pH-Controlled Selective Separation of Neodymium (III) and Samarium

Oct 16, 2015 - ILs were used to separate Nd(III) and Sm(III) from transition metals in solutions .... Ahmed A. Galhoum , Khalid M. Hassan , Osman A. D...
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Research Article pubs.acs.org/journal/ascecg

pH-Controlled Selective Separation of Neodymium (III) and Samarium (III) from Transition Metals with Carboxyl-Functionalized Ionic Liquids Yuehua Chen, Huiyong Wang, Yuanchao Pei, Jiao Ren, and Jianji Wang* Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China S Supporting Information *

ABSTRACT: The recovery of rare-earth metals from waste materials is very important due to the risks associated with having a low supply of them in the future. In this work, hydrophobic 1alkylcarboxylic acid-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids, [(CH2)nCOOHmin][Tf2N] (n = 3, 5, 7), were synthesized and used to separate neodymium (III) from Fe(III) and samarium (III) from Co(II) in aqueous solutions. The factors affecting the solvent extraction process such as phase volume ratio, contact time, pH value of the aqueous phase, alkyl chain length of the ionic liquids, and temperature of the system were examined systematically. It was found that the maxmium extraction efficiency of the investigated metal ions was as high as 99%, and Sm(III) and Nd(III) could be selectively separated from Co(II) and Fe(III), with separation factors of 104−105, by simply modulation of the aqueous phase pH. After extraction, about 97% of the metal ions could be stripped from ionic liquid phase in a single stripping step by using dilute aqueous HCl or oxalic acid, and the ionic liquids would be recovered and reused in the next extraction process. These results indicate that the ionic liquids developed here are useful for the selective recovery of rare-earth metals from NdFeB and SmCo permanent magnets. KEYWORDS: pH-controlled selective separation, Carboxyl-functionalized ionic liquid, Rare earth metal, Solvent extraction, Transition metal

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INTRODUCTION Due to their excellent magnetic performance, neodymium− iron−boron (NdFeB) and samarium−cobalt (SmCo) magnets have been widely applied in fields such as computer peripherals, electric motors, and wind turbines.1,2 With increasing demand for permanent magnets in the world, it is anticipated that the supply of Nd and Sm will face a crisis. From the viewpoint of sustainable development, the key to solving supply problems is the recovery of Nd and Sm from waste rare-earth materials.2,3 It is known that NdFeB magnet contains about 59 wt % Fe and 26 wt % Nd, and a SmCo magnet consists of about 76 wt % Co and 24 wt % Sm.4,5 Thus, it is important to recover Nd and Sm from NdFeB and SmCo magnets, respectively. As far as the separation techniques are concerned, liquid−liquid extraction is always found to be a simple and effective way for the separation of rare-earth metal from common metals.6−8 However, in the conventional liquid−liquid extraction, water-immiscible organic solvents are often used as diluents, which are usually flammable, volatile, and hazardous to the environment. Therefore, development of sustainable and efficient techniques is greatly necessary for the separation of the neodymium and samarium from transition metals such as iron and cobalt. © XXXX American Chemical Society

