Solvent extraction of lithium from aqueous solution using non-fluorinated functionalized ionic liquids as extraction agents

Solvent extraction of lithium from aqueous solution using non-fluorinated functionalized ionic liquids as extraction agents

Separation and Purification Technology 172 (2017) 473–479 Contents lists available at ScienceDirect Separation and Purification Technology journal h...

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Separation and Purification Technology 172 (2017) 473–479

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Solvent extraction of lithium from aqueous solution using non-fluorinated functionalized ionic liquids as extraction agents Chenglong Shi a,b,c, Yan Jing a,b, Jiang Xiao a,b,c, Xingquan Wang a,b,c, Ying Yao a,b, Yongzhong Jia a,b,⇑ a

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 c University of Chinese Academy of Sciences, 100049 Beijing, China b

a r t i c l e

i n f o

Article history: Received 3 June 2016 Received in revised form 21 August 2016 Accepted 29 August 2016 Available online 3 September 2016 Keywords: Solvent extraction Ionic liquids Lithium

a b s t r a c t Two non-fluorinated ionic liquids (ILs), namely tetrabutylammonium bis(2-ethylhexyl)-phosphate, [N4444][DEHP], and tetraoctylammonium bis(2-ethylhexyl)-phosphate, [N8888][DEHP], were synthesized and fully characterized by 1H and 13C NMR spectroscopy. These functionalized ILs were subsequently evaluated in terms of their ability to extract lithium ions from aqueous solution. The extraction efficiencies of these functionalized ILs for lithium ions were measured as a function of various extraction parameters, including aqueous acidity, temperature and extractant concentration. The extracted species were determined using a slope analysis method, which indicated that the ILs formed a 1:1 (metal:ligand) complex. Fourier transform infrared spectroscopy studies were conducted to better understand the nature of the extracted species. We also investigated the stripping of the lithium ions from the organic phase and the re-use of the ionic liquid in a subsequent extraction step. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Lithium, as the lightest metal element, is widely used in various fields, including lithium batteries, aerospace, ceramics, polymers and metal additives [1–5]. It is envisaged that the demand for lithium metal will increase considerably over the next decade with the continued development of modern industry. Lithium resources exist naturally in two forms: as a mineral and as a dissolved salt. The limited availability of lithium from mineral resources has steered the industry towards the use of liquid lithium resources, which have been estimated to contain >85% of the world’s recoverable lithium [6]. Lithium ions can be recovered via a number of different processes, including precipitation, ion exchange, adsorption, extraction and supported liquid membrane [7–11]. Among these processes, solvent extraction is considered to be one of the most powerful techniques, offering several advantages over the other approaches, including continuous operation and the use of relatively simple equipment on the both laboratory and industrial scales. One of the most commonly used systems for the extraction of lithium ions is tributyl phosphate (TBP)/kerosene-FeCl3 [12–14]. In this particular system, FeCl3 acts as a co-extraction agent, where it plays a key role in the extraction of lithium ions. However, this ⇑ Corresponding author at: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, 810008 Xining, China. E-mail address: [email protected] (Y. Jia). http://dx.doi.org/10.1016/j.seppur.2016.08.034 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

system must be used under strongly acidic conditions to avoid the hydrolysis of residual ferric ions in the aqueous phase. The use of this system therefore results in the generation of large amounts of acidic wastewater during the extraction and stripping process, representing a serious environmental pollution issue. Furthermore, the strongly acidic conditions required by this system may result in severe corrosion to the equipment. The development of innovative and green extraction processes for the extraction of lithium ions are therefore highly desired to address growing concerns about the environmental impact and safety issues associated with existing processes. Ionic liquids (ILs) are organic salts composed entirely of ions that have melting points near or below room temperature. ILs have attracted considerable interest from researchers working towards the development of solvent extraction processes because of their many useful solvent properties, including their high thermal stability, negligible volatility, high selectivity and high solvent extraction efficiency [15–17]. ILs can be used to extract metals by simply adding conventional molecular extractants to commercially available ILs. The molecular extractants used in these systems interact with the metal ions to form hydrophobic complexes that readily dissolve in the hydrophobic IL phase. Most of the ILs typically used for solvent extraction processes contain fluorinated anions, such as hexafluorophosphate (PF 6 ) and bis(trifluoromethyl sulfonyl)imide (NTf 2 ) anions, because these ILs tend to be immiscible with water, even in combination with small cations

