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Separation factor of lanthanoids by solvent extraction of their ternary complexes
132
Journal of Alloys and Compounds, 192 (1993) 132-134 JALCOM 2108
Separation factor of lanthanoids by solvent extraction of their ternary complexes Junji Noro Research Department, Nissan ARC Ltd., Natsushima Yokosuka, Kanagawa 237 (Japan) Tatsuya Sekine Department of Chemistry, Science University of Tokyo, Kagurazaka, Shinjuku, Tokyo 162 (Japan)
Abstract The solvent extraction of praseodymium (III) and neodymium (III) (denoted by M 3÷) with 2-thenoyltrifluoroacetone (Htta) into carbon tetrachloride was studied in the absence and presence of tetrabutylammonium ions (tba÷). The extraction with Htta was greatly enhanced by the addition of tba ÷, and this fact was explained in terms of the formation of a ternary complex, M(tta)4-tba +. The neutral metal chelate was extracted into the organic phase and associated with ion pairs of the r¢agents, tta-tba ÷, in the organic phase. It was concluded that the degree of association of Pr(tta)3 with tta-tba + (the ion pair) is slightly greater than that of Nd(tta)a.
1. Introduction
Kex3,0= [M(tta)3lors[M 3+ ] - 1 [ t t a - ] - 3
The solvent extraction of europium (III) with benzoyltrifluoroacetone (Hbfa) into carbon tetrachloride was greatly enhanced by the addition of bulky cations, such as tetrabutylammonium ion [1]. This was elucidated by the extraction of the anionic chelate complex as ion pairs, Eu(bfa)4-tba ÷, which was extracted much better than the neutral chelate, Eu(bfa)3. In the present paper, the solvent extraction of praseodymium (III) and neodymium (III) with Htta into carbon tetrachloride in the absence and presence of tba ÷ was studied. The separation of these metal ions be extraction in the presence of tba ÷ was compared with those by the conventional extraction of the neutral chelates. The data were analyzed statistically in the same m a n n e r as in the authors' previous study [1].
2. Statistical details In the present paper, any chemical species in the organic phase is denoted by the subscript "org" and those in the aqueous phase are shown without any subscript. The volumes of the two liquid phases are assumed to be the same. The equilibrium for the extraction of M 3 + with Htta in the organic phase can be written as: M 3+ + 3 t t a - . 0925-8388/93/$6.00
" M(tta)3(or~)
(1)
When the anionic chelate, M(tta)4-, is extracted as ion pairs with tba +, the equilibrium can be written as: M 3+
+4tta- +tba + .
" M(tta)4-tba~+org)
K~4.1 = [M(tta),- tba +]or,[M3+ ]- l[tta- ]-4[tba + ]- 1
(2)
However, the extraction of the ion pairs can also be given in the form that the neutral chelate, given by eqn. (1), is extracted first and then associated with the ion pairs of the reagents, t t a - t b a +, in the organic phase. The entire extraction equilibrium can be written as: M(tta)3forg)+ tta-tbaf+rg) .
" M(tta)4-tba~org)
Korg= [M(tta)4-tba + ]org[M(tta)a]or,-l[tta-tba +]org- 1
(3)
The anions of the chelating extractant, t t a - , and the bulky cations, tba +, would be extracted as ion pairs, t t a - t b a ÷, and the following equation can be written for the extraction equilibrium: tta- +tba ÷ .