Ionic liquids (ILs) are a family of organic salts being composed of organic cations and inorganic or organic anions. Due to their unique properties, such as negligible volatility, low flammability, low melting point (99.9%), Nd(NO3)3·6H2O (>99.0%), Co(NO3)3· 6H2O (>99.9%), Fe(NO3)3·9H2O (>99.9%), oxalic acid dehydrate (>99.5%), hydrochloric acid (37%), and lithium bis(trifluoromethylsulfonyl)imide (LiTf2N) (99%) were obtained from Aladdin Chem. Co. Ltd. 8-Bromooctanoic acid (97%) was from Alfer Aser Co. Ltd. Analytical grade hydrogen nitrate (65%) and sodium hydroxide used to adjust pH of the aqueous phases were acquired from Shanghai Chem. Co. All the chemicals except for N-methylimidazole were used as received without further purification. Water used in the experiments was prepared by redistillation of deionized water. In order to reduce the experimental error, stock solutions of the metal ions were prepared by dissolving their nitrate salts in aqueous solutions. Dilute solutions were prepared by appropriate dilution of the stock solution. Synthesis and Characterization of the ILs. 1-Alkylcarboxylic acid-3-methylimidazolium bromides were prepared according to the procedure described in literature,26 and then equimolar amount of lithium bis(trifluoromethylsulfonyl)imide and 1-alkylcarboxylic acid-3methylimidazolium bromide was added into 50 mL water and stirred overnight at room temperature. The ionic liquid phase was separated from the aqueous phase and washed with amounts of ice-cold water to remove the bromide impurity until no AgBr precipitate was detected in aqueous phase by the addition of a few drops of aqueous silver nitrate. Finally, the ionic liquids were dried under vacuum at 65 °C in the presence of P2O5. 1H and 13C NMR spectra of the ILs were recorded on a Bruker AV-400 spectrometer, thermal decomposition temperature (Td) of the ILs was determined by a Netzsch STA 449c thermal gravimetric analyzer (TGA), and glass transition temperature (Tg) of the ILs was determined by a Netzsch DSC 204 F1 differential scanning calorimeter (DSC). The detailed data were presented in the Supporting Information. The viscosities of the ILs were measured by a Lovis 2000 ME Rolling-ball Microviscometer (Anton Paar, Austria). Furthermore, the contents of residual bromide in the ILs were determined by the procedure reported by Seddon and co-workers.27 It was found that the bromide content was less than 0.006 wt %. Extraction Experiments. The extraction experiments were carried out at 25.0 °C, and the experimental temperature was controlled by circulating water from a thermostat. Taking into account the hygroscopicity of the ILs (see their solubility in water in Table S1), the presaturated ILs with water were used instead of dry ionic liquids to prevent the further uptake of water during the extraction process. At the same time, viscosity of ionic liquids was significantly decreased, which was beneficial to the mass transfer of solutes. Typically, 5 mL of aqueous solution containing a certain concentration of metal ion was mixed with 0.5 mL of IL, and the mixture was vigorously stirred for 60 min to ensure extraction equilibrium. Then, the aqueous phase was separated by centrifugation at 2800 rpm for 10 min. The concentrations of Fe(III) and Co(II) in aqueous phases were determined by atomic absorption analysis (AAS, PerkinElmer, AA700, USA); whereas the concentrations of Sm(III) and Nd(III) in aqueous phases were determined by inductive coupled plasma mass spectrometry (ICP-MS, PerkinElmer, ELAN DRC-e, USA). Then the metal ion concentrations in IL phase were calculated by mass balance. For the determination of thermodynamic parameters associated with the extraction, variable temperature measurements were conducted. The concentration values of metal ions were measured in triplicate with uncertainty of 5%. The pH values of the aqueous phase were determined by a model pHSJ-4F pH meter (Leici, Shanghai, China), which was calibrated daily by the standard buffer solutions with pH 4.00 and 6.86. For the stripping of metal ions from ILs phases after extraction, the IL phase loaded with metal ions was separated from the aqueous phase and then contacted with 3 mL of aqueous acid solution at different concentrations. The mixture of aqueous HCl (HNO3 or oxalic acid) and the IL containing metal ions was stirred with magnetic stirrer for 60 min to ensure the attainment of extraction equilibrium. The subsequent procedures for the determination of metal ions concentrations in aqueous solution and in IL phase were the same as described above for metal ions extraction.

