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Enhanced separation of trivalent lanthanoids by solvent extraction with 18-crown-6 and EDTA complexonate
0039-9140/92 SO0 + 0.00
Tahnta, Vol. 39, No. 3, pp. 211-214, 1992 Printed in Great Britain. All rights racrwd
Copyright0 1992 Pcrgamon Prcsa pk
ENHANCED SEPARATION OF TRIVALENT LANTHANOIDS BY SOLVENT EXTRACTION WITH 18-CROWN-6 AND EDTA COMPLEXONATE ROBERT FRAZIERand C. M. WAI*
Department of Chemistry, University of Idaho, Moscow, Idaho 83843, U.S.A. (Receiued 12 August 1991. Accepted 29 August 1991) W-The selectivities during solvent extraction of lanthanoids with macrocycles can be modified with complexonates in the aqueous phase. In the CBSC of solvent extraction of lanthanoids with 18-crown-6 and trichloroacetic acid (TCA), addition of EDTA to the aqueous phase enhances the selectivities of lanthanoids by 3-7 times compared to those without the complexonate. This is due to the fact that the stability of lanthanoid-EDTA complexes increases in the opposite direction to the crown-TCA complexes across the lanthanoid series. The selectivities observed in this system are among the largest reported for the light lanthanoids. The effect of the complexonate on lanthanoid extraction can be explained by a simple model presented in this paper.
Macrocyclic polyethers (crown ethers) are known to form stable complexes of different with trivalent lanthanoids.’ stoichiometry The stability of crown complexes in solvent extraction depends generally on the following four factors: crown cavity to cation diameter ratio, the counter anion, ligand flexibility, and the type of donor atoms (N, 0 or S) in the macrocyclic host.2 Using 18-crown-6 to extract lanthanoid ions in aqueous solution with trichloroacetate (TCA) as counter anions, selectivities as good as the best commercial extractant, di-2-ethylhexyl-phosphoric acid (HDEHP) were obtained by Samy et aL3 The relative stability constants of the lanthanoid complexes in this case decrease by three orders of magnitude from La’+ to Eu3+. The large selectivity observed in this macrocyclic system has been attributed to the steric hindrance of the lanthanoid complex with TCA. The decrease in stability across the lanthanoid series with 18-crown-dTCA complexes is significant, because it is opposite to many known lanthanoid complexonates in aqueous solution. Therefore, by choosing a proper complexonate, the two opposite trends may compliment each other resulting in enhanced selectivity for the extraction of lanthanoids by l%crown-6. It is known that lanthanoid-EDTA complexes show increased stabilities from La’+ to Lu3+ and the complexation depends on PH.~ This paper reports *Author for correspondence.
the enhanced lanthanoid selectivity in solvent extraction, using 18crown-6 as an extractant, TCA as the anion and EDTA as a complexonate with 1,2-dichloroethane solvent. Hydrogen ion is used as a displacer to release lanthanoids complexed with EDTA in the aqueous phase. EXPERIMENTAL
The 18-crown-6 extractant was purchased from Aldrich. The EDTA was Aldrich gold label and dried four days at 80” according to Schwartzenbach.5 The La, Eu and Lu solutions were purchased as standard solutions from Aldrich and Thiokol. Lanthanoid solutions of Ce, Nd and Ho were prepared from their nitrates obtained from Aldrich. Praseodymium was prepared from its oxide obtained from Baker and Adamson. The solvent, 1,Zdichloroethane, was purchased from EM Science. Demineralised water was used in all experiments. Radioisotope tracer techniques were used in the extraction experiments. Usually, lanthanoid solutions were irradiated in a TRIGA nuclear reactor with a steady flux of 6 x lOI n.cm-2.sec- ’ for one hour. An aliquot of this radioisotope solution was placed in a Pyrex tube (50 ml), containing TCA, 18-crown-6, EDTA, and lanthanoids of known concentration in a total aqueous volume of 10 ml with a phase ratio (organic/aqueous) of 1.0. The pH of the solution was adjusted with lithium hydroxide or 211
