Extractive separation of trivalent lanthanide metals with a combination of Di(2-ethylhexyl)phosphoric acid and 1,10-phenanthroline

Talanta ELSEVIER

Talanta 44 (1997) 365-371

Extractive separation of trivalent lanthanide metals with a combination of Di(2-ethylhexyl)phosphoric acid and 1,10-phenanthroline Md. Hasan

Zahir, Yoshitaka

Masuda

*

Dit:ision qf Science qf Materials', Graduate School (!{"Science and Technology, Kobe University, Rakkodai Nada-ku. Kobe 657, Japan Received 12 January 1996; received in revised form 14 August 1996: accepted 16 August 1996

Abstract

The equilibrium extraction behavior of a series of trivalent lanthanide ions (Ln 3 + ) using a chloroform Kerosine solution containing Di(2-ethylhexyl)phosphoric acid, combined with an adductant, 1,10-phenanthroline monohydrate (phen), was studied. The enhancement of the extraction by addition of such a neutral adductant is explained in terms of the extraction of the quaternary complex, M(HX2)3(phen)2, in addition to the neutral complex, M(HX2) 3, into the organic phase. The stoichiometry, extraction constants and separation factors of these systems were determined. The extraction constants of these systems partially follow the order of the atomic numbers. The synergistic extraction constants increased in the other Gd > Er > Ho > Eu > Ce > La > Pr and the highest separation factor was observed for Er Ho (2.09). pH~ 2 values were also obtained. In this synergistic extraction system, both the extraction equilibrium constants and the separation factors were found to be greater than those of commercial extractants. ~ 1997 Elsevier Science B.V.

Keywor&': Di(2-ethylhexyl)phosphoric acid; Lanthanides; 1,10-Phenanthroline: Solvent extraction

1. Introduction The separation of trivalent lanthanides by solvent extraction is still an interesting and formidable problem. As a part of a systematic evaluation of the use of chelating extractants in extracting and separating trivalent lanthanides, the equilibrium extraction behavior of a series of

* Corresponding author. Fax: + 81 78 8030722.

representative lanthanide ions with chloroform containing one of several kinds of ligands alone or combined with adduct-forming agents has been studied in detail [1-6]. Many acidic organophosphorus compounds have been studied for the extraction of lanthanide metals [7 10]. One of the most important acidic organophosphorus compounds is Di(2-ethylhexyl)phosphoric acid (HDEHP), whose extraction behavior has been described by Marcus et al. [10]. H D E H P and its analogs, in organic solvents of low polarity, are present as dimers [10,11].

0039-9140/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII $0039-9 140(96)02067-X

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M.H. Zahir, Y. Masuda / Talanta 44 (1997) 365 371

In synergistic systems, the extracting power of the mixture exceeds the sum of the extracting powers of its components. This phenomenon greatly enhances extraction or synergism with a mixture of extractants and has attracted considerable attention in recent years. Handley and Dean [12,13] examined Di(2-ethylhexyl)phosphorodithioic acid ( H D E H P D T ) as a metal extractant. According to their results, americium and europium could be extracted into dodecane fairly well with H D E H P D T . That a 'hard' metal ion such as Eu 3 + can be extracted with a 'soft' ligand such as D E H P D T was reminiscent of our findings that a nitrogen ligand such as 1,10-phenanthroline (phen) seemed to bond with lanthanide chelates at least as readily as with the hydrated lanthanide ions [3]. Komatsu and Freiser [14] have reported extensive investigations on the solvent extraction of trivalent lanthanide metal ions (Ln 3 + ) with mixed ligands. They described the adduct formation of La(III), Pr(III), Eu(III), Ho(III) and Yb(III) with bis(2,4,4-trimethylpentyl)phosphinic acid (HBTMPP), tri-N-octylphosphine oxide (TOPO), octyl(phenyl) -N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and methylenebis(diphenylphosphine) oxide (MBDPO) in chloroform. The equilibrium extraction behavior of series of representative trivalent lanthanide ions (La, Pr, Eu and Yb) was studied with either chloroform solutions of selected 1-phenyl-3-methyl-4-acyl-5-pyrazolones (acyl =decanoyl, phenacetyl, 3-phenylpropionyl and p-tert-butylbenzoyl) (HP) alone or these solutions in combination with phen, TOPO or methyltrioctylammonium chloride (R3 R' NC1) [15]. Extraction of La(III) and Y(III) [16] and Eu(III) [17] with H D E H P was carried out by Peppard and co-workers, and the extraction behavior of the metals was clarified. Nitrogen bases such as 1,10-phenanthroline, which have high proton affinity, are appropriate auxiliary ligands. This paper reports a study of adduct formation reactions between the lanthanides and auxiliary ligands aimed at obtaining a better understanding of the extractive separation of lanthanides. The role of the diluent was