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[18–22]. However, it is well known that the extraction mechanisms of ILs can differ from those observed in molecular solvents. In fact, ion exchange is often involved in the extraction mechanisms of ILs, which can lead to the loss of some of the components of the ILs. This process can also lead to the contamination of the water phase with hydrofluoric acid, following the hydrolysis of  the fluorinated anions (NTf 2 , PF6 ). These issues can therefore complicate efforts to regenerate and reuse ILs, thereby limiting their recyclability. Subsequent developments in this area have led to the introduction of functionalized ionic liquids (FILs) or task-specific ionic liquids (TSILs), where a metal-coordinating group is attached to the cationic or the anionic part of the IL. FILs have been successfully applied as extractants for the separation of metal ions from aqueous medium [23–26]. For example, Onghena and Binnemans [27] synthesized betainium bis(trifluoro methylsulfonyl)imide as a new FIL, and studied its ability to coordinate scandium(III). The results revealed that this FIL allowed for the rapid extraction of Sc(III) ions from water, even at room temperature, with good extraction efficiencies (>95%). Similarly, Sun and Waters [28] synthesized five FILs and evaluated their use as extractants for the separation of rare earth elements. These novel FILs were reported to be highly efficient extraction extractants and showed excellent selectivity towards rare earth ions. Fang et al. [29] reported the development of a new thiol-FIL for the extraction of Cd(II). This system exhibited several good characteristics, including fast adsorption equilibrium and high efficiency, as well as, the interference-free extraction of trace Cd(II). Here, we have examined the extraction of lithium ions using a solvent extraction system containing a novel non-fluorinated IL consisting of the functionalized IL extractant tetrabutylammonium bis(2-ethylhexyl)-phosphate or tetraoctylammonium bis(2-ethylhexyl)-phosphate in methylbenzene. The effects of several experimental parameters, including the aqueous acidity, temperature and extractant concentration, were also evaluated in terms of their impact on the extraction behavior of the lithium ions. Fourier transform infrared (FTIR) spectroscopy was used to investigate the interactions between the ligands and the lithium ions. Thermodynamic analysis was used to determine the thermodynamic parameters of the lithium ion extraction reaction. The stripping of the lithium ions from the loaded organic phase was also investigated as well as the recyclability and re-usability of the organic phase. 2. Experimental 2.1. Materials and apparatus The chemicals, tetrabutylammonium chloride (99% purity), tetraoctylammonium bromide (99% purity) and bis(2-ethylhexyl)phosphoric acid (99% purity), were purchased from Aladdin Industrial Corporation (China) and used without further purification. Lithium chloride (>99%) used in this study were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). The extraction experiments were performed in a THZ-82A thermostatic water bath oscillator (Changzhou, China). TG16-WS high-speed centrifuge (Hunan, China) was employed for sufficient disengagement of the organic phase and aqueous phase. 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, and the concentration in organic phase was obtained from by mass balance. 1H NMR and 13 C NMR spectra of ILs were recorded on an Inova-400 spectrometer (Agilent, USA) and CDCl3 was used as the solvent for recording the NMR spectra. Fourier transform infrared spectroscopy (FTIR) measurements were performed on a Thermo Nicolet Corporation 670 Spectrometer (USA). Distilled water was used to prepare the