bis(trifluoromethylsulfonyl) amide ([P2225][TFSA]). Yoon et al.23 conducted the extraction of five rare earth metals including neodymium and samarium from aqueous solution by bis(2ethylhexyl)phosphoric acid (DEHPA) in the IL 1-Cn-3methylimidazolium hexafluorophosphate (n = 2, 4) or 1butyl-4-methylpyridinium hexafluorophosphate. In the above investigations, ILs were usually employed as a solvent or a diluent for a molecular extractant. However, the IL components may be lost to the water phase by ion exchange to contaminate the aqueous phase since degradation of ILs is very difficult in water.24 In addition, in the consequent stripping procedure, it is necessary to add other agent to decompose the complex formed by metal and extractant for the recovery of the metal ions and the extractant, which makes the process to be complicated. In order to resolve these problems, Binnemans and his co-workers5 applied ionic liquid trihexyl(tetradecyl)phosphonium chloride ([P66614][Cl]) to separate transition metal ions from neodymium and samarium in highly concentrated aqueous HCl solution without adding any extractant or organic diluents. Recently, they also used trihexyl(tetradecyl)phosphonium nitrate ([P66614][NO3]) to extract samarium (III) or lanthanum (III) from cobalt or nickel based on inner salting-out effect of a highly concentrated metal nitrate aqueous phase.25 This is a significant progress although the separation process involves some disadvantages such as strongly acidic condition (9 M HCl), higher extraction temperature (60 °C), and highly concentrated salt in aqueous phase.5,25 Therefore, it is necessary to develop new functional ionic liquids for the selective separation of Nd and Sm from transitional metals to overcome these shortcomings. In this work, a series of carboxyl-functionalized imidazolium ionic liquids, 1-alkylcarboxylic acid-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(CH2)nCOOHmim][Tf2N] (n = 3, 5, 7), were designed, synthesized, and used for the separation of Nd(III) from Fe(III) and Sm(III) from Co(II) in aqueous solution only by modulating the acidity of aqueous phase. The chemical structure of these ILs was shown in Figure 1. The effect of the related extraction parameters such as phase

Figure 1. Structure of the ionic liquids used in this work.

volume ratio, contact time, the alkyl chain length of the ILs, the pH values of aqueous phase and temperature of the system on the extraction efficiency was investigated in detail. The extraction mechanism was also studied by FT-IR spectra. Moreover, the stripping of metals from the ILs phases was investigated together with the recycling and reuse of the ILs. It is worthy to mention that for the solvent extraction systems with ILs reported here, the experimental condition was mild (at 25 °C), no salt, diluent, and other extraction agents were necessary, and no IL components were lost in the extraction process by ionic exchange. Therefore, such advantages are significant, and the findings would have great potential for the separation and recycling of the rare-earth metals from wasted permanent magnets of NdFeB and SmCo by using simple ILbased liquid−liquid extraction technology.

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EXPERIMENTAL SECTION

Materials. N-methylimidazole (99%) was purchased from Linhai Kaile Chem. Co. and distilled before use. Ethyl-4-bromobutyrate B

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering The distribution ratio (D), separation factor (SF), extraction efficiency (E), and stripping efficiency (S%) were calculated, respectively, by using the following equations:

D=

Vaq C i − Cf × Cf VIL

SF =

(1)

C i − Cf × 100% Cf

E(%) = D1 D2

S(%) =

[M]aq,R Vaq,R [M]IL,R VIL,R

phases was presented in Figure 2. It can be seen that the extraction efficiency (E %) of all the metal ions first increased

(2)

(3)

× 100% (4)

where Ci and Cf stand for the metal ion concentrations in aqueous phase before and after extraction, Vaq and VIL are the volumes of aqueous phase and the ILs, and D1 and D2 are the distribution ratio of metal ion 1 and 2, respectively. [M]aq,R represents the equilibrium concentration of metal in the stripping acid and [M]IL,R is the initial concentration of metal in the IL phase. Vaq,R and VIL,R are the volume of the stripping acid and the IL phase, respectively.

Figure 2. Extraction efficiency (E %) of Fe(III), Nd(III), Sm(III), and Co(II) at different equilibrium pH values of the aqueous phase. [M] = 100 μg/mL; IL2, 0.5 mL.

