The Separation of Cerium(IV) from Nitric Acid Solutions Containing

Moreover, the stripping of Ce(IV) from IL phase was also investigated. The Ce(IV) in IL phase can be quantitatively recovered by water. Recycle test i...
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Ind. Eng. Chem. Res. 2008, 47, 2349-2355

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SEPARATIONS The Separation of Cerium(IV) from Nitric Acid Solutions Containing Thorium(IV) and Lanthanides(III) Using Pure [C8mim]PF6 as Extracting Phase Yong Zuo, Yu Liu, Ji Chen, and De Q. Li* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Graduate School, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China

The extraction behavior of Ce(IV) along with Th(IV) and Ln(III) (Ln ) Ce, Gd, Yb) nitrate by pure ionic liquid, [C8mim]PF6, was investigated. [C8mim]PF6 alone showed good extraction ability for Ce(IV), while it was slight for Th(IV) and negligible for Ln(III). The extraction behavior of Ce(IV) by [C8mim]PF6 was particularly studied, and the most probable extraction mechanism proposed was the anion exchange mechanism. Moreover, the stripping of Ce(IV) from IL phase was also investigated. The Ce(IV) in IL phase can be quantitatively recovered by water. Recycle test indicated that IL loss could be avoided via adding [C8mim+] to the initial Ce(NO3)4 solutions before extraction and/or using PF6- solutions for stripping. Inspired by the proposed extraction mechanism, a new kind of Ce(IV)-doped IL was prepared to give further elucidation for the anion exchange mechanism. The result indicated that pure [C8mim]PF6 may act as extracting phase for separating Ce(IV) from nitric acid solutions containing Th(IV) and other trivalent lanthanides. 1. Introduction The development of ionic liquids (ILs) as solvent for extraction has gone its way for nearly one decade since Rogers et al. first applied [C4mim]PF6 as solvent for the extraction of organic benzene derivatives in 1998.1 Now new work on this area is going on.2-8 The reason why people have so much interest in ionic liquids is due to its unique properties such as low vapor pressure, nonflammability, high thermal stability, wide liquid range, wide electrical window, and tunable hydrophobicity. Yet the loss of IL due to the ion exchange mechanisms and the difficulties in recovering metal ions from IL phase were the two main limitations for utilizing ILs in separation applications.7,9 These limitations were especially serious for alkyl imidazolium hexafluorophosphate, one of the most commonly used ILs. Moreover, the partial hydrolysis of the IL anion, that is, PF6-, in acid solution often makes the extraction systems rather complex.10 More work needs to be done to solve these problems. As Cocalia et al. mentioned in their review, efforts could be made to use the differences of ILs from organic solvents for achieving more advantageous separations.11 It is known that half of the amount of the rare earth element in bastnasite is cerium. Now cerium is widely used in various fields including catalysts, scintillation material, polishing powder, and ceramic technology.12-16 Many cerium separation schemes were reported in previous work,17-19 among which solvent extraction was proved to be an efficient method for recovering cerium from bastnasite leaching liquor.20 In our recent work, a kind of neutral organophosphate, that is, di(2ethylhexyl) 2-ethylhexyl phosphonate (DEHEHP), was also employed in cerium(IV) recovery, and a satisfactory result was obtained.21-25 Nevertheless, the seeking for a more efficient and environment friendly process is never-ending. ILs as potential “green solvents” attracted much of our attention, although it may have some limitations as discussed above. * To whom correspondence should be addressed. Tel.: +86-43185262036. Fax: +86-431-85698041. E-mail: [email protected].

