Ind. Eng. Chem. Res. 2004, 43, 6703-6707
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Synergistic Extraction of Lanthanum(III) from a Nitrate Medium by Mixtures of 1-Phenyl-3-methyl-4-benzoyl-pyrazalone-5 and Triisobutylphosphine Sulfide Qiong Jia,*,† Qingkun Shang,‡ and Weihong Zhou† College of Chemistry, Jilin University of Nanling Campus, Changchun, 130022, P. R. China, and College of Chemistry, Northeast Normal University, Changchun, 130024, P. R. China
This paper presents the results of the extraction of lanthanum(III) by mixtures of 1-phenyl-3methyl-4-benzoyl-pyrazalone-5 (HPMBP) and triisobutylphosphine sulfide (TIBPS, B) from a nitrate medium. The mixtures have evident synergistic effects on lanthanum(III). La(III) is extracted as La(NO3)2‚PMBP‚B by the mixture instead of La(PMBP)3‚HPMBP, which is extracted by HPMBP alone. The equilibrium constants and the thermodynamic functions, i.e., ∆H, ∆S, and ∆G, are determined. The extraction of other rare earth elements, Nd(III) and Dy(III), by mixtures of HPMBP and TIBPS is also studied. The synergistic effect on Nd(III) and Dy(III) is not so high as that on La(III). The possibility of separating La(III), Nd(III), and Dy(III) is discussed according to the separation factors. Introduction In recent years, the extraction and separation of trivalent lanthanides and actinides are of great relevance and interest with the increasing demand for these elements and their compounds. Much attention has been paid to the synergistic extraction of lanthanide and actinide elements by chelating reagents as extractants, especially in a combination of β-diketones and organophosphorus esters1-4 or β-diketones and ammonium salts,5,6 such as 1-phenyl-3-methyl-4-benzoylpyrazalone-5 (HPMBP) or 2-thenoyltrifluoroacetone (HTTA) and tributyl phosphate (TBP), tri-n-octyl phosphine oxide (TOPO), primary amine, or quaternary ammonium salts, etc. Among the various chelating agents being tried for the extraction of metal ions HPMBP has been found to extract metal ions from more acidic solutions than HTTA (pKa ) 6.25) because of its lower pKa value (4.01).7 The synergistic extraction of trivalent lanthanides by mixtures of HPMBP and organophosphorus extractants, e.g., di(1-methylheptyl) methyl phosphonate, 2-ethylhexylphosphonic acid mono2-ethylhexyl ester (HEH/EHP), and bis(2,4,4-trimethylpentyl) monothiophosphinic acid (Cyanex302), has been studied.8-10 The extraction mechanism and the extracted species are determined. Triisobutylphosphine sulfide (TIBPS) is a phosphinebased extractant developed by Cyanamid with the following structure:
Its commercial availability since the mid 1980s has resulted in a number of more recent investigations into * To whom correspondence should be addressed. E-mail:
[email protected]. † Jilin University of Nanling Campus. ‡ Northeast Normal University.
the extraction of silver,11,12 gold,13 mercury,14 palladium,15 and divalent metals.16 There are also some papers about the synergistic or antagonistic extraction of metal ions by mixtures of TIBPS and other extractants such as trifluoroacetone and carboxylic acids.17,18 In the present work, the synergistic extraction of La3+ by HPMBP and TIBPS from a nitrate medium is studied. The extracted complexes and their formation constants are determined together with the effects of the aqueous acidity, the ratio of the extractants, and experimental temperature. The extraction of Nd3+ and Dy3+ is also studied and the possibilities of separating La3+, Nd3+, and Dy3+ are provided according to the separation factors. Experimental Section Materials. HPMBP with a purity greater than 99% was supplied by Shanghai Chemistry Reagent Factory. TIBPS (trade name Cyanex471X) was kindly supplied by Cytec Canada, Inc. and used without any further purification. The extractants were dissolved in benzene to the required concentration. Stock solutions of rare earths were prepared from their oxides (purity g99.99%) by dissolving in concentrated nitric acid and diluting with distilled water. The concentrations of rare earth ions were determined by a spectrophotometer using Arsenazo III as an indicator. All extraction experiments were performed at constant ionic strength (1.0 M NaNO3). All the other reagents were of analytical grade. Apparatus. A pHS-3C digital pH meter made by Shanghai Rex Instruments Factory was used for pH measurements. A UV-Vis-NIR recording spectrophotometer of Model UV-365 made by Shimadzu Company was employed for absorption measurements. Methods. For the equilibrium experiments, equal volumes (5 mL each) of aqueous and organic phases were shaken for 30 min, which was determined to be sufficient for equilibrium attainment in preliminary experiments. The solutions were then settled and separated by gravity. Temperature was kept constant
10.1021/ie049831u CCC: $27.50 © 2004 American Chemical Society Published on Web 09/16/2004
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Figure 1. Extraction of La3+ by HPMBP and HPMBP + TIBPS. [La3+] ) 1.510 × 10-3 mol/L, pH ) 2.97, m ) 1.0 mol/L, [HPMBP](o) + [TIBPS](o) ) 0.10 mol/L.
at 293 ( 1 K except for the temperature experiments. After the split of the phases, the concentration of metal ions in the aqueous phase was determined and that in the organic phase by material balance. These concentrations were used to obtain the distribution ratio, D.
Figure 2. Relationship between distribution ratio D12 and pH. [La3+] ) 1.510 × 10-3 mol/L, m ) 1.0 mol/L, [HPMBP](o) ) [TIBPS](o) ) 0.05 mol/L.
expressed as K12
La(NO3)3(a) + xHPMBP(o) + yB(o) 798 La(NO3)3-x‚(PMBP)x‚By(o) + xH+ (a) + xNO3(a) (3)
Results and Discussion Extraction of La3+ by HPMBP or TIBPS Alone. Liu et al.19 has studied the solvent extraction of La3+ by HPMBP and assumed that the extraction of La3+ can be expressed by the following equation: n+ M(a)
The distribution ratio, D12, of the mixing system should be described as
log D12 ) x log[HPMBP](o) + y log[B](o) + xpH + log K12 - x log[NO3 ](a) (4)
+ (n + s)HPMBP(o) T MPMBPn‚sHPMBP(o) + nH+ (a)
After n and s had been determined, the reaction equation could be expressed as K1
+ La3+ (a) + 4HPMBP(o) 798 La(PMBP)3‚3H(a)
(1)
where “a” and “o” denote aqueous and organic phase, respectively. The relationship between the distribution ratio D1 and the extraction constant K1 is described as follows:
log D1 - 3pH ) 4 log[HPMBP](o) + log K1
Figure 2 shows the plots of log D12 versus pH at fixed concentrations of HPMBP and TIBPS, giving a straight line with a slope of about 1.0. At fixed aqueous acidity and concentration of the other extractant, the plots are linear with slopes of about 1.0 for both [TIBPS](o) and [HPMBP](o) (Figure 3). The x, y values and the extracted complex can thus be determined. Therefore, the synergistic extraction reaction, eq 3, can be rewritten as K12
La(NO3)3(a) + HPMBP(o) + B(o) 798 La(NO3)2‚PMBP‚B(o) + H+ (a) + NO3(a) (5)
(2)
The experiments of La3+ extraction by HPMBP from a nitrate medium have been repeated in the present paper to obtain the extraction constant log K1, which was calculated as -7.36 ( 0.20. The extraction of La3+ by TIBPS alone is carried out under similar experimental conditions to those of extraction by HPMBP. The experimental data show that the extraction of La3+ by TIBPS alone is negligibly small. According to HSAB (hard-soft acid-base) theory,20 rare earth ions are hard acids but TIBPS is a sulfide ligand that has a soft adduct atom. It can extract the hard acid cation, La3+, weakly as expected. Extraction of La3+ by Mixtures of HPMBP and TIBPS. The distribution ratios of La3+ when extracted by mixtures of HPMBP and TIBPS in benzene are given in Figure 1, showing that there is an evident synergistic effect. The synergistic enhancement factor, Dmax/ (DHPMBP + DTIBPS), is calculated according to Xu et al.’s method21 to be 1.82. The synergistic extraction of La3+ by mixtures of HPMBP and TIBPS from a nitrate medium can be
The value of log K12 is calculated as -3.13 ( 0.04. It can be concluded from eq 5 that TIBPS replaces the molecules of the chelated β-diketone and NO3- is coextracted to form the final complex. Wharf et al.17 studied the extraction of Eu3+ by mixtures of TIBPS and HTTA from perchloric acid solutions and determined the extracted complex as [M(TTA)2(ClO4)(TIBPS)]. Our result is similar to their work. Since one TIBPS molecule is extracted to La(NO3)2‚PMBP‚B, we suggest the possible structures of La(PMBP)3‚HPMBP and La(NO3)2‚PMBP‚B as shown in Figure 4. From eqs 1 and 5, the following formation reaction can be obtained: Kf
La(PMBP)3 + HPMBP(o) + B(o) + 2HNO3(a) 798 La(NO3)2‚PMBP‚B(o) + 3HPMBP (6) Kf )
[La(NO3)2‚PMBP‚B](o)‚[HPMBP]3(o) La(PMBP)3‚HPMBP(o)‚[B](o)‚[HNO3]2(a)
)
K12 K1 (7)
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Figure 3. Relationship between distribution ratio D12 and equilibrium concentration of TIBPS and HPMBP. [La3+] ) 1.510 × 10-3 mol/L, pH ) 2.97, m ) 1.0 mol/L. (a) ∆, [HPMBP](o) ) 0.02 mol/L; O, [HPMBP](o) ) 0.05 mol/L. (b) ∆, [TIBPS](o) ) 0.02 mol/ L, O, [TIBPS](o) ) 0.05 mol/L.
where Kf denotes the formation constant and can be calculated as log Kf ) (-3.13) - (-7.36) ) 4.23. Kf is much higher than the equilibrium constants K1 and K12, which indicates that the extracted complex of La3+ by HPMBP alone, La(PMBP)3‚HPMBP, is prone to react with TIBPS to form the final complex La(NO3)2‚ PMBP‚B. Influence of Temperature. The plots of the distribution ratios D12 of La3+ by mixtures of HPMBP and TIBPS versus 1000/T at different temperatures (2050 °C) at fixed aqueous acidity and concentrations of HPMBP and TIBPS are shown in Figure 5, giving a slope of 1.82. The change of enthalpy of the reaction, ∆H, can thus be calculated as ∆H ) -34.83 kJ‚mol-1 according to the equation
∆ log D12/∆(1/T) ) -∆H/2.303R The sign of ∆H is negative, indicating that the synergistic extraction is exothermic driven. ∆G and ∆S of the extraction of La3+ by mixtures of the HPMBP and TIBPS system at 20 °C can thus be calculated as
∆G ) -RT ln K12 ) 17.57 kJ‚mol-1 ∆G ) ∆H - T∆S f ∆H - ∆G ∆S ) ) -178.75 J‚K-1‚mol-1 T
Figure 4. Structures of the extracted complexes. (a) La(PMBP)3‚ HPMBP; (b) La(NO3)2‚PMBP‚B.
Figure 5. Relationship between distribution ratio D and temperature. [La3+] ) 1.510 × 10-3 mol/L, pH ) 2.97, m ) 1.0 mol/L, [HPMBP](o) ) [TIBPS](o) ) 0.05 mol/L.