RESULTS AND DISCUSSION Table S2 shows thermal decomposition temperature, glass transition temperature, and viscosity at 25 °C for the ILs investigated in this work. It was shown that the prepared ILs exhibited super thermal stability from 370 to 407 °C, and they are really room temperature ionic liquids with glass transition temperatures ranging from −52 to −58 °C. The viscosities of these ILs at 25 °C were 1049.7, 851.9, and 902.0 cP for IL1, IL2, and IL3, respectively. They are highly viscous liquids at room temperature due to the formation of hydrogen bond between carboxyl group on the cation and the anion [Tf2N]−.28 In addition, it can be seen from Table S1 that these ILs are highly hydrophobic at room temperature. Thus, they are good candidate for the use in liquid−liquid solvent extraction. Effects of Phase Volume Ratio and Contact Time on the Extraction Efficiency. A series of experiments were performed to find out the optimum extraction phase volume ratio. It was shown that the best extraction efficiency of metal ions was observed at a volume ratio, 10:1, of an aqueous phase to an IL2 phase for Fe(III) extraction, while 8:1 was the optimum volume ratio for Sm(III) extraction. Considering the fact that for Sm(III) extraction, the difference in extraction efficiency was small at the volume ratio of 10:1 and 8:1, volume ratio of 10:1 was selected to investigate the extraction process of metal ions in order to use the smallest amount of ionic liquids. Such a volume ratio was also chosen by Dai and coworkers29,30 to extract metal ions using IL−water systems. The effect of contact time on extraction efficiency was investigated at 25 °C to confirm how long the extraction process takes to reach equilibrium. The results showed that the extraction efficiency of Fe(III) and Sm(III) was almost not changed after contacting of 40 min under stirring. Therefore, 60 min was selected to ensure the attainment of extraction equilibrium. Effect of pH on the Extraction Efficiency and the Selective Separation of Rare Earth Metals. In order to investigate the potential of the ILs as extractants and the influence of pH value on the extraction efficiency, the extraction of Fe(III), Nd(III), Sm(III), and Co(II) was carried out by IL2 from aqueous solution. The change in the extraction efficiency of these metal ions with the equilibration pH value of aqueous

steeply and then slowed down with the increase of aqueous phase pH value, and the extraction efficiency was as high as 99% at the optimal pH conditions. The observed pH dependence of extraction efficiency was similar to the extraction of metal ions by the traditional carboxyl acid such as Naphthentic and Versatic 10 acids dissolved in organic solvents.31,32 Similar trend was also found in the extraction of rare-earth metal by N,N-dioctyldiglycol amic acid (DODGAA) in the IL 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Cnmim][Tf2N].33 It is suggested that the driving force for the extraction of metal ions by traditional carboxyl acids is the favorable interaction between metal ion and oxygen atom of carboxylic group of the carboxyl acids. Whether the same driving force works for the extraction by this IL? In order to verify this point, we also conducted a series of extraction experiments of Fe(III) and Sm(III) directly by using 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C6mim][Tf2N]) under the same experimental conditions. It was found that Fe(III) and Sm(III) were not significantly extracted by the IL, implying that the extraction of Fe(III) and Sm(III) by [(CH2)5COOHmim][Tf2N] may be ascribed to the interaction of carboxyl in IL2 with the metal ions. After extraction, it may be supposed that metal-complexes were dissolved in a mass of IL phase. In order to further confirm the interaction of carboxylic functional group with the metal ions, Fourier transform infrared (FTIR) spectra were performed for the IL2 before and after the extraction of 10 mg/mL Fe(III), and the results were shown in Figure 3. It is clear that before extraction, the peak at 1710 cm−1 could be assigned to the stretching vibration of CO of the IL dimmer due to the formation of hydrogen bonding among carboxylic groups of the IL.34 After extraction, the strength of the peak at 1710 cm−1 decreased significantly whereas the new peaks were observed at 1591 and 1434 cm−1, which correspond to the strong antisymmetric vibration and weak symmetric vibration of O−C−O group, respectively.35 This suggests that the proton of the carboxylic groups in the IL dimmer was replaced by Fe(III) and then the complex of IL−Fe(III) was formed. Therefore, it can be concluded that the driving force for the extraction of Fe(III) by the IL is ascribed to the strong

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C

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering M3aq+ + 3[(CH 2)5 COOHmim][Tf 2N]IL

= M[((CH 2)5 COO−mim+)3 ][Tf 2N]3,IL + 3H+aq

(5)