With the increasing interest from all over the world in ILs, many other properties of ILs such as toxicity, biodegradability,26,27 and radiochemical stability28 are being understood now. All of this information gives us strong support and great inspiration for the application of ILs in separation. The initial intent of this work is to study the separation behavior of Ce(IV), Th(IV), and lanthanides(III) from nitric acid medium by DEHEHP and [C8mim]PF6 combination as extracting phase because of the satisfactory behavior of DEHEHP in n-heptane. Interestingly, we found that [C8mim]PF6 alone has good extractability for Ce(IV) in nitric acid medium and shows high selectivity on Ce(IV), Th(IV), and Ce(III) with the increasing acidity. To our knowledge, free trivalent lanthanide ions and some actinide ions such as UO22+ and Th4+ can hardly be extracted (D < 1) by pure imidazolium-based hexafluorophosphate ILs in most cases.2,29,30 The extraction of Ce(IV) by pure IL has not been found in any reported literature. So in this Article, we give an investigation on the extraction of Ce(IV), Th(IV), and Ln(III) (Ln ) Ce, Gd, Yb) from nitric acid medium by pure IL, [C8mim]PF6. The extraction mechanism for Ce(IV) was particularly studied for its remarkable extraction ability. 2. Experimental Section 2.1. Materials. The IL, [C8mim]PF6 (see Figure 1), was synthesized with the modified methods described in the literature.2,31 Briefly, it involves refluxing a mixture of equal molar amounts of 1-methylimidazole with 1-chlorooctane for 72 h at 70 °C. The resulting product was washed three times with ethyl acetate and evaporated under vacuum for several days to remove ethyl acetate. A nearly quantitative yield of 1-octyl-3-methylimidazolium chloride ([C8mim]Cl) was obtained. [C8mim]PF6 was then prepared by adding 60% hexafluorophosphoric acid in a 1.1:1 molar ratio to a solution of [C8mim]Cl in water. The mixture was stirred for 12 h, and the bottom layer of ionic liquid was separated and washed several times with distilled water until the pH of the upper phase was ca. 7. Finally, the IL was evaporated to remove moisture at 70 °C on a vacuum line for 4 h. The overall yield was about 60%. Purity characterization

10.1021/ie071486w CCC: $40.75 © 2008 American Chemical Society Published on Web 03/01/2008

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Figure 1. Structure of 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim]PF6).

Figure 2. UV-vis spectra of [C8mim]PF6 prepared in our laboratory.

The stock solutions of Ce(NO3)4 were prepared according to the following procedure: first, CeO2 (purity >99.99%) was dissolved overnight by the proper amount of HNO3 and H2O2 mixtures with several drops of HF added into the solutions; then excessive concentrated ammonia was added into the obtained Ce(NO3)3 solutions to convert Ce3+ to Ce(OH)3; after that excessive H2O2 was dropped into the ammonia-Ce(OH)3 mixtures and then the mixtures were kept at 70-80 °C with agitation for several hours; and then the mixtures were boiled for several minutes to wipe off excessive H2O2; finally, the obtained Ce(OH)4 was filtered and the cake was dissolved by excessive 3 mol L-1 HNO3 immediately. The ultimately obtained stock solutions of Ce(NO3)4 had a concentration of [Ce(IV)] ) 0.527 mol L-1 with an oxidation rate ([Ce(IV)]/ [Ce]total) over 95%. The stock solutions of Th(IV) and Ln(III) were directly prepared from Th(NO3)4‚4H2O and Ln(NO3)3 of analytical grade, respectively. All other reagents were of analytical grade. 2.2. Methods. All of the extraction experiments were performed by contacting 1 mL of IL phase and 5 mL of aqueous phase solutions for 30 min at 25 ( 1 °C in a vibrating mixer. After that the mixture was settled and centrifuged at 2000 rpm for 10 min. Next, the metal ion concentration in the upper phase (aqueous phase) was determined by the following methods. All of the metal ions were examined as individual element in this study. Ce(IV) was determined by titration with standard (NH4)2Fe(SO4)2 solutions using 1,10-phenanthroline as indicator.21 Thorium(IV) was determined by spectrophotometric analysis with Arsenazo III under the wavelength of 660 nm. Trivalent lanthanides such as Ce(III), Gd(III), and Yb(III) were titrated by standard EDTA using xylenol orange as indicator.2 The distribution ratios of the metal ions between the IL phase and the aqueous phase were defined as follows:

DM )

Figure 3. Decomposition of [C8mim]PF6 with increasing HNO3 concentration.