ROBERT Fx~~tea and C. M. WAI
212
nitric acid and measured with an Orion model 701A digital analyzer and Orion 8103 semimicro combination electrode. The mixture was shaken for 45 min, which was determined to be equilibrium by a series of time dependent experiments, at 25.0 f 0.5”, using a Burrell model 75 wrist-action shaker. After phase separation, samples of the organic and aqueous phases were taken with disposable pipettes and counted in polyvials on a large volume EG&G Ortec Ge(Li) detector with a resolution of 2.3 keV at 1332 keV. The following radioisotopes and characteristic gamma energies were used for the identification and quantification of the lanthanoids: lWLa (40.2 hr, 328,487,815, 1596 keV), 14’Ce (32.5 d, 145 keV), ‘43Ce(33.7 hr, 293 keV), ‘42Pr (19.2 hr, 1575 keV), 14’Nd (11.1 d, 91, 531 keV), ‘52mEu(9.3 hr, 121, 344, 841 keV), ‘&Ho (26.8 d, 82 keV), and “‘Lu (6.7 d, 208 keV). The multichannel analyzer was an EG&G ADCAM model 950A. EG&G software was used to set visually-adjusted peak and background regions. A program was written to reduce data, correcting exactly for decay during counting. The details of neutron irradiation and gamma spectroscopy are described elsewhere.6 RESULTS AND DISCUSSION
Preliminary experiments showed the lanthanoid-EDTA complexes were insoluble in 1,2dichloroethane. Calculations of the effective stability constants of La and Lu with EDTA also indicated that the lanthanoid ions would be released in the pH region 2-3. The distribution coefficients(D) of La3+, Ce3+, Pr3+, Nd3+, Eu3+, Ho3+ and Lu3+ in the pH range 1.9-2.8 and the experimental conditions are given in Table 1. The percent error is one-sigma counting statistic.
0 8
J 1.!30
2.m
2.50
3.w
PH
Fig. 1. Graph of log D versus pH. Experimental conditions: FCA] = 1.1M [18-C-6] = O.O17M,Ln’+ = 1 x 10-‘&f [EDTA] = O.OOZM,25 f O.S”, solvent = l,Zdichloroethane, phase ratio = 1.0.
The D values for Ho are very small (< 10m3) with relatively large errors and those for Lu are virtually unmeasurable. A significant observation of this system is the large selectivities for the light lanthanoids, e.g. the selectivities of La/Cc, La/Pr and La/Eu are 10.2, 101 and 1.04 x 104, respectively, at pH 2.74. These values are 3 to 7 times greater than the best selectivities reported by Samy et ~1.~ A graph of log D us. pH for Las+, Ce3+, Pr’+, Nd3+ and Eu3+ is shown in Fig. 1. The graph is linear in the pH region 2.1-2.8, which can be accounted for by the following model. The complexation of a cation (M”+) with a macrocycle (L) and a counter anion (A- ) can be generally expressed by the following equation: M”++mL+nA--ML The extraction equation 2. K =
constant
A mn K
is
(1) given
~~~~~,1,,/~~“+1~~1”~~-1
by
(2)
or K = D’/[L]“[A-]
(3)
Table 1. Distribution coefficients of lanthanoids with number of measurements (in parentheses) and per cent error
La Ce Pr Nd Eu Ho
pH 1.94
pH 2.11
pH 2.23
pH 2.42
pH 2.74
pH 2.84
4.58 (4) 2.8% 1.65 (2) 8.3% 0.457 (1) 3.8% 0.121 (2) 7.8% 3.13 x 10-3 (2) 2.0% 4.65 x 10-‘(l) 35%
6.49 (3) 2.4% 1.73 (2) 6.0% 0.342 (1) 3.9% 0.0748 (2) 9.5% 1.09 x 10-j (3) 3.9%
5.53 (4) 2.0% 1.07 (2) 5.2% 0.183 (1) 4.2% 0.0202 (2) 24% 4.31 x lo-‘(3) 7.2%
4.15 (4) 1.9% 0.613 (2) 6.2% 0.0729 (1) 6.0% 0.0148 (2) 28% 2.17 x lo-‘(2) 18%
1.75 (7) 1.6% 0.172 (4) 6.7% 0.0173 (1) 11.9% 3.97 x 10-J (1) 50% 1.69 x lo-‘(2) 26%
1.34 (3) 1.4% 0.118 (2) 5.9%
Experimental conditions: [TCA] = l.lM, [18-C-6] = O.O17M, Ln3+ = 1 x lo-‘M dichloroethane, temp = 25.0 f 0.5”.