It can be seen from this equation that when aqueous phase pH value increases, the chemical equilibrium shifts to right, resulting in the increase of [M((CH2)5COOmim)3]3+ concentration and thus the higher D value. Compared to the ion exchange occurred in the extraction of metal ions by traditional ILs, the major advantage of the proton exchange mechanism in the ILs extraction process is that the loss of carboxylicfunctionalized IL to aqueous phase can be avoided in the exchange process. Due to the difference in the interaction of metal ions with carboxylic group of the IL, the optimal pH values for the extraction process are different. It can be seen form Figure 2 that Fe(III), Nd(III), and Sm(III) were almost completely extracted by IL2 at pH 2.51, 4.72, and 4.65, respectively. However, the extraction efficiency of Co(II) was only about 8% at pH 4.95. Therefore, it is possible to separate Fe(III) from Nd(III) and Sm(III) from Co(II) in aqueous solution through modulation of the pH value. To study the performance of IL2 to selectively separate Fe(III) from Nd(III) and Sm(III) from Co(II), two aqueous solutions of Fe(III) + Nd(III) and Sm(III) + Co(II) were prepared by adding their nitrate salts in the purified water, in which the concentration of Fe(III) and Nd(III) was 100 and 37 μg/mL, and that of Sm(III) and Co(II) was 100 and 325 μg/mL, respectively. Such mass ratios of Fe(III) to Nd(III) and Sm(III) to Co(II) in aqueous solution were selected according to the corresponding mass ratio found in Nd2Fe14B and Sm2Co17 magnets.5 It was found that the separation factor was 1.1 × 105 for Fe/Nd mixture at pH 2.51 and 2.1 × 104 for Sm/Co mixture at pH 4.65. Such high separation factors indicate that IL2 can be used to selectively separate Nd and Sm from Nd2Fe14B and Sm2Co17 magnets, respectively. Effect of the Concentration of Metal Ions on the Extraction Efficiency. The effect of metal ions concentration on the extraction efficiency was shown in Figure 5. It can be seen that the ionic liquid phase could be loaded with higher amounts of Fe(III) and Sm(III) although the extraction efficiency of metal ions declined slightly with the increase of metal ions concentration in aqueous phase. At the same time, it is interesting to find that the equilibrium pH values of aqueous phase also decreased to a different extent with increasing metal ions concentration in aqueous phase. According to eq 5, the increase of metal ion concentration in aqueous phase would increase the proton concentration in the aqueous phase. This suggests that the metal ions extraction by a carboxylic functionalized ionic liquid is in a cation exchange mechanism where proton of the ILs was replaced by metal ion in the extraction process. The increase of proton concentration in aqueous phase would accordingly suppress more metal ions to be extracted in the ionic liquid phase. Therefore, the abovementioned results indicate that aqueous phase pH value played a decisive role in the extraction of metal ions using carboxylic functionalized ionic liquids. Effect of Alkyl Chain Length of the ILs on the Extraction Efficiency. In order to examine the effect of alkyl chain length of the ILs, the extraction efficiency of Fe(III) and Sm(III) by IL1, IL2, and IL3 were determined and the results were shown in Figure 6 as a function of the equilibrium pH value of aqueous phase. It is clear that both metals could be

Figure 3. FTIR spectra of IL2 before and after extraction of Fe(III).

interaction between metal ion and oxygen atom of carboxylic group in the IL. In addition, it is interesting to note that there exists a linear relationship between logarithm of the distribution ratio D of the metal ions and the equilibration pH values of aqueous phase (shown in Figure 4). A similar linear relationship was reported

Figure 4. Linear plots of log D versus equilibrium pH values of the aqueous phase for the extraction of Fe(III) and Sm(III) with IL2. Fe(III), 100 μg/mL; Sm(III), 100 μg/mL; IL2, 0.5 mL.

for the extraction of metal ions by carboxylic acids in organic solvent or by the ionic liquid betainium bis(trifluoromethylsulfonyl)imide ([HBet][Tf2N]).32,36,37 The values of the slopes obtained from the linear plots were 3.04 for Fe(III) and 3.08 for Sm(III), indicating that three protons were released to the aqueous phase from the IL phase when one metal ion was extracted into the IL phase. Based on the obtained slope value, we can also infer that the coordination number of the IL with the extracted metal ion should be 3. Therefore, the extraction mechanism of the metal ions by the IL is the cation exchange mechanism where the proton of [(CH2)5COOHmim]+ was replaced by metal ion, and [M((CH2)5COOmim)3]3+ was formed in the extraction process. Thus, the extraction process can be described in term of the following equation: D

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 6. Extraction efficiency (E %) of Fe(III) and Sm(III) by IL1, IL2, and IL3 at different equilibrium pH values of the aqueous phase. Fe(III), 100 μg/mL; Sm(III), 100 μg/mL; IL, 0.5 mL.