aqueous solutions in all experiments. All other chemicals used in this study were of analytical grade. 2.2. Synthesis of ionic liquids The FILs were prepared using a combination of ion-exchange and neutralizing reactions. The first step of the synthesis of [N4444][DEHP] involved the ion exchange of chloride ions present in tetrabutylammonium chloride, [N4444][Cl], by hydroxide ion to form [N4444][OH]. This was done by equilibrating a solution of [N4444][Cl] (1 eq.) in dichloromethane with an aqueous 2 mol L1 NaOH solution (1.5 eq.) for 5 h. The aqueous phase was removed, and the organic phase was equilibrated again with a fresh sodium hydroxide solution. This procedure was repeated several times until the chloride content in organic phase was negligible (checked with an acidified silver nitrate solution). The second step involved refluxing the [N4444][OH] with bis(2-ethylhexyl)phosphoric acid (DEHPA) in 1:1 mol ratio for about 10 h. The lower aqueous phase was separated and the organic phase was washed with distilled water several times. The solvent and water were removed with a rotary evaporator, and the product was dried at 50 °C under vacuum for 4 h to yield [N4444][DEHP] as a viscous liquid. The FIL, [N8888][DEHP], was synthesized by stirring a solution of [N8888][Br] (1 eq.) in dichloromethane with an aqueous 2 mol L1 NaOH solution (1.5 eq.) for 5 h. After phase disengagement, the organic phase was equilibrated again with a fresh sodium hydroxide solution. This procedure was repeated several times until the chloride content in organic phase was negligible (checked with an acidified silver nitrate solution). Then, the resulting [N4444] [OH] (1 eq.) was refluxed with DEHPA (1 eq.) in dichloromethane medium for 10 h. The lower aqueous phase was separated and the organic phase was washed with distilled water several times. The product was evaporated to remove the solvent and water using a rotary evaporator. Then, the product was dried at 50 °C under vacuum for 4 h to yield [N8888][DEHP] as a viscous liquid. Fig. 1 showed the structure of the synthesized ILs and their precursors. NMR spectral data of synthesized ILs are given below. [N4444][DEHP]. 1H NMR (CDCl3, ppm): 0.78 (m, 12H, 4CH3), 0.91 (m, 12H, 4CH3), 1.18–1.37 (m, 32H, 16CH2), 1.56 (m, 2H, 2CH), 3.22 (m, 8H, 4NCH2), 3.61 (m, 4H, 2OCH2); 13C NMR (CDCl3, ppm): 10.41 (4CH3), 13.55 (2CH3), 14.10(2CH3), 19.08 (4CH2), 22.66 (2CH2), 23.36 (2CH2), 24.07(4CH2), 28.52 (2CH3), 29.61 (2CH2), 39.93 (2CH), 57.96(4NCH2), 66.89 (2OCH2). [N8888][DEHP]. 1H NMR (CDCl3, ppm): 0.84 (m, 24H, 8CH3), 1.17–1.31 (m, 56H, 28CH2), 1.34 (m, 2H, 2CH), 1.60 (m, 8H, 4CH2), 3.25 (m, 8H, 4NCH2), 3.60 (m, 4H, 2OCH2). 13C NMR (CDCl3, ppm): 11.12 (2CH3), 14.06 (6CH3), 22.60–23.49 (8CH2), 28.51– 29.74 (22CH2), 30.23 (2CH2), 41.15 (2CH), 54.58 (4NCH2), 69.37 (2OCH2). 2.3. Extraction experiments All the extraction studies were carried out at 293 K, with the exception of the experiments at variable temperatures for determination of the thermodynamic parameters, where the temperature was varied over the range 293–333 K. The extraction of lithium ions was studied by equilibrating 5 mL methylbenzene containing the FILs with 2.5 mL of the aqueous phase at the desired acidity. Extractions were performed by intensive shaking of the extraction mixture for 30 min. Separation of the phases was assisted by centrifugation for 10 min at 5000 rpm. After phase disengagement, the aqueous phase was properly diluted and the concentration of lithium ions was measured using an ICP-AES. The concentration of metal ions in organic phase was calculated from mass balance. For stripping experiments, the loaded organic phase was scrubbed with the dilute hydrochloric acid solution a few times. In the next