each, [EDTA] = O.O02M, solvent = 1,2-
Separation
of trivalent lantbanoidsby solventextraction
where D’ is the distribution coefficient without EDTA. If two cations M, and M, have the same stoichiometry in the extraction process, the selectivity a is related to the relative extraction constants K, and K2 by the following equation. a = KJK, = D2/D,
(4)
If a complexonate such as EDTA (YHd) is present in the aqueous phase, lanthanoid ions can displace one or more protons in the complexonate to form different water-soluble complexes. In the pH range 2-3, H.,Y and H3Yare the two EDTA species present in water with the latter being the predominant one and having almost constant concentration in this region. The reaction of lanthanoid ions with the complexonate can be represented by the following reactions: Ln3+ + H,Y- = LnH*Y+ + H+
(5)
Ln3+ + H,Y- = LnHY + 2H+
(6)
Ln3+ + H,Y- = LnY- + 3H+
(7)
These reactions form.
are of the following general
Ln3+ + H,Y- = LnHo _n3Y(2-r’)+ n’H+ (8) The distribution coefficient in the presence of complexonate is given by the following equation. D = [LnL,A,],,/([Ln3+]
+ [LnH,_,,.Y”-“‘)I) (9)
=
[LnmA,l,/[Ln3+I(1+ W-13Y-I/[H+l”) (10)
= D’/( 1 + K’[H,Y - ]/[H+ I”‘)
(11)
where D’ is the distribution coefficient without complexonate and K’ is the equilibrium constant for reaction (8). Since excess EDTA is used, [LnH, _ ..Yc2-“‘)]/[Ln3+ ] or K’[H,Y- ]/ [H+]“’ B 1, and equation (11) can be simplified as the following. D = D’/(K’fJ-l,Y-]/[H+ I”)
(12)
Therefore, log D =log D’-
log K’[H3Y-] - n’pH (13)
In the pH range 2-3,[A-] is practically a constant because the pK, value of TCA is very small, about 0.65. Since [L] (1.7 x 10A2M) is much larger than [Ln3+] (1 x lo-‘M) in our experiments, it can also be considered as a con-
213
stant. Consequently, D’ should be a constant in this pH range, as observed experimentally by Samy et al.3 It was pointed out previously that [H,Y-] is also near a constant in this pH range. Therefore, log D should be linearly related to pH as shown in equation (14). log D = constant - n’pH
(14)
According to Fig. 1, the slopes for La3+, Ce3+, Pr’+ and Eu3+ are - 1.2, - 1.6, -2.1 and - 2.8, respectively, indicating less protonated complexes are favored with the heavier lanthanoids. Consequently, the differences in relative D values increase with pH as shown in Fig. 1. At pH 2.7, the a value for La3+/Ce3+ is 10.2 and for La3+/PIJ+ is 101. With each unit increase in the 2 number of the lanthanoids, the D value appears to decrease by approximately one order of magnitude from La’+ to Nd3+ at pH 2.8. We did not extend our experiments at pH higher than 3 because the D values for most of the lanthanoids studied became so small and could not be measured accurately. According to equation (12) the selectivity with complexonate (a) is enhanced by the factor (K;/K;)([H+ ]“j-“i) with respect to (a’), the selectivity without complexonate, for two metals as shown in equation (15). a = a’(K;/K;)([H+]“i-“i)
(1%
Since lanthanoid ions form protonated complexes with EDTA and their stability constants are not available in the literature, it is not possible to calculate the selectivity with equation (15). However, it is known that at pH > 3, lanthanoids form unprotonated complexes (LnY-) with EDTA. Using the stability constants of LnY- from Martell and Smith’ and the D values obtained without EDTA as given in Table 2, the calculated selectivity of La/Cc (ah&, and Ce/Pr (klpr) are 11 f 1 and 12 f 1, respectively. Our measured values at pH 3.1 were 15 & 1 for (aLa& and 15 f 3 for (atim), in reasonable agreement with the calculated values. A comparison of the D values obtained without complexonate is given in Table 2. Data reported by Samy et al., as taken from a figure,3 are also given in the table. Their D values are for slightly different TCA and 18crown-6 concentrations and should be about 20% greater than ours. After normalization, our data without EDTA agree well with those reported by Samy et ~1.~ within experimental uncertainties. Furthermore, our experimental
Roumar FRAZIBRand C. M. WAI
214
Table 2. Distribution coefllcients of lanthanoids without EDTA complcxonate at 25.0 f 0.5” D values Literature*
La’+
1.6 2.0 0.56 0.10 0.004
Ce’+ Pr’+ Nd3+ EU’+
This workt 5.8 1.6 0.36 0.096 0.0039
l[TCA] = l.OM,
[l&crown-61 = O.OlM, Ln’+ = 1 x 10-sM each, pH 3.0. (Ref. 3) t[TCA] = 0.9M, [l&crown-61 = 0.01 lhf, Ln’+ = 1 x lo-‘M each , pH 3. 1.