also examined for the evaluation of the association constant.

2. Experimental 2. I. Apparatus

Extraction was carried out in a Taiyo M incubator at 25 + 0.1°C. Ultraviolet and visible absorption spectra were measured with a Shimadzu self-recording spectrophotometer (Model 240UV-Vis) with 10 mm optical path glass cells. The pH of the aqueous phase was measured with a Hitachi Horiba M-7II pH-meter. A Kokusan H200 centrifuge was also used for rapid and complete separation of the phases. 2.2. Materials

A 1 x 10 -2 standard solution of lanthanide nitrates was prepared by dissolution of a suitable amount of the pure oxide (99.99%) (gadolinium was purchased from Santoku Chemicals, Osaka, Japan, hydrated Ce(NO3)3 from Wako Pure Chemical Industries, Osaka, Japan, and other rare earths from Nacalai Tesque, Kyoto, Japan) in a small volume of concentrated nitric acid, followed by dilution with distilled water. The solution was then heated in order to remove any excess acid. The solution was diluted to 100 ml with distilled water and the concentration was determined as 1 x 10-3 M. The solution was standardized complexometrically at pH 5.1-5.6 with xylenol orange as metal indicator [18]. The prepared solution was then stored in a polyethylene bottle. 2.3. Preparation o f Di(2-ethylhexyl)phosphoric acid stock solution

For the preparation of a stock solution of H D E H P , 348 cm 3 of chemically pure H D E H P (DP-8R, Daihachi Chemicals) were placed in a clean, 1 dm 3 volumetric flask and diluted to volume with chloroform-kerosine so that the solution was 1 tool din-3. This solution was washed three times with 500 cm 3 of 6 tool dm 3 HC1 and three times with 500 cm 3 of distilled water, after

M.H. Zahir, Y. Masuda /Talanta 44 (1997) 365 371 Table 1 P e r c e n t a g e e x t r a c t i o n o f r a r e e a r t h m e t a l ions b y H D E H P

367

in the p r e s e n c e o f p h e n ~

Solvent

L a 3+

C e 3+

Pr 3+

S m 3+

E u 3+

G d 3+

T b 3+

Dy ~ ~

H o 3+

E r 3+

Y b ~+

Lu ~*

Y~ +

CHCI 3 Kerosine

91.91 65.02

99.92 73.03

84.58 57.69

99.53 72.64

97.20 70.31

98.99 72.10

91.21 64.32

98.28 71.39

93.81 66.92

98.71 71.82

84.50 57.61

79,33 52.44

93.27 66.38

" [ M e t a l n i t r a t e ion] = 1 x 10 4 M; [ligand] = 2 x 1 0 - 3 M: [phen] = 2 x 10 " M: [succinic acid] = 2 x 10 ~ M: 250( `

which the solution was left overnight. Since water is slightly soluble in H D E H P , the final H D E H P contains a small amount (1-2%) of water, which can be removed by using a rotating evaporator at 50°C and 15 m m H g pressure [8]. A 0.01 M phen solution was prepared by dissolving 1.9823 g of 1,10-phenanthroline monohydrate (Nacalai Tesque) in chloroform and diluting it to 100 cm 3 with the same solvent. A 0.001 M Arsenazo III solution was prepared by dissolving 0.0776 g of the reagent (Dojin Chemicals) in 100 cm 3 of distilled water. This solution was freshly prepared each week. Chloroform was used after distillation. All other chemicals were of analytical grade.

