The synergistic extraction of heavy rare earth elements using EHEHP-type and BTMPP-type functional ionic liquids

Abstract

As for the supply risk and application in clean energy, heavy rare earth elements (HREEs) are more important than light rare earth elements (LREEs). In this paper, the synergistic extraction based on 2-ethyl(hexyl) phosphate mono-2-ethylhexyl ester and the bis(2,4,4-trimethylpentyl) phosphate acid base coupling of bifunctional ionic liquids (ABC-BILs) for HREEs separation is reported. Extractabilities of the novel systems were better than those using mixed 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) and bis(2,4,4-trimethylpentyl) phosphinic acid (HBTMPP). Also, the stripping property of the ABC-BIL based synergistic extraction system for HREE was better than that of mixed HEH[EHP] and HBTMPP. Advantages of the novel synergistic system based on ABC-BILs have revealed potential applications for the HREEs separation industry.

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Introduction

Rare earth elements (REEs) are critical for many advanced materials because of their unique electronic, optical, and magnetic properties. REEs can be divided into the “cerium group” (or light REEs: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, and gadolinium), and the “yttrium group” (or heavy REEs: yttrium (Y), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu)). The development of novel REEs separating and reclaiming techniques have aroused worldwide interest.^1–3 2-Ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP], P507) has been one of the most commonly used extractants in the Chinese REEs separation industry since the 1980s.^4,5 However, the heavy rare earth elements (HREEs) extracted by HEH[EHP] are difficult to be stripped because of the stronger complexation between HEH[EHP] and HREEs. Because of its higher pK_a value, the extraction and stripping of REEs using bis(2,4,4-trimethylpentyl)phosphinic acid (HBTMPP, Cyanex 272) can take place at lower aqueous acidities than those of HEH[EHP].⁶ To overcome the problems of HEH[EHP] for HREE stripping, the combined extraction system composed of HEH[EHP] and HBTMPP is applied in the HREEs separation industry. The synergistic effect in solvent extraction by the combination of two extractants for enhancing extraction efficiency has been extensively studied.⁷ Mixtures of HEH[EHP] and HBTMPP showed an evident synergistic effect when they were used to extract Yb(III) from chloride solution, and the maximal synergistic enhancement of 1.86 could be obtained when the ratio of HEH[EHP] to HBTMPP was 1 : 1.⁸ The extraction of Y(III) from a chloride medium using a mixture of HEH[EHP] and HBTMPP in kerosene was also studied. The extraction mechanism was determined using the method of slope analysis. From the maximum synergistic enhancement factor, R (calculated based on R = D_mix/(D_HEH[EHP] + D_HBTMPP), a D value of 2.94 obtained from [Y³⁺]_(o)/[Y³⁺]_(a)) at a HEH[EHP]/HBTMPP molar ratio of 1 : 1, the extraction mechanism was found to be cation exchange.⁹ In additional, Liao et al.¹⁰ reported that HEH[EHP]–HBTMPP impregnated resin could increase the distribution ratios and improve the saturated extraction capacities of HREEs.

