal volumes of the desired aqueous chloride solution. Then 5 ml of the preequilibrated organic solution and 5 ml of the aqueous solution were pipetted 155
156
W.J. McDOWELL et al.
into screwcapped vials. After 0.1 ml of the 2"~Amtracer solution had been added, the phases were equilibrated on a verticalrotating mixer for 1 hr at room temperature. Separate experiments had established that 1 hr was more than adequate to ensure that equilibrium had been attained. The vials were centrifuged to effect phase separation, providing this was necessary for clean phase separation after prolonged standing. Finally, samples (0.1 ml) of each phase were withdrawn and analyzed by liquid scintillation counting. Liquid scintillation counting. The organic- and aqueous-phase samples from the above extractions were dissolved in a scintillation liquid which contained 80g of naphthalene, 5g of 2,5-diphenyloxazole (PPO), and 50 rng of bis[2-(5phenyloxazolyl)]benzene (POPOP) dissolved in a diluent mixture consisting of 385 ml of p-xylene, 385 ml of dioxane and 230 ml of ethanol. All these chemicals were either reagent or scintillation grade. The alpha activity of Z4~Amin each sample was measured on a Packard Tri-Carb Liquid Scintillation Spectrometer (Model No. 3214) with appropriate window settings to include the alpha peak and to minimize background. Water extraction. The extraction of water by 0.5 M TOPO and 1.0 M Adogen 464 in p-xylene from aqueous LiC1 solutions was studied by employing a tritium tracer technique. Aliquots (0.1 ml) of the tritium solution (-109 cpm/ml) were added to aqueous LiCI solutions, and the tritium beta activity of each solution was carefully measured by a standard liquid scintillation procedure. Then the solutions were equilibrated with the organic phases, and samples (0.1 ml) of each organic phase were analyzed for tritium beta activity. Quenching corrections were made on both the initial aqueous- and organic-phase samples. The weights of water and LiCI in the aqueous solutions were determined by density measurements and by chloride analyses utilizing the Mohr method [20]. Lithium extraction. The extraction of lithium by 0.5 M TOPO and 1.0M Adogen 464 was examined from 0.5 and 10M LiCl solutions both in the presence and absence of 0.0125 M EuCI3. The extractions were carried out according to the procedure outlined earlier. The organic phases were analyzed for lithium via flame photometry by the staff of the Analytical Chemistry Division of ORNL. The extent of Eu(III) extraction was determined with ~"Eu tracer in extraction experiments identical to, but separate from, those carried out for lithium analysis. The organic-phase Eu(III) concentration was calculated from the results of Eu(III) extraction measurements and the initial aqueous EuCI3 concentration. Eu(III) isotherm study. Aqueous solutions of 5 M LiCI were prepared with EuCI3 concentrations varying from 2× 10 6 to 1× 10-'M and with '~4Eu tracer (~10~cpm/ml). Separate 5-ml aliquots of each aqueous solution were equilibrated, as described above, with equal volumes of 1.0 M Adogen 464 and 0.5 M TOPO, each in p-xylene. Samples (l ml) of each phase were withdrawn for gamma-activity analysis. The concentration of Eu(III) in each phase was calculated from results of the extraction measurements and from the initial aqueous EuCI3 concentration. Reagent dependence o[ Am(III) extraction. Extraction of Am(Ill) from 1 and 5 M LiC1 solutions was carried out as described previously, with TOPO solutions of 0.01-0.5 M and Adogen 464 solutions of 0.02-1.0M, all in p-xylene. Samples (0.2ml) of each phase were analyzed by liquid scintillation counting as described above. RESULTS AND DISCUSSION
Theory of neutral species extraction. The extraction coefficient of a metal ion M c÷ by a neutral species extractant is given by E -
- [MCI
~
EC~ c
c
] o r g /~__N° [MCI n c - .
.
.
.