pletely, then the strip liquor was allowed to evaporate. The residue was decomposed and diluted, then the p H was adjusted to 2.5. The solution was transferred into a 10 cm 3 volumetric flask, 0.5 cm 3 of the Arsenazo III was added and the mixture was diluted to volume with distilled water. The absorbance was measured at 650 nm. Neither the extractant nor its complexes in chloroform showed appreciable absorption in the visible region, hence Arsenazo lII was used in the dual role of a calorimetric reagent and a scavenger for the lanthanides.

3. Results and discussion 2.4. Soh,ent extraction procedure The distribution experiments were performed at room temperature. An aliquot (10 cm 3) of the aqueous solution containing the metal ion (1 x 10 4 M) was placed in a stoppered 50 cm 3 glass tube. After the addition of 10 cm 3 of synergistic mixture containing the extractant solution (2 x 10 ~ M) and neutral adductant solution (2 x 10 ~ M), the mixture was shaken for 10 min at 200 strokes rain t at 25 + 0°C, which was sufficient for equilibration. The mixture was then centrifuged at 2000 rpm for 5 min and the p H of the aqueous phase was measured. The metal content in the aqueous phase was determined spectrophotometrically by the Arsenazo III method [19], as was the lanthanide concentration in the organic phase following back-extraction into hydrochloric acid. The concentration of metal ion in the organic phase was determined after back-extraction into 6 M hydrochloric acid for 30 min; 5 cm 3 of strip liquor thus obtained were transferred in to a separating funnel and washed once with 5 c m 3 of pure chloroform to remove the free ligand corn-

3. I. EJ]ect of pH on lanthanide(III) extraction systems containing HDEHP and adduetant into chloroform-kerosine First, we determined the percentage extraction of metal nitrates at p H 3-3.5 into chloroform (shown in Table 1). The effective extraction of lanthanides with H D E H P in the presence of phen occurs at aqueous phase p H values ranging from 3.00 to 3.35 for La 3+, 3.10 to 3.45 for Ce 3+, 3.15 to 3.40 for Pr 3+, 2.89 to 3.20 for Sm 3+, 2.99 to 3.25 for Eu 3 +, 2.77 to 3.00 for Gd 3 +, 2.85 to 3.20 for Tb 3 - , 2.76 to 3.25 for Dy 3+, 2.88 to 3.10 for Ho 3~, 2.90 to 3.10 for Er ~+, 2.99 to 3.30 tbr Yb 3~, 2.80 to 3.20 for Lu 3+ and 2.77 to 3.15 for Y~+. The logarithmic distribution coefficients diminish monotonically with increasing aqueous acidities (at pH > 6.0), implying that the extractions are dominated by an ion-exchange reaction in which hydrogen is liberated. Gaikwad and D a m o d a r a n [20] studied the extraction behavior of Ho(III) with (2-ethylhexylphosphoric acid mono-2-ethylhexyl ester) ( E H P N A ) and they con-

368

M.H. Zahir, Y. Masuda / Talanta 44 (1997) 365-371

cluded that there is no clear separation of phases at higher pH values. The percentage extraction of Ln 3 + reaches the maximum in the case of phen adduct formation at pH 2.9-3.50.

