Lithium isotope separation by liquid-liquid extraction using ionic liquid system containing dibenzo-14-crown-4

Lithium isotope separation by liquid-liquid extraction using ionic liquid system containing dibenzo-14-crown-4

Journal of Molecular Liquids 224 (2016) 662–667 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevie...

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Journal of Molecular Liquids 224 (2016) 662–667

Contents lists available at ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Lithium isotope separation by liquid-liquid extraction using ionic liquid system containing dibenzo-14-crown-4 Chenglong Shi a,b,c, Yongzhong Jia a,b, Jiang Xiao a,b,c, Xingquan Wang a,b,c, Ying Yao a,b, Yan Jing a,b,⁎ a b c

Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Chinese Academy of Sciences, 810008 Xining, China University of Chinese Academy of Sciences, 100049 Beijing, China

a r t i c l e

i n f o

Article history: Received 4 August 2016 Received in revised form 12 October 2016 Accepted 16 October 2016 Available online 18 October 2016 Keywords: Ionic liquids Solvent extraction Dibenzo-14-crown-4 Lithium isotopes

a b s t r a c t In this paper, a novel liquid-liquid extraction system was investigated for the selective separation of lithium isotopes using ionic liquid (IL) and dichloroethane as extraction solvent and dibenzo-14-crown-4 (DB14C4) as extractant. Results showed that the separation performance of DB14C4 has been improved after the ILs were involved in the extraction process. The maximum single stage separation factor α for 6Li/7Li obtained in the present study was 1.021. The light lithium isotope 6Li and the heavy lithium isotope 7Li were concentrated in organic phase and solution phase, respectively. The order of the single stage separation factor obtained in different lithium salts was LiI N LiBr N LiClO4 N LiCl. From the temperature dependence data, the thermodynamic functions values of the lithium isotope exchange reaction were calculated. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Natural lithium is composed of two stable isotopes, 6Li and 7Li, which play important roles in nuclear science and industry [1]. The tritium needed as a fuel for fusion reactors is produced by the neutron capture reaction of 6Li in tritium breeding materials. The heavier isotope of lithium, 7Li, is used as pH controller of coolants in nuclear fission reactors [2]. Nowadays, there have been various methods, such as amalgam, electromigration, membrane separation, laser, chromatography and solvent extraction, which were developed to separate lithium isotope [3–10]. The only method that was applied to a large-scale lithium isotope separation is the amalgam method. Although this method was attractive, benefiting by its large single-stage separation factor (αmax = 1.054 ± 0.002), the use of toxic mercury might result in severely biological and environmental problems. Solvent extraction is considered one of the most powerful techniques for lithium isotopes separation, offering several advantages such as high selectivity, possibilities of operations in a continuous mode and use of relatively simple equipment at both laboratory and industrial scales. The crown ethers and cryptands have been proven to be effective extractants in liquid-liquid extraction systems of lithium isotopes separation. Nishizawa and his coworkers [11] ever determined the single-stage separation factors for lithium salt - cryptand (2B,2,1) two-phase chemical exchange systems, and it was 1.039 for LiCl at ⁎ Corresponding author at: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail address: [email protected] (Y. Jing).

http://dx.doi.org/10.1016/j.molliq.2016.10.081 0167-7322/© 2016 Elsevier B.V. All rights reserved.