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Fig. 1. Structures of the precursors and various dialkylphosphate ILs.

step, the organic phase was scrubbed with an aqueous NaOH solution to transform DEHPA back into the anionic form. The organic phase was then dried for reuse in the next cycle. The distribution ratio (D) is the ratio of the concentration of the metal ion M in the organic phase to the concentration of the metal ion in the aqueous phase, at equilibrium:



½Morg Co  Ce Vaq ¼  ½Maq Ce Vorg

ð1Þ

where Co and Ce (mg L1) are the initial and equilibrated concentrations of metal ions in the aqueous phase, respectively. Vaq and Vorg (mL) represent the volume of the aqueous phase and organic phase, respectively. The extraction efficiency (E) is defined as the amount of metal extracted to the organic phase over the total amount of metal in both phases and is given by the following expression:

E ð%Þ ¼

Co  Ce  100 Co

ð2Þ

3. Results and discussion

Fig. 2. Effect of the HCl concentration on the extraction of Li+ from aqueous medium. Aqueous phase: [Li+] = 0.5 g L1 and [HCl] = 0–0.5 mol L1; [FIL] = 1.06 mol L1; temperature = 293 K.

3.1. Effect of HCl concentration The extraction behaviors [N4444][DEHP] and [N8888][DEHP] for lithium ions were evaluated using various concentrations of hydrochloric acid (0–0.5 mol L1) in the aqueous phase. As shown in Fig. 2, increasing the HCl concentration led to a decrease in the extraction efficiency (E). This decrease could be attributed to the preferential extraction of the acid over the extraction of the lithium ions, with the HCl molecules then blocking the coordinating sites of the [DEHP] anions. These results therefore indicated that the aqueous solutions could be used for extracting directly without adding acid to the aqueous phase. To develop a better understanding of the extraction behavior of DEHPA, we investigated the extraction of lithium ions from aqueous HCl using a dilute solution of DEHPA in methylbenzene (Fig. 2). The results revealed that DEHPA exhibited only negligible extraction activity for all of the HCl concentrations tested in the current study. The poor extraction efficiency of DEHPA could be attributed to the torsion angle of DEHPA being larger than that of the corresponding [DEHP] anion. The larger torsion angle of DEHPA means that this molecule would have to undergo considerable conformational changes to form the metal-solvate complex. This result therefore indicates that the

[DEHP] anions are more likely to form complexes with metal ions than DEHPA. The differences observed in the extraction behaviors of [N4444][DEHP], [N8888][DEHP] and DEHPA highlight the differences typically observed in the extraction properties of ILs and molecular extractants. Despite using the same extracting moiety, the extraction behavior of these FILs indicated that there existed an inner synergism between the quaternary ammonium cation and deprotonated organic anion. As a result, the extraction efficiencies of these FILs for Li+ were much better than that of the mixture of their corresponding precursors. Under strong acidic conditions, the FILs would most likely be transformed to a mixture of [Nxxxx][Cl] (x = 4,8) and DEHPA. The inner synergism will be lost and the extraction efficiencies decreased. Furthermore, the results in Fig. 2 showed that the extraction efficiency of [N4444][DEHP] for lithium ions was superior to that of the [N8888][DEHP]. This phenomenon could be attributed to the different structures of the cations. Notably, the extraction efficiency of the two FILs decreased as the length of the alkyl chain attached to the cation increased, indicating that steric hindrance could be playing a role in the extraction of the lithium ions.