lop k-1
Fig. 3. Graph of log (D)-nlog[H+] versus log(A-1. Experimental conditions: [18-C-6] = O.O17M, Ln3+ = 1 x lo-‘M each, [EDTA] = O.OOZM,25 f 0.5”, solvent = 1,2dichloroethane, phase ratio = 1.O.
results indicate that in the absence of EDTA, the D value for La’+ decreased by nearly a factor of 40 from pH 2 to 0.7. Other lanthanoids such as PlJ+ , Nd3+, Eu3+ and Ho3+ showed a similar decrease in D at low pH. This decrease at low pH suggests that the lanthanoid-1% crownd-TCA complex in 1,Zdichloroethane can be back-extracted with an acid solution. Experimentally, we have verified that lanthanoids extracted into 1,2dichloroethane in this system can be stripped from the organic phase with 0.3M nitric acid. This reversibility is important for the recovery of lanthanoids in practical applications. The model given has also been experimentally verified as follows: Substituting K[A- ]“[L]” [equation (3)] for D' in equation (12) and taking the log gives the following linearization. log(D/[H+l”‘)=logK+nlog[A-] + mlog [L] - log K’[H,Y-]
(16)
Figure 2 shows the variation of log(D/[H+]“‘) with respect to log[L] in the pH region 2.1-2.9 using the n’ values determined from Fig. 1. As
shown in Fig. 2, La3+, Ce3+ and PIJ+ gave linear plots. The slope for La3+ was 1.1 and the slopes for Ce3+ and P?+ were 1.0, suggesting a 1: 1 complex for the lanthanoids with 18-crown-6. Figure 3 offers further support, showing a linear log(D/[H+]“‘) 11s.log[A-] plot. Slopes of 2.5 for La3+ and Ce3+ were obtained, which imply three TCA anions in the complex. Samy ef al.’ obtained slopes of 2.15 and 2.41 for Las+ and Ce3+ respectively, without EDTA.’ They explained’ these values as being a result of the decreasing activity coefficient with ionic strength. In conclusion, EDTA showed enhanced selectivities for lanthanoids in solvent extraction with 18-crown-6 and TCA. The effect of the complexonate on lanthanoid extraction can be explained by a simple model presented in this paper. The selectivities observed in this system are among the largest reported in the literature for the light lanthanoids. Acknowledgements-This work was supported, in part, by the Idaho EPSCoR Program of the National Science Foundation (Grant No. RII-8902065) and by the Idaho State Board of Education.
REFERENCES
Fig. 2. Graph of log (0) - n’log[H+] versus log[18-C-61. Experimental conditions: FCA] = 0.93M, Ln’+ = 1 x lo-‘A4 each, @DTA] = O.OOZM,25 f 0.5”, . solvent ._ = 1,2_dichloroetbane, phase ratio = 1.0.
1. J. G. Bunzli and D. Wessner, Coord. Chem. Rev., 1984, 60, 191. 2. I. M. Kolthoff, Anal. Chem., 1979, 51, IR. 3. T. M. Samy, N. Suzuki and H. Imura, J. Radioanal. Nucl. Chem. Letters, 1988, 126, 153. 4. T. Moeller, Inorganic Chemistry, An Advanced Textbook, Wiley, New York, 1965. 5. G. Schwarxenbach and H. Flaschka, Complexometric Titration, Methuen & Co., London, 1969. 6. W. M. Mok and C. M. Wai, Anal. Chem., 1987,59,233. 7. A. E. Martell and R. M. Smith, Critical Stabi!ity Constants, Vol. I: Amino Acids, Plenum Press, New York, 1974.