293 K. They also investigated the lithium isotope separation by liquidliquid extraction with benzo-15-crown-5 (B15C5) ether in chloroform, while the separation factors were 1.002, 1.014 and 1.026 at 298 K for Cl−, Br− and I− as counter anions, respectively [12]. Demin's group [13] studied the extraction characteristics of chloroform-water system using lithium tetrafluoroborate as extracted salt and B15C5 as an extractant. In their research, the magnitude of separation factor for 6 Li\\7Li pair was 1.030. It was found that when the B15C5 was used as extracting agent in 1,1,7-trihydrododecafluoroheptanol system to extract lithium chloride, the magnitude of separation factor for 6Li\\7Li pair was 1.024 [14]. Obviously, the liquid-liquid extraction method using the B15C5 and cryptand (2,2,1) exhibited a good separation efficiency for lithium isotope. However, there exist some problems: (a) the severe consume of B15C5 and the cryptand (2,2,1) after countercurrent multi-stage extraction which may attribute to their high water solubility; (b) the large cost and synthetic difficulty of the cryptand. To ameliorate these problems, a new macrocyclic ligand, dibenzo-14crown-4 (DB14C4) was developed for the lithium isotope separation. Due to its high molecular symmetry and low polarity of DB14C4, it will have small water solubility so that the loss will significantly decrease. In addition, the cavity size of DB14C4 is close to the ionic diameter of lithium ion, which will exhibit good selectivity and separation performance. In order to develop an environmentally friendly lithium isotope system, a burgeoning green organic solvent, ionic liquid (IL), has been used in our experiment. ILs have attracted increasing attention in various fields because they have some unique characteristics such as negligible vapor pressures, nonflammability, high polarity, high thermal stability

C. Shi et al. / Journal of Molecular Liquids 224 (2016) 662–667

and tunable properties [15–20]. In recent years, several imidazoliumbased ILs have also been employed for lithium isotope separation with remarkably high extraction performance [21–24]. In the present study, the imidazolium-based IL, 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim] [NTf2]), was used as a green solvent for the lithium isotope separation. At the same time, however, we added some conventional molecular solvent, dichloroethane, into the organic phase. We took two main reasons into consideration. Firstly, the DB14C4 was partially soluble in pure IL, which is unfavorable to the extraction process. Secondly, the high viscosity of pure ILs resulted in low rate of mass transfer and difficulty in separate-phase. In our studies, the effects of the concentration of extractant, the type of counteranion of lithium salts and extraction temperature on the single stage separation factor of lithium isotopes were explored. The stripping of lithium from the loaded organic phase was also investigated. 2. Experimental 2.1. Materials and apparatus ILs, [C4mim][NTf2], were procured from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (Lanzhou, China). The chemicals dichloroethane (N99%, Aladdin Industrial Corporation, China), lithium chloride (N99%, Sinopharm Chemical Reagent Co., Ltd., China), lithium bromide (N 99%, Sinopharm Chemical Reagent Co., Ltd., China), lithium iodide (N99%, Aladdin Industrial Corporation, China), and lithium perchlorate (N99%, Aladdin Industrial Corporation, China) were used as received, without further purification. All other reagents were of analytical grade and obtained from Tianjin Kermel Chemical Reagent Co. Ltd. (Tianjin, China). A THZ-82A thermostatic water bath oscillator (Changzhou, China) was performed for extracting lithium isotope separation. A TG16-WS high-speed centrifuge (Hunan, China) was employed for sufficient disengagement of the organic phase and aqueous phase. Lithium isotopic ratio was determined by a double focusing inductively coupled plasma mass spectrometry (Neptune Plus, Thermo Fisher Scientific, Bremen, Germany) using sample-standard bracketing method. Two percent HNO3 solution and Li2CO3 were used as blank and lithium isotope standard substance for correcting machinery bias, respectively. Thermo scientific iCAP 6500 series inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to determine the concentrations of metal ions in aqueous phase. Fourier transform infrared spectroscopy (FTIR) was recorded in the range of 600–4000 cm− 1 on a Bruker VERTEX 70 spectrometer (Bruker, Germany). 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on an Inova-400 spectrometer (Agilent, USA) in CDCl3.

