Synergistic Effect of AcidâBase Coupling Bifunctional Ionic Liquids in

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The Synergistic Effect of Acid-base Coupling Bifunctional Ionic Liquids in Impregnated Resin for Rare Earth Adsorption zeyuan zhao, Xiaoqi Sun, Yamin Dong, and Yanliang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01253 • Publication Date (Web): 01 Jan 2016 Downloaded from http://pubs.acs.org on January 2, 2016

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ACS Sustainable Chemistry & Engineering

The Synergistic Effect of Acid-base Coupling Bifunctional Ionic Liquids in Impregnated Resin for Rare Earth Adsorption

Zeyuan Zhao, Xiaoqi Sun*, Yamin Dong, and Yanliang Wang

Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen, 361021, China

*Author for Correspondence: Xiaoqi Sun E-mail: [email protected] Tel.: +86 592 6376370; Fax: +86 592 6376370;

ABSTRACT The synergistic effect produced by ionic liquid extractants in the field of adsorption was first reported in this article. The data from this work show that distribution coefficient and synergistic enhancement coefficient of Lu(III) extracted by [P66614][EHEHP] and [N1888][BTMPP] in impregnated resin are pronounced higher than those in solvent extraction. The synergistic interplay of combined acid-base coupling bifunctional ionic liquids (ABC-BILs) is the key to the higher adsorption efficiencies of REEs. Moreover, no third phase was observed in the adsorption 1

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systems. The elimination of third phase from ABC-BIL is a remarkable advantage of adsorption over solvent extraction in the present study. This paper reveals efficient and environment-friendly potentials in both of academic research and industrial application for REEs adsorption.

KEYWORDS: Rare earth; Impregnated resin; Adsorption; Synergistic effect; Third phase INTRODUCTION The importance of rare-earth elements (REEs) in the global economy is booming as they are used in numerous advanced technologies, such as magnets, lightings, sensors, lasers, electronics, batteries, catalysts, alloys and communications.1,2 Solvent extraction has been the most common industrial separation technology for REEs in the past decades.3 Synergistic extraction is an important research field in solvent extraction. When a combination of two extractants yields partitioning that is greater than the sum of their individual contributions, the system is synergistic. The enhanced extraction is most often attributed to the combined coordinating/solvating abilities of the two extractants.4 The synergistic effect contributes to increase the extractability and selectivity of mixed extractants. Up to now, many investigations on synergistic extraction for REEs separation have been conducted. The synergistic extraction of Nd using mixture of triisooctylamine and bis(2,4,4-trimethylpentyl) monothiophosphinic acid from chloride solution was studied. The mixture may be of practical importance for the separation of Lu, Yb from light REEs.5 A systematic liquid-liquid extraction of La and Nd from aqueous nitric acid solution using a mixture of trioctylphosphine 2


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oxide and trialkylphosphine oxide in kerosene was investigated, which was applied to separate Y from La and Nd from Egyptian monazite.6 The solvent extraction of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y from weak acidic hydrochloric

acid

solution

into

an

organic

phase

containing

4-benzoyl-3-methyl-1-phenylpyrazol-5-one and neutral tridentate organophosphorus ligand has also been studied.7 Solvent extraction is expensive on a large scale and results in huge environmental problems, because the toxic organic diluents and modifiers are used widely.8 The separation and purification of REEs by solvent extraction require the treatment of a large volume of hazardous VOC (volatile organic compound) solvents.9 Due to flammable VOC solvents are concerned, solvent extraction may be dangerous. The advantages of adsorption make it to be a very competitive alternative to solvent extraction, i.e., simplicity, flexibility, cost effectiveness, ease of operation and low consumption of organic reagent.10 A summary of published adsorptions of f-elements using impregnated resins is provided as follows. The removal of Am from aqueous solution was

studied

by

2-ethylhexyl

phosphonic

acid

mono-2-ethylhexyl

ester

(HEH[EHP])-impregnated macroporous polymeric bead. The synthesized polymeric bead shows a great potential for effective removal of Am from low level nuclear waste stream.11 A previous study investigated the separation of La and Ce using impregnated resin containing HEH[EHP].12 The removal of U and Th from aqueous solution was studied by impregnated resin containing carminic acid.10 The impregnation of o-phenylene dioxydiacetic acid into Amberlite XAD-2000 was also 3