of [C8mim]PF6 by 1H NMR (neat sample, dried, ppm) resulted in a spectrum containing the following peaks, δ 0.86 (t, 3H, CH2CH3), 1.26 (m, 10H, CH2CH2CH2), 2.50 (m, 2H, CH2CH2CH2), 3.00 (s, 3H, N-CH3), 4.14 (t, 2H, N-CH2-CH2), 7.69 (t, 1H, CH), 7.76 (t, 1H, CH), 9.09 (s, 1H, N-CH-N), and the corresponding 13C NMR was δ 13.84, 22.12, 25.54, 28.40, 28.53, 29.43, 31.24, 35.65, 40.12, 48.88, 122.23, 123.56, 136.53. The UV-vis spectrum of [C8mim]PF6 (diluted with ethanol 15 times) was shown in Figure 2 according to Burrell’s suggestion.32 As all our extraction experiments were carried out by contacting IL phase with acidic solutions, the decomposition of [C8mim]PF6 with increasing concentration of nitric acid should be noticed. A series of 1 mL of pure [C8mim]PF6 and 5 mL of nitric acid solutions were fully mixed for an hour, and then the fluoride ions in the aqueous solution were determined (Figure 3). Obvious decomposition of [C8mim]PF6 was found when [HNO3] > 5 mol L-1 according to Figure 3. To avoid the impact of the decomposition on our experiment, most of our experiments were carried out with [HNO3] < 1.5 mol L-1 (the investigation of the acidity dependence is an exception, yet the highest acidity is still lower than 4 mol L-1).

Ci - Cf volume of aqueous phase ‚ Cf volume of IL phase

where Ci and Cf refer to the initial and final (equilibrated) concentrations of metal ions in the aqueous phase. Preexperiment indicated that Ce(IV) can hardly be reduced by pure [C8mim]PF6 and the extraction of Ce(III) by pure IL is negligible. So when we studied the extraction of Ce(IV) by IL alone, the extraction of Ce(III) (content Th(IV) . Ce(III). The result suggests that IL alone may serve as extracting phase for the separation of Ce(IV) from nitric acid medium containing Th(IV) and other trivalent lanthanides. Before we accepted this hypothesis, further investigations about the extraction behavior of metal ions by pure IL should be made. 3.2. Extraction Mechanism of Ce(IV) Nitrate by Pure [C8mim]PF6. As to our knowledge, most of the extraction mechanisms in metal ions/IL systems are ion-exchange mechanisms.11 So at first the impact of the IL cation (i.e., [C8mim+]) and IL anion (i.e., PF6-) on the extraction of Ce(IV) by pure [C8mim]PF6 was investigated. For comparison, the impact of

Figure 5. Impact of the initial concentrations of [C8mim]Cl, KCl, KPF6, and HF on the extraction of Ce(NO3)4 by pure [C8mim]PF6. [Ce(NO3)4]ini ) 0.0275 mol L-1, [HNO3]ini ) 1.36 mol L-1.

KCl and HF in initial aqueous solutions was also examined. The nearly horizontal line of KCl in Figure 5 indicated that K+ and Cl- have no influence on the extraction of Ce(IV), while HF shows great impact on the extraction of Ce(IV) by pure [C8mim]PF6 and this phenomenon may be caused by the formation of the cationic species, Ce(HF)n4+,33 which may have less chances to enter the IL phase. The lines of [C8mim]Cl and KPF6 in Figure 5 indicate that [C8mim+] and PF6- have an obviously positive and negative impact on the extraction of Ce(IV) by pure [C8mim]PF6, respectively. The opposite impact of IL cation and anion on the extraction of Ce(IV) by pure [C8mim]PF6 indicates that the most probable extraction mechanism may be the anion exchange mechanism. As shown in eq 1, the increasing [C8mim+] concentration may prompt the formation of [C8mim]n‚Ce(NO3)m in the IL phase and thus increase the partition of Ce(IV) to the IL. On the other hand, the increasing [PF6-] may diminish the [C8mim+] in the aqueous phase and inhibit the formation of [C8mim]n‚Ce(NO3)m.