Ionic liquids (ILs) are salts with a low melting point (below 100 °C). The uses of ILs in synthesis, catalysis, separation, and electrochemistry have been widely investigated.¹¹ IL-based extraction is a separation strategy that uses ILs instead of volatile organic compounds as diluents and/or extractants. The unique and efficient properties of ILs, such as low volatility, low combustibility, wide liquid range, and thermal stability, make them particularly suitable for use in solvent extraction.¹² Some interesting results on IL-based extraction for HREE separation have been reported. The solvent impregnated resin containing Cyanex 923 (alkyl phosphine oxides) and imidazolium IL was studied to separate Y from Ho, Er, Yb by adding a water soluble complexing agent.¹³ A carboxyl-functionalized IL, betainium bis(trifluoromethylsulfonyl)imide was investigated for the selective extraction of yttrium oxide from lamp phosphor waste.¹⁴ The solvent extraction systems based on a combination of imidazolium ILs and 1-methylimidazole or 2-methylimidazole were applied to extract Y.¹⁵ The diluent influence from imidazolium, ammonium and phosphonium ILs on the extraction of Ho, Y, and Yb using Cyanex 923 were studied.¹⁶ Some ammonium ILs were used as extractants and investigated for separation of Ho, Er, Tm, Yb, Lu using a ‘trivalent actinide lanthanide separation with phosphorus reagent extraction from aqueous complexes’ (TALSPEAK) like process.¹⁷ The interesting and effective HREEs separation processes mentioned previously reveal that IL-based extraction is a promising technology for industrial applications. To break the hydrogen bonds in the dimers of acidic extractants, the extractants always require saponification by NH₃·H₂O, sodium hydroxide, or calcium hydroxide in the REE separation industry. However, the saponification processes release the resulting ammonia-nitrogen, Na⁺ or Ca²⁺ wastewater to the environment. It has been reported that the Chinese REE industry produces over 20 million tons of wastewater annually. Developing a sustainable extraction technology based on acid base coupling of bifunctional ionic liquids (ABC-BIL) offers a promising strategy to avoid the saponification wastewater pollution.¹⁸ In recent papers, the adjustable synergistic effects between ABC-BILs for REE separation were reported.¹⁹ Furthermore, the synergistic extraction between ABC-BIL and a molecular extractant for lanthanide separation was also studied.²⁰ To develop an efficient separation technology for HREEs, the novel synergistic extraction systems using 2-ethyl(hexyl) phosphate mono-2-ethylhexyl ester (EHEHP)-type ABC-BIL and bis(2,4,4-trimethylpentyl) phosphate (BTMPP)-type ABC-BIL were first studied in this research.

Experimental

Reagents

Chemicals and reagents. Tetrabutylammonium chloride ([N₄₄₄₄]Cl) and trioctylmethylammonium bromide ([N₁₈₈₈]Br) were purchased from Yixing Kailida Chemical Co., Ltd (China). An anion exchange resin [Dowex Monosphere 550A (OH)] was obtained from the Dow Chemical Company. HEH[EHP] was purified by washing with 2% sodium carbonate, 0.2 mol L⁻¹ sulfuric acid and distilled water. Bis(2,4,4-trimethylpentyl) phosphinic acid (HBTMPP, Cyanex 272) was supplied by Cytec Industries Inc. ¹H-nuclear magnetic resonance and ¹³C-nuclear magnetic resonance (NMR) spectra of the ABC-BILs were obtained in deuterated chloroform using an Avance III-500 spectrometer (Bruker). The extracting phases were prepared by dissolving the extractants in heptane. All the rare earth oxides were purchased from the Fujian Changting Golden Dragon Rare-Earth Co., Ltd (China). Stock solutions of REEs were prepared by dissolving their oxides (>99.99%) in concentrated hydrochloric acid (HCl) and then diluting with distilled water. The detailed compositions of the REE solutions are given in the captions of the figures in this paper. An iCAP 6500 series inductively coupled plasma-atomic emission spectrometer (ICP-AES; Thermo Scientific) was used to determine the concentrations of REEs in the aqueous phase. Infrared (IR) spectra were measured using a iS50 Fourier-transform IR spectrometer (Nicolet). Fig. 1 shows the structures and abbreviations of different extractants used in this study.

Fig. 1 Structures of the extractants investigated in this study.

Synthesis of ABC-BILs

All the ABC-BILs were prepared using the combination of ion-exchange and a neutralizing reaction.^{17 1}H-NMR and ¹³C-NMR of the prepared ABC-BILs are given in the ESI.†

[N₄₄₄₄][EHEHP]. A solution of [N₄₄₄₄]OH in 60 mL of methanol was prepared from 2.8 g of [N₄₄₄₄]Cl (0.01 mol) using a Dowex Monosphere 550A (OH) anion exchange resin. HEH[EHP] (3.065 g, 0.01 mol) was added to the [N₄₄₄₄]OH solution. The mixture was then stirred at room temperature for 6 h until the solution became neutral. The methanol and water were distilled off using a RV10 rotary evaporator (IKA), and the product was dried at 70 °C under vacuum for 12 h to yield [N₄₄₄₄][EHEHP] as a viscous liquid (5 g, yield: 91.4%).

