Asymmetric Membrane Containing Ionic Liquid [A336][P507] for the

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A novel asymmetric membrane containing ionic liquid [A336][P507] for the preconcentration and separation of heavy rare earth Lutetium Li Chen, and Ji Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00141 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 26, 2016

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ACS Sustainable Chemistry & Engineering

A novel asymmetric membrane containing ionic liquid [A336][P507] for the preconcentration and separation of heavy rare earth Lutetium Li Chena,b and Ji Chena,* a

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun, 130022, P. R. China. b

University of Chinese Academy of Sciences, Beijing, 100039, P. R. China.

KEYWORDS: Polymer inclusion membrane, ionic liquid, heavy rare earth, separation, preconcentration

ACS Paragon Plus Environment

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ABSTRACT

A novel polymer inclusion membrane (PIM) was prepared for the preconcentration and separation of heavy rare earth element (HREE) Lutetium (Lu) from dilute solution. The membrane was composed of poly(vinylidene fluoride) (PVDF) as matrix and bifunctional ionic liquid

extractant

[tricaprylmethylammonium][di-(2-ethylhexyl)orthophosphinate]

([A336][P507]) as carrier, without any plasticizer. Weak physical effect existed between PVDF chain segment and [A336][P507] but no chemical interaction. Structure characterization and transport experiment both proved the membrane an asymmetric structure and LuCl3 would transport faster from surface with small pores to large pores. YbCl3 was transported much faster than LuCl3 which was different in liquid-liquid extraction. Preconcentration experiment realized a great acceleration on the transport of LuCl3. And the PIM was proved stable and durable. All these would provide a novel approach to the preconcentration and separation of Lu from dilute solution like leachate of southern ion adsorbed rare earth deposit.


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INTRODUCTION Rare earth elements (REEs) have been served human society for years in a variety of high-tech fields like luminescence, electronics, magnetism, catalysis, therapeutic application and so on.1-3 Although China has massive rare earth resources, not all the REEs are abundant. The world’s largest rare earth mineral, Baotou baiyun obo mine is mainly made up of light rare earth elements (LREEs) while the HREEs are rare.4 HREEs exist in southern ion adsorbed rare earth deposit with complete partition but its leaching grade is only about 0.05 wt.%-0.3 wt.% REO.5,6 So the concentration of HREEs in the leachate is extremely low. In addition the separation between HREEs is not as easy as LREEs for many traditional extractants,7 so it is with the stripping process. Among all the HREEs, Lu is the rarest (except for promethium which is an artificial radioactive element). Despite of classic applications, more attentions are paid on medical scintillation crystals like Ce: Lu2SiO5 (LSO) or Ce: LuAlO3 (LuAP) 8,9 nowadays which are used in positron emission tomography (PET) technology in medical imaging. More attentions should be paid on the preconcentration and separation of high purity Lu from dilute leachate solution of southern ion adsorbed rare earth deposit to reduce the cost and expand the application of PET technology in medical field in China. Membrane separation process has been developed prosperously as a promising alternative to conventional solvent extraction10 recent years. Its continuous and simultaneous extraction and stripping of chemical species made it being a highly integrated technology in water treatment11,12. In addition it requires less amount of extractant and organic solvent, low energy consumption and no phase separation problem. Previously, in our published articles,13,14 we


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described a novel PIM for the transport of Cr(VI), which stimulated our new study on the transport of REEs across such kind of PIMs. Very few articles have been reported on the application of PIM technology for the separation process of REEs.15,16 PVDF was used as membrane material in this study because of its thermal stability, chemical resistance, high hydrophobicity and mechanical strength.17 Room temperature ionic liquid with negligible vapor pressure, high stability and environmentally friendliness makes it being a promising alternative of volatile organic solvent. They have been widely applied in metal ions separation either as diluents or as extractants.18 [A336][P507] was applied as carrier in this study. It was the most researched one of the [A336]-based ionic liquid series developed by our laboratory.19 Systematic investigation on the extraction and separation of LuCl3 by [A336][P507] had already been made.20 Considering the imidazolium ionic liquids (ILs) used as plasticizer before13,14 were expensive and preliminary experiments showed this kind of plasticizer had no positive influence on the transport of LuCl3, we developed a more simple and cheaper PIM without any plasticizer. Our study was aimed at the preconcentration and separation of Lu by this PVDF-[A336][P507] membrane. Surface and internal structures were characterized to get a comprehensive understanding of the PIM. Different conditional parameters of the transport process for LuCl3 were studied. We also tested the stability and reusability of the PIM. All these provide us a prospective research on the treatment of dilute leachate from southern ion adsorbed rare earth deposit. EXPERIMENTAL SECTION Materials and reagents. Solef® PVDF-1008 was purchased from Solvay online. Aliquat336 was kindly supplied by the Cytec Canada Inc. P507 (>95%) was purchased from Luoyang Aoda


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Chemical Co., Ltd. [A336][P507] was synthesized according to published method.19 Stock solutions of trivalent REs were prepared via dissolving their oxides (>99.99%) in hydrochloric acid and diluted with deionized water. Other reagents employed in this work were of analytical grade. The structures of PVDF, Aliquat336, P507 and [A336][P507] were illustrated in Figure 1.