.n
]a q
,
(1)
where ft, = [MCI, c "]/[MC+][C1-]", E c is the extraction
coefficient of the neutral species MCIc, and N is the number of ligands in the most highly coordinated complex[21]. In systems where the ionic strength is varied considerably, the mean ionic activity of the chloride salt act may be substituted for [CI ] in eqn (1), and the 13, values become "effective stability constants" as defined by/3* = [MCl,C-"]/[MC+][ao]" [22]. It can be shown from eqn (1) that the extraction of a metal ion which forms aqueous cationic, neutral, and anionic complexes should exhibit a plot of IogE vs log [CI-] with an "inverted-U" shape. A region of positive slope results at low ligand concentrations, where the metal ion is predominantly uncomplexed. A maximum, or plateau, is seen where the neutral complex predominates, and a negative slope is found at ligand concentrations where anionic complexes exist in high concentrations [21]. Am(III) extraction results. The results of Am(III) extraction from HC1 and LiCI solutions are shown in Fig. 1 as plots of the log of the extraction coefficient, E, vs the log of the mean ionic activity of LiC1 or HCI. The necessary activity coefficients and density data required for the calculation of the activities were obtained from standard sources[23-26]. The curves for TOPO and Adogen 464 extractions of Am(III) from HC1 solutions exhibit the previously discussed "inverted-U" shape, but the maxima are not at the same HCI activity. Thus, it is probably not valid to interpret these maxima as indicating the existence of neutral and anionic complexes. As concluded in previous TOPO-HNO3 extraction studies[2], it is likely that the decrease in extraction at high HCI concentrations is due to competition between HCl and Am(III) for the organic extractant. The Am(III) extractions by TOPO and Adogen 464 from LiCI solutions were much greater than from HCI, and these extraction curves do not show any decreasing portion to suggest significant concentrations of neutral or anionic aqueousphase complexes. However, results of additional studies (described below) carried out on the TOPO and Adogen 464 systems suggest that these extraction data cannot be used to indicate the type of complex species present in the aqueous phase. Reagent dependence studies. The extraction of a trivalent actinide, M 3+, by a neutral species extractant such as TOPO can be expressed as 3+
M(~ + (n - 3)H~a)+ n Cl~a~+ yTOPO~o) = MC1, H,-3"yTOPO~o),
(2)
where n is the number of CI ions in the extracted species, y is the number of extractant molecules associated with each M 3÷ species, the subscripts a and 0 represent the aqueous and organic phases, respectively, and possible polymerization is neglected. The concentration quotient Q for this reaction can be written as [MCI, H, 3' y TOPO]o Q = [M3+], [H+]o" 3[C1-]," [TOPO],~ E = [H+]a"-3[C1-]o"[TOPO]J '
(3)
where the extraction coefficient E = [MCI, H,_3-yTOPO]o/[M3+]a. When [H÷]~ and [C1-] are held constant, it can be shown from eqn (3) that a plot of log E vs log[TOPO] should have a slope of y. For equilibrium methods, Adogen 464 chloride can be considered a neutral species extractant and, therefor-e, can also be treated according to eqn (2)[11, 27].
157
Phusphine oxide and quaternary ammonium extraction of americium(Ill) - - ] ] -T ....
I
Iil
I
7-T
.........
1o~ L
t i
I I
,
2 i
'
! i
i
I
~0°5 L-
t I !1111 ~
~
~ - ~
•
z; ADOGEN 4 6 4 - L C • TOPO - HCI • ADOGEN 464 HC 2
1:? } F
•
~0- t
A /
¢D
g x
J
i¢d i
t0 2 5
•
*0 3 i
5 •
• 5.O'M L:CI •tO-M LIC!
•
I
~d ~
2 i
i
/
"
]
•
: lii:
_~.__LIi !