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and the solvent was removed by rotary evaporator. It was found that the reaction was concentrated to black oil. The oil was dissolved in dichloromethane and washed with 1 mol L−1 HCl and water. Then, the combined organic layers were concentrated under reduced pressure to give a residue. The ethyl acrylate was added to the residue and it was filtered to give a filter cake. The DB14C4 was obtained as a white solid using methanol recrystallization. 2.3. Lithium isotope separation Organic phase was prepared by dissolving appropriate amounts of DB14C4 in 5 mL organic solvent which containing IL and dichloroethane. For all extraction experiments, the volume ratio of IL to dichloroethane was 1:1 in the organic phase. For comparison with the performance of IL, pure dichloroethane (5 mL) containing DB14C4 was also prepared by the same procedures. Aqueous phases were prepared by dissolving each of the lithium salt in demineralized water. The extraction experiments were performed at certain temperature by contacting the organic phase and aqueous phase and vibrating for 30 min. Separation of the phases was assisted by centrifugation in a high-speed centrifuge that accompanied with lithium isotope exchange reaction in the interface between the organic phase and the aqueous phase. After phase disengagement, the aqueous phase was properly diluted and the concentration of lithium ions was measured using a ICPAES. The extraction efficiency (E), and the distribution ratio (D) were calculated according to the following equations: Eð%Þ ¼



Co −Ce  100 Co

½Liorg ½Liaq

¼

Co −Ce Vaq  Ce Vorg

ð1Þ

ð2Þ

where Co and Ce (mg L−1) are the initial and equilibrated concentrations of lithium ion in the aqueous phase, respectively. Vaq and Vorg (mL) represent the volume of the aqueous phase and organic phase, respectively. Lithium ion in the organic phase was back-extracted into the aqueous phase using 0.5 mol L−1 HCl solution. The ratio of isotopic 6Li/7Li in the solution could be measured by inductively coupled plasma mass spectrometry. The single stage separation factor of lithium isotope, α, is defined by Eq. (3): α¼

6  7  Li = Li org ð½6 Li=½7 LiÞaq

ð3Þ

where [6Li]/[7Li] represents the isotopic ratio. The subscripts of org and aq refer to the organic phase and aqueous phase, respectively.

2.2. Synthesis of DB14C4 DB14C4 was prepared following the procedures as illustrated in Scheme 1. A solution of pyrocatechol (1.00 eq) in n-butanol (1.20 L) was stirred at 288 K under nitrogen atmosphere in a three-necked flask. Then LiOH·H2O (1.00 eq) was added to the reaction. After stirred for 40 min, the reaction was heated to 373 K. Subsequently, 50% of 1, 3dibromopropane (1.00 eq) was added drop wise over a period of 40 min. Then the reaction was heated to 393 K and reflux for 2 h. After reaction is finished, it is naturally cooled to room temperature

3. Results and discussion 3.1. Characterization of DB14C4 The FTIR spectrum of DB14C4 was shown in Fig. 1. 1592, 1506 cm−1 were ascribed to the skeletal vibration of benzene ring. The peaks at 2914 and 2867 cm−1 were identified to be the asymmetric stretching vibration of C\\H bond of the crown ring. The symmetric vibration of

Scheme 1. Synthesis route of DB14C4.

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temperature on the single stage separation factor of lithium isotopes were discussed as follows. 3.2.1. Effect of the concentration of crown ether The nature of the extracted species, from available literature on analogous extraction systems [24], has been indicated in Eq. (4). Furthermore, the number of DB14C4 molecules involved in the metal ion extraction can be easily found out by carrying out Li+ extraction studies at varying DB14C4 concentrations. þ − þ − þ − Liþ Y− ðaqÞ þ nLðorgÞ þ C4 mim XðorgÞ →ðLiLn Þ XðorgÞ þ C4 mim YðorgÞ

ð4Þ

where Li+ Y−(aq), L(org), n, and (LiLn )+ X −(org) denoted the lithium salt in the aqueous phase, the DB14C4 dissolved in organic phase, the number of crown ether molecules composing a Li-crown complex and formed complexes in organic phase, respectively. The counter anions of the extracted Li(DB14C4) + cations were exchangeable with the anions of ILs. According to the Eq. (4), the apparent equilibrium constant (Ke) of the extraction reaction can be expressed as: Fig. 1. FT-IR spectrum of DB14C4.