Figure 1. Structures of PVDF, Aliquat336, P507 and [A336][P507]. Membrane preparation. PVDF (1.0g) was dissolved in N,N-dimethyacetamide (DMAc, 10.0 mL) at 80℃ for nearly 1 h, then [A336][P507] was added. The mixture was stirred at 40℃ for 5h to get a homogeneous solution. After being treated in a sonic bath for 10 minutes, it was applied to casting membranes on a glass plate with 5.5 cm diameter by a spin coater in nitrogen atmosphere at room temperature. The nascent membrane was immersed into deionized water for 6h, and washed several times in order to remove DMAc. Then the membrane was dried for 24h, and cut into a suitable size (diameter 3 cm) for transport experiment. Membrane characterization. Thermogravimetric analysis was achieved by a Pyris Diamond TG/DTA with temperature heating from ambient to 800℃ at 10℃/min in air atmosphere. The porous structure of the membrane was observed via a field-emission SEM (Hitachi S-4800). The membrane was sputtered with a layer of platinum. AFM (Bruker Dimension ICON) was applied to observe the surface morphology of the PIM under tapping mode. Attenuated total reflection


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infrared spectroscopy was determined by a Bruker VERTEX 70 (Germany). Differential scanning calorimeter analysis was carried out by a PerkinElmer Thermal Analysis DSC7. The membrane was experienced from heating, cooling and heating again at a rate of 20℃/min in the temperature range of 20-200℃ .

Transport experiment. All transport experiments were performed at 30 ± 0.2℃ (except for temperature experiment) in a two compartment membrane cell of 100 mL each, and the membrane was sandwiched on the circular window (diameter 3 cm) to separate the two compartments. The effective membrane contact area was 7.07 cm2. The transport apparatus was shown in Figure 2. Feed and stripping solutions were placed in each compartment, and both compartments were stirred at 450 rpm by stirrers (except for stirring speed experiment). HCl was chosen as stripping solution.

Figure 2. Illustration of transport apparatus. The permeability coefficient (P, µm s-1) of RECl3 in the membrane was obtained by the following equation:

ln (

c ) = − kt ci

(1)


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Where ci (mol L-1) and c (mol L-1) are the concentration of RECl3 in the feed phase at initial time and selected time, respectively. k is the rate constant (s-1), and t is the selected time of transport (s). To calculate k values, the plots of ln(c/ci) versus time were demonstrated. The relationship of ln(c/ci) versus time was linear, which was confirmed by high values of determined coefficients (r2), i.e., ≥ 0.98. P was calculated as follow:

P = −(

V )k A

(2)

Where V is the volume of the aqueous phase (mL), and A is the area of the effective membrane (cm2). The initial flux (Ji, µmol m-2 s-1) was determined by the following equation:

J i = Pci

(3)

RESULTS AND DISCUSSION Membrane characterization. Figure 3 shows that PVDF membrane thermally degrades from nearly 440℃ and carbonizes to ashes at around 570℃, while [A336][P507]’s degradation occurs from 80℃ to 280℃. PIM degraded in two steps. First weight losses, [A336][P507]’s degradation from 160℃ to 280℃, in the central and marginal zone of the membrane were 29.3 wt.% and 49.7 wt.%, respectively. They were both not consistent with the amount added in the membrane (37.5 wt.%) during the preparation. The increased amount of carrier from centre to surrounding area of the membrane was the result of the centrifugal force via spin-casting method. Despite these small surrounding areas, which would be cut off before the transport, carrier [A336][P507]


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was evenly distributed in the whole central area. This could be proved by testing different areas in the effective contact area of the PIM by TGA.

Figure 3. TGA thermogram of PVDF membrane, [A336][P507] alone, central and marginal zone of a 62.5 wt.% PVDF - 37.5 wt.% [A336][P507] membrane. Long dashed line for [A336][P507], solid line for the marginal zone of the PIM, dotted line for the central zone of the PIM, short-short dashed line for PVDF membrane. Morphology of the membrane usually determines the distribution of carriers and affects the membrane transport efficiency. Figure 4 shows the morphology of the membrane containing 37.5 wt.% [A336][P507] by SEM. The air side of the membrane was distributed with uniform round pores (Figure 4A) with average diameter around 10 µm, while the surface to glass side was distributed with much smaller sized pores (Figure 4B). The cross section image of the membrane (Fig. 4C) was a sponge-like structure which was in accordance with references.21 Immersion precipitation during the preparation of the PIM is the reason of this asymmetric structure.17


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Figure 4. SEM images of the central area of the PVDF membrane containing 37.5 wt.% [A336][P507]. (A) air side of the membrane, (B) glass side of the membrane, (C) cross section of the membrane. AFM images of both surfaces (Figure 5) of the PVDF membrane including 37.5 wt.% [A336][P507] are taken in a three-dimensional format of 1 µm × 1 µm. Dark red regions stood for [A336][P507] included in PVDF support. It was clear that ionic liquids were homogeneously distributed over the entire scan size on both sides. Distribution of the dark red regions on the glass side (Figure 5B) was obviously more densely than that of air side (Figure 5A). Roughnesses (Ra) of the air and glass sides are 15.0 nm and 14.3 nm, respectively. This slight difference in roughness may cause the different transport properties along with the different pore sizes.22,23


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Figure 5. AFM images of the surface of the central area of the membrane with 37.5 wt.% [A336][P507]. (a) air side of the membrane, (b) glass side of the membrane. In order to get a further understanding on the conformation of ionic liquid and PVDF chain segment, attenuated total reflection infrared spectroscopy (ATR-FTIR) and differential scanning calorimetry (DSC) were applied. ATR-FTIR results are listed in Table 1. The characteristic peak of [A336][P507] are P=O stretching vibration peak near 1200 cm-1 and P-O stretching vibration peak near 1045 cm-1. The P=O stretching vibration peak was covered by C-F stretching vibration peak near 1172 cm-1 after the ionic liquid was included into the PVDF. P-O peak could be distinguished clearly which was proved invariable with each side of the PIM or the PIM after transport. We could draw the conclusion that ionic liquid had no chemical interaction with the PVDF matrix from the results of ATR-FTIR. Table 1. ATR-FTIR characteristic peak value of the membranes.

C-F st / cm-1

P=O st / cm-1

P-O st / cm-1


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PVDF membrane

1172

-

-

[A336][P507]

-

1200

1046

PIM-to-air side

1175

-

1045

PIM-to-glass side

1171

-

1041

PIM-to-air side after transport

1173

-

1043

PIM-to-glass side after transport

1175

-

1045

Figure 6 shows the crystallization curves of PVDF membrane and PIM containing [A336][P507] in cooling process. The crystallization temperature of PVDF increased from 126.9℃ to 131.3℃ and the crystal enthalpy value decreased from 42.49J g-1 to 27.16J g-1. It is clear that with the addition of ionic liquid, the crystalline properties of PVDF improved. There is weak physical effect between PVDF chain segment and ionic liquid [A336][P507] but no chemical interaction. [A336][P507] acts like small molecular nucleating agent so that PVDF chain segments could arrange around it into the crystal lattice orderly. This heterogeneous nucleation effect improved PVDF’s crystalline properties as a result. 24-26


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Figure 6. DSC curves of the membranes. Solid line for the membrane with 37.5 wt.% [A336][P507], dotted line for PVDF membrane. Transport mechanism. The transport mechanism was illustrated in Figure 7. As been confirmed by reference,20 the extraction mechanism for [A336][P507] was main neutral complexing. LuCl3 reacts with three [A336][P507] molecules on the extraction side of the PIM to form a complex LuCl3•3[A336][P507], then this complex diffuses across the membrane under the driving force of concentration gradient of LuCl3 •3[A336][P507], and releases LuCl3 at the stripping side under high acidity in stripping compartment.

Figure 7. Illustration of the main transport mechanism of LuCl3 by PIM. Conditional experiments on the transport of LuCl3. The transport of LuCl3 will be influenced by many facts. Here we take membrane composition, thickness, stirring speed, temperature and stripping acidity into consideration. Figure 8(a) shows that with the increasing of [A336][P507] content from 23.1 wt.% to 37.5wt.%, the transport of LuCl3 increases quickly. The permeability coefficient P increased from 1.19 µm s-1 to 2.80 µm s-1, and the maximum P obtained at 60.0 wt.% [A336][P507] was 3.55 µm s-1.


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Figure 8(b) shows that the thinner the membrane is, the faster the transport of LuCl3 will be. The thinnest membrane with the thickness of 58 µm had the fastest P 2.80 µm s-1, while membrane with thickness 115 µm was only 1.80 µm s-1. However, membrane will become easy cracking if they are too thin, so we finally chose a thickness around 100µm to obtain a durable application. It is clear that the two stagnation layers beside the membrane will be thinning if the feed and stripping solutions are under vigorous stirring. This can be proved from Figure 8(c). The maximum P was obtained under the stirring speed of 450 rpm. No increasing in P had been observed when the stirring speed was higher than 450 rpm, which indicated the transport in that zone may be chemically controlled. The effect of temperature was studied in Figure 8(d). The transport of LuCl3 increases quickly with the increasing of temperature. So moderately increasing the temperature could improve the permeability of the membrane and we finally chose 30℃.

The optimum stripping acidity is explored by membrane transport, which is shown in Figure 8(e). With the increasing of HCl concentration, the transport of LuCl3 increases dramatically at first. Then there is a little decrease during the transport when HCl concentration is higher than 1.0 mol L-1. In addition, the stripping acidity should not be too high to protect the IL [A336][P507]. So we finally chose 1.0 mol L-1 HCl for the transport study.


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Figure 8. Effect of (a) membrane composition; (b) thickness; (c) stirring speed; (d) temperature and (e) stripping acidity on the transport of LuCl3. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF (except for Figure 7 a), thickness 101 µm (except for Figure 7 b), feed solution: 7.5×10-4 mol L-1 LuCl3, pH 4.0, stripping solution: 1.0 mol L-1 HCl (except for Figure 7 e), stirring speed 450 rpm (except for Figure 7 c), 303K (except for Figure 7 d). Different transport properties of the two surfaces. As been proved by SEM and AFM images, this PIM was an asymmetry structure. So there must be differences on the transport of LuCl3 from one side to another. The effect of different surfaces is shown in Figure 9. The value of P from membrane-to-glass side to air side is 5.24 µm s-1 while the transport from membrane-to-air side to glass side is only 3.88 µm s-1. This result was in line with reference27 which implied that LuCl3 would be more suitable to transport from surface with small pores to large pores in purpose of avoiding the overcrowding and retention of complexes inside the membrane.


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Figure 9. Effect of different surfaces on the transport of LuCl3. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 98 µm, feed solution: 7.5×10-4 mol L-1 LuCl3, pH 2.84, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. Solid circle for membrane-to-glass surface, empty circle for membrane-to-air surface. Further exploration had been done to study the different transport properties of the two surfaces. LuCl3 fixed in the membrane during the transport was calculated by mass balance. It is obvious in Figure 10 that more LuCl3 is detained in the membrane during the first 4 h transport from membrane-to-air side to membrane-to-glass side. And there was still more than 4 mmol L-1 LuCl3 left in the membrane after 11 h transport.


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Figure 10. Effect of different surfaces on the concentration of LuCl3 fixed in the membrane. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 98 µm, feed solution: 7.5×10-4 mol L-1 LuCl3, pH 2.84, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. Solid circle for membrane-to-glass surface, empty circle for membrane-to-air surface. Preconcentration and Separation for LuCl3. As mentioned in the introduction, how to deal with the HREEs in dilute solution from the leachate of southern ion adsorbed rare earth deposit was a difficult problem. We tried to solve it via our PIM from the view of saving organic reagents. Improvement was made on the transport apparatus to obtain smaller volumes of stripping solution by adding some neatly cutting glass plates with invariable and measurable volumes. The transport result is shown in Figure 11. LuCl3 transported across the membrane was obviously faster with less stripping solution. The amount of Lu3+ in stripping compartment with 80 mL stripping solution was almost twice that in 100 mL stripping solution.


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Figure 11. Effect of different stripping solution volume on the transport of LuCl3. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 98 µm, feed solution: 5.0×10-4 mol L-1 LuCl3, pH 2.80, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. Square symbol for the stripping solution volume of 80 mL, upper triangle symbol for the stripping volume of 90 mL and lower triangle symbol for the stripping volume of 100 mL. However concentration gradient of the complex formed within the membrane, which is the driving force enabling the transport, made it difficult for this PIM to transport more LuCl3 after the concentration is equal in both compartments.28,29 That is to say, we can only accelerate the transport of 50% LuCl3 in feed solution. The purpose of preconcentration could not be reached if the volume of stripping solution is more than half of the feed solution. More changes should be designed upon the apparatus to get a much smaller volume of stripping solution. Some novel devices30 had already been reported for our reference. As for further experiment, we could design and make multi-stage transport apparatus to realize the total separation and preconcentration.


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Figure 12. Transport of YbCl3 and LuCl3 by the PIM. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 98 µm, feed solution: 7.8×10-4 mol L-1 RECl3, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. Solid lines for RECl3 concentration in feed compartment, middle dashed lines for RECl3 concentration in stripping compartment. Solid circle for the concentration of YbCl3, empty circle for the concentration of LuCl3. The selectivity of this PIM on the separation of YbCl3 and LuCl3 has been shown in Figure 12. The decrease of LuCl3 concentration in feed compartment was slower than that of YbCl3. On the other hand, the increase of LuCl3 concentration in stripping compartment was slower too. Compared with liquid-liquid sequence,20 it is interesting to find that the transport of YbCl3 is easier than LuCl3. This abnormal extraction order may caused by the more difficult stripping of LuCl3, which made the transport driving force, i.e., concentration gradient of complex formed within the membrane, being weaker. This phenomenon would benefit the separation between Yb and Lu since it increased the difference between these two similar elements.


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Stability and reusability of the membrane. Repeated runs of transport experiments were demonstrated to get the stability behavior of the PIM. Figure 13 shows that the initial flux Ji remains constant after 10 cycles (4 h each) of transport experiments. The reusability of this membrane was improved much better than that with plasticizer [C8mim][BF4] because of the avoid of decomposition of [BF4]-.12,13 In addition, PVDF would be chemically attacked by sodium hydroxide solution,16 so this PVDF-based PIM is more suitable for the transport of REs other than Cr(VI) because the stripping solution for Cr(VI) is sodium hydroxide solution.

Figure 13. Repeated runs of LuCl3 transport across the PIM. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 99 µm, feed solution: 7.5×10-4 mol L-1 LuCl3, pH 2.84, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. Thickness from the central of the membrane did not change much around the initial 99 µm (Figure 14). A little loss in thickness from 99 µm to 96.4 µm was observed after 10 times cycle.


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These results proved this novel PIM with the addition of [A336][P507] as carrier will be suitable for further long term application.

Figure 14. Thickness from the central of PIM after repeated runs of LuCl3 transport. Membrane composition: 37.5 wt.% [A336][P507] - 62.5 wt.% PVDF, thickness 99 µm, feed solution: 7.5×10-4 mol L-1 LuCl3, pH 2.84, stripping solution: 1.0 mol L-1 HCl, stirring speed 450 rpm, 303K. CONCLUSIONS A novel PIM containing [A336][P507] aiming at the preconcentration and separation of LuCl3 was prepared, characterized and tested. No plasticizer was applied. Weak physical effect, most likely the heterogeneous nucleation effect, existed between PVDF chain segment and [A336][P507] but no chemical interaction. The PIM was characterized as an asymmetric structure from one side to another. LuCl3 was more suitable to be transported from the surface with small pores to large pores so that the overcrowding and retention of complexes inside the


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membrane could be avoided. Abnormal transport sequence was found which expanded the differences between of YbCl3 and LuCl3. The acceleration of the transport was obvious and promising in the preconcentration experiment. Stability and reusability of the membrane were proved well. All these would provide a new approach to dealing with HREEs in the dilute leachate from southern ion adsorbed rare earth deposit. AUTHOR INFORMATION Corresponding Author *Tel.: +86-431-8526-2646; E-mail: [email protected]. ACKNOWLEDGMENT The authors wish to thank the National Basic Research Program of China (2012CBA01202), the National Natural Science Foundation of China (51174184) and the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-201-1). REFERENCES (1) Zaimes, G. G.; Hubler, B. J.; Wang, S.; Khanna, V. Environmental Life Cycle Perspective on Rare Earth Oxide Production. ACS Sustainable Chem. Eng. 2015, 3, 237-244. (2) Yang, D.; Ma P.; Hou, Z.; Cheng, Z.; Li, C.; Lin J. Current advances in lanthanide ion (Ln3+)-based upconversion nanomaterials for drug delivery. Chem. Soc. Rev. 2015, 44, 1416. (3) Fricker, S. P. The therapeutic application of lanthanides. Chem. Soc. Rev. 2006, 35, 524-533. (4) Chen, Z. Global rare earth resources and scenarios of future rare earth industry. J. Rare Earth 2011, 29 (1), 1-6.


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Table of Contents

A novel asymmetric membrane containing ionic liquid [A336][P507] for the preconcentration and separation of heavy rare earth Lutetium Li Chen and Ji Chen* A novel asymmetric membrane containing ionic liquid was prepared for the preconcentration and separation of heavy rare earth Lutetium from dilute solution.


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Asymmetric Membrane Containing Ionic Liquid [A336][P507] for the

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