Tahnta, Vol. 39, No. 3, pp. 211-214, 1992 Printed in Great Britain. All rights racrwd
Copyright0 1992 Pcrgamon Prcsa pk
ENHANCED SEPARATION OF TRIVALENT LANTHANOIDS BY SOLVENT EXTRACTION WITH 18-CROWN-6 AND EDTA COMPLEXONATE ROBERT FRAZIERand C. M. WAI*
Department of Chemistry, University of Idaho, Moscow, Idaho 83843, U.S.A. (Receiued 12 August 1991. Accepted 29 August 1991) W-The selectivities during solvent extraction of lanthanoids with macrocycles can be modified with complexonates in the aqueous phase. In the CBSC of solvent extraction of lanthanoids with 18-crown-6 and trichloroacetic acid (TCA), addition of EDTA to the aqueous phase enhances the selectivities of lanthanoids by 3-7 times compared to those without the complexonate. This is due to the fact that the stability of lanthanoid-EDTA complexes increases in the opposite direction to the crown-TCA complexes across the lanthanoid series. The selectivities observed in this system are among the largest reported for the light lanthanoids. The effect of the complexonate on lanthanoid extraction can be explained by a simple model presented in this paper.
Macrocyclic polyethers (crown ethers) are known to form stable complexes of different with trivalent lanthanoids.’ stoichiometry The stability of crown complexes in solvent extraction depends generally on the following four factors: crown cavity to cation diameter ratio, the counter anion, ligand flexibility, and the type of donor atoms (N, 0 or S) in the macrocyclic host.2 Using 18-crown-6 to extract lanthanoid ions in aqueous solution with trichloroacetate (TCA) as counter anions, selectivities as good as the best commercial extractant, di-2-ethylhexyl-phosphoric acid (HDEHP) were obtained by Samy et aL3 The relative stability constants of the lanthanoid complexes in this case decrease by three orders of magnitude from La’+ to Eu3+. The large selectivity observed in this macrocyclic system has been attributed to the steric hindrance of the lanthanoid complex with TCA. The decrease in stability across the lanthanoid series with 18-crown-dTCA complexes is significant, because it is opposite to many known lanthanoid complexonates in aqueous solution. Therefore, by choosing a proper complexonate, the two opposite trends may compliment each other resulting in enhanced selectivity for the extraction of lanthanoids by l%crown-6. It is known that lanthanoid-EDTA complexes show increased stabilities from La’+ to Lu3+ and the complexation depends on PH.~ This paper reports *Author for correspondence.
the enhanced lanthanoid selectivity in solvent extraction, using 18crown-6 as an extractant, TCA as the anion and EDTA as a complexonate with 1,2-dichloroethane solvent. Hydrogen ion is used as a displacer to release lanthanoids complexed with EDTA in the aqueous phase. EXPERIMENTAL
The 18-crown-6 extractant was purchased from Aldrich. The EDTA was Aldrich gold label and dried four days at 80” according to Schwartzenbach.5 The La, Eu and Lu solutions were purchased as standard solutions from Aldrich and Thiokol. Lanthanoid solutions of Ce, Nd and Ho were prepared from their nitrates obtained from Aldrich. Praseodymium was prepared from its oxide obtained from Baker and Adamson. The solvent, 1,Zdichloroethane, was purchased from EM Science. Demineralised water was used in all experiments. Radioisotope tracer techniques were used in the extraction experiments. Usually, lanthanoid solutions were irradiated in a TRIGA nuclear reactor with a steady flux of 6 x lOI n.cm-2.sec- ’ for one hour. An aliquot of this radioisotope solution was placed in a Pyrex tube (50 ml), containing TCA, 18-crown-6, EDTA, and lanthanoids of known concentration in a total aqueous volume of 10 ml with a phase ratio (organic/aqueous) of 1.0. The pH of the solution was adjusted with lithium hydroxide or 211