Ke ¼ Ar\\O\\C were at 1250, 1123 and 1049 cm−1. The peak at 734 cm−1 was assigned to the out-plane flexural vibration of C\\H bond of the benzene ring [25]. These results indicate that crown ethers have been successfully synthesized. To further confirm the above observations, the chemical structure of DB14C4 was characterized by 1H NMR as illustrated in Fig. 2. The chemical shift at 6.85 ppm (designated as c and d) could be assigned to the methylene protons of benzene ring. The peaks at 2.21 and 4.49 ppm (designated as a and b) were attributed to the ring of the crown ether. The results obtained from 1H NMR confirm the observations from FTIR.

3.2. DB14C4 for lithium isotope separation It is known that the size selectivity of crown ethers for metal ions is based upon the number of ether oxygen atoms in the ring. The ionic diameters of lithium ion and DB14C4 were 1.20 and 1.24 Å, respectively. However, the ionic diameter of B15C5 was 1.70 Å. It was therefore envisaged that the DB14C4 could make complex with the lithium ions since their cavity size was closer to the ionic diameter of lithium ions. The effects of the concentration of crown ether, the type of counter anions of lithium salts and extraction

    ðLiLn Þþ X− org C4 mimþ Y− org   ½LiYaq ½Lnorg C4 mimþ X− org

K e ¼ DLi

½C4 mimþ Y‐ org n ½Lorg ½C4 mimþ X‐ org

ð5Þ

ð6Þ

The logarithmic expression of Eq. (6) was:     logDLi þ log C4 mimþ Y− org − log C4 mimþ X− org ¼ nlog½Lorg þ logK e

ð7Þ

The values of log [C4mim+ Y−]org and log [C4mim+ X−]org could be obtained based on the initial concentration of IL and the extraction efficiency of lithium ions. A variation of the extraction efficiency of lithium ions as a function of the crown ether concentration in the organic phase showed that the extraction efficiency increased with increasing the concentration of the DB14C4 (Fig. 3). As shown in Fig. 4, the slope ratio of ([log D Li + log [C 4 mim+ Y−] org − log [C4mim+ X−]org]) versus log [L] org curve was 1.14, suggesting that one molecule of the crown ether was involved during the extraction process. Lithium ions in aqueous phase were extracted into the organic phase with DB14C4 through the process showed in Eq. (4). Besides, the lithium isotope exchange equilibrium between organic phase and aqueous phase will reach quickly within 1 min [21].

Fig. 2. 1H NMR spectrum of DB14C4 in CDCl3.

C. Shi et al. / Journal of Molecular Liquids 224 (2016) 662–667

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Table 1 Effect of the concentration of crown ether on lithium isotope separation.a Concentration of DB14C4/mol L−1

Single stage separation factor (α)

Abundance of 6Li in Enrichment isotope organic phase/% in organic phase

0.050 0.100 0.125 0.150 0.175 0.200

1.004 1.006 1.007 1.009 1.010 1.013

7.577 7.596 7.603 7.611 7.617 7.634

6

Li Li Li 6 Li 6 Li 6 Li 6 6

a The initial concentrations of LiCl in aqueous phase is 0.5 mol L−1. The abundance of 6Li in original lithium salt was 7.563%.

Fig. 3. Variation of percentage extraction efficiency as a function of the ratio of the concentration of DB14C4. [DB14C4] = 0.05–0.20 mol L−1; [LiCl] = 0.5 mol L−1; T = 293 K.

Table 1 showed the effect of the concentration of crown ether on lithium isotope separation. It can be seen from Table 1 that the single stage separation factor increased from 1.004 to 1.013 with an increase of the concentration of crown ether from 0.050 to 0.200 mol L− 1. At the same time, the abundance of 6Li in organic phase increased from 7.577% to 7.634%. For comparison, the lithium isotope separation performance of crown ether in pure dichloroethane was conducted under the same conditions. The magnitude of separation factor α for 6Li\\7Li pair was 1.009. Meanwhile, the single stage extraction efficiency of lithium ions obtained was 6.658%. However, when the IL was added to the organic phase, the magnitudes of separation factor and extraction efficiency increased to 1.013 and 12.391%, respectively. The results indicated that the separation performance of DB14C4 has been improved after the ILs were involved in the extraction process. To get a deep insight into the unique extraction behavior, the extraction and isotope exchange mechanism were elucidated in Fig. 5.

3.2.2. Effect of the kind of lithium salt Due to the greasy exterior of DB14C4, the complex cation formed at the interface of the aqueous and organic phase was lipophilic in

the present study. Furthermore, the complex cation must accompany the anion to be extracted into the organic phase for preserving electrical neutrality. Therefore, the lithium isotopes separation was influenced by the affinity of the anion to the organic solvent. The lithium salts used included LiCl, LiClO4, LiBr, and LiI. It can be found from Table 2 that the order of the single stage separation factor was LiI N LiBr N LiClO4 N LiCl. This result obtained was in agree with the previous extraction work of Nishizawa [12]. The maximum lithium isotopes single-stage separation factor of 1.021 was observed with the counter anion of I− 1. At the same time, the abundance of 6 Li in organic phase reached 7.666% in the LiI solution. These results can be understood by the hard and soft of acids and bases (HSAB) theory and the electrostatic interaction of counter anions. On one − hand, the order of the softness character is I− N Br− N ClO− 4 N Cl . The crown ether must exclude the water molecules to make an inner sphere coordinating complex. The softer anion has the greater affinity to the organic phase and Li+–crown ether complex is more easily formed. On the other hand, as the anionic radius decreased, the charge density of counter anions increased accordingly. The small anions with higher surface charge density lead to stronger electrostatic interactions with Li + , which will reduce the coordination bond of the Li+-crown ether complex. 3.2.3. Thermochemical consideration The separation capability of the lithium isotopes by the solvent extraction can be evaluated by the free energy changes ΔG° (6Li/7Li) corresponding to the following chemical exchange equilibrium: 6

7 þ  þ − − Liþ Y− Li  DB14C4 ðNTf 2 ÞðorgÞ → 6 Li  DB14C4 ðNTf 2 ÞðorgÞ ðaqÞ þ þ − þ 7 Li YðaqÞ

ð8Þ

where Y − denoted the counter anion of lithium salt and the subscripts (aq) and (org) represent the ions found in the aqueous and in the organic phase, respectively. The equilibrium constant K of Eq. (8) is represented as follows: h

ih i  þ  7 þ  − 6 þ Li  DB14C4 ðNTf 2 ÞðorgÞ 7 Liþ Y− Li Li ðaqÞ ih i ¼  þ ðorgÞ  þ ðaqÞ K¼h þ 7 6 − þ − Li ðorgÞ Li ðaqÞ ð7 Li  DB14C4Þ ðNT f 2 ÞðorgÞ 6 Li YðaqÞ 6

ð9Þ

Besides, according to the Eq. (3), the single stage separation factor of lithium isotope, α, can be expressed as: α¼

Fig. 4. ([log DLi + log [C4mim+ Y−]org − log [C4mim+ X−]org]) versus log [DB14C4] (the initial concentrations of LiCl in the aqueous phase is 0.5 mol L−1, [L]org is the concentrations of DB14C4 in organic phase).

6  7  Li = Li ðorgÞ ð½6 Li=½7 LiðaqÞ

6 ¼

Liþ

7 Li

 ðorgÞ

7

Liþ



ðaqÞ  6 Liþ ðorgÞ ðaqÞ

þ



¼K

ð10Þ

Therefore, the equilibrium constant K of the lithium isotope exchange reaction was the same with the single stage separation factor α [12,26], as can be seen from the above equations. According to the van't Hoff equation and other thermodynamic equations, the following equation can be obtained and the change in enthalpy ΔH°

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Fig. 5. Illustration of the mechanism of the reaction between [Li+](aq) and [DB14C4](org) and the isotope exchange reaction between [6Li+](aq) and [7LiL+](org).