[N₁₈₈₈][EHEHP]. A solution of [N₁₈₈₈]OH in 40 mL of methanol was prepared from 2.69 g of [N₁₈₈₈]Br (0.006 mol) using a Dowex Monosphere 550A (OH) anion exchange resin. HEH[EHP] (1.83 g, 0.006 mol) was added to the [N₁₈₈₈]OH solution. The mixture was then stirred at room temperature for 6 h until the solution became neutral. The methanol and water were distilled off using a RV10 rotary evaporator, and the product was dried at 70 °C under vacuum for 12 h to yield [N₁₈₈₈][EHEHP] as a viscous liquid (3.71 g, yield: 95.8%).

[N₄₄₄₄][BTMPP]. A solution of [N₄₄₄₄]OH in 60 mL of methanol was prepared from 2.78 g of [N₄₄₄₄]Cl (0.01 mol) using a Dowex Monosphere 550A (OH) anion exchange resin. HBTMPP (2.904 g, 0.01 mol) was added to the [N₄₄₄₄]OH solution. The mixture was then stirred at room temperature for 6 h until the solution became neutral. The methanol and water were distilled off using a RV10 rotary evaporator, and the product was dried at 70 °C under vacuum for 12 h to yield [N₄₄₄₄][BTMPP] as a viscous liquid (4.9 g, yield: 92.5%).

[N₁₈₈₈][BTMPP]. A solution of [N₁₈₈₈]OH in 40 mL of methanol was prepared from 2.69 g of [N₁₈₈₈]Br (0.006 mol) using a Dowex Monosphere 550A (OH) anion exchange resin. HBTMPP (1.74 g, 0.006 mol) was added to the [N₁₈₈₈]OH solution. The mixture was then stirred at room temperature for 6 h until the solution became neutral. The methanol and water were distilled off using a RV10 rotary evaporator, and the product was dried at 70 °C under vacuum for 12 h to yield [N₁₈₈₈][BTMPP] as a viscous liquid (3.7 g, yield: 93.7%).

Extraction and stripping experiments

The extraction experiments were performed by contacting 5 mL of heptane containing the extractant with 5 mL of aqueous REE solution for 30 min in a vibrating mixer at 30 °C. The aqueous phase was separated, and concentration of the REE was determined using ICP. The stripping experiment was conducted by contacting 4 mL of heptane containing the extractant with 4 mL of acid with different concentration for 60 min in a vibrating mixer at 30 °C. After centrifugation at 5000 rpm for 5 min, the concentration of REE in the stripping acid was determined. The concentration of REE in the IL phase was calculated by using the mass balance. The distribution coefficient (D_M), separation factor (β), synergy coefficient (R) and stripping ratio (S) are defined as follows:where [M]_t and [M]_a represent the initial and final concentration of REE in aqueous phase, respectively. D_a is the distribution ratio from one extractant for REE, D_b is the distribution ratio from the other extractant for REE, D_mix is the distribution ratio from their mixture for REE. [M]_aq,a is the equilibrium concentration of REE in stripping acid and [M]_org,t is the initial concentration of REE in the extracting phase. D₁ and D₂ are the distribution ratio of REE 1 and REE 2, respectively. All the concentration values of the REEs were measured in duplicate and the uncertainty was within 5%.

Results and discussion

The synergistic extraction of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] for Lu(III)

The synergistic extractions of Yb(III)⁸ and Y(III)⁹ using HBTMPP and HEH[EHP] were studied. In this research, the synergistic extraction of Lu(III) using combined ABC-BILs, i.e., [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] was investigated first. As shown in Fig. 2, the extraction of 0.003 mol L⁻¹ Lu(III) using [N₄₄₄₄][EHEHP], [N₄₄₄₄][BTMPP], and their mixture were compared when the concentration of sodium chloride (NaCl) in the REE solution was 0.5 mol L⁻¹ and the pH value of the solution was 2.09. Distribution ratios of Lu(III) extracted using the combined ABC-BILs were bigger than those extracted using individual ABC-BIL with the same conditions. The distribution ratios of Lu(III) in the combined ABC-BILs system changed with the different mole fraction of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] in the total fixed concentration of 0.015 mol L⁻¹. It is worth mentioning that the distribution ratios of Lu(III) decreased as the mole fraction of [N₄₄₄₄][EHEHP] was decreased and the mole fraction of [N₄₄₄₄][BTMPP] was increased. This effect can be attributed to the fact that the extractability of [N₄₄₄₄][EHEHP] for Lu(III) is stronger than that of [N₄₄₄₄][BTMPP] for Lu(III). The different extractabilities of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] for Lu(III) are given in Table 1.