ROBERT Fx~~tea and C. M. WAI
212
nitric acid and measured with an Orion model 701A digital analyzer and Orion 8103 semimicro combination electrode. The mixture was shaken for 45 min, which was determined to be equilibrium by a series of time dependent experiments, at 25.0 f 0.5”, using a Burrell model 75 wrist-action shaker. After phase separation, samples of the organic and aqueous phases were taken with disposable pipettes and counted in polyvials on a large volume EG&G Ortec Ge(Li) detector with a resolution of 2.3 keV at 1332 keV. The following radioisotopes and characteristic gamma energies were used for the identification and quantification of the lanthanoids: lWLa (40.2 hr, 328,487,815, 1596 keV), 14’Ce (32.5 d, 145 keV), ‘43Ce(33.7 hr, 293 keV), ‘42Pr (19.2 hr, 1575 keV), 14’Nd (11.1 d, 91, 531 keV), ‘52mEu(9.3 hr, 121, 344, 841 keV), ‘&Ho (26.8 d, 82 keV), and “‘Lu (6.7 d, 208 keV). The multichannel analyzer was an EG&G ADCAM model 950A. EG&G software was used to set visually-adjusted peak and background regions. A program was written to reduce data, correcting exactly for decay during counting. The details of neutron irradiation and gamma spectroscopy are described elsewhere.6 RESULTS AND DISCUSSION
Preliminary experiments showed the lanthanoid-EDTA complexes were insoluble in 1,2dichloroethane. Calculations of the effective stability constants of La and Lu with EDTA also indicated that the lanthanoid ions would be released in the pH region 2-3. The distribution coefficients(D) of La3+, Ce3+, Pr3+, Nd3+, Eu3+, Ho3+ and Lu3+ in the pH range 1.9-2.8 and the experimental conditions are given in Table 1. The percent error is one-sigma counting statistic.
0 8
J 1.!30
2.m
2.50
3.w
PH
Fig. 1. Graph of log D versus pH. Experimental conditions: FCA] = 1.1M [18-C-6] = O.O17M,Ln’+ = 1 x 10-‘&f [EDTA] = O.OOZM,25 f O.S”, solvent = l,Zdichloroethane, phase ratio = 1.0.
The D values for Ho are very small (< 10m3) with relatively large errors and those for Lu are virtually unmeasurable. A significant observation of this system is the large selectivities for the light lanthanoids, e.g. the selectivities of La/Cc, La/Pr and La/Eu are 10.2, 101 and 1.04 x 104, respectively, at pH 2.74. These values are 3 to 7 times greater than the best selectivities reported by Samy et ~1.~ A graph of log D us. pH for Las+, Ce3+, Pr’+, Nd3+ and Eu3+ is shown in Fig. 1. The graph is linear in the pH region 2.1-2.8, which can be accounted for by the following model. The complexation of a cation (M”+) with a macrocycle (L) and a counter anion (A- ) can be generally expressed by the following equation: M”++mL+nA--ML The extraction equation 2. K =
constant
A mn K
is
(1) given
~~~~~,1,,/~~“+1~~1”~~-1
by
(2)
or K = D’/[L]“[A-]
(3)
Table 1. Distribution coefficients of lanthanoids with number of measurements (in parentheses) and per cent error
La Ce Pr Nd Eu Ho
pH 1.94
pH 2.11
pH 2.23
pH 2.42
pH 2.74
pH 2.84
4.58 (4) 2.8% 1.65 (2) 8.3% 0.457 (1) 3.8% 0.121 (2) 7.8% 3.13 x 10-3 (2) 2.0% 4.65 x 10-‘(l) 35%
6.49 (3) 2.4% 1.73 (2) 6.0% 0.342 (1) 3.9% 0.0748 (2) 9.5% 1.09 x 10-j (3) 3.9%
5.53 (4) 2.0% 1.07 (2) 5.2% 0.183 (1) 4.2% 0.0202 (2) 24% 4.31 x lo-‘(3) 7.2%
4.15 (4) 1.9% 0.613 (2) 6.2% 0.0729 (1) 6.0% 0.0148 (2) 28% 2.17 x lo-‘(2) 18%
1.75 (7) 1.6% 0.172 (4) 6.7% 0.0173 (1) 11.9% 3.97 x 10-J (1) 50% 1.69 x lo-‘(2) 26%
1.34 (3) 1.4% 0.118 (2) 5.9%
Experimental conditions: [TCA] = l.lM, [18-C-6] = O.O17M, Ln3+ = 1 x lo-‘M dichloroethane, temp = 25.0 f 0.5”.
each, [EDTA] = O.O02M, solvent = 1,2-
Separation
of trivalent lantbanoidsby solventextraction
where D’ is the distribution coefficient without EDTA. If two cations M, and M, have the same stoichiometry in the extraction process, the selectivity a is related to the relative extraction constants K, and K2 by the following equation. a = KJK, = D2/D,
(4)
If a complexonate such as EDTA (YHd) is present in the aqueous phase, lanthanoid ions can displace one or more protons in the complexonate to form different water-soluble complexes. In the pH range 2-3, H.,Y and H3Yare the two EDTA species present in water with the latter being the predominant one and having almost constant concentration in this region. The reaction of lanthanoid ions with the complexonate can be represented by the following reactions: Ln3+ + H,Y- = LnH*Y+ + H+
(5)
Ln3+ + H,Y- = LnHY + 2H+
(6)
Ln3+ + H,Y- = LnY- + 3H+
(7)
These reactions form.
are of the following general
Ln3+ + H,Y- = LnHo _n3Y(2-r’)+ n’H+ (8) The distribution coefficient in the presence of complexonate is given by the following equation. D = [LnL,A,],,/([Ln3+]
+ [LnH,_,,.Y”-“‘)I) (9)
=
[LnmA,l,/[Ln3+I(1+ W-13Y-I/[H+l”) (10)
= D’/( 1 + K’[H,Y - ]/[H+ I”‘)
(11)
where D’ is the distribution coefficient without complexonate and K’ is the equilibrium constant for reaction (8). Since excess EDTA is used, [LnH, _ ..Yc2-“‘)]/[Ln3+ ] or K’[H,Y- ]/ [H+]“’ B 1, and equation (11) can be simplified as the following. D = D’/(K’fJ-l,Y-]/[H+ I”)
(12)
Therefore, log D =log D’-
log K’[H3Y-] - n’pH (13)
In the pH range 2-3,[A-] is practically a constant because the pK, value of TCA is very small, about 0.65. Since [L] (1.7 x 10A2M) is much larger than [Ln3+] (1 x lo-‘M) in our experiments, it can also be considered as a con-
213
stant. Consequently, D’ should be a constant in this pH range, as observed experimentally by Samy et al.3 It was pointed out previously that [H,Y-] is also near a constant in this pH range. Therefore, log D should be linearly related to pH as shown in equation (14). log D = constant - n’pH
(14)
According to Fig. 1, the slopes for La3+, Ce3+, Pr’+ and Eu3+ are - 1.2, - 1.6, -2.1 and - 2.8, respectively, indicating less protonated complexes are favored with the heavier lanthanoids. Consequently, the differences in relative D values increase with pH as shown in Fig. 1. At pH 2.7, the a value for La3+/Ce3+ is 10.2 and for La3+/PIJ+ is 101. With each unit increase in the 2 number of the lanthanoids, the D value appears to decrease by approximately one order of magnitude from La’+ to Nd3+ at pH 2.8. We did not extend our experiments at pH higher than 3 because the D values for most of the lanthanoids studied became so small and could not be measured accurately. According to equation (12) the selectivity with complexonate (a) is enhanced by the factor (K;/K;)([H+ ]“j-“i) with respect to (a’), the selectivity without complexonate, for two metals as shown in equation (15). a = a’(K;/K;)([H+]“i-“i)
(1%
Since lanthanoid ions form protonated complexes with EDTA and their stability constants are not available in the literature, it is not possible to calculate the selectivity with equation (15). However, it is known that at pH > 3, lanthanoids form unprotonated complexes (LnY-) with EDTA. Using the stability constants of LnY- from Martell and Smith’ and the D values obtained without EDTA as given in Table 2, the calculated selectivity of La/Cc (ah&, and Ce/Pr (klpr) are 11 f 1 and 12 f 1, respectively. Our measured values at pH 3.1 were 15 & 1 for (aLa& and 15 f 3 for (atim), in reasonable agreement with the calculated values. A comparison of the D values obtained without complexonate is given in Table 2. Data reported by Samy et al., as taken from a figure,3 are also given in the table. Their D values are for slightly different TCA and 18crown-6 concentrations and should be about 20% greater than ours. After normalization, our data without EDTA agree well with those reported by Samy et ~1.~ within experimental uncertainties. Furthermore, our experimental
Roumar FRAZIBRand C. M. WAI
214
Table 2. Distribution coefllcients of lanthanoids without EDTA complcxonate at 25.0 f 0.5” D values Literature*
La’+
1.6 2.0 0.56 0.10 0.004
Ce’+ Pr’+ Nd3+ EU’+
This workt 5.8 1.6 0.36 0.096 0.0039
l[TCA] = l.OM,
[l&crown-61 = O.OlM, Ln’+ = 1 x 10-sM each, pH 3.0. (Ref. 3) t[TCA] = 0.9M, [l&crown-61 = 0.01 lhf, Ln’+ = 1 x lo-‘M each , pH 3. 1.
lop k-1
Fig. 3. Graph of log (D)-nlog[H+] versus log(A-1. Experimental conditions: [18-C-6] = O.O17M, Ln3+ = 1 x lo-‘M each, [EDTA] = O.OOZM,25 f 0.5”, solvent = 1,2dichloroethane, phase ratio = 1.O.
results indicate that in the absence of EDTA, the D value for La’+ decreased by nearly a factor of 40 from pH 2 to 0.7. Other lanthanoids such as PlJ+ , Nd3+, Eu3+ and Ho3+ showed a similar decrease in D at low pH. This decrease at low pH suggests that the lanthanoid-1% crownd-TCA complex in 1,Zdichloroethane can be back-extracted with an acid solution. Experimentally, we have verified that lanthanoids extracted into 1,2dichloroethane in this system can be stripped from the organic phase with 0.3M nitric acid. This reversibility is important for the recovery of lanthanoids in practical applications. The model given has also been experimentally verified as follows: Substituting K[A- ]“[L]” [equation (3)] for D' in equation (12) and taking the log gives the following linearization. log(D/[H+l”‘)=logK+nlog[A-] + mlog [L] - log K’[H,Y-]
(16)
Figure 2 shows the variation of log(D/[H+]“‘) with respect to log[L] in the pH region 2.1-2.9 using the n’ values determined from Fig. 1. As
shown in Fig. 2, La3+, Ce3+ and PIJ+ gave linear plots. The slope for La3+ was 1.1 and the slopes for Ce3+ and P?+ were 1.0, suggesting a 1: 1 complex for the lanthanoids with 18-crown-6. Figure 3 offers further support, showing a linear log(D/[H+]“‘) 11s.log[A-] plot. Slopes of 2.5 for La3+ and Ce3+ were obtained, which imply three TCA anions in the complex. Samy ef al.’ obtained slopes of 2.15 and 2.41 for Las+ and Ce3+ respectively, without EDTA.’ They explained’ these values as being a result of the decreasing activity coefficient with ionic strength. In conclusion, EDTA showed enhanced selectivities for lanthanoids in solvent extraction with 18-crown-6 and TCA. The effect of the complexonate on lanthanoid extraction can be explained by a simple model presented in this paper. The selectivities observed in this system are among the largest reported in the literature for the light lanthanoids. Acknowledgements-This work was supported, in part, by the Idaho EPSCoR Program of the National Science Foundation (Grant No. RII-8902065) and by the Idaho State Board of Education.
REFERENCES
Fig. 2. Graph of log (0) - n’log[H+] versus log[18-C-61. Experimental conditions: FCA] = 0.93M, Ln’+ = 1 x lo-‘A4 each, @DTA] = O.OOZM,25 f 0.5”, . solvent ._ = 1,2_dichloroetbane, phase ratio = 1.0.
1. J. G. Bunzli and D. Wessner, Coord. Chem. Rev., 1984, 60, 191. 2. I. M. Kolthoff, Anal. Chem., 1979, 51, IR. 3. T. M. Samy, N. Suzuki and H. Imura, J. Radioanal. Nucl. Chem. Letters, 1988, 126, 153. 4. T. Moeller, Inorganic Chemistry, An Advanced Textbook, Wiley, New York, 1965. 5. G. Schwarxenbach and H. Flaschka, Complexometric Titration, Methuen & Co., London, 1969. 6. W. M. Mok and C. M. Wai, Anal. Chem., 1987,59,233. 7. A. E. Martell and R. M. Smith, Critical Stabi!ity Constants, Vol. I: Amino Acids, Plenum Press, New York, 1974.