Figure 5. Extraction efficiency (E %) of Fe(III) and Sm(III) and equilibrium pH values of aqueous phase as a function of metal ion concentrations. IL2, 0.5 mL.

with the increase of alkyl chain length. Therefore, in the range of low pH value, the extraction efficiency of the metal ions decreases with elongation of the alkyl chain length in cation. However, in the range of high pH value, the concentration of [CnCOO−mim+] is much higher than that of the metal ions, and most of the metal ions have been extracted by the ILs. As a result, almost the same extraction efficiency for the metal ions could be obtained by the ILs with different alkyl chain lengths. This indicates that the ability to release proton rather than hydrophobicity of the ILs may have an important impact on the extraction efficiency of metal ions. Thus, the extraction efficiency of metal ions from aqueous solutions can be simply regulated and controlled by changing alkyl chain length of the ILs. Effect of Temperature. Here, IL2 was used as an example to assess the influence of system temperature on the extraction of Fe(III) and Sm(III), and the experiments were carried out in the temperature range from 15.0 to 65.0 °C. In order to keep the other experimental condition unchanged except for the variable of temperature, the constant amount of NaOH solution was introduced into the mixture of metal and IL2, and then the mixture was vigorously stirred for 60 min at a given temperature to attain extraction equilibrium. Figure 7 shows the variety of logarithm of the distribution ratio D for the metals with reciprocal of the temperature. It was shown that the log D values increased with the increase of temperature, indicating that the extraction process was endothermic and the rise of temperature is favor to the extraction of metal ions.

extracted effectively by these ILs, and the maximum extraction efficiency could be as high as 99%. In addition, the extraction efficiency increased with the decrease of the alkyl chain length at lower pH values, while almost no change was observed at higher pH values. Evidently, the difference in the extraction efficiency of the two metals is related with the alkyl chain length of cation of the ILs. In general, hydrophobicity of the ILs increases with increasing alkyl chain length of the cation under the condition of the same anion. If hydrophobicity of the ILs plays a decisive role in the extraction of these metals, the extraction ability of the ILs would vary in the order: IL1 < IL2 < IL3. However, opposite result was obtained. Meanwhile, it is interesting to find that the aqueous solution became acidic when the carboxyl-functionalized ILs with hydrophobic [Tf2N] anion was contacted with pure water. The pH values were determined to be 2.58, 2.89, and 2.99 for such aqueous solutions at 25 °C which was in equilibrium with IL1, 2, and 3, respectively. This indicates that the proton on the carboxylic group of the ILs would dissociate in aqueous phase, and the ability to dissociate proton decreased with the increase of alkyl chain length of the ILs. According to the cation exchange mechanism mentioned above, the metal ions can be better extracted by the ILs with an easily released proton on the carboxylic group of their cations. Based on our previous report, the inductive effect of the alkyl chain in the cation would decrease the dissociation ability of proton of carboxylic group.26 Such an effect becomes smaller E