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The stoichiometry of the complex was determined using a slope method according to Eq. (7). Fig. 3 shows the variation in the distribution ratio of lithium ions as a function of the concentration of the FIL in the organic phase. These data revealed that the

distribution ratio of lithium ions increased as the concentration of FIL in the organic phase increased. Linear regression analysis of the extraction data resulted in a slope of about 1 for both of the FILs, suggesting that one molecule of FIL formed a complex with a single lithium ion during the extraction process (i.e., a 1:1 complex). Although the number of ligands present in the extracted species can be determined using the slope analysis method described above, further analysis is required to determine the nature of the coordination in these complexes. FTIR spectroscopy is sensitive to small differences in the chemical microenvironment of complex systems, and can be used to characterize different functional group information. These measurements were carried out to clarify the existence of interactions between the lithium ions and the FILs. The FTIR spectra of organic phase before and after the extraction process were recorded and the results are shown in Fig. 4. The peaks at 2960 and 2874 cm1 were considered to be the asymmetric and symmetrical stretching vibration of the ACH3 groups of the [N4444]+ cations, respectively. The peak at 1083 cm1 was attributed to the symmetric stretching vibration of the CAN bonds of the [N4444]+ cations, whereas the peak at 1463 cm1 was assigned to the in-plane flexural vibration of the ACH2 groups. The FTIR results clearly showed that the positions of most of the characteristic peaks of the cations of the ILs were unchanged after the extraction, suggesting that they were not involved in the extraction reaction. The [DEHP] anions give a characteristic vibrational band for their [email protected] group at 1227 cm1. The results revealed that the loading of the organic phase with Li+ ions lead to a considerable shift in the [email protected] stretching vibration from 1227 to 1211 cm1. The shift in the [email protected] frequency with respect to the net [DEHP] content was about 16 cm1, which indicated that the lithium ion was coordinated to the oxygen atom of the [email protected] group. We also observed a considerable decrease in the intensity of the [email protected] group after the extraction process. This decrease in the intensity was mainly attributed to the reduction in the electron density during the formation of the [email protected] ? Li coordination bond. Notably, an intense band appeared at 1035 cm1 following the extraction process, which was attributed to the symmetrical stretching frequency of the PAOAC bond of the [DEHP] anion in the organic phase. As seen in Fig. 4, the presence of lithium ions in the organic phase did not lead to any changes in the FTIR vibration bands of the PAOAC bond. This result therefore demonstrated that the loading of the lithium ions had a slight impact on the stretching vibration of the PAOAC bond.

Fig. 3. Plot of log DLi as a function of log [FIL]. Aqueous phase: [Li+] = 0.5 g L1; temperature = 293 K; organic phase: [FIL] = 0.43–1.27 mol L1.

Fig. 4. FTIR spectra of the organic phase before and after extraction. (a) The organic phase before extraction; (b) the organic phase after extraction.

3.2. Extraction mechanism Given that the non-fluorinated bis(2-ethylhexyl)phosphate ILs prepared in this study performed well for the extraction of lithium ions, we proceeded to investigate their extraction mechanism. A comparison with analogous extraction systems from the literature [30] suggested that the current extraction reaction could be expressed as shown in Eq. (3). To guarantee electronic neutrality, it was envisaged that only one chloride ion could be involved in the extraction process. The number of FIL molecules involved in the metal ion extraction could be easily determined by conducting Li+ extraction studies at varying FIL concentrations. 

þ

LiðaqÞ þ n½Nxxxx ½DEHPðorgÞ þ ClðaqÞ ! LiCl  n½Nxxxx ½DEHPðorgÞ

ð3Þ

where [Nxxxx][DEHP], n and {LiCln[Nxxxx][DEHP]} represent the FIL extractant [N4444][DEHP] or [N8888][DEHP], the number of FIL molecules involved in the metal ion extraction and the complexes formed in the organic phase, respectively. Besides, the subscripts aq and org denote the aqueous phase and organic phase, respectively. According to Eq. (3), the two-phase extraction equilibrium constant (K) could be given as follows:



½LiCl  n½Nxxxx ½DEHPðorgÞ

ð4Þ



þ

½Li ðaqÞ ½Nxxxx  DEHPnðorgÞ ½Cl ðaqÞ

The DLi value, which represents the distribution ratio of Li+ ions in the organic and aqueous phases can be determined as follows:

DLi ¼

½LiCl  n½Nxxxx ½DEHPðorgÞ

ð5Þ

þ

½Li ðaqÞ

Eq. (4) can therefore be modified as follows:



DLi  ½Nxxxx  DEHPnðorgÞ ½Cl ðaqÞ

ð6Þ

Taking the logarithm and rearranging the resulting equation would give the following: 

log DLi ¼ log K þ n log½Nxxxx  DEHPðorgÞ þ log ½Cl ðaqÞ

ð7Þ

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3.3. Determination of thermodynamic parameters The effects of the temperature on the extraction efficiencies of methylbenzene solutions of [N4444][DEHP] and [N8888][DEHP] for lithium ions from aqueous phase were evaluated using a thermostatic water bath oscillator at temperatures in the range of 293–333 K (Fig. 5). The results revealed that the extraction efficiencies of these FILs decreased with increasing temperature. The enthalpy change (DH°) during an extraction can be determined based on the slope of a plot of log D versus 1000/T (K1) using the van’t Hoff equation:

log D ¼ 

DH o 1  þC 2:303R T

ð8Þ

where R is the universal gas constant and C is the integration constant. In this study, the integration constant was assumed to be constant at a particular temperature under the experimental conditions. The enthalpy changes of the [N4444][DEHP] and [N8888][DEHP] extraction systems were found to be 30.83 and 21.25 kJ mol1, respectively, highlighting the exothermic nature of the extraction reaction. The overall enthalpy change during the solvent extraction process represents the sum of several factors, including: changes in the enthalpy owing to the dehydration of the metal ion; the formation of a complex between the metal ion and the extractant; the

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dissolution of the metal ion/ligand complex in the organic phase; and the rearrangement of the organic phase to attain a stable configuration. The [N8888][DEHP] metal complex would be more lipophilic than the [N4444][DEHP] metal complex because of the longer alkyl chains on the ammonia cation. The greater lipophilicity of this system would result in more energy being released during the dissolution of the Li+-[N8888][DEHP] complex into the organic phase than in case of the Li+-[N4444][DEHP] complex. However, the rearrangement of the Li+-[N8888][DEHP] complex in the organic phase would require more energy than that of Li+-[N4444][DEHP] complex. The overall entropy change for the extraction would therefore be negative for both FILs, although the value for [N4444] [DEHP] would be more negative than that of [N8888][DEHP]. The change in Gibbs free energy (DG°) can be calculated from Eq. (9):

DGo ¼ 2:303RT log K

ð9Þ

The log K value was obtained from Fig. 3. The changes in the Gibbs free energy of the [N4444][DEHP] and [N8888][DEHP] extraction systems were found to be 91.06 and 66.21 kJ mol1, respectively, indicating that these processes were energetically favored and more spontaneous in the forward direction. The change in the Gibbs energy for the complexation of Li+ with [N4444][DEHP] was more negative than that of [N8888][DEHP], showing that the complex formation of [N4444][DEHP] was thermodynamically more favorable than that of [N8888][DEHP]. The change in entropy (DS°) at a fixed temperature can be evaluated using Eq. (10):

DSo ¼

DHo  DGo T

ð10Þ

The DS° value for the [N4444][DEHP] and [N8888][DEHP] extraction systems were determined to be 89.30 and 5.97 J K1 mol1, respectively. The fact that these values are negative indicated that the degree of order had increased during the extraction process. 3.4. Extraction of lithium ions from multi-metal-ion solutions The extraction performance of the DEHP-type FILs was explored for alkali metal ions. The concentration of each metal ion (Li+, Na+, K+, Rb+, and Cs+) in aqueous solution was 0.5 g L1. Fig. 6 showed the distribution ratios for alkali metal ions extracted by [N4444] [DEHP] and [N8888][DEHP] in methylbenzene. As can be seen in

Fig. 5. (a) Effects of the temperature on the extraction efficiencies of [N4444][DEHP], [N8888][DEHP] for Li+; (b) plot of log D versus 1000/T for the extraction of Li+ ions. Aqueous phase: [Li+] = 0.5 g L1; organic phase: [FIL] = 1.06 mol L1; temperature = 293–333 K.