(6Li/7Li) can be calculated from the slope of the line determined by plotting log α versus 1/T. loga ¼ −

ΔH ° 1  þC 2:303R T

ð11Þ

where R is the universal gas constant and C is the integration constant. The free energy change ΔG° (6Li/7Li) and entropy change ΔS° (6Li/7Li) of the lithium isotope exchange reaction could also be calculated by applying the well-known thermochemical equations: ΔG ° ¼ −2:30RT logK ¼ −2:30RT loga

ð12Þ

ΔH ° −ΔG ° ΔS ¼ T °

ð13Þ

It can be found from Table 2 that the maximum lithium isotopes single stage separation factor was observed with the counter anion of I −. Therefore, various thermodynamic parameters of the LiIDB14C4/IL/C2H4Cl2 system were obtained and the temperature was varied over the range 293–323 K. Fig. 6 showed that log α versus 1000/T yielded a straight line with a slope of 0.0061 equal to − ΔH°/2.303R. So the lithium isotope exchange process was exothermic with ΔH° (6Li/7Li) = − 116.798 J mol− 1. In addition, as shown in Fig. 6, the separation factor increased with the decrease of operating temperature. Thus, 293 K of operating temperature was used for lithium isotope separation to obtain a relatively high separation factor. ΔG° (6 Li/7 Li) value was found to be − 50.635 J mol− 1 , so the reaction was spontaneous and beneficial to the extraction separation of the lithium isotope.

of lithium ions into the aqueous phase was possible within fourth stages. No trace amount of lithium ions was detected in the acid of the fifth back-extraction. After complete stripping of the lithium ions, the organic phase was scrubbed thoroughly with distilled water before its use in the subsequent extraction step. Compared with the first lithium isotope separation, no significant change of extraction efficiency and lithium isotope separation factor was observed. These results showed that this extraction system for lithium isotopes separation was recyclable and reusable. 4. Conclusions In this paper, the crown ether, DB14C4, was successfully synthesized and they have exhibited good selectivity and separation performance for lithium isotopes. Adding IL into the organic phase could improve the separation factor and extraction efficiency. Due to the high molecular symmetry and low polarity of DB14C4, it will be difficult to dissolve in water and the loss will be significantly reduced during the multi-stage extraction process. The single stage separation factor increased with an increase in the concentration of crown ether and the lithium ion were extracted by formation of [Li·DB14C4]+. Among a variety of lithium salts, LiI is most applicable due to the high distribution coefficient for the liquid-liquid extraction and moderately great separation factor. It was also found that the lighter isotope, 6 Li, was enriched in the organic phase, whereas the heavy one, 7Li, was concentrated in the solution phase. So our present work highlights the potential of DB14C4 as extractant for lithium isotopes separation in IL systems.

3.3. Stripping studies After the extraction separation of lithium isotopes, it is necessary to strip the lithium ions back into an aqueous phase and to recycle the organic phase for further use. The stripping was carried out using 0.5 mol L− 1 hydrochloric acid and complete back extraction Table 2 Effect of the kinds of lithium salts on lithium isotope separation.a Kinds of lithium ions

Abundance of 6Li in original lithium salt/%

Abundance of Li in organic phase/%

Single stage separation factor (α)

Enrichment isotope in organic phase

LiCl LiClO4 LiBr LiI

7.563 7.577 7.554 7.559

7.634 7.647 7.666 7.676

1.013 1.014 1.018 1.021

6

6

Li Li Li 6 Li 6 6

a The initial concentration of lithium salt in aqueous phase was 0.5 mol L−1, and the concentration of crown ether in organic phase was 0.2 mol L−1.

Fig. 6. Plot of log α versus 1/1000 T for lithium isotope separation. [DB14C4] = 0.2 mol L−1; [LiI] = 0.5 mol L−1; T = 293–323 K.

C. Shi et al. / Journal of Molecular Liquids 224 (2016) 662–667

Acknowledgments This research was supported by the National Natural Science Foundation of China (U1407717 and U1407205). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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