~o 4 I0 2
2
~0-t
5
2
5
~__~LAt
t00
2
5
~0~
,5,DOGEN 4.64 MOLARITY
Fig. 2. The dependence of Am(IIl) extraction on the concentration of Adogen 464 in p-xylene at 1.0 and 5.0 M LiC1.
r
I
~dsI ~Gz
t lll,ltil
lo-1
i lll;ilh Jllilll , IIII[UL~i ~oo Io 1 ~oz MEAN IONIC ACTIVITY OF LICI OR HCI
I Jill ~o3 2
Fig. l. The extraction of Am(Ill) as a function of the mean ionic activity of LiC1 or HC1.
! [111111
I
I IIIIIII
!
2_-
5 --
Plots of log E vs log [extractant] for the extraction of Am(lII) from 1.0 and 5.0M LiCI by Adogen 464 and TOPO are shown in Figs. 2 and 3, respectively. The slopes of 1.0 in Fig. 2 indicate that the extracted species is R4NAmC14, where R4N ~ represents the quaternary ammonium cation. This result is consistent with quaternary ammonium reagent dependence studies of Am(IIl) extraction in the SCN- system[13,14] and in the NO, system[15], where values of y = 1.0 were also obtained. With tertiary amines, complexes containing two molecules of alkylammonium salt per metal atom have been reported in the extraction of actinides from concentrated aqueous chloride solutions [5-7, 27]. The value of y found for the TOPO extraction was not constant over the 1-5 M LiCI concentration range, Nonintegral values of 1.2 and 2.7 were obtained with 1.0 and 5.0M LiCI, respectively. Such nonintegral values may indicate either the simultaneous existence of species with different y values or variations in the organic-phase activity coefficients, or both. However, these results do suggest that the species extracted from 1.0 and 5.0 M LiCI are predominantly AmCh.TOPO and AmC13.3TOPO, respectively. As the water activity is lowered by increasing the LiC1 concentration from 1 to 5M, it is possible that the water coordination of the extracted Am(III) complex decreases with a concomitant increase in TOPO coordination. The AmC13.3TOPO species is in agreement with y values of - 3 found for trivalent actinide extraction by TOPO in the NO3 system[l, 3]. Eu(IH) extraction isotherm. The variation of Eu(IlI)
Z 2 400
= _Z _ -
5
~, _~ 2 ~ t0 -t ~, 5 g r,='~ 2 n
~0-2 5
,~
ITI
_=
&
/
--
2 tO - 3 5 L 2
--
PE = 2 . 7
/
/
/
t0 -2
LiCI
II 4. O-M
LiCI
A ~OPE = ~ 2
/ [ I 1~411H
~0 - 4
• 5.O-M
I I IIIlill
t0 -~
i t0 0
IiL t0 ~
TOPO M O L A R I T Y
Fig. 3. The reagent dependence of Am(llI) extraction by TOPO solutions in p-xylene from 1-0 and 5.0 M LiC1.
158
W. J. McDOWELLet al.
extraction from 5 M LiCI with varying aqueous Eu(III) concentration was studied to determine whether the extracted Eu(III) species, and thus the Am(Ill) species by analogy, has the same degree of association in the aqueous and organic phases. Europium(Ill) extraction coefficients were determined over the Eu(III) concentration range of 2× 10-6 to 1 × 10-'M, and the equilibrium concentration of Eu(III) in each phase was calculated. Plots of log [Eu(III)]o vs log [Eu(III)], for the extraction by 1.0 M Adogen 464 and 0.5 M TOPO are given in Fig. 4. Each of the extraction isotherms in Fig. 4 has a slope of 1.0, which indicates that the Eu(III) species have the same degree of association (presumably monomeric) in both the aqueous and organic phases. 164 ~_ 5
I
I [ [11111
] i I IIII1~,,~1
Z~ t.0-M ADOGEN 464
I I II1~
/ 6 ¢
/a
~d 5
_
~
5
O
"
166 _z
z
-510-7
I 2
I
I IIIIII 5 tO-6
I 2
I
I [11111 5 10-5
I 2
]
I IIIJ 5 10-4
AQUEOUS Eu(III ) MOLAR ITY
Fig. 4. Extraction isotherms for Eu(III) extraction from 5 M LiCI by 1.0 M Adogen464 and 0.5 M TOPO, both in p-xylene. Water extraction. In order to utilize the TOPO and
Adogen 464 systems in studies of aqueous complex equilibria, the organic-phase composition must remain reasonably constant. Therefore, the variation in the extraction of water by 0.5 M TOPO and 1.0 M Adogen 464, both in p-xylene, was examined as a function of aqueous LiCl concentration. The results, in terms of moles of water extracted per mole of the organic reagent, are shown in Fig. 5. Water activities were obtained by graphical interpolation of published values[23]. The organic-phase water is considered to be bound largely by the extractant, since Roddy[28] has found that p-xylene extracts only 0.0183 mM of water per ml of p-xylene from pure water. These results show decreased water extrac-
7
tO
0.96
0.92
o
~
z
WATER ACTIVITY 07t
0.28
6 a:
o
t 0
~
4 5 6 7 L~ClMOLARITY
8
9
to
Fig. 5. The water content of 1.0 M Adogen 464 and 0.5 M TOPO, both in p-xylene,as a function of LiCImolarity.