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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is favorable at given conditions. Based on the principle of hard− soft acid base interaction, the positive values of both ΔHt and ΔSt are the characteristics of the interaction between hard metal ion and hard ligand (O, N donor) to form complexes in solvent extraction.41,42 Therefore, it can be deducted that in the extraction process of the metal ions by IL2, Fe(III), and Sm(III) (as hard acid) can form extractable complexes with [(CH2)5COO−mim+] (as hard base). In addition, because the value of |TΔSt| is bigger than that of |ΔHt|, the extraction process of metals ions by IL2 is a entropy-driven process. According to the report of Komasawa, liquid−liquid solvent extraction process for metal ions is mainly composed of the following two steps: (i) the hydrate metal ion is combined with the anion to produce a neutralized outer-sphere complex and (ii) the ligand displaces the coordinated water molecules of the metal ion to form an inner-sphere complex, which is soluble in organic phase in a stable configuration.43 Based on eq 5, in the IL-based extraction systems, the anion is [Tf2N]− and the ligand should be zwitterionic [(CH2)5COO−mim+]. The formation of inner-sphere complex is generally regarded as a dehydration process of metal ion, which increases the randomness of the system and leads to a positive entropy contribution (ΔSd > 0). However, the formation of metal− anion complex decreases the order of the system and results in a negative entropy value (ΔSc < 0) due to formation of the new binding between the cation and the anion. From such a analysis, it seems clear that the formation of inner-sphere complex ([M(CH2)5COO−mim+]3+) plays a predominant role in the IL-based extraction of metal ions, which is also verified from our FTIR spectrum. Stripping of the Metals and Recycling of the ILs. For a sustainable extraction and separation process, it is important to recover metal ions from the ILs phase and recycle the ILs. Considering the fact that acidity of aqueous phase has a remarkable influence on the extraction efficiency of metal ions, the stripping of metal ions from the IL phase can be affected by the pH value of stripping solution. Therefore, a series of stripping tests with different concentrations of HCl were conducted to separate metal ions from the IL phase. It can be seen from Figure 8 that the stripping percentage of the metal ions greatly increases with increasing acidity of aqueous solution in the low concentration range of HCl. About 96.8% of Fe(III) can be stripped from the IL phase in a single step by using a 0.4 M aqueous HCl solution, and the single step stripping efficiency of Sm(III) is 97.1% by using 0.1 M aqueous HCl. The residual metal ions in the ILs could be then completely stripped in the second stripping step. In addition, aqueous HNO3 solution could be employed to strip metal ions from ionic liquids and the similar results were obtained.

Figure 7. Relationship between log D and 1000/T for the extraction of Fe(III) and Sm(III). Fe(III), 100 μg/mL; Sm(III), 100 μg/mL; IL2, 0.5 mL.

From the viewpoint of thermodynamics, the extraction of a metal ion from aqueous solution to IL phase can be regarded as a transfer process of the metal ion from the aqueous to the IL phases. Thus, we investigated the Gibbs energies of the transfer of Fe(NO3)3 and Sm(NO3)3 from aqueous phase to the IL phase. The Gibb’s energy of the transfer process at a given temperature, ΔGt, was calculated by the following equation:38,39 ΔGt = −RT ln Kx = −RT[ln D + ln(Vm,IL /Vm,aq)]

(6)

where Kx is the extraction equilibrium constant expressed in mole fraction scale, and Vm,IL and Vm,aq stand for the molar volume of the IL and water, respectively. The molar volume data of water at different temperatures could be obtained from the density and molar mass data of water.40 Because no density data have been reported for IL2 at different temperatures in literature, we determined these data by using an Anton Paar DMA 60/602 digital densimeter. According to these density data and the molar mass of IL2, the molar volumes of IL2 were calculated at different temperatures. During the extraction process, the enthalpy change can be calculated by using van’t Hoff equation: ΔHt = −2.303Rδ log D /δ(1/T )

(7)

Then the change in entropy (ΔSt) can be obtained by eq 8: ΔGt = ΔHt − T ΔSt

(8)

The experimental D values and the transfer thermodynamic data were listed in Table 1. It was shown that all the ΔGt values are negative, whereas both values of ΔHt and ΔSt are positive. The negative values of ΔGt suggest that the extraction process

Table 1. Transfer Thermodynamic Data for Fe(NO3)3 and Sm(NO3)3 from Aqueous to the Ionic Liquid Phases Fe(NO3)3 T (°C)

D

ΔG (kJ·mol−1)

15.0 20.0 25.0 35.0 45.0 55.0 65.0

36.5 58.1 91.5 146 198

−9.3 −10.0 −10.6 −11.5 −12.3

360

−13.9

ΔH (kJ·mol−1)

37.2

Sm(NO3)3 TΔS (kJ·mol−1)

D

ΔG (kJ·mol−1)

46.6 47.2 47.9 48.8 49.5

194 197 208 247 270 277 302

−11.1 −11.3 −11.5 −12.2 −12.7 −13.1 −13.6

51.1 F

ΔH (kJ·mol−1)