3.2. Percentage extraction The order of extraction with 2-thenoyltrifluoroacetone : (1-(2-thienyl)-4,4,4-trifluorobutane1,3-dione) (HTTA) in the presence of a bidentate heterocyclic amine, phen or 2,9-dimethyl-l,10phenanthroline (dmp), was scandium(III)> lutetium(III) > europium(III) > neodymium(III) > praseodymium(III) >lanthanum(III), but enhancement of the extraction by tba + (tetrabutylammonium cation) was nearly the same with lanthanum(III) to europium(III); enhancement was negligible in the case of scandium(III) delated [21]. The results (shown in Table 1) were compared with the previous data concerning the synergistic enhancement of these metal complexes; the similarity of ternary complex extraction and synergistic extraction was also considered. In the case of Ce 3 +, Eu 3 +, Er 3 + and Gd 3 + the extraction of M(HX2)3B 2 is much greater than that of M(HX2)3, to the point where extraction of M(C104)(HX2)3 is also negligible. It is concluded that synergistic extraction systems in which adduct formation occurs enhance the separation capability. In a mixed complex system containing 7-dodecenyl-quinolin-8-ol (DDQ), the donating ability of quinolin-8-ol (HQ) is not sufficient to form an adduct [4]. The effect of phenanthroline on the extraction of Eu(III) with DDQ-8 (quinolinol mixtures) was examined and it appeared that the formation with mixed complexes with a mixture of two chelating extractants and adduct formation with mixed complexes further enhance the separation capability in the lanthanide series. On the other hand, in the case of both La 3 + and Pr 3 +, the extraction of Ln(HX2)3B2 take place in a similar pH region, resulting in the extraction behavior shown in Fig. 1. In Table 1, the enhancement of the extraction of La 3 + and Pr 3 + is not very marked, and the slopes for the straight portion are 2.7 for La 3+ and Pr 3+. Obviously, lanthanides are extracted by a different mechanism under these conditions.

3.3. Slope analysis A traditional and effective means of obtaining both stoichiometric and equilibrium constant information about extraction processes, slope analysis, is based on an examination of the logarithmic variation of the distribution ratio, D, with relevant experimental variables. The log-log plots of the extraction in the form of D vs. a concentration variable indicate the stoichiometry of the formation of the extractable complex and thus leads to the derivation of a suitable equilibrium expression and then to the calculation of equilibrium constants. Plots of log D vs. variables such as the pH of the aqueous phase and the logarithm of the concentration of [(HX)2] were constructed [22]. Straight lines with a slope of + 3 for lanthanides were obtained with a low extent of metal extracted. Thus the extraction reaction can be written a s Ln 3 + + 3(HX)2¢o)~ Ln(HX2)3(o) + 3H +

(1)

Examination of Eq. (1) leads to the conclusion that the slope of a log D vs. pH plot should be + 3, indicating that three hydrogen ions are released in the extraction of the metal ion (the subscript (o) designates concentration in the organic phase). The extraction constant, Kex, for this reaction 2

dEr

1.5 -

Eu

Ce

0.5-

0 2.75

3

3,25

3J5

3.75

pH Fig. 1. Distribution constants for lanthanides between chloroform and the aqueous phase as a function of pH in the aqueous phase. A q u e o u s phase: [Ln 3+] = 1 x 10 4 M; succinic acid = 2 x 10 3 M. Organic phase: H D E H P = 2 × 1 0 - ~ M; [ p h e n ] = 2 x 10 3 M.

369

M.H. Zahir, Y. Masuda / Talanta 44 (1997) 365 371

2-

The results o f the extraction o f lanthanides using a mixture o f H D E H P and phen are shown in Table 1, where it is seen from the lower pHj,~ (pH o f 50% extraction) values that highly enhanced extractions are achieved in the presence o f as little as 10 3 M auxiliary reagent. According to Eq. (4), the values for log KexIm were evaluated as 6.22, 5.81, 5.33, 5.15, 4.90, 3.95 and 3.95 for G d 3 ~ , E r 3 + , H o 3 + , E u 3+ Ce 3 + , L a 3~ a n d P r 3 +