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used for the sorption of U and Th.13 The extractant-impregnated resin was developed using brilliant green and Amberlite XAD-7 resin, which showed superior binding affinity for Cr in the presence of co-existing ions.14 Ionic liquids (ILs) are composed of cations and anions, and their melting points are generally below 100OC.15 Some properties of ILs make them particularly suitable for separation processes, i.e., low volatility and combustibility, wide liquid range, thermal stability, functional group, high conductivity, and wide electrochemical window.16,17 The advantages of ILs mentioned above contribute to be used in preparing sustainable adsorption materials. Also, adsorption materials containing ILs are preferred because of their well performances. [C8mim][PF6] containing Cyanex923 was immobilized on XAD-7 resin for adsorption of REEs, which contributed to improve mass transfer efficiency and increase selectivity of Cyanex923 in [C8mim][PF6].18 Several chromatography resins containing TODGA and IL were studied for selective sorption of trivalent f-elements over hexavalent uranyl ions and other fission product elements.19 Ordered N-methylimidazolium functionalized mesoporous silica anion exchanger was synthesized by co-condensation of tetraethoxysilane with 1-methyl-3-(triethoxysilylpropyl)imidazolium chloride as highly efficient anion exchanger of Cr.20 IL functionalized multi-walled carbon nanotube was studied as a super sorbent for Cr(VI)/Cr(III) adsorption from aqueous solution.21 Recently, some common industrial extractants for REEs were developed to acid-base coupling bifunctional ionic liquid extractants (ABC-BILs),22 and the synergistic effects between ABC-BILs in solvent extraction for REEs have been 4


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studied.23 More than 1018 ILs can be prepared through different combinations of substitution patterns and ion choices.16 As reported in our previous paper, the development of ABC-BIL contributes to reduce the pollution of million tons of saponification wastewater annually in industrial REEs extraction.22 However, the application of ABC-BIL in solvent extraction concerns massive hazardous VOC. Thus, the impregnated resin containing ABC-BIL is prepared in this study for REEs adsorption. Though the adsorptions of metal ions have been widely investigated, no synergistic effect of ligands in adsorption material containing IL for metal ion has been reported to our knowledge. In this paper, the synergistic effect of ABC-BILs in impregnated resin is first investigated for REEs adsorption. MATERIALS AND METHODS Reagents The anion-exchange resin (Dowex Monosphere 550A (OH)) was purchased from Dow Chemical Company. Trihexyl(tetradecyl) phosphonium chloride (CyphosIL101, [P66614]Cl) and bis(2,4,4-trimethylpentyl)phosphonic acid (Cyanex272, HBTMPP) were

obtained

from

Cytec

Industries

Inc.

without

further

purification.

2-ethylhexylphosphonic acid and mono-(2-ethylhexyl) ester (P507, HEH[EHP]) was purified by washing with 2% Na2CO3, 0.2 mol/L H2SO4 and deionized water. Individual REE stock solutions were prepared by dissolving the corresponding oxides (> 99.99%, Fujian Changting Golden Dragon Rare-Earth Co. Ltd) with hydrochloric acid and diluted with deionized water. Trihexyl(tetradecyl) phosphonium mono-(2-ethylhexyl) 2-ethylhexyl phosphonate ([P66614][EHEHP]) and 5



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trioctylmethylammonium bis(2,4,4-trimethylpentyl) phosphonate ([N1888][BTMPP]) were prepared using the combination of ion-exchange and neutralizing reactions.3,23 The detailed synthetic steps of [P66614][EHEHP] and [N1888][BTMPP] are given in the supporting information. The structures of XAD-7 resin and ABC-BILs used in this study are shown in Figure 1.

CH3 *

CH3 H2 C

C C

C

O

C

O

O

R

R

O

O

C O *

C

C H2 C

C

CH3

CH3

*

C2H5 H2 C C C4H9 C6H13 C6H13 O H P P H2 H O C C C4H9 C14H29 C6H13 O

O

C2H5

[P66614][EHEHP] CH3 C8H17

O *

n

C8H17

CH3 O H2C C H N P H HC C C8H17O 2 CH3

CH3 H2 C

C

H2 C

C CH3

CH3 CH3 CH3 CH3

[N1888][BTMPP]

XAD-7 Resin

Figure 1. The chemical structures of [P66614][EHEHP], [N1888][BTMPP] and XAD-7 resin used in this study. Instrumentation 1

H and 13C NMR spectra of [P66614][EHEHP] and [N1888][BTMPP] were recorded

in CDCl3 with an AV III-500 BRUKER spectrometer. Elemental analysis of nitrogen element in [N1888][BTMPP] was performed on a Vario EL Cube elementary analysis instrument. Infrared spectra were obtained on a Nicolet IS 50 infrared spectrometer in the range of 4000-500 cm-1. EDS was performed with SU1510 produced by HITACHI. Nitrogen adsorption was test with ASAP2020 M+C produced by Micromeritics. A pHS-3C digital pH meter (Shanghai Rex Instruments Factory) was 6


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used for the initial pH measurements. Thermo scientific ICP 6500 series inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to determine the concentration of REE in aqueous phase. Impregnated Resins Preparation. Before the impregnation process, 100 g Amberlite XAD-7 resin was washed with about 200 ml ethanol each time for three times with continuous stirring. The resin was filtered, and dried in the oven at 100 °C for 1 hour. Then the resin was dried at 110 °C under vacuum for 12 h. After that, a known amount of clean resin was used for the impregnation process. The dry resin was impregnated in acetone solution containing ABC-BIL, stirred for 12 h at room temperature. Subsequently, the acetone was distilled off with a RE-52AA rotary evaporator at 110 °C under vacuum for 12 h. The impregnation process of resin for further adsorption study is similar to those reported in previous papers.24,25 Adsorption and Extraction Studies. In the adsorption experiment, 4 ml of aqueous phase containing Lu(III) and impregnated resins were mixed and shaken in an equilibrium tube with a mechanical shaker for 60 mins. To make a comparison, identical extractants were dissolved in toluene, and the organic phase was shaken with aqueous phase on the same condition of adsorption. Sodium chloride was used to keep constant ionic strength in the adsorption and extraction. The experiments were maintained at 303 ± 1 K unless otherwise stated. The aqueous solution was separated from the impregnated resin and organic phase by centrifugation at 2500 rpm for 10 mins. After phase separation, 7



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ICP-AES was used to determine the concentration of REE in the aqueous phase. The concentration of REE in impregnated resin and organic phase were calculated by mass balance. In both of the adsorption and extraction studies, the extraction efficiency (E), distribution coefficient (D), synergistic enhancement factor (R), the amount of adsorption (qe) are defined as the following Equation (1) to Equation (4): E% =

D=

R=

[ M ]t − [ M ]a ×100% [ M ]t

[ M ]t − [ M ]a [ M ]a

（1）

（2）

Dmix Da + Db

（3）

qe = (C0 − Ce ) × V/M

（4）

Where [M]t and [M]a represent the initial and final concentration of REE in aqueous phase, Da is the distribution coefficient from one extractant for REE, Db is the distribution coefficient from the other extractant for REE, Dmix is the distribution coefficient from their mixture for REE. C0 and Ce are the initial and equilibrium concentrations of metal ion, V is the volume of metal ion solution, and M is dry weight of impregnated resin. All the concentration values of REEs were measured in duplicate with the uncertainty within 5%.

RESULTS AND DISCUSSION Characterization of Impregnated Resin. To study the effect of impregnated ABC-BIL on the XAD-7 resin, specific surface areas, pore volumes and pore sizes of the blank XAD-7 resin and impregnated resin containing 0.26 mmol/g [P66614][EHEHP] and 0.26 mmol/g [N1888][BTMPP] were 8


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compared. The unit of mmol/g is used in this paper to indicate the amount of ABC-BIL in impregnated resin, for example, 0.26 mmol/g [P66614][EHEHP] mentioned above refers to 1 g impregnated resin contain 0.26 mmol [P66614][EHEHP]. As shown in Table 1, the specific surface area of XAD-7 resin decreases from 482.06 m²/g to 121.02 m²/g after the ABC-BILs are impregnated. The process also leads to the decrease of pore volume from 0.818 cm3/g to 0.531 cm3/g. As previously mentioned on impregnated resin containing IL, the IL initially forms a thin layer that cover internal surface of the resin at low IL loading. Then the IL fills the smallest mesopores (

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