n[C8mim]PF6 + Ce(NO3)mn- H [C8mim]n‚Ce(NO3)m(IL) + nPF6- (1) Theory analysis about the anion exchange mechanism of the extraction of Ce(IV) nitrate by [C8mim]PF6 is also possible. To our knowledge, anionic metal ion complexes are more likely to be extracted into ILs phase than are metal cations.8 The anionic hexanitratocerate ion may stably exist in some salts such as K2Ce(NO3)6 and (NH4)2Ce(NO3)6.34 The hexanitratocerate ion in the solid salts is an icosahedron coordination compound with 12 oxygen atoms (coming from 6 bidentate ligands, NO3-) surrounding the central cerium atom. This hexanitratocerate ion may even exist in nitrate-rich solution.35 So it is very possible that Ce(IV) may be extracted into IL phase as anionic nitratocerate form. While this anionic coordination compound can hardly form between NO3- and Th4+ or Ce3+ (in some extreme conditions such as in very high-concentration nitric acid solutions, anionic nitrate species of thorium or trivalent lanthanides may be formed36,37), this may be the reason why far less Th(IV) and Ce(III) can be extracted by pure [C8mim]PF6 in our examined acidity range. The negative impact of the HF on the extraction of Ce(IV) by [C8mim]PF6 (shown in Figure 5) may be caused by the formation of cationic species Ce(HF)n4+, which may not favor the anion exchange mechanism. This observation is coincident with Vidal et al.’s work where zinc that exists as ZnCl42- in 3 mol L-1 NaCl solutions is easier to extract by [C6mim]PF6 than is copper, which exists as cationic

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Figure 6. Dependence of aqueous ∆[P] on the concentration of Ce(IV) in the IL phase. [HNO3]ini ) 1.36 mol L-1, [Ce(NO3)4]ini ) 0-0.09 mol L-1, [P]0 refers to the concentration of aqueous total phosphorus when [Ce(NO3)4]ini ) 0.

Figure 8. IR spectra of ILs. (A) Pure [C8mim]PF6; (B) [C8mim]PF6 loaded Ce(NO3)4; and (C) new IL obtained by contacting [C8mim]Cl with Ce(NO3)4 solutions.

Figure 9. Back extraction of Ce(IV) from IL phase by water, 0.02 mol L-1 KPF6, and 1% H2O2-0.1 mol L-1 HNO3. IL phase/aqueous phase ) 1 mL/5 mL, [Ce(IV)]IL ) 0.0710 mol L-1. Figure 7. Dependence of DCe(IV) on the equilibrated concentration of nitrate ions. [Ce(NO3)4]ini ) 0.0268 mol L-1, [HNO3]ini ) 1.36 mol L-1, [NaNO3]ini ) 0-0.7 mol L-1.

or neutral species such as Cu2+, CuCl+, and CuCl2 in 0-4.5 mol L-1 NaCl solutions.8 Two additional investigations were carried out to give experimental support for the anion exchange mechanism. As the IL anion, PF6-, may be exchanged into the aqueous phase according to the proposed extraction mechanism for Ce(IV), the total phosphorus in the equilibrated aqueous phase was determined using ICP-MS. Figure 6 shows that with increasing Ce(IV) concentrations in IL phase, the phosphorus (exists as PF6- and part of its hydrolytic form) in the aqueous phase increases almost linearly. This observation indicated that more PF6- would be exchanged into the aqueous solutions when more Ce(IV) entered the IL phase. The other investigation supporting the anion exchange mechanism is the need of NO3- for the formation of Ce(NO3)mnin the IL phase. Figure 7 gives a strong dependence of the extraction of Ce(IV) on the aqueous NO3- concentration. This observation is coincident with the acidity study shown in Figure 4. The result proved that Ce(IV) was not extracted into the IL phase as free Ce4+ but as a coordination compound of Ce4+ and NO3-, that is, Ce(NO3)mn-. More direct evidence for the formation of the nitratocerate will be given via IR study for the loaded IL (Figure 8B). As compared to pure [C8mim]PF6 (Figure 8A), a new absorption peak at 1037 cm-1 (symmetrical stretch vibration of bidentate NO3-) was observed, indicating the existence of bidentate NO3- in the IL phase.

Table 2. Concentrations of Ce and NO3- in Water-Stripped Solutions [Ce]/mol L-1

[NO3-]/mol L-1

[NO3-]:[Ce]

0.01403 0.01604 0.02116 0.02360

0.08488 0.09750 0.1279 0.1424

6.07 6.06 6.04 6.03

The stripping of Ce(IV) from loaded IL phase and the recycling of the IL for Ce(IV) extraction were carried out because of the potential application of pure [C8mim]PF6 in Ce(IV) separation process as discussed above. The stripping of 0.0710 mol L-1 Ce(IV) in IL was carried out using pure water, 0.02 mol L-1 KPF6, and 1% H2O2-0.1 mol L-1 HNO3 mixed solutions as stripping phase, respectively. Almost all of the Ce(IV) in IL could be quantitatively stripped by pure water and the other two solutions (Figure 9). The easy stripping of Ce(IV) in IL may be explained by the hydrolysis of Ce(NO3)mnin dilute, low-acidity solutions.35 The analysis of the concentrations of total cerium and nitrate ions in the water-stripped solutions gives a result of [Ce]:[NO3-] ) 1:6 (see Table 2). This data again proved the formation of anionic coordination compound Ce(NO3)62- in the IL phase. Thus the extraction reaction could be given as eq 2. The Kex of the reaction may be calculated as

Kex )

DCe(IV)‚[PF6-]2 ) [[C8mim]PF6]‚[Ce4+]‚[NO3-]6 [IL]2‚[NO3-]6 [[C8mim]2Ce(NO3)6]‚[PF6-]2

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Figure 11. Photo of the new IL obtained by contacting [C8mim]Cl with Ce(NO3)4 solutions.

Figure 10. Recycle test for the extraction of Ce(IV) by pure [C8mim]PF6. [Ce(NO3)4] ) 0.0268 mol L-1, [HNO3] ) 1.36 mol L-1. (A) The loaded IL was stripped by 5 mL of water. (B) The loaded IL was stripped by 5 mL of 0.02 mol L-1 KPF6. (C) 0.02 mol L-1 [C8mim]Cl was added into Ce(NO3)4 solutions, and the loaded IL was stripped like (B). (D) The loaded IL was stripped by 5 mL of 1% H2O2-0.1 mol L-1 HNO3 mixed solutions.

using the data in Figure 7. The average log Kex was about -2.70.

2[C8mim]PF6 + Ce4+ + 6NO3- H [C8mim]2‚Ce(NO3)6(IL) + 2PF6- (2) 3.3. Recycle Test. Recycle test for the extraction of Ce(IV) by pure [C8mim]PF6 was carried out using the above three kinds of strippants. The extraction percentages of 5 mL of 0.0268 mol L-1 Ce(NO3)4 by 1 mL of [C8mim]PF6 were given in Figure 10. Also, the IL was recycled as extraction f stripping f washing f extraction 10 times. An obvious decreasing extraction capacity was observed when just water or H2O2-HNO3 was used as strippants (see plots A and D in Figure 10). This observation could be explained by the ion exchange mechanism shown in eq 2 where IL anion was lost when anionic cerium complex was extracted into the IL phase. The stripping of Ce(IV) also causes the loss of IL cation, which can be shown in eq 3 according to the proposed mechanism.

[C8mim]2‚Ce(NO3)6(IL) f Ce4+ + 6NO3- + 2[C8mim+](aq) (3) So if only water or H2O2-HNO3 was used as strippants, part of IL will be lost to the aqueous solution after one cycle was finished. The relative lower capacity of the IL recycled using H2O2-HNO3 as strippant than that using water may be caused by the reductive effect of the residual H2O2 in the IL, which may convert part of Ce4+ to Ce3+ in the extraction of the next cycle. When KPF6 solutions were used as strippant, the loss of IL cation in the stripping could be avoided (eq 4).

[C8mim]2‚Ce(NO3)6(IL) + 2PF6- f Ce4+ + 6NO3- + 2[C8mim]PF6(IL) (4) This reaction could be proved by plot B in Figure 10 where 0.02 mol L-1 KPF6 was used as strippant and has a higher capacity as compared to water (plot A). The lost IL anions (eq 2) could be fetched up by adding KPF6 in stripping (eq 4). An interesting test was carried out in the recycle test (plot C in Figure 10). Before the extraction step in every cycle, [C8mim]-

Cl was added into the aqueous Ce(NO3)4 solutions (this may cause an increasing extraction of Ce(IV) by IL according to Figure 5). The extraction reaction in eq 2 can then be changed as:

Ce4+ + 6NO3- + 2[C8mim+](aq) H [C8mim]2‚Ce(NO3)6(IL) (5) So the loss of IL anion in eq 2 could be avoided via adding [C8mim]Cl to the Ce(NO3)4 solutions. If KPF6 was used as strippant, no IL would be lost; however, more [C8mim]PF6 would form during the stripping step (eq 4). The slightly increasing extraction capacity of IL after being recycled 10 times (plot C in Figure 10) is evidence for the above analysis. Inspired by eq 5, a new kind of IL, that is, [C8mim]2[Ce(NO3)6], may be expected to be directly synthesized via contacting [C8mim]Cl with Ce(NO3)4 solutions. The highsymmetry pseudo-spherical anion, Ce(NO3)62-, which is very similar to PF6- and BF4-, is quite possible for formation of ILs.11 A simple test was conducted via dropping 2 mL of pure [C8mim]Cl into 8 mL of Ce(NO3)4 solutions ([Ce(IV)] ) 0.527 mol L-1, [HNO3] > 7 mol L-1) in a stoppered tube. Caution: White fume (may be HCl) was emitted during this process. After being fully reacted for several minutes, a red viscous liquid formed at the bottom (Figure 11). IR spectra of this new IL were given in Figure 8C. As compared to [C8mim]PF6 (Figure 8A), typical absorptions of PF6- at 559 and 838 cm-1 disappeared, while the absorptions about [C8mim+] remained (1165, 1571, 2857, 2928, 3116, 3156 cm-1). Otherwise, the characteristic band of Ce(NO3)62- may be observed in spectrum C (two broad peaks at 1393 and 3401 cm-1 and typical absorption of bidentate NO3- at 826, 1038, 1393 cm-1). The newly obtained IL is stable while contacting with HNO3 solutions but may totally decompose in pure water due to the hydrolysis of Ce(NO3)62- in dilute solution as discussed above. Further investigation on this Ce(IV)doped IL on its preparation and characterization is undergoing in our laboratory. It should be emphasized here that the formation of the new IL, may be [C8mim]2[Ce(NO3)6], strongly supports the anion exchange mechanism of the extraction of Ce(IV) by pure [C8mim]PF6 from nitric acid solutions as shown in eq 2. 4. Conclusion In this study, pure IL [C8mim]PF6 has been demonstrated to have high selectivity on the extraction of Ce(IV), Th(IV), and Ln(III) from nitric acid medium. The separation coefficient between Ce(IV) and Th(IV) was obtained and was similar to that using DEHEHP/n-heptane as extraction phase. The unique

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extraction behavior of Ce(IV) by pure [C8mim]PF6 is mainly due to the formation of Ce(NO3)62- in nitric acid solutions, which is likely to form new IL with [C8mim+] cation. The easy stripping of Ce(IV) by water from IL phase can be explained by the hydrolysis of Ce(NO3)62- in dilute solution. Although the anion exchange mechanism may result in IL loss during the extraction process, the recycle test of IL demonstrated that the IL loss could be controlled via adding [C8mim+] to the initial Ce(NO3)4 solutions and/or using KPF6 solution as strippant. This is an interesting phenomenon: [C8mim]PF6 acted as liquid anion exchanger during the extraction process, and then the anion could be recovered in stripping process. A Ce(IV)-doped IL was obtained via contacting [C8mim]Cl with Ce(NO3)4 solutions inspired by the extraction study, returning to giving further elucidation for the proposed mechanism. So, we can give a preliminary conclusion that it is quite possible that [C8mim]PF6 alone may act as extracting phase or anion exchanger for the recovery of Ce(IV) from nitric acid medium containing Th(IV) and Ln(III). However, as fluorine exists in bastnasite, the nitric acid leaching liquor of the roasted bastnasite contains the proper amount of HF, which may restrain the extraction of Ce(IV) by pure [C8mim]PF6. In this situation, an extractant may be employed to solve this problem. The extraction behavior of Ce(IV), Th(IV), and Ln(III) by the combination of DEHEHP and [C8mim]PF6 has been determined in our laboratory and will be discussed in future work. Acknowledgment This project was supported by the “Hundreds of Talents Program” from the Chinese Academy of Science, Natural Science Foundation of China (50574080), and Distinguished Young Scholar Foundation of Jilin Province (20060114). Literature Cited (1) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Room Temperature Ionic Liquids as Novel Media for “Clean” Liquid-Liquid Extraction. Chem. Commun. 1998, 1765-1766. (2) Sun, X. Q.; Wu, D. B.; Chen, J.; Li, D. Q. Separation of Scandium(III) from Lanthanides(III) with Room Temperature Ionic Liquid Based Extraction Containing Cyanex 925. J. Chem. Technol. Biotechnol. 2007, 82, 267-272. (3) Germani, R.; Mancini, M. V.; Savelli, G.; Spreti, N. Mercury Extraction by Ionic Liquids: Temperature and Alkyl Chain Length Effect. Tetrahedron Lett. 2007, 48, 1767-1769. (4) Li, Z. J.; Wei, Q.; Yuan, R.; Zhou, X.; Liu, H. Z.; Shan, H. X.; Song, Q. J. A New Room Temperature Ionic Liquid 1-Butyl-3-Trimethylsilylimidazolium Hexafluorophosphate as a Solvent for Extraction and Preconcentration of Mercury with Determination by Cold Vapor Atomic Absorption Spectrometry. Talanta 2007, 71, 68-72. (5) Poole, C. F. Applications of Ionic Liquids in Extraction, Chromatography, and Electrophoresis. In AdVances in Chromatography; Grushka, E., Grinberg, N., Eds.; CRC/Taylor & Francis: Boca Raton, 2007; Vol. 45, pp 89-124. (6) Sun, X. Q.; Peng, B.; Chen, J.; Li, D. Q.; Luo, F. An Effective Method for Enhancing Metal-Ions’ Selectivity of Ionic Liquid-Based Extraction System: Adding Water-Soluble Complexing agent. Talanta 2008, 74, 1071-1074. (7) Jensen, M. P.; Neuefeind, J.; Beitz, J. V.; Skanthakumar, S.; Soderholm, L. Mechanisms of Metal Ion Transfer into Room-Temperature Ionic Liquids: The Role of Anion Exchange. J. Am. Chem. Soc. 2003, 125, 15466-15473. (8) Vidal, S. T. M.; Correia, M. J. N.; Marques, M. M.; Ismael, M. R.; Reis, M. T. A. Studies on the Use of Ionic Liquids as Potential Extractants of Phenolic Compounds and Metal Ions. Sep. Sci. Technol. 2004, 39, 21552169. (9) Dietz, M. L. Ionic Liquids as Extraction Solvents: Where Do We Stand? Sep. Sci. Technol. 2006, 41, 2047-2063. (10) Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Ionic Liquids Are not Always Green: Hydrolysis of 1-Butyl-3-Methylimidazolium Hexafluorophosphate. Green Chem. 2003, 5, 361-363.

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ReceiVed for reView November 1, 2007 ReVised manuscript receiVed December 27, 2007 Accepted January 26, 2008 IE071486W