tion, with increasing LiC1 concentration, of 33 and 34%, respectively, for the 1.0 M Adogen 464 and 0.5 M TOPO systems. Thus, it seems unlikely that the organic-phase activity coefficients remain constant. Lithium extraction. Marcus[17] has concluded that lithium is coextracted from LiCI solutions in a 1:1 ratio with macro amounts of Am(III) by tri-n-octylamine hydrochloride. However, Havelka and Duyckaerts[29] failed to confirm the coextraction of lithium with macro amounts of Am(III). In an effort to resolve this question, we investigated the extraction of lithium by 0.5 M TOPO and 1-0 M Adogen 464 in the presence as well as in the absence of macro concentrations of Eu(III), the lanthanide analogue of Am(Ill). The results of this study are listed in Table 1. First, the extremely high variation in lithium extraction from 0.5 to 10 M LiC1 by each reagent should be noted. At 10 M LiC1, the extracted lithium is a significant molar percentage of the organic reagent concentration. A similar result was obtained by Seeley and Crouse[30] for the extraction of lithium by 0.1 M Aliquat 336, a quaternary ammonium salt that has essentially the same composition as Adogen 464. Secondly, the addition of 0.0125 M EuCI3 to each aqueous solution resulted in a decrease in lithium extraction by both Adogen 464 and TOPO. These results are not consistent with the extraction of a mixed Li-Eu complex. However, this conclusion cannot be applied directly to Marcus' work [17] since the extractants are not identical. Table 1. The extraction of lithium by 1.0 M Adogen464 and 0.5 M TOPO as a function of aqueous LiC1and EuCI3concentration Aqueous Phase a
Per
Mole of Ado~en 464
Per Mole of TOPO
0.5 M LiCI, 0.0125 M EuCI 3
3.93 x 10 -3
1.07 x 10-5
0.5 M LICI
4.44 x 10-3
1.27 x 10-5
i0 M LiCI, 0.0125 M EuC13
0.654
0,206
10 M LiCl
0.746
0.354
i0 M LiCI, 0.2 M HCI[30]
0.7
aThe LICI solutions in this work were maintained at a pH of i.
CONCLUSIONS In the extraction of americium from both 1 M and 5 M LiCI by Adogen 464 in p-xylene, the organic-phase species is ILNAmCI4 (where R4N+ is the quaternary ammonium ion). Extraction of americium by TOPO from the same lithium chloride concentrations, however, appeared to be primarily AmC13.TOPO from 1 M LiC1 and AmCI3.3TOPO from 5 M LiCI, with nonintegral slopes suggesting that mixtures of species were present or that the nature of the organic phase was changing with lithium chloride concentration, or both. Europium has the same degree of association (probably monomeric) in both aqueous and organic phases, suggesting the same for its actinide analogue, americium. The large changes in extracted water and lithium chloride observed in both extractant systems as the aqueous lithium chloride concentration increased preclude conventional use of these systems for investigating the aqueous-phase chloride complex equilibria at high aqueous chloride concentrations. Acknowledgements--One of the authors (H.D.H.) would like to
acknowledge financial support from a National Defense Education Act Title IV Fellowship from the University of Tennessee
Phosphine oxide and quaternary ammonium extraction of americium(Ill) (Knoxville) and support from the Oak Ridge National Laboratory during the summer of 1970. REFERENCES
1. K. Watanabe, J. Null. S¢i. Tech. I, 39 (1964). 2. B. Martin, D. W. Ockenden and J. K. Foreman, J. Inorg. Nucl. Chem. 21, 96 (1961). 3. J.GoffartandG.Duyckaerts,Anal.Chem.Acta46,91(l%9). 4. J. Goffart, European Atomic Energy Community Report EUR-2297.e, pp. 15-26. Li6ge, Belgium (1%5). ~,. F. L. Moore. Anal. Chem. 33, 748 (1961). 6. R. D. Baybarz and B Weaver, U.S. Atomic Energy Commission Report ORNL-3185. Oak Ridge, Tennessee (1961). -7. R D. Baybarz, B. Weaver and H. B. Kinser, Nucl. Sci. Engr. 17, 457 (1963). g. A. van Dalen and W. Kraak, European Atomic Energy Community Report EUR-2640.e, pp. 1-8. Petten, Netherlands (1966). 9. W. Miiller, European Atomic Energy Community Report EUR-2297.3, pp. 3-14. Liege, Belgium (1%5). Ill. H. C. Jain and G. Duyckaerts, lnorg. Nucl. Chem. Lett. 5,553 11969). 1 I. W. Mtiller, Actinides Rev. 1, 71 (1%7). 12, F L. Moore, Anal. Chem. 36, 2158 (1964), 13. P Th. Gerontopulos, L. Rigali and P. G. Barband. Radiochimica Acta 4, 75 (1965). 14. E A. Huff..L Chromatog 27, 229 (1967).
JINC Vol 38, No. I ~ K
159
15. E. P. Horwitz, C. A. A. Bloomquist, L. J. Sauro and D. J. Henderson, J. lnorg. Nucl. Chem. 28. 2313 (1966). 16. H. D. Harmon, J. R. Peterson and W. J. McDowell. hTorg. Nucl. Chem Lett. 8, 57 (1972). 17. Y. Marcus, J. lnorg. Nucl. Chem. 28, 209 (1966) 18. M. Shiloh and Y. Marcus, J. lno~e. Nucl. Chem. 28, 7!725 (1966). 19. Bernard Marin, CEA-R-3803 (Thesis). b\S. Atomic Energy Commission Report ORNL-tr-2342 (June 1%9), 20. R. B. Fischer, Quantitative Chemical Analysis. 2rid Edn. pp. 281-284. Saunders, London (1962). 21. F. J. C. Rossotti and H. Rossotti, The Determination o/ Stability Cunstants. pp. 203-205. McGraw-Hill, New York {1961). 22. Y. Marcus, gec. Chem. Progr. 27, 105 11966). 23. R. A. Robinson, Trans. Faraday Soc. 41,756 11945). 24. H. S. Harned and R. W. Ehlers. J. Am. Chem. Soc. 55. 2179 t1963t. 25. G. Akerl6f and J. W. Teare, J. Am. Chem. Soc. 59. 1855 (1937). 26. E. W. Washburg (Editor), International Critical Tables. Vo[. lIl, pp. 54, 77. McGraw-Hill, New York (19261. 27. W. Mfiller, Chem. Ztg. 95(11), 490 (1971). 28. J. W. Roddy, Oak Ridge National Laboratory, Oak Ridge. Tennessee, personal communication (Aug. 1969). 29. S. Havelka and G, Duyckaerts, personal commtmication. 1%6; W. Miiller, Actinides Rev. 1, 71 (1967). 30. F. G. Seeley and D. J. Crouse. J. Chem. Engng Data 1 I, 424 (1966).