Fig. 2 Distribution ratios of Lu(III) extracted by [N₄₄₄₄][EHEHP], [N₄₄₄₄][BTMPP], and their mixture. Lu(III) = 0.003 mol L⁻¹, pH = 2.09, NaCl = 0.5 mol L⁻¹.

Table 1 Distribution ratios and synergistic enhancement coefficients of Lu(III) extracted by [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP]

C[N4444][EHEHP] (mol L^−1)	C[N4444][BTMPP] (mol L^−1)	D1	D2	D12	R
0.0135	0.0015	2.56	0.01	5.00	1.94
0.012	0.003	1.65	0.02	4.70	2.82
0.0105	0.0045	1.19	0.06	4.28	3.43
0.009	0.006	0.84	0.09	3.75	4.05
0.0075	0.0075	0.52	0.16	3.38	4.99
0.006	0.009	0.34	0.31	3.25	4.97
0.0045	0.0105	0.21	0.48	3.19	4.63
0.003	0.012	0.14	0.81	2.93	3.07
0.0015	0.0135	0.06	0.97	2.02	1.97

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The synergistic enhancement coefficient, R, was calculated according to the method of Xu et al.²¹ Table 1 shows the variation of R with the different ratios of [N₄₄₄₄][EHEHP] : [N₄₄₄₄][BTMPP]. The biggest synergistic coefficient of 4.99 was obtained with the mole fraction of X_{[N4444][EHEHP]} : X_{[N4444][BTMPP]} = 0.0075 mol L⁻¹ : 0.0075 mol L⁻¹. The ratio reveals that the synergistic effect between the ABC-BIL-based system is the strongest when the concentrations of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] are the same. The second largest synergistic coefficient of 4.97 was obtained with the mole fraction X_{[N4444][EHEHP]} : X_{[N4444][BTMPP]} = 0.006 mol L⁻¹ : 0.009 mol L⁻¹. The third largest synergistic coefficient of 4.63 was obtained with the mole fraction X_{[N4444][EHEHP]} : X_{[N4444][BTMPP]} = 0.0045 mol L⁻¹ : 0.0105 mol L⁻¹. Although the extractabilities of [N₄₄₄₄][BTMPP] for Lu(III) were lower than those of [N₄₄₄₄][EHEHP], the mole fractions mentioned previously indicate that [N₄₄₄₄][BTMPP] was more important than [N₄₄₄₄][EHEHP] for the synergistic effects between the ABC-BILs.

Extraction mechanism

As mentioned previously, it was revealed that [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] had a synergistic effect on the extraction of Lu(III). Because the primary extraction mechanism of ABC-BIL for REE has been indicated to be ion association,²² the extraction equilibrium between Lu(III), the chloride ion, and the ABC-BILs in this study can be expressed by eqn (5).where M represents Lu(III), the subscript (org) denotes the organic phase, the subscript (aq) denotes the aqueous phase.

D and the extraction equilibrium constant (K_ex) can be obtained using eqn (6) and (7), respectively.

Modification of eqn (7) leads to eqn (8).

According to eqn (9), the logarithm of D can be deduced as follows:

log D = log K_ex + x log [N₄₄₄₄][EHEHP]_org + y log [N₄₄₄₄][BTMPP]_org + z log [C1⁻]_aq (9)

The complex stoichiometries were determined using a slope method according to eqn (9). As can be seen in Fig. 3, the higher [N₄₄₄₄][EHEHP] concentration contributed to the extraction of 0.003 mol L⁻¹ Lu(III) when the concentration of NaCl in REE solution was 0.5 mol L⁻¹ and the pH value of the solution was 2.09. At the fixed [N₄₄₄₄][BTMPP] concentration of 0.0045 mol L⁻¹, the linear relationship between log D and log C_{[N4444][EHEHP]} was obtained with a slope of 1.11 when the [N₄₄₄₄][BTMPP] concentration was changed from 0.003 mol L⁻¹ to 0.006 mol L⁻¹. The slope reveals that the stoichiometry of [N₄₄₄₄][EHEHP] with Lu(III) is 1 : 1.