Extraction of Nd3+ and Dy3+ by Mixtures of HPMBP and TIBPS. Figures 6 and 7 show the extraction of Nd3+ and Dy3+ from a nitrate medium by HPMBP or/and TIBPS. The experimental results indicate that the extraction of Nd3+ and Dy3+ by TIBPS alone is negligibly small like that of La3+. The HPMBP + TIBPS system has weak synergistic effects on Nd3+ while neither evident synergistic nor antagonistic effects on Dy3+, that is, the extraction of Dy3+ by
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Conclusions
Figure 6. Extraction of Nd3+ by HPMBP and HPMBP + TIBPS. [Nd3+] ) 1.440 × 10-3 mol/L, pH ) 2.86, m ) 1.0 mol/L, [HPMBP](o) + [TIBPS](o) ) 0.10 mol/L.
The synergistic extraction of La3+ by mixtures of HPMBP and TIBPS from a nitrate medium has been studied. The mixtures have evident synergistic effects on La3+ and the synergistic enhancement factor is calculated to be 1.82. The extracted complex by the mixtures is determined to be La(NO3)2‚PMBP‚B. The equilibrium constants, formation constants, and thermodynamic functions are determined, showing that the synergistic extraction is exothermic driven. The extraction of Nd3+ and Dy3+ by mixtures of HPMBP and TIBPS has also been investigated and compared with that of La3+. The synergistic effect on the extraction of La3+ is highest among the three cations. In addition, the extraction of La3+, Nd3+, and Dy3+ by HPMBP alone follows the order Dy3+ > Nd3+ > La3+. According to separation factors, the mixtures have no better separation abilities for rare earth elements than HPMBP. Acknowledgment The authors are very grateful to Dr. Donato of Cytec Canada for supplying Cyanex 471X. We also acknowledge the experimental assistance and valuable discussions from Professor Li D. Q. of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Literature Cited
Figure 7. Extraction of Dy3+ by HPMBP and HPMBP + TIBPS. [Dy3+] ) 1.410 × 10-3 mol/L, pH ) 3.13, m ) 1.0 mol/L, [HPMBP](o) + [TIBPS](o) ) 0.10 mol/L. Table 1. DHPMBP+TIBPS/DHPMBP Values of La3+, Nd3+, Dy3+ XHPMBP:
0.2
0.4
0.5
0.6
0.8
0.9
1.0
La3+
2.94
Nd3+ Dy3+
1.97
2.38 2.30 1.80
2.40 2.22 1.14
2.14 1.83 1.03
1.82 1.32 0.90
1.64 1.10 1.15
1 1 1
Table 2. Separation Factors for La3+, Nd3+, Dy3+ by HPMBP + TIBPS System separation factors (Di/DLa) extractant
Nd3+/La3+
Dy3+/La3+
HPMBP HPMBP + TIBPS
9.88 6.61
69.26 48.50
mixtures of HPMBP and TIBPS is like that by HPMBP alone. The values of DHPMBP+TIBPS/DHPMBP are calculated as shown in Table 1. Furthermore, it can be seen from Figures 1, 6, and 7 that the extraction ability of the three cations follows the order Dy3+ > Nd3+ > La3+ by HPMBP alone. This order can also be explained by HSAB theory. 20 Rare earth ions are hard acids while HPMBP is a hard base. For the hard acids, Dy3+, Nd3+, and La3+, the ionic radii follow the order of Dy3+ > Nd3+ > La3+, resulting in the same order of hardness. Therefore, when the three ions react with the hard base, HPMBP, the extraction ability also follows this order. The differences of the extraction of the rare earth elements by mixtures of HPMBP and TIBPS can be considered to separate La3+, Nd3+, and Dy3+ from each other. The separation factors among La3+, Nd3+, and Dy3+ are given in Table 2. It is clear that there is no potential for a decent rare earth elements separation with mixtures of HPMBP and TIBPS.
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Received for review March 2, 2004 Revised manuscript received July 5, 2004 Accepted July 21, 2004 IE049831U