TΔS (kJ·mol−1)

7.7

18.8 19.0 19.2 19.8 20.4 20.8 21.3

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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used to selectively separate Fe(III) from Nd(III) and Sm(III) from Co(II) by tuning aqueous phase pH value. Additionally, the extraction efficiency of metal ions was found to increase with the decrease of alkyl chain length of the ILs at lower pH values, while it became constant at higher pH values. This suggests that the ability to release proton rather than hydrophobicity of the ILs may have an important impact on the extraction efficiency of the metal ions. FTIR spectra investigation suggested a cation exchange mechanism, and the extraction process was understood from transfer thermodynamic parameters of the rare-earth nitrates from aqueous solution to the ILs phase. The major advantage of the present extraction procedure is that the metals can be selectively separated, with separation factor of 104−105, only by modulation of the acidity of aqueous phase without adding any diluents or extraction agents. The stripping of metal ions from the ILs into aqueous phase could be easily achieved by using diluted aqueous HCl, HNO3 or oxalic acid solution. The IL was regenerated at the same time and could be reused in the next extraction process. It is expected that the hydrophobic carboxylic-functionalized ILs reported in the present work would be promising for the use in the recovery of rare earth metals from waste permanent magnets without using any organic solvents and other extractants.

Figure 8. Stripping of Fe(III) and Sm(III) from the ionic liquid phase with different concentrations of aqueous HCl. IL2, 0.5 mL.

It is evident from eq 5 that the increase of H+ concentration in the stripping solution would shift the equilibrium to the left, and the metal ion in the IL phase will be exchanged with the H+ ions in the aqueous phase, and then released into water. Therefore, we can rationalize the stripping results by the fact that when acid was added to the loaded IL phase, metal complex in the IL may be decomposed to form free metal ion and ligand, leading to the regeneration of the cation [(CH2)5COOHmim]+. Importantly, compared with the stripping procedure reported in the literature,5 no chelating agents were necessary during the stripping progress described here except for the addition of a little amount of HCl, which may be the advantage of the present IL based extraction. In addition, the loss of H[Tf2N] formed in the stripping process can be avoid because the regenerated cation [(CH2)5COOHmim]+ can interact with [Tf2N]− anion to form the IL again. Therefore, the stripping of metal ion and the regeneration of IL2 can be achieved at the same time. After stripping, the ionic liquid could be reused for the next extraction process of metal ions. In the case of Fe(III) extraction, we investigated the recycling and reuse of the IL2. It was found that more than 85% of extraction efficiency was still remained after four cycles. In addition, about 99.7% Fe(III) could be stripped from the IL phase in a single step by using 0.01 mol/L oxalic acid aqueous solution. This is due to the fact that Fe(III) could react with oxalic acid to form Fe(III) oxalate complex which is soluble in the water phase.37 It was also found that white precipitate was formed when aqueous oxalic acid solution was mixed with the ionic liquid loaded Sm(III), indicating that the stripping of Sm(III) could also be achieved by the formation of polymeric precipitates between Sm(III) and oxalic acid. Actually, by using 0.002 mol/L oxalic acid aqueous solution, about 99.8% Sm(III) could be stripped from the IL phase in a single step.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00742. 1 H NMR and 13C NMR data, thermal properties of the ILs, mutual solubility of the ILs and water at 25 °C, and stripping percentage (S%) of Fe(III) and Sm(III) from the ionic liquid phase with different concentrations of HCl at 25 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 373 3325805. Fax: +86 373 3329030. E-mail address: [email protected] (J.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21273062, 21133009).

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REFERENCES

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CONCLUSION In this work, 1-alkylcarboxylic acid-3-methylimidazolim bis(trifluoromethylsulfonyl)imide ILs were synthesized, characterized and used to extract Fe(III), Nd(III), Sm(III), and Co(II) from aqueous solutions. It was found that in the extraction process, the ILs could act as both the organic phase and the extractant. The extraction efficiency of all the investigated metal ions increased with the increase of aqueous phase pH value in a different pH range, and the maximum extraction efficiency was high as 99%. Thus, these ILs can be G

DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acssuschemeng.5b00742 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX