Synergic Extraction of Lanthanoids by Mixtures of LIX 54 (High Molecular Weight -Diketone) and Bidentate Neutral Amines

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1 2002 The Japan Society for Analytical Chemistry 19 Synergic Extraction of Lanthanoids by Mixtures of LIX 54 (High Molecular Weight -Diketone) and Bidentate Neutral Amines Shigeto NAKAMURA,* Shuichi TAKEI,** and Kenichi AKIBA** *Hachinohe National College of Technology, Tamonoki, Hachinohe , Japan **Institute for Advanced Materials Processing, Tohoku University, Katahira-2, Aoba, Sendai , Japan The synergic extraction of lanthanoids has been investigated using mixtures of high molecular weight β-diketone, LIX 54 (HA; major component, 1-phenyl--isoheptyl-1,-propanedion) and bidentate neutral ligands (S) in toluene. The distribution behavior of bidentate amines (2,2 -bipyridine (bpy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10- phenanthroline (dmp)) was investigated and their related equilibrium constants were evaluated. The synergic effect is produced by the formation of such adduct complexes as MA S. The extraction constants with LIX 54 alone and those in the presence of bidentate ligands were determined for lanthanoid-series elements. (Received September 17, 2001; Accepted November 14, 2001) Introduction Lanthanoid elements are ordinarily stable as trivalent ions in ueous solutions. These ions are extracted into the organic phase with a variety of extractants having oxygen donor atoms, such as β-diketones and acidic organophosphorus compounds, forming neutral metal compounds. Extracted metal chelate compounds are electrically neutral, and sometimes their coordination sites are unsaturated with extractant molecules. In the presence of neutral ligands, such as tributylphosphate (TBP), the extraction is extremely enhanced by a synergic effect, forming adduct complexes saturated with neutral ligands. Although the extraction of metal complexes is greatly enhanced by a synergic effect, the separation of desired metal ions becomes inferior in the usual cases with monodentate neutral ligands. Thus, the mutual separation of lanthanoids elements has been rather limited. On the other hand, in the extraction of lanthanoids with a specific β-diketone, large enhancements were produced not only in their distribution ratios, but also in their separation factors in the presence of bidentate neutral amines, such as 1,10-phenanthroline. 1 5 High molecular weight extractants are suitable for industrial use in the separation and purification of metals, with a depression of the soluble loss of the extractant in the ueous phase. LIX 54, a water-insoluble phenyl alkyl β-diketone, is a commercially available chelating extractant; the active component of LIX 54 is 1-phenyl--isoheptyl-1,-propanedion. 6 LIX 54 has extracted a series of lanthanoids(iii), while low extraction ability was observed. 7 The present study dealt with the synergic extraction of a series of lanthanoid elements with a high molecular weight β-diketone (LIX 54) and bidentate neutral amines (2,2 -bipyridine (bpy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10- phenanthroline (dmp)) as additive ligands. The extracted adduct complexes were clarified and their formation constants were determined. Experimental Reagents The concentration of LIX 54 (Henkel Corporation) was determined by the acid-base titration of an LIX54 ethanol solution with a sodium hydroxide solution. Bidentate neutral amines (2,2 -bipyridine (bpy), 1,10-phenanthroline (phen), and 2,9-dimethyl-1,10-phenanthroline (dmp)) were obtained from Wako Pure Chemical Ind., Ltd. Toluene was used as an inert diluent for these extractants. The lanthanoid solutions were obtained by diluting standard solutions of lanthanoids for atomic absorption analysis (Wako). Preparation of radioisotopes Lanthanoid oxides (La 2O, Tb 4O 7, Tm 2O and Lu 2O ) were sealed in a quartz ample and irradiated for 6 h by neutrons with a flux of n/cm 2 in a JRR- or JRR-4 reactor of the Japan Atomic Energy Research Institute. The radioisotopes ( 140 La (t 1/2 = 40.2 h), 160 Tb (t 1/2 = 72.4 d), 170 Tm (t 1/2 = d) and 177 Lu (t 1/2 = 6.7 d)) were produced through (n, γ) reactions. Distribution of amines A toluene solution containing a bidentate amine was equilibrated with an ueous solution of 0.1 M (M = mol dm ) sodium nitrate for 1 h at 25 C. Since the absorption of amines at nm overlapped with that of toluene, the amine concentration in the organic phase was determined as protonated species after stripping into a 0.1 M hydrochloric acid solution. The absorption for amines in the ueous phase overlapped with that for NO. In order to avoid this interference, the amine in the ueous phase, after distribution equilibrium, was recovered into fresh toluene with a known yield at a high ph by adding a sodium hydroxide solution, and then stripped into 0.1 M hydrochloric acid solution to measure the absorbance. The molar-absorption coefficients of protonated amines were calculated as at 00 nm for bpy, at 270 nm for phen, and 1100 at 281 nm for dmp.

2 20 ANALYTICAL SCIENCES MARCH 2002, VOL. 18 Table 1 Partition coefficients and acid-dissociation constants of amines Bidentate amine log K HS log P S bpy phen dmp Fig. 1 Distribution of bidentate amines between toluene and a 0.1 M NaNO ueous solution., phen;, bpy;, dmp. The open and closed symbols are for the absence and presence of 0.1 M LIX 54, respectively. Extraction of lanthanoids An ueous solution of 10 5 M lanthanoid labeled with the desired radioisotope was shaken with a toluene solution of extractants for 1 h at 25 C. The ueous ph values were controlled with 10 M acetic acid, 5 10 M piperazine-1,4- bis(2-ethnesulfonic acid) (PIPES; Wako) or sodium hydroxide. The ionic strength of the ueous phase was kept at 0.1 with NaNO. After phase separation, aliquots of two phases were pipetted out, and then their γ-radioactivities were measured by a counter with a well-type NaI (Tl) scintillator (OKEN, Model RC-101A). When radioisotopes were not available, the concentrations of the lanthanoids were determined by ICP-AES (PERKIN ELMER, Optima 00XL). 7 Results and Discussion Distribution equilibrium of amines Neutral bidentate amines distribute between the organic and ueous phases, and have an acid-base equilibrium in the ueous phase. The association between a neutral ligand and an acidic extractant sometimes occurs in the organic phase. The equilibrium concentration of each free amine in the organic phase is significant for analyzing the extraction equilibria of metals. Here, the distribution behavior of neutral ligands was examined and related equilibrium constants were evaluated. The variation in the distribution ratios of amines is illustrated as a function of the ph in Fig. 1. The open symbols are related to the distribution ratio (D S) of sole amine, and are almost constant at high-ph regions where only neutral species exist. In the acidic region, the distribution ratios increase with the ph values following straight lines with a slope of one, where protonated amine species are predominant. The distribution ratio of the amine (S) can be expressed as [S] org D S = =, (1) [S] 1+ [H + + [HS + ] ] /K HS where P S and K HS denote the partition coefficient of amine and the acid dissociation constant of the protonated one, respectively. The values of P S and K HS were evaluated by a least-squares fitting method based on Eq. (1), as shown in Table 1. The solid lines indicate the calculated values from Eq. (1), and are in good agreement with the experimental plots. The distribution ratios of amines in the presence of LIX 54 are also illustrated in Fig. 1 as closed symbols. These plots almost P S follow the calculated lines, indicating that no interaction exists between the chelating extractant and the neutral amines. The values of P S for the amines obtained in this work are very close to the corresponding values using benzene as a diluent. 8 This is predictable from the regular solution theory, 9 since the solubility parameters of benzene and toluene are close to each other. The values of P S are found to be in the order bpy > dmp > phen. The lower value for phen, which has a larger molecular weight than bpy, seems to indicate a large interaction with water molecules in the ueous phase. The equilibrium concentration of free amine in the organic phase in synergic extraction experiments was calculated from the initial concentration ([S] init) and these equilibrium constants as [S] init [S] org =. (2) 1 + (1 + [H + ] /K HS)/P S Effect of the ph on the extraction of lanthanoids The extraction behavior of lanthanoids with LIX 54 and three kinds of bidentate amines was examined by varying the ueous ph value. The distribution ratios of the lanthanoids are illustrated as a function of ph in Fig. 2: (a) for La of a light lanthanoid, (b) for Tb of a middle lanthanoid, and (c) for Lu of a heavy lanthanoid. A plot for the extraction with LIX 54 alone consists of a straight line with a slope equal to, indicating that protons are released in the extraction equilibrium. The distribution ratios by the mixtures with bidentate amines are greatly enhanced by a synergic effect, and follow almost parallel lines with slopes close to, except for the Tb-phen and Lu-phen systems, in which the equilibrium concentration of phen in the organic phase below ph 5 increase with increasing ph. Effect of the LIX 54 concentration The distribution ratios of lanthanoids are illustrated as a function of the LIX 54 concentration in Fig.. These plots for La, Tb, and Lu in the absence and presence of neutral bidentate amines almost follow straight lines with slopes of around, indicating that the number of LIX 54 molecules in the extracted species remains unaltered for all lanthanoids. Extraction equilibrium with β-diketone (HA) alone is expressed by M + + HA org MA,org + H +. () The extraction constant is given by K ex = ] org[h + ]. (4) [M + ] [HA] org The synergic extraction equilibrium with LIX 54 and bidentate amine in the form of the MA ns adduct is expressed by M + + HA org + ns org MA ns org + H +. (5)

3 21 Fig. 2 Distribution ratios of (a) La, (b) Tb, and (c) Lu with 0.1 M LIX 54 in the absence and presence of 0.01 M amine as a function of the ph., LIX 54 alone;, phen;, bpy;, dmp. Fig. Distribution ratios of (a) La, (b) Tb, and (c) Lu in the absence and presence of 0.01 M amine as a function of the LIX 54 concentration., LIX 54 alone: La, ph 7.4; Tb, ph 6.4; Lu, ph 5.8., phen: La, ph 5.8; Tb, ph 5.5; Lu, ph 5.5., bpy: La, ph 7.1; Tb, ph 5.7; Lu, ph 5.5., dmp: Tb, ph 6.. The synergic extraction constant is given by K exs = ns] org[h + ]. (6) [M + ] [HA] org [S] n org Effect of the bidentate amine concentration A large synergic enhancement in the extraction of lanthanoids by a mixture of LIX 54 and bidentate amines is produced owing to the formation of adduct complexes with neutral ligands. The distribution ratios of lanthanoid in the extraction with LIX 54 alone and in a synergic extraction are written as K D 0 = = ex [HA] ] org org (7) [M + ] [H + ] and D= ] [HA] =K ex 1+ ΣK exs,n[s] n org + Σ ns] org org org. [M + ] [H + ] K ex (8) When both the concentration of LIX 54 and the ph in the extraction with LIX 54 alone are the same as those in the synergic extraction, D/D 0 is given as D D 0 ΣK exs,n[s] n org K ex = 1 + = 1 + Σβ s,n[s] orgn, (9) where β s,n is the adduct formation constant in the organic phase corresponding to the following equilibrium: MA,org + ns org MA ns org, (10) K β s,n = ns] org exs,n =. (11) ] org [S] n org Kex Figure 4 shows logarithmic plots of D/D 0 for Tb against the neutral ligand concentration in the organic phase. These log log plots give straight lines with a slope of 1, indicating the formation of a 1:1 adduct complex of MA S. The solid lines in this figure represent the calculated values from Eq. (9) using a least-squares method. Similar plots for La having a larger ionic radius and Lu having a smaller size follow straight lines of slope of 1, as shown in Fig. 5. Thus, only one molecule of bidentate amine adds to the β-diketone chelate irrespective of the sizes of the lanthanoids.

4 22 ANALYTICAL SCIENCES MARCH 2002, VOL. 18 Fig. 4 Effect of the bidentate amine concentration on the synergic enhancement in the extraction of Tb with 0.1 M LIX 54., phen;, bpy;, dmp. Fig. 5 Effect of the phen concentration on the synergic enhancement in the extraction of La and Lu with 0.1 M LIX 54., La;, Lu. Table 2 Ln(III) Extraction constants, synergic extraction constants and adduct formation constants in the LIX 54 system LIX 54 alone bpy phen dmp log K ex α log K exs,1 α log βs,1 log K exs,1 α log βs,1 log K exs,1 α log βs,1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Mean value of α Regularities in synergic extraction The equilibrium constants related to synergic extraction by LIX 54 and bidentate amines were evaluated from the extraction behavior of a series of lanthanoids. The extraction constants with LIX 54 alone, and the synergic extraction constants in the presence of each bidentate amine and the desired adduct formation constants are summarized in Table 2. Then, the separation factor (α) between adjacent lanthanoids is defined as the ratio of the extraction constants or the synergic extraction constants. The mean values of α between adjacent lanthanoids in synergic systems are not improved compared with that for the LIX 54 alone. The variations in log K ex and log K exs,1 are illustrated in the order of the atomic number of lanthanoids in Fig. 6. On the whole, the K ex value has a tendency to increase along with the atomic number, though some deviations due to a tetrad effect 10 are observed. In synergic extraction, the K exs,1 value has a trend to roughly increase with the atomic number similarly to the K ex value, and the synergic extraction constants increase in the order dmp < bpy < phen for middle-to-heavy lanthanoids. The adduct formation constants are summarized against the atomic number of the lanthanoids in Fig. 7. The β s,1 values for

5 2 Fig. 6 Variation in the extraction constants and the synergic extraction constants with the atomic number of the lanthanoids. K ex:, LIX 54 alone. K exs,1:, phen;, bpy;, dmp. Fig. 7 Variation in the adduct formation constants with atomic number of the lanthanoids., phen;, bpy;, dmp. phen are much larger than those for bpy and dmp. The large synergic enhancement with phen compared with bpy may be attributable to its high bacisity and its larger activity coefficient in the organic phase. 2 The β s,1 values for bpy and phen roughly increase with the atomic number of light-to-middle lanthanoids, except for La. This trend is different from that for a monodentate ligand, such as TBP, 11 in which a synergic enhancement is ordinarily suppressed with decreasing size of the lanthanoid. The β s,1 values, however, tend to decrease with a further increase in the atomic number of heavy lanthanoids, similarly to monodentate ligands. The β s,1 values in synergic extraction with the β-diketone having a fluoroalkyl group, such as 2-thenoyltrifluoroacetone or pivaloyltrifluoroacetone in the presence of phen, increased with increasing atomic number of light-to-middle lanthanoids, and were almost unaltered or somewhat increased for heavy lanthanoids. 2,5 In contrast, the β s,1 values for acetylacetone (Hacac) or benzoylacetone (Hba) having higher complexing ability in the presence of phen or bpy were almost unaltered, or somewhat increased, for light and middle lanthanoids, and decreased for heavy lanthanoids. 1,2,12 These trends cannot be completely elucidated, but the coordination ability of β-diketone to the central metal seems to further affect the coordination of a neutral ligand. 2 LIX 54 has no fluoroalkyl group, and seems to rather resemble Hacac and Hba concerning the complexation properties. In the case of dmp having methyl groups exerting a steric hindrance, the β s,1 values are rather low compared with the use of phen. The β s,1 values for dmp are larger than those for bpy for light lanthanoids of large sizes, while those for dmp decrease for heavy lanthanoids of small sizes with a large steric hindrance, and are smaller than those for bpy at middle and heavy lanthanoids. These tendencies in adduct formation reflect the overall synergic extraction of lanthanoids, while an apparent increase in the K exs,1 values with atomic number mainly depends on a large increase tendency of the K ex values. Acknowledgements This work has been supported by the Inter-University Program for the Joint Use of JAERI Facilities. References 1. S. Nakamura and N. Suzuki, Inorg. Chim. Acta, 1986, 114, S. Nakamura and N. Suzuki, Polyhedron, 1986, 5, S. Nakamura and N. Suzuki, Polyhedron, 1988, 7, S. Nakamura and N. Suzuki, Anal. Chim. Acta, 1992, 270, S. Nakamura and N. Suzuki, Bull. Chem. Soc. Jpn., 199, 66, W. Mickler, E. Uhlemann, R. Herzschuh, B. Wenclawiak, and L. Plaggenborg, Sep. Sci. Technol., 1992, 27, S. Nakamura, Y. Surakitbanharn, and K. Akiba, Anal. Sci., 1989, 5, S. Nakamura, H. Imura, and N. Suzuki, Inorg. Chim. Acta, 1985, 110, S. Nakamura, H. Imura, and N. Suzuki, Inorg. Chim. Acta, 1985, 109, D. F. Peppard, G. W. Mason, and S. Lewey, J. Inorg. Nucl. Chem., 1969, 1, L. Farbu, J. Alstad, and J. H. Augstson, J. Inorg. Nucl. Chem., 1974, 6, S. Nakamura and N. Suzuki, J. Radioanal. Nucl. Chem., 1986, 99, 145.