" tta-tba~or~)
K~xio,p~ir= [tta- tba ÷ ]org[tta- ] - l[tba ÷ ] -1
(4)
When t t a - forms complexes in the aqueous phase, the distribution ratio of metal ions can be written as: Do = [M(tta)3]org/([M3+] + [Mtta 2+] + [M(tta)2 +] +...) = K~x3,0[tta- ]3/(1 +/31[tta -] + ~[tta - ]2 +... )
(5) (6)
where /3n is the stability constant of the nth complex in the aqueous phase:
© 1993- Elsevier Sequoia. All rights reserved
J. Noro, T. Sekine / Solvent extraction of lanthanoids from ternary complexes /3n = [ M ( t t a ) 3 - " ] [ M 3+ ] - l [ t t a - ] - "
(7)
T h e following general equations can also be written when the bulky cation, tba +, is added: D = ([M(tta)3]org + [M(tta)4- tba + ]org)/([M 3 + ] + [Mtta 2+] + [M(tta)2 +] + . . . )
(8)
= (Kex3,0[tta- ]3 + gex4,1 [tta-14[tba + 1)/(1 + 131 × [ t t a - ] +/32[tta-12 + . . . )
(9)
W h e n the t t a - concentration in the a q u e o u s p h a s e in the absence of tba ÷ is identical with that in the p r e s e n c e of tba +, the following equations can be int r o d u c e d f r o m eqns. (1)-(4), (6) and (9)
D/Do = 1 + Kex4,i Kex3,o- l [ t t a - ][tba ÷ ]
(10)
= 1 + KorgKexion palr[tta--][tba +]
133
3. Experimental details All the e x p e r i m e n t a l work was carried out at 298 K in a t h e r m o s t a t t e d room. T h e aqueous phase was 0.1 m o l d m -3 sodium nitrate solution and was buffered by acetate. T h e organic phase was carbon tetrachloride containing 0.1 mol dm -3 Htta. T h e concentration of the m e t a l ion in both phases was lower than 1 x 10 - 4 tool din-3. T h e two phases were agitated for 1 h and centrifuged off. T h e m e t a l concentrations in both phases were d e t e r m i n e d by inductive coupled plasma atomic emission s p e c t r o m e t r y ( I C P - A E S ) . T h e concentration of ion pairs of t t a - and tba + in the organic p h a s e was d e t e r m i n e d by colorimetry after the t t a - ions were exchanged with the equivalent a m o u n t of picrate ions,
4. Results and discussion
(11)
w h e r e D/Do indicates the i m p r o v e m e n t of the rate of extraction. As seen f r o m eqn. (10), D/Do is d e p e n d e n t on the quantity [ t t a - ] [ t b a +] w h e n tba + is added. Furt h e r m o r e , as seen from eqn. (11), D/Do can be given clearly on the basis of the association of the extracted neutral chelate with the ion pair of the reagents, t t a - t b a +, in the organic phase.
T h e extraction constant of reagents, t t a - and tba +, as ion pairs into the organic phases, Kexionpair, w a s TABLE 1. Equilibrium constants for the solvent extraction of Pr 3+ and Nd 3+
Pr + Nd 3+
Log Kex3,0
LogK~x4,1
Log Korg
Log /31
13.2 13.5
23.8 24.0
8.5 8.4
4.3 4.4
3
ca)
,.3 -1
-2 -6
-5
-4
Log [tta] Fig. 1. Distribution ratio as a function of the concentration of tta- in the aqueous phase. Organic phase: carbon tetrachloride containing 0.1 tool d m -3 Htta. Aqueous phase: 0.1 mol d m -3 sodium nitrate solution without tba ÷ (closed symbols) or 1.0 × 10-3 mol dm -3 tba ÷ at initial (open symbols). Pr (III) (0, O) and Nd (III) (A, A).
0 -11
J°#,'*°°
I
-10
-9
-8
Log [tta] [tba+ ] Fig. 2. Increase in the distribution ratio as a function of the quantity [tta-][tba +] in the aqueous phase. Organic phase: carbon tetrachloride initially containing 0.1 tool dm -3 Htta. Aqueous phase: 0.1 tool dm -3 sodium nitrate solution of the designated pH for the metal ion with various amounts of tba +. Pr (III) at pH 3.7 (©), Nd (III) at pH 3.7 (A).
134
J. Noro, T. Sekine / Solvent extraction of lanthanoids from ternary complexes
determined to be 1019. Figure 1 gives the extraction curves of the metal ions with 0.1 mol d m - 3 Htta (at initial) in the organic phase in the absence and presence of tba ÷. From the statistical analysis of the data in the absence and presence of tba ÷ in Fig. 1, on the basis of eqns. (6) and (9), it was concluded that the first complex Mtta 2÷ in the aqueous phase should be taken into account, while higher complexes were negligible. The values of the extraction constants, Kox3.0 in eqn. (1) and Kox4,1 in eqn. (2), and stability constant, /31 in eqn. (7), obtained by using a least squares program are listed in Table 1. The stability constant, Kor~ in eqn. (3), was calculated from these constants~ the values are also listed in Table 1. With the addition of a certain amount of tba ÷, D~ Do cannot be compared at various tta- concentrations, since the slopes in Fig. 1 are varied with and without tba ÷. Figure 2 gives D/Do as a function of [tta-][tba ÷ ] at equilibrium. In this way, D/Do of two metal ions
can be compared even when the tta- ion concentration is different as seen from eqn. (11). The separation factors (Ke~.n(Nd)/Ke~m.n(vO)calculated from the constants in Table 1 were 0.3 and 0.2 for the extracted species of neutral and ternary complexes, respectively. Thus the separation of the two metal ions is slightly impaired if an effective extraction method is employed. Similar observations were found in a previous study [2]; the separation was poorer when lanthanoid ions were extracted as adduct chelates with Htta and tributylphosphate(tbp), M(tta)3(tbp)2, compared with when they were extracted as simple chelates M(tta)3. References 1 J. Noro and T. Sekine, Bull. Chem. Soc. Jpn., 65 (1992) 1910. 2 T. Sekine and D. Dyrssen, J. Inorg. Nucl. Chem., 29 (1967) 1481.
Journal of Alloys and Compounds, 192 (1993) 132-134 JALCOM 2108
Separation factor of lanthanoids by solvent extraction of their ternary complexes Junji Noro Research Department, Nissan ARC Ltd., Natsushima Yokosuka, Kanagawa 237 (Japan) Tatsuya Sekine Department of Chemistry, Science University of Tokyo, Kagurazaka, Shinjuku, Tokyo 162 (Japan)
Abstract The solvent extraction of praseodymium (III) and neodymium (III) (denoted by M 3÷) with 2-thenoyltrifluoroacetone (Htta) into carbon tetrachloride was studied in the absence and presence of tetrabutylammonium ions (tba÷). The extraction with Htta was greatly enhanced by the addition of tba ÷, and this fact was explained in terms of the formation of a ternary complex, M(tta)4-tba +. The neutral metal chelate was extracted into the organic phase and associated with ion pairs of the r¢agents, tta-tba ÷, in the organic phase. It was concluded that the degree of association of Pr(tta)3 with tta-tba + (the ion pair) is slightly greater than that of Nd(tta)a.
1. Introduction
Kex3,0= [M(tta)3lors[M 3+ ] - 1 [ t t a - ] - 3
The solvent extraction of europium (III) with benzoyltrifluoroacetone (Hbfa) into carbon tetrachloride was greatly enhanced by the addition of bulky cations, such as tetrabutylammonium ion [1]. This was elucidated by the extraction of the anionic chelate complex as ion pairs, Eu(bfa)4-tba ÷, which was extracted much better than the neutral chelate, Eu(bfa)3. In the present paper, the solvent extraction of praseodymium (III) and neodymium (III) with Htta into carbon tetrachloride in the absence and presence of tba ÷ was studied. The separation of these metal ions be extraction in the presence of tba ÷ was compared with those by the conventional extraction of the neutral chelates. The data were analyzed statistically in the same m a n n e r as in the authors' previous study [1].
2. Statistical details In the present paper, any chemical species in the organic phase is denoted by the subscript "org" and those in the aqueous phase are shown without any subscript. The volumes of the two liquid phases are assumed to be the same. The equilibrium for the extraction of M 3 + with Htta in the organic phase can be written as: M 3+ + 3 t t a - . 0925-8388/93/$6.00
" M(tta)3(or~)
(1)
When the anionic chelate, M(tta)4-, is extracted as ion pairs with tba +, the equilibrium can be written as: M 3+
+4tta- +tba + .
" M(tta)4-tba~+org)
K~4.1 = [M(tta),- tba +]or,[M3+ ]- l[tta- ]-4[tba + ]- 1
(2)
However, the extraction of the ion pairs can also be given in the form that the neutral chelate, given by eqn. (1), is extracted first and then associated with the ion pairs of the reagents, t t a - t b a +, in the organic phase. The entire extraction equilibrium can be written as: M(tta)3forg)+ tta-tbaf+rg) .
" M(tta)4-tba~org)
Korg= [M(tta)4-tba + ]org[M(tta)a]or,-l[tta-tba +]org- 1
(3)
The anions of the chelating extractant, t t a - , and the bulky cations, tba +, would be extracted as ion pairs, t t a - t b a ÷, and the following equation can be written for the extraction equilibrium: tta- +tba ÷ .
" tta-tba~or~)
K~xio,p~ir= [tta- tba ÷ ]org[tta- ] - l[tba ÷ ] -1
(4)
When t t a - forms complexes in the aqueous phase, the distribution ratio of metal ions can be written as: Do = [M(tta)3]org/([M3+] + [Mtta 2+] + [M(tta)2 +] +...) = K~x3,0[tta- ]3/(1 +/31[tta -] + ~[tta - ]2 +... )
(5) (6)
where /3n is the stability constant of the nth complex in the aqueous phase:
© 1993- Elsevier Sequoia. All rights reserved
J. Noro, T. Sekine / Solvent extraction of lanthanoids from ternary complexes /3n = [ M ( t t a ) 3 - " ] [ M 3+ ] - l [ t t a - ] - "
(7)
T h e following general equations can also be written when the bulky cation, tba +, is added: D = ([M(tta)3]org + [M(tta)4- tba + ]org)/([M 3 + ] + [Mtta 2+] + [M(tta)2 +] + . . . )
(8)
= (Kex3,0[tta- ]3 + gex4,1 [tta-14[tba + 1)/(1 + 131 × [ t t a - ] +/32[tta-12 + . . . )
(9)
W h e n the t t a - concentration in the a q u e o u s p h a s e in the absence of tba ÷ is identical with that in the p r e s e n c e of tba +, the following equations can be int r o d u c e d f r o m eqns. (1)-(4), (6) and (9)
D/Do = 1 + Kex4,i Kex3,o- l [ t t a - ][tba ÷ ]
(10)
