Solvent impregnated resin prepared using task-specific ionic liquids for rare earth separation

JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009, p. 932

Solvent impregnated resin prepared using task-specific ionic liquids for rare earth separation SUN Xiaoqi (ᄭᰧ⧺)1, JI Yang (㑾ᴼ)1,2, CHEN Ji (䰜㒻)1, MA Jiutong (偀⥪ᔸ)2 (1. State Key Laboratory of Rare Earth Resource Utilization, Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China; 2. College of Chemistry, Jilin University, Changchun 130061, China) Received 20 April 2009; revised 19 August 2009

Abstract: A novel bifunctional task-specific ionic liquid (TSIL), i.e. [trialkylmethylammonium][sec-nonylphenoxy acetate] ([A336] [CA-100]) was impregnated on intermediate polarized XAD-7 resin, and the prepared solvent impreganated resin (SIR) was studied for rare earth (RE) separation. Adsorption ability of the SIR was indicated to be obviously higher than that prepared by [A336][NO3] because of the functional anion of [A336][CA-100]. Adsorption kinetics, adsorption isotherm, separation and desorption of the SIR were also studied. Keywords: task-specific ionic liquid; solid-liquid extraction; rare earths

Ionic liquid (IL) is a kind of booming “designer solvents” for a multitude of applications[1]. Their physicochemical properties can be easily tuned by changing combinations of cations and anions. The non-inflammability and non-volatility of ILs provide environment-friendly advantages for using them as replacement to volatile organic compounds (VOCs) in solvent extraction. Moreover, the higher extraction efficiencies of some IL-based extraction system than those of VOC-based system have brought them more attention since 2000s[2]. Dai et al. described the large distribution ratio for strontium ion offered by ILs than conventional organic solvent[3]. Nakashima et al. found that octyl(phenyl)N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) had higher extraction efficiencies for Ce3+, Eu3+ and Y3+ in IL than in dodecane[4]. Shimojo revealed that the extraction performance of calix[4]-arene for silver ion was greatly enhanced by dissolution in ILs compared to that in chloroform[5]. Stepinski et al.[6] indicated the markedly synergistic interactions between dicyclohexano-18-crown-6 (DCH18C6) and tri-n-butyl phosphate (TBP) in [Cnmim][Tf2N] for strontium extraction. Due to the development of IL, TSIL has been regarded as a kind of functional material. The TSILs contain functional groups for task-specific purpose in their cation and/or anion, which can be “tailor-made” for some separation processes[7].

Recently, ammonium-type ILs have attracted considerable attention[8]. Their costs are much lower than those of imidazolium-type IL[9]. Moreover, the toxicity of ammonium-type IL is lower than that of imidazolium-type IL[10]. In addition, some quaternary ammonium salts have good applied background in solvent extraction[11]. Quaternary ammonium salts possess nitrogen donors to complex with RE, accordingly, they can be regarded as a kind of TSILs. Undoubtedly, such advantages of ammonium-type ILs are valuable to IL-based extraction for RE separation. In this work, we further enhanced the extraction ability of ammonium-type IL by combining a carboxylic acid group as its anion, thus, a novel bifunctional task-specific ionic liquid (TSIL), i.e. [trialkylmethylammonium] [sec-nonylphenoxy acetate] ([A336][CA-100]) was prepared. However, bigger viscosity of the TSIL is a disadvantage for its application in the extraction, which results in lower rate of mass transfer. Solid-liquid extraction seems to be a more advisable method for its application in separation since the macroporous resins containing TSIL within their lattice can afford more chelating sites. Thus, [A336][CA-100] is impregnated on intermediate polarized XAD-7 resin. To reveal adsorption ability of the material, the solvent impregnated resin (SIR) prepared using [A336][CA-100] was studied for rare earths (RE) separation.