Fig. 3 Effect of [N₄₄₄₄][EHEHP] concentration on the extraction of Lu(III). [N₄₄₄₄][BTMPP] = 0.0045 mol L⁻¹, Lu(III) = 0.003 mol L⁻¹, pH = 2.09, NaCl = 0.5 mol L⁻¹.

As indicated in Fig. 4, the [N₄₄₄₄][BTMPP] concentration is very important to the Lu(III) extraction. In this study, the concentration of [N₄₄₄₄][EHEHP] in the extraction phase was 0.0045 mol L⁻¹, the concentration of Lu(III) in the aqueous phase was 0.003 mol L⁻¹, the concentration of NaCl in the aqueous phase was 0.5 mol L⁻¹ and the pH value of the solution was 2.09. When the [N₄₄₄₄][BTMPP] concentration was changed from 0.003 mol L⁻¹ to 0.0075 mol L⁻¹, the linear relationship between log D and log C_{[N4444][BTMPP]} was obtained with a slope of 0.96. The relationship indicates that the stoichiometry of [N₄₄₄₄][BTMPP] with Lu(III) is also 1 : 1. According to the electroneutrality principle, the cation is extracted with a neutral extractant and the anion is co-extracted because of the electroneutrality. The stoichiometry of chloride can thus be concluded to be 3.

Fig. 4 Effect of [N₄₄₄₄][BTMPP] concentration on the extraction of Lu(III). [N₄₄₄₄][EHEHP] = 0.0045 mol L⁻¹, Lu(III) = 0.003 mol L⁻¹, pH = 2.09, NaCl = 0.5 mol L⁻¹.

The synergistic extraction equation of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] for Lu(III) can be represented by the following equation:

Lu_aq³⁺ + [N₄₄₄₄][EHEHP]_org + [N₄₄₄₄][BTMPP]_org + 3C1_aq⁻ ⇌ [N₄₄₄₄]₂Lu[EHEHP][BTMPP](C1)₃ (10)

To further investigate the extraction mechanism, IR spectra were used to analyze the extraction phase before and after Lu(III) extraction. As shown in Fig. 5, the bands at 1187 cm⁻¹ (a) and 1143 cm⁻¹ (b) in [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] can be assigned to the P=O stretching vibration. The bands at 1143 cm⁻¹ of the P=O stretching vibrations in the combined ABC-BIL system were replaced by the band at 1163 cm⁻¹ (d) after extraction. The P=O stretching vibration of 1163 cm⁻¹ (d) after Lu(III) extraction is different to the individual [N₄₄₄₄][EHEHP] stretching vibration of 1187 cm⁻¹ (a), the individual [N₄₄₄₄][BTMPP] stretching vibration of 1143 cm⁻¹ (b) and the mixture of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] stretching vibration of 1143 cm⁻¹ (c). Because the spectra of individual [N₄₄₄₄][BTMPP] (b) and its mixture with [N₄₄₄₄][EHEHP] (c) have the same stretching vibration of 1143 cm⁻¹, they cannot be differentiated using the IR spectra. However, the different shifts in the spectra (a–d) reveal that there are pronounced interactions between the P=O bands from [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] with Lu(III), which confirm the participation of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] in the formation of the Lu(III) extraction complex. As can be seen in the figures from Fig. 2 to Fig. 4, both [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] reveal effective extractabilities for Lu(III). The extraction experiments further confirmed the IR results, i.e., the participation of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] in the extraction of Lu(III). In addition, the hydrogen bonds in the dimers of the acidic extractant can be fully broken when the extractant is prepared as ABC-BIL.²² The elimination of the dimers contributes to bring the extractability of the acidic extractant into full play, which may be an important reason for the stronger synergistic effect between the ABC-BILs.

Fig. 5 IR transmittance spectra of the different extraction systems. (a) [N₄₄₄₄][EHEHP], (b) [N₄₄₄₄][BTMPP], (c) combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], (d) combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] loaded with Lu(III).

The cations of the ABC-BILs had an important effect on the synergistic effects between different ABC-BILs.²⁰ In this study, distribution ratios of 0.003 mol L⁻¹ of Lu(III) extracted by the three extraction systems were compared: (1) mixture of HEH[EHP] and HBTMPP, (2) mixture of [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], (3) mixture of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP]. The concentration of the individual extractant in the combined system was 0.0036 mol L⁻¹, the concentration of NaCl in the aqueous phase was 0.5 mol L⁻¹ and the pH value of the solution was 2.09. As can be seen in Fig. 6, the extractabilities of the combined ABC-BILs were better than those of the mixed HEH[EHP] and HBTMPP at a pH of 2.09 and 4.6.

Fig. 6 A comparison of distribution coefficients from different extraction systems on the extraction of Lu(III). (1) mixture of HEH[EHP] and HBTMPP, (2) mixture of [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], (3) mixture of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], Lu(III) = 0.003 mol L⁻¹, NaCl = 0.5 mol L⁻¹. (a) pH = 2.09, HEH[EHP] = HBTMPP = [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = 0.0075 mol L⁻¹, (b) pH = 4.6, HEH[EHP] = HBTMPP = [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = 0.0036 mol L⁻¹.

Fig. 7 shows the synergistic coefficients of the combined extraction systems for Lu(III). Using the same conditions as the experiment shown in Fig. 6, the synergistic coefficients of the combined ABC-BILs are bigger than those of mixed HEH[EHP] and HBTMPP. As for the [N₄₄₄₄]-based system and the [N₁₈₈₈]-based system, the synergistic coefficients of Lu(III) extracted by the [N₄₄₄₄]-based system are larger than those extracted by the [N₁₈₈₈]-based system. The results mentioned previously indicate that the synergistic effect from the combined EHEHP-type ABC-BIL and the BTMPP-type ABC-BIL with a smaller cation for Lu(III) is stronger than those with a larger cation. Such tendency is the same as that reported in some other synergistic extraction system between ABC-BILs.²⁰

Fig. 7 The comparison of synergistic coefficients from different extraction systems on the extraction of Lu(III). (1) mixture of HEH[EHP] and HBTMPP, (2) mixture of [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], (3) mixture of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], Lu(III) = 0.003 mol L⁻¹, NaCl = 0.5 mol L⁻¹. (a) pH = 2.09, HEH[EHP] = HBTMPP = [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = 0.0075 mol L⁻¹, (b) pH = 4.6, HEH[EHP] = HBTMPP = [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = 0.0036 mol L⁻¹.

Stripping test

To investigate the stripping properties of the extraction systems mentioned previously for Lu(III), the Lu(III) loaded extracting phases were prepared using HEH[EHP], combined HEH[EHP] and HBTMPP, combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP]. The stripping of Lu(III) using HCl was studied at the different acidities from 0.01 mol L⁻¹ to 0.6 mol L⁻¹. As can be seen in Fig. 8, the stripping ratios of Lu(III) in the systems gradually increased as the acidities were increased. The HEH[EHP] system could be fully stripped by 0.4 mol L⁻¹ HCl. Unlike the HEH[EHP] system, the loaded Lu(III) in the combined HEH[EHP] and HBTMPP system could be stripped using 0.14 mol L⁻¹ HCl. The better stripping properties of the combined HEH[EHP] and HBTMPP for REE than those of HEH[EHP] can be attributed to the lower activation energy of the combined extractants.²³ In this study, the extracted Lu(III) using combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] could be fully stripped when the concentration of HCl was 0.05 mol L⁻¹.

Fig. 8 The stripping of Lu(III) from different extraction systems using HCl from the Lu(III) loaded organic phase. Lu(III) = 3 × 10⁻³ mol L⁻¹, NaCl = 0.5 mol L⁻¹, pH = 2.19.

Extraction and separation of mixed HREEs

The cation radii for Ho, Er, Tm and Y are 0.90 Å, 0.89 Å, 0.88 Å and 0.89 Å, respectively.²⁴ Because the cation radius of Y is quite similar to those of Ho, Er and Tm, Y together with Tb, Dy, Ho, Er, Tm, Yb, Lu are called the HREEs. To investigate the synergistic effects from the ABC-BILs for the mixed HREEs, the distribution coefficients of Ho(III), Y(III), Er(III), Tm(III), Yb(III), and Lu(III) were extracted using combined HEH[EHP] and HBTMPP, combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], combined [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP] and the results were compared. All the concentrations of individual extractants in the extraction phase was 0.0075 mol L⁻¹. The concentrations of individual REE ions and NaCl in the aqueous phase were 0.0005 mol L⁻¹ and 0.5 mol L⁻¹, respectively. The pH values of aqueous phases were 2.09 and 4.6. As shown in Fig. 9, the distribution coefficients of all the HREEs extracted by the combined ABC-BILs were bigger than those extracted by the combined HEH[EHP] and HBTMPP. Also, the extractabilities of the combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] for the REEs were bigger than those of the combined [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP]. The effects of cations in ABC-BILs were the same as for previous results from individual ABC-BIL-based extraction systems, i.e., the ABC-BILs with shorter carbon chains contribute to the extractabilities of REEs because of the smaller steric hindrance of the extractants. The extraction sequence of REEs from the combined ABC-BILs was Ho(III) < Y(III) < Er(III) < Tm(III) < Yb(III) < Lu(III). The sequence is mainly attributed to the coordination abilities of functional groups in the ABC-BILs with the HREEs. As the ionic radius of the REE decreases, the coordination strength of the ABC-BIL with REE increases, and thus, the extractability of the ABC-BIL for REE increases.

Fig. 9 Distribution coefficients of HREEs from (1) combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], (2) combined [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], (3) combined HEH[EHP] and HBTMPP for HREEs at the pH value of 2.09 (a) and 4.6 (b). [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = HEH[EHP] = HBTMPP = 0.0075mol L⁻¹, [Ho(III)] = [Y(III)] = [Er(III)] = [Tm(III)] = [Yb(III)] = [Lu(III)] = 0.0005 mol L⁻¹, [NaCl] = 0.5 mol L⁻¹, pH = 4.6.

Er(III), Tm(III), Yb(III), Lu(III) are the last four elements in the lanthanide group, and the separation of Er(III), Tm(III), Yb(III), Lu(III) is difficult because of their similar physical and chemical properties. As can be seen in Fig. 10, the separation of Er(III), Tm(III), Yb(III), Lu(III) by combined HEH[EHP] and HBTMPP, combined [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] were studied at the different pH values of 2.09 and 4.6. At the pH of 2.09, separation factors of the three extraction systems were similar. At the pH value of 4.6, separation factors of the combined HEH[EHP] and HBTMPP for HREEs were better than those of the combined ABC-BILs system.

Fig. 10 Separation factors of HREEs from (1) combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP], (2) combined [N₁₈₈₈][EHEHP] and [N₁₈₈₈][BTMPP], (3) combined HEH[EHP] and HBTMPP for HREEs at the pH value of 2.09 (a) and 4.6 (b). [N₄₄₄₄][EHEHP] = [N₄₄₄₄][BTMPP] = [N₁₈₈₈][EHEHP] = [N₁₈₈₈][BTMPP] = HEH[EHP] = HBTMPP = 0.0075 mol L⁻¹, Ho(III) = Y(III) = Er(III) = Tm(III) = Yb(III) = Lu(III) = 0.0005 mol L⁻¹, NaCl = 0.5 mol L⁻¹, pH = 4.6.

Conclusion

In this study, the synergistic extraction of EHEHP-type ABC-BIL and BTMPP-type ABC-BIL for separation of REEs is reported for the first time. The largest synergistic coefficient of 4.99 for Lu(III) was obtained when the concentrations of [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] were the same. The primary mechanism of the synergistic extraction based on ABC-BILs for REEs was found to be ion association. The extractabilities of the combined ABC-BILs were better than those of the combined HEH[EHP] and HBTMPP, furthermore, the synergistic coefficients of the combined ABC-BILs were larger than those of the combined HEH[EHP] and HBTMPP. The synergistic effect from the EHEHP-type ABC-BIL and the BTMPP-type ABC-BIL with a smaller cation for Lu(III) was stronger than that with the larger cation. Full stripping acidity of combined [N₄₄₄₄][EHEHP] and [N₄₄₄₄][BTMPP] for Lu(III) was lower than those of the combined HEH[EHP] and HBTMPP. The lower stripping acidity contributes to decreasing the cost and pollution of the method. In summary, the ABC-BIL-based synergistic extraction mentioned previously reveals efficient and environmentally friendly advantages for the HREE separation.

Acknowledgements

This work was supported by ‘Hundred Talents Program’ from the Chinese Academy of Sciences and ‘Xiamen Double Hundred Plan’. The authors wish to thank Cytec Industries Inc., for supplying Cyanex 272.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05884h

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