2 0.5

0 -3.4

-3.2

-3

-2.8

-2'.6

-2.4

Iogl(HX)~l Fig. 2. Distribution constants for lanthanides as a function of HDEHP concentration in the presence of phen in the organic phase. Aqueous phase: [Ln3+] = I × 10-4 M; succinic acid = 2 x 10 ~ M. Organic phase: [phen]=2 x 10 3 M; H D E H P = 2 x 10 3 M. K~x = [Ln 3 + (HX2)3]o[H + ]3/[Ln3 + ][(HX)2] 3

(2)

or

log K,x = log D , , - 3pH - 3 log[(HX)2]

(3)

As mentioned above, the adduct formation o f t r i s - D E H P A complexes o f Ln 3 + with the nitrogen-containing chelating agent phen with a representative =N C - C - N : d o n o r g r o u p was studied. It was f o u n d that H D E H P complexes showed an exceptionally large tendency to form adducts with the nitrogen-containing the chelating agent phen. The data support the conclusion that the plot o f log D vs. log[(HX)2]o exhibits a slope o f + 3 (Fig. 2), that the plot o f log D / D o vs. log[phen]o is linear with a slope o f 2, indicating that two molecules o f phen are included in the extracted species [22], where D represents the distribution ratio with time o f adduct formation (Fig. 3) and that the plot o f log D vs. p H also has a slope o f + 3 (Fig. 1). Thus. Ln 3 + + 3(HX):/o I + 2B(,,) -~ Ln(HX2)3B2(o) + 3H +

(4) log D / D o = log K ~ x ( m / K ~ + n log[B]o

(5)

where K~(m represents the extraction constant with the a d d u c t formation and K,x the extraction constant with H D E H P .

respectively. As seen from the above data, remarkable extraction efficiencies and selectivities were observed. This strongly indicates a new possibility that the separation efficiency can be improved, even in the synergistic solvent extraction system. However, H D E H P in a synergistic extraction system has proved to be an efficient extractant because o f its higher separation factors for consecutive elements. A l t h o u g h heavy rare earths were selectively extracted with these extractants, the extraction sequence o f yttrium with respect to rare earths was Dy > Y > Lu. (Table 1 ). Generally, in the chelate extraction o f lanthanides; the extraction constant increases with increasing atomic n u m b e r [23]. This general tendency was also observed in the present system. The extraction constant increases as the atomic 0.8La

0.6"3

+

Er

"~ 0.4-

0.2-

0 -3.4

-3.2

-3 -i.8 log[phen]

-2.6

-2.4

Fig. 3. Plots of log D/D o vs. log[phen] at constant HDEHP and pH (3.10). Aqueous phase: [Ln3+ ] = I × 10- 4 M; succinic acid=2× 10 3 M. Organic phase: H D E H P = 2 x 10 ~ M: [phen]= 2 x 10 ~ M.

M.H. Zahir, Y. Masuda / Talanta 44 (1997) 365-371

370

Table 2 Summary of results for extraction of lanthanides with H D E H P and phen as neutral ligand a Element

Slope b

pH~ 2

La Ce Pr Eu Gd Ho Er

2.74 3.00 2.80 3.15 3.00 3.05 3.22

3.15 2.9 3.15 2.76 2.1 2.7 2.55

± 0.11 b + 0.09 _+ 0.03 4- 0.07 _+ 0.31 _+ 0.02 + 0.07

a [ H D E H P ] ( o ) = 2 × 10 - 3 M; [phen](o) = 2 × 10 3 M in c h l o r o form.

Mean + S.D. (n = 2). p H ] / 2 is the p H v a l u e at w h i c h log D = 0.

number increases until G d 3 +, and then it starts to decrease. It is interesting that the extraction constants Kex(m of La 3 + and Pr 3 + are much smaller than those of the other six trivalent metal complexes with the same ligand and the Kex(B) values of the quaternary compounds from the neutral complex of europium(III), holmium(III) and erbium(III) are similar to each other, although the values for the cerium(III) complexes are slightly smaller than those for the other complexes. It is very interesting that the values of log Ke×(m with H D E H P are very large compared with those with H D E H P alone in chloroform. This is because the steric hindrance of the synergistic mixture is low. In Table 2, the values of pHi~ 2 show that lanthanide(III) can be extracted at slightly lower pH in the system with H D E H P alone than in the mixed system under the same extraction conditions and may be influenced by the distribution constant of the L n ( H X 2 ) 3 complex. In the present study, chloroform was employed as the solvent. It was observed that the extraction constant K~x(m, given by Eq. (5), is more than one order of magnitude larger in chloroform than in kerosine. Fig. 4 shows the log K~(m values for the systems studied here as a function of atomic number of the lanthanide metal ions. It is interesting that the relationship is linear up to G d 3 + and suggests that the slope might provide a good way of describing the selectivity of extractants used for t h e Ln 3 + group.

3.4. Separation factor The separation factor (SF) is the ratio of the distribution coefficients of two solutes measured under the same conditions. By convention, SF is greater than unity and a synonym for it is the separation coefficient. Mathematically it can be written as SF=logD,/togDii, where the subscripts I and II refer to two distinct metal ions. In the study of metal ion separations, the dependence of the separation factors on metal ion concentrations, aqueous acidities, organic phase extractant concentrations, temperature of extractions and adduct concentration was investigated in order to determine the optimum experimental conditions in order to achieve the greatest separation factors. For evaluation of the selectivity of H D E H P , we compared the separation factors for different pairs of lanthanide(III). The values of Iog(Dt/DII ) show that the separation of lanthanides becomes better only for Er Ho, G d - L a and not for G d - C e . However, there is a very small gain or no gain in the separation of C e - H o and Eu La, as shown by the values of log (DI/ DII ) in Table 3. All experiments were performed in the absence of perchlorate ion. In the absence of the perchlorate ion, the slope of the log D vs. pH lines was found to be 2.97, but when the perchlorate ion was present the slope was 1.98 [24]. Thus in the 6.5

Gd 6-

5.5-

s~

5-

Ce n/~E~

'-~ 4.5: 4-

3.5 Atomic number Fig. 4. Relationship between log K,~(m and the atomic number of lanthanide ions. Organic phase containing HDEHP with phen, in the absence of sodium perchlorate.

M.H. Zahir, Y. Masuda / Talanta 44 (1997) 365 37/

Table 3 Logarithmic separation factors in the extraction of lanthanide(Ill) with HDEHP and 1,10-phenanthroline Separation

Log (separation factor)

Er Ho Gd La Ho - La Ce Ho Eu La Gd - Ce

2.09 1.84 1.79 1.75 1.45 1.26

evaluation and characterization of organic solvents.

References

case of lanthanide(III) ion extraction, the following equation can be written: Ln 3 + + 3(HX)2~o ~+ 2phen --* Ln(HXz)3¢o)(phen)2 + 3 H +

371

(6)

A l t h o u g h all the s y s t e m s e x a m i n e d e x h i b i t excellent e x t r a c t a b i l i t y , t h e selectivities are i n f e r i o r in case o f L a ( I I I ) a n d P r ( I I I ) . It is n o t a b l e t h a t a d d i t i o n o f p h e n i m p r o v e s the s e p a r a t i o n o f t h e h e a v i e r l a n t h a n i d e s by v i r t u e o f a s u r p r i s i n g inc r e a s e in the e x t r a c t a b i l i t y o f t h e l i g h t e r m e t a l (Ce 3 + ) to a g r e a t e r e x t e n t t h a n t h o s e o f t h e h e a v i e r m e t a l s . H e n c e s u c h a s y s t e m w o u l d be o f p r a c t i c a l v a l u e in e x t r a c t i n g t h e l a n t h a n i d e s as a group.

Acknowledgements The present report is part of a project financed by the Ministry of Education, Science and Culture, Government of Japan (No. 06241250) for the development of new preparation methods for rare earth compounds and (No. 05303004) for the

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