Foundation item: Project supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences, National Natural Science Foundation of China (50574080, 20901073), National Key Technology R&D Program of China (2006BAC02A10) and Distinguished Young Scholar Foundation of Jilin Province (20060114) Corresponding author: CHEN Ji (E-mail: [email protected]) DOI: 10.1016/S1002-0721(08)60365-8

SUN Xiaoqi et al., Solvent impregnated resin prepared using task-specific ionic liquids for rare earth separation

1 Experimental 1.1 Reagents and materials Aliquat336 was purchased from Aldrich. The TSILs used in this study were [tricaprylmethylammonium chloride][nitrate] ([A336][NO3]) and [trialkylmethylammonium][sec-nonylphenoxy acetate] ([A336][CA-100]), Amberlite XAD-7 was purchased from Aldrich with surface area 450 m2/g and pore volume 1.14 ml/g. Stock solutions of Sc(III), Y(III), Eu(III) and Ce(VI) were prepared by dissolving their oxides (99.9%) in the concentrated HNO3. All other reagents used were of analytical grade and purchased from commercial sources. 1.2 Instrumentations

Thirdly, [A336][CA-100] was prepared by combining 250 ml [A336][OH] (0.11 mol/L) and 6.96 g CA-100 (mole ratio is 1.1:1). The mixture was vigorously agitated for 12 h at 50 °C under reflux and left to settle. After a while, an aqueous layer formed at the bottom. The upper phase was poured into a vacuum rotatory evaporator (353 K, 20 mbar, 60 min) to remove the residual water and isopropanol. The final yield of [A336][CA-100] was indicated to be 78.69%. 1 H NMR: 0.86 (t, J=1.2 Hz, 9H), 1.18(m, 3H), 1.26 (m, 30H), 1.33 (m, 16H), 1.61 (m, 6H), 3.26 (s, 3H), 3.378– 3.419 (t, J=8.0 Hz, 6H), 4.44 (s, 2H), 6.892 (d, J=7.2 Hz, 2H), 7.125(m, 2H). 13 C NMR: 13.81, 22.35, 26.11, 26.78, 27.46, 28.81, 29.03, 29.38, 31.62, 35.34, 41.31, 43.18, 48.47, 51.73, 54.03, 61.05, 62.04, 68.09, 113.84, 126.81, 139.06, 156.64, 173.12.

To confirm the structure and the purity of [A336][NO3] and [A336][CA-100], 1H and 13C NMR spectra were obtained in CDCl3 with a Bruker AV 600 NMR spectrometer. The concentrations of rare earth ions were measured on a Shimadzu UVmini-1240 UV-visible spectrophotometer with Arsenazo(III) spectrophotometric method. A PHS-3C digital pH meter made by Shanghai Rex Instruments Factory was used for pH measurements.

1.4 Preparation of the SIRs

1.3 Preparation of TSILs

1.5 Adsorption experiments

The [A336][NO3] and [A336][CA-100] were prepared with acid/base neutralization reaction. Firstly, sodium alkoxide was prepared by combining 6.39 g (0.278 mol) sodium and 125 ml isopropanol for 3 h. 112.36 g (0.278 mol) Aliquat 336 was dissolved in 500 ml of isopropanol, and added dropwise into the isopropanol solution containing sodium alkoxide. The solutions were stirred for 4 h at 50 °C. The mixture was centrifuged at 8000 r/min for 10 min to remove the white precipitate of sodium chloride. Then the filtrates were shaken with equal volume of DI water for half an hour to get [A336][OH] by the hydrolysis of [A336][OR]. The yield of [A336][OH] prepared by isopropanol were 88.42%. Secondly, [A336][NO3] was prepared by combining 250 ml [A336][OH] (0.12 mol/L) and 1.74 ml HNO3 (15.69 mol/L) (mole ratio is 1.1:1). The mixture was vigorously agitated for 4 h at 50 °C under reflux and left to settle. After a while, an aqueous layer was formed at the bottom. The upper phase was poured into a vacuum rotatory evaporator (353 K, 20 mbar, 60 min) to remove the residual water and isopropanol. The final yield of [A336][NO3] was indicated to be 79.14%. 1 H NMR: 0.88 (t, J=4 Hz, 9H), 1.27–1.36(m, 42H), 1.66 (s, 6H), 3.19 (s, 3H), 3.33 (t, J=8.0 Hz, 6H). 13 C NMR: 13.88, 22.17, 22.47, 25.66, 26.16, 27.21, 28.87, 29.23, 31.48, 31.67, 32.70, 48.23, 61.47.

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To remove impurities, XAD-7 resins were washed with absolute ethanol for 3 times (20 min/each time). After filtering, the resins were left out till dry overnight at 50 °C to remove excess ethanol. 4 g of fresh XAD-7 resin was placed in 0.25 g/ml chloroform containing TSILs for 12 h. The resins were separated through a porous filter using a vacuum pump, washed with water and dried at 50 °C.

Adsorption experiments were conducted to evaluate adsorption properties of the SIRs. In the equilibrium experiments, 5 ml of aqueous phase containing Sc(III) and 0.1 g SIR were mixed and shaken in equilibrium tubes with a JINGBO KS mechanical shaker for 60 min at a rate of 300 times/min. The mixtures were then centrifuged for 3 min to separate completely. For desorption experiment, the SIR loaded with metal ion was placed in 5 ml nitrate acid and shaken for 1 h. The adsorption experiment and desorption experiment were all conducted at 298 K. Metal ion concentration was determined with inductively coupled plasma spectroscopy (Thermo iCAP 6000 ICP-OES). The amounts of adsorption (q), distribution ratio (D), separation factor (ȕ), and desorption ratio (Ds) were defined as the following equations: (1) V q (C 0  C ) x M (2) C0  C e V D x Ce M (3) D1

E

Ds =

D2 C e' (C0– Ce )

+ 100 %

(4)

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where C0 and Ce are the initial and equilibrium concentration of metal ion (mol/l), and C ʾe is the equilibrium metal ion concentration in elution medium (mol/l). V is the volume of metal ion solution (l), and M is dry weight of the SIRs (g).

2 Results and discussion 2.1 Effect of equilibrium time on extraction efficiency

To guarantee enough shaking time for the extraction equilibrium, the effect of equilibrium time on amounts of adsorption were measured. As shown in Fig. 1, the amounts of adsorption increase within 30 min, then come to the maximum and keep constant. The equilibrium was thus determined to be 60 min to get enough time for adsorption equilibrium. In addition, qe of the SIR prepared by [A336][CA-100] is obviously bigger than that prepared by [A336][NO3]. Such a phenomenon may be attributed to the higher adsorption ability of [A336][CA-100] because of its functional anion. Since [A336][CA-100] is a novel bifunctional TSIL and it possesses stronger complexing ability with Sc(III), the SIR prepared using [A336][CA-100] for Sc(III) extraction and separation was further studied.

Fig. 1 Structure of [A336][NO3] (a) and [A336][CA-100] (b)

Fig. 2 Sc(III) uptake kinetics by SIRs prepared by [A336][NO3](ƽ) and [A336][CA-100](Ƶ) ([Sc(III)] =8×10–4 mol/L, MSIR=0.1 g, V=5 ml)

JOURNAL OF RARE EARTHS, Vol. 27, No. 6, Dec. 2009

2.2 Adsorption isotherm

The Langmuir isotherm described monolayer coverage of adsorbate over specific homogeneous sites within an adsorbent[12]. (5) qe=qmaxbCe/(1+bCe) where Ce is the equilibrium concentration (mol/L), qe is the amount of metal ion adsorbed at specified equilibrium (mmol/g), and qmax and b are the Langmuir constants related to adsorption capacity and adsorption energy. Linear form of the Langmuir model could be described by the equation: 1 1 1 1 = + (6) qmax qe qmax b C e In this study, Langmuir isotherm was applied to analyze the relationship between the Sc(III) concentration and the adsorption capacity of the SIRs. The uptake isotherms of Sc(III) were obtained in a wide range of initial Sc(III) concentrations varying from 0.7 to 1.2 mmol/L, and the pH value was adjusted to nearly 5.30. As shown in Fig. 3, experimental data of the SIRs are well fitted to the Langmuir plots. qmax value of the SIR was estimated to be 0.042 mmol/g. Freundlich isotherm suggested that the sorption energy exponentially decreased on the completion of sorptional centers of an adsorbent and described heterogeneous systems[13]. (7) qe=KfCe1/n where Ce is the equilibrium metal ion concentration in solution (mol/l), and qe is the amount of metal ion adsorbed at specified equilibrium (mmol/g). Kf and 1/n are the Freundlich constants characteristics of the system, indicating the adsorption capacity and adsorption intensity, respectively. A linear form of the Freundlich model can be obtained by taking logarithms of Eq. (7). 1 lnq e = lnK f + (8) lnC e n

Fig. 3 Langmuir adsorption isotherm of the SIR for Sc(III) (MSIR= 0.1 g, V=5 ml)

SUN Xiaoqi et al., Solvent impregnated resin prepared using task-specific ionic liquids for rare earth separation

As can be seen in Fig. 4, the Freundlich model fitted well in Ce range of the SIR. Since the correlation coefficient of freundlich model (R2=0.99) was higher than that of Langmuir isotherm (R2=0.975), the Freundlich equation was more applicable to describe sorption data of the SIR. 2.3 Competitive adsorption behavior of the RE ions

To confirm the validity of the SIR for rare earth separation, the dependence of qe value of Sc(III), Ce(III), Eu(III) and Y(III) on the acidity was studied. As can be seen in Fig. 5, the separation factors of Sc(III) to Y(III), Eu(III), and Ce(III) could arrive to 16.49, 62.73, and 87.12, respectively, which indicated that the Sc(III) can be effectively separated from Y(III), Eu(III) and Ce(III). The competitive adsorption behavior of the RE ions can be attributed to the different complexation abilities of [A336][CA-100] with Y(III), Eu(III), Ce(III) and Sc(III).

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potential application of the SIR. As mentioned above, the adsorption capacity of the SIR was strongly affected by the acidity, which offered a possibility for desorption. Thus, the desorption of Sc(III) was carried out by HNO3 at 25 °C. To optimize the concentration of HNO3 required for a quantitative stripping of the loaded Sc(III), serial experiments were carried out with varying HNO3 solution concentration from 0.01 to 0.3 mol/L. Since H+ ion possesses higher affinity with SIR than that of metal ions, the Sc(III) could be effectively desorbed by HNO3 at 0.1 mol/L.

2.4 Desorption characteristics

Desorption character is an important factor for evaluating

Fig. 6 Desorption of Sc(III) by HNO3

3 Conclusions

Fig. 4 Freundlich adsorption isotherm of the SIR for Sc(III) (MSIR= 0.1 g, Va=5 ml)

The SIRs prepared by [A336][NO3] and [A336][CA-100] were comparatively studied in this paper. The SIR prepared using [A336][CA-100] exhibited higher adsorption ability due to the functional cation and anion of [A336][CA-100]. The adsorption capacity of the SIR and Sc(III) concentration was fitted to Freundlich isotherm rather than Langmuir isotherm. The SIR can be used to separating Sc(III) from Ce(III), Eu(III) and Y(III) by adjusting the aqueous acidity. Its desorption character revealed potential application of the SIR for rare earth separation.

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Fig. 5 Extraction of Sc(III), Ce(III), Eu(III) and Y(III) by SIR from nitric acid media ([Sc(III)]=[Ce(III)]=[Eu(III)]=[Y(III)]=8×10–4 mol/L, MSIR=0.1 g, Va=5 ml)

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