Fig. 6. Extraction efficiencies of alkali metal ions from aqueous solution with FILs. Aqueous phase: [Li+] = [Na+] = [K+] = [Rb+] = [Cs+] = 0.5 g L1; organic phase: [FIL] = 1.06 mol L1; temperature = 293 K.

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The organic phase was then washed with distilled water and dried for reuse in the next cycle. In the current extraction system, the cations and [DEHP] anions of the ILs were very hydrophobic because of their long alkyl chains. It was therefore envisaged that the loss of IL to the aqueous phase during the stripping stage would be negligible. Pleasingly, we did not observe any discernible decrease in the extraction efficiency of the recycled organic phase after the recycling stage, with the value remaining quite close to that of the first extraction stage. This indicated that the organic phase was recyclable and reusable. A flow sheet was proposed for a process to extract lithium ions from aqueous solution by these FILs extraction systems (Fig. 8). 4. Conclusions

Fig. 7. Percentage stripping of lithium from the loaded organic phase. Aqueous phase: 0.5 mol L1 hydrochloric acid; temperature = 293 K; phase volume ratio = 1.

Fig. 6, the distribution ratios decreased with increase the atomic number of the alkali metal. The decrease in distribution ratios could be due to the coordination abilities of the functional groups in the [DEHP] anions. The ionic radius sequence of the alkali metal ions was Cs+ > Rb+ > K+ > Na+ > Li+. As the ionic radius decreased, the surface charge density of the metal ions increased. The small cations with higher surface charge density lead to stronger electrostatic interactions and to the formation of more stable species with the anion [DEHP] anions.

3.5. Stripping and reusability studies The observation that the extraction efficiencies of [N4444] [DEHP] and [N8888][DEHP] for lithium ions decreased with increasing acid concentration suggested that the loaded metal ions could be stripped from the organic phase using an acidic solution. To evaluate this possibility, we conducted a stripping study using 0.5 mol L1 HCl as a stripping solution. The results revealed that the lithium stripping percentage increased as the number of stripping stages increased, and that the complete stripping of lithium was possible within four stages (Fig. 7). After the back extraction steps, the organic phase was washed three to four times with a 0.5 mol L1 NaOH solution to fully deprotonate the IL extractant.

We have prepared two FILs, [N4444][DEHP] and [N8888][DEHP], bearing bis(2-ethylhexyl)phosphate anions and evaluated their extraction efficiencies for the liquid–liquid extraction of lithium ions from aqueous solution. The extraction efficiencies of [N4444] [DEHP] and [N8888][DEHP] for lithium ions decreased as the acidity of the aqueous phase increased. Furthermore, increasing the length of the alkyl chain of the cation led to a decrease in the extraction ability of these FILs, which could be attributed to an increase in steric hindrance. The mechanism for the extraction of lithium ions using these FILs was investigated by slope analysis and FTIR spectroscopy. The extraction stoichiometry indicated the formation of a 1:1 complex between the FILs and the lithium ions. Thermodynamic calculations were conducted to determine the thermodynamic parameters of the lithium ion extraction reaction. The DH° and DG° values for these systems were both found to be negative, highlighting the exothermic and spontaneous nature of the extraction process. The lithium ions were completely stripped from the loaded organic phase under relatively mild conditions without the need to add a complex-forming agent to the aqueous phase. These new FIL extraction system could also be regenerated and reused for subsequent extractions without any loss discernible loss in their extraction efficiency. These results therefore demonstrate that FILs can be used to broaden the potential utility of liquid– liquid extraction processes and establish efficient and environment friendly processes for the separation of valuable metal ions. Acknowledgments This work was supported by the National Natural Science Foundation of China (U1407717 and U1407205).

Fig. 8. Flow sheet for the extraction of lithium ions from aqueous solution with the FILs/methylbenzene system.

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