= 1 + KorgKexion palr[tta--][tba +]
133
3. Experimental details All the e x p e r i m e n t a l work was carried out at 298 K in a t h e r m o s t a t t e d room. T h e aqueous phase was 0.1 m o l d m -3 sodium nitrate solution and was buffered by acetate. T h e organic phase was carbon tetrachloride containing 0.1 mol dm -3 Htta. T h e concentration of the m e t a l ion in both phases was lower than 1 x 10 - 4 tool din-3. T h e two phases were agitated for 1 h and centrifuged off. T h e m e t a l concentrations in both phases were d e t e r m i n e d by inductive coupled plasma atomic emission s p e c t r o m e t r y ( I C P - A E S ) . T h e concentration of ion pairs of t t a - and tba + in the organic p h a s e was d e t e r m i n e d by colorimetry after the t t a - ions were exchanged with the equivalent a m o u n t of picrate ions,
4. Results and discussion
(11)
w h e r e D/Do indicates the i m p r o v e m e n t of the rate of extraction. As seen f r o m eqn. (10), D/Do is d e p e n d e n t on the quantity [ t t a - ] [ t b a +] w h e n tba + is added. Furt h e r m o r e , as seen from eqn. (11), D/Do can be given clearly on the basis of the association of the extracted neutral chelate with the ion pair of the reagents, t t a - t b a +, in the organic phase.
T h e extraction constant of reagents, t t a - and tba +, as ion pairs into the organic phases, Kexionpair, w a s TABLE 1. Equilibrium constants for the solvent extraction of Pr 3+ and Nd 3+
Pr + Nd 3+
Log Kex3,0
LogK~x4,1
Log Korg
Log /31
13.2 13.5
23.8 24.0
8.5 8.4
4.3 4.4
3
ca)
,.3 -1
-2 -6
-5
-4
Log [tta] Fig. 1. Distribution ratio as a function of the concentration of tta- in the aqueous phase. Organic phase: carbon tetrachloride containing 0.1 tool d m -3 Htta. Aqueous phase: 0.1 mol d m -3 sodium nitrate solution without tba ÷ (closed symbols) or 1.0 × 10-3 mol dm -3 tba ÷ at initial (open symbols). Pr (III) (0, O) and Nd (III) (A, A).
0 -11
J°#,'*°°
I
-10
-9
-8
Log [tta] [tba+ ] Fig. 2. Increase in the distribution ratio as a function of the quantity [tta-][tba +] in the aqueous phase. Organic phase: carbon tetrachloride initially containing 0.1 tool dm -3 Htta. Aqueous phase: 0.1 tool dm -3 sodium nitrate solution of the designated pH for the metal ion with various amounts of tba +. Pr (III) at pH 3.7 (©), Nd (III) at pH 3.7 (A).
134
J. Noro, T. Sekine / Solvent extraction of lanthanoids from ternary complexes
determined to be 1019. Figure 1 gives the extraction curves of the metal ions with 0.1 mol d m - 3 Htta (at initial) in the organic phase in the absence and presence of tba ÷. From the statistical analysis of the data in the absence and presence of tba ÷ in Fig. 1, on the basis of eqns. (6) and (9), it was concluded that the first complex Mtta 2÷ in the aqueous phase should be taken into account, while higher complexes were negligible. The values of the extraction constants, Kox3.0 in eqn. (1) and Kox4,1 in eqn. (2), and stability constant, /31 in eqn. (7), obtained by using a least squares program are listed in Table 1. The stability constant, Kor~ in eqn. (3), was calculated from these constants~ the values are also listed in Table 1. With the addition of a certain amount of tba ÷, D~ Do cannot be compared at various tta- concentrations, since the slopes in Fig. 1 are varied with and without tba ÷. Figure 2 gives D/Do as a function of [tta-][tba ÷ ] at equilibrium. In this way, D/Do of two metal ions
can be compared even when the tta- ion concentration is different as seen from eqn. (11). The separation factors (Ke~.n(Nd)/Ke~m.n(vO)calculated from the constants in Table 1 were 0.3 and 0.2 for the extracted species of neutral and ternary complexes, respectively. Thus the separation of the two metal ions is slightly impaired if an effective extraction method is employed. Similar observations were found in a previous study [2]; the separation was poorer when lanthanoid ions were extracted as adduct chelates with Htta and tributylphosphate(tbp), M(tta)3(tbp)2, compared with when they were extracted as simple chelates M(tta)3. References 1 J. Noro and T. Sekine, Bull. Chem. Soc. Jpn., 65 (1992) 1910. 2 T. Sekine and D. Dyrssen, J. Inorg. Nucl. Chem., 29 (1967) 1481.