Boosting the extraction of rare earth elements from chloride medium by novel carboxylic acid based ionic liquids

Article abstract of DOI:10.1016/j.molliq.2021.115549

Two novel carboxylic acid based ionic liquids (ILs), benzyltributylammonium myristic acetate ([N_444Bn][MA]) and benzyltributylammonium dodecanedioic acetate ([N_444Bn]₂[DDA]), have been rationally designed and successfully synthesized for the extraction of yttrium (denoted as Y(III)) from a chloride aqueous solution and for the separation of mixed rare earth elements (REEs). Comparison studies of the extraction performances of [N_444Bn][MA] and [N_444Bn]₂[DDA] were carefully investigated under different conditions (e.g., equilibrium time, solution acidity, salting-out agent concentration, and REEs concentration). The results indicated that the extraction efficiency of Y(III) reached 87.18% after 2 min using [N_444Bn]₂[DDA], whereas it required 10 min to reach an extraction efficiency of 47.37% using [N_444Bn][MA], mainly due to the larger electronegativity of the incorporated dicarboxylates in the anions of [N_444Bn]₂[DDA], which enhanced its coordination with Y(III) compared to [N_444Bn][MA]. However, [N_444Bn][MA] illustrated better selective separation behavior than [N_444Bn]₂[DDA] in mixed REEs solutions (including La, Nd, Eu, Ho, Yb and Y), and the selective factors of Yb and La were 16.21 and 3.30, respectively. Furthermore, the ion association mechanism, evolved from the slope analysis and FT-IR results, contributed to avoiding the saponification procedures and resultant saponification wastewater during the extraction process. In addition, the back-extraction experiments indicated that ~97% Y(III) can be stripped from [N_444Bn]₂[DDA] and [N_444Bn][MA] using 1.0 M and 0.4 M HCl, respectively, demonstrating that these ILs can be further used in large-scale applications for the separation and recovery of REEs.

Full text of DOI:10.1016/j.molliq.2021.115549

Journal of Molecular Liquids 329 (2021) 115549
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Boosting the extraction of rare earth elements from chloride medium
by novel carboxylic acid based ionic liquids
b
b
b
a,b,
Yu Deng a, Yigang Ding a, , Zhong Huang , Ying Yu , Jun He , Yi Zhang
⁎
⁎
a
Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key Laboratory for Novel Reactor and Green Chemical Technology, School of Chemical Engineering and Pharmacy,
Wuhan Institute of Technology, Wuhan 430074, China
b
Hubei Xiangyun Chemical Co., Ltd., Wuxue 435405, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel carboxylic acid based ionic liquids (ILs), benzyltributylammonium myristic acetate ([N444Bn][MA])
and benzyltributylammonium dodecanedioic acetate ([N444Bn]2[DDA]), have been rationally designed and suc-
cessfully synthesized for the extraction of yttrium (denoted as Y(III)) from a chloride aqueous solution and for
the separation of mixed rare earth elements (REEs). Comparison studies of the extraction performances of
[N444Bn][MA] and [N444Bn]2[DDA] were carefully investigated under different conditions (e.g., equilibrium time,
solution acidity, salting-out agent concentration, and REEs concentration). The results indicated that the extrac-
tion efficiency of Y(III) reached 87.18% after 2 min using [N444Bn]2[DDA], whereas it required 10 min to reach an
extraction efficiency of 47.37% using [N444Bn][MA], mainly due to the larger electronegativity of the incorporated
Received 13 November 2020
Received in revised form 11 January 2021
Accepted 30 January 2021
Available online 2 February 2021
Keywords:
Ionic liquids
Rare earth
dicarboxylates in the anions of [N444Bn]2[DDA], which enhanced its coordination with Y(III) compared to [N444Bn
]
Solvent extraction
Separation
[MA]. However, [N444Bn][MA] illustrated better selective separation behavior than [N444Bn]2[DDA] in mixed REEs
solutions (including La, Nd, Eu, Ho, Yb and Y), and the selective factors of Yb and La were 16.21 and 3.30, respec-
tively. Furthermore, the ion association mechanism, evolved from the slope analysis and FT-IR results, contrib-
uted to avoiding the saponification procedures and resultant saponification wastewater during the extraction
process. In addition, the back-extraction experiments indicated that ~97% Y(III) can be stripped from [N444Bn 2
]
[DDA] and [N444Bn][MA] using 1.0 M and 0.4 M HCl, respectively, demonstrating that these ILs can be further
used in large-scale applications for the separation and recovery of REEs.
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
electronic structures and physicochemical properties [8]. To date, a se-
ries of molecular extractants have been employed for REEs separation,
Rare earth elements (REEs) are a group of scandium, yttrium and 15
lanthanides in the periodic table. Due to their unique role in many high-
technology industries and the increasing global demand, the separation
and recovery of REEs have attracted tremendous attention [1]. In recent
decades, many kinds of techniques have been developed for REEs sepa-
ration, such as extraction, adsorption, and membrane separation [2–7].
Solvent extraction is still one of the most widely used methods in the
rare earth industry owing to its facile operation, large treatment capac-
ity, fast extraction rate and remarkable separation ability [8,9].
such as ammonium, phosphoric acid, acetic acid, phosphine oxide and
phosphate [11–15]. Unfortunately, the extraction performance of or-
ganic phosphonic acid and carboxylic acid still suffers from unsatisfac-
tory capacity due to the intermolecular hydrogen bonds between
dimers. In addition, the hydrogen ions released from acidic extractants
in the ion-exchange mechanism also obviously increase the acidity of
the aqueous phase and thereby decrease extraction efficiency [16]. To
overcome these disadvantages, saponified acidic extractants developed
by neutralizing acidic extractants with aqueous ammonia have exhib-
ited improved extraction capacities. Nevertheless, the resultant ammo-
nium ion in the aqueous phase can cause serious pollution [17].
Therefore, exploring green strategies for rare earth extraction is highly
desirable but still presents challenges.
REEs are naturally distributed together in geological minerals, in-
cluding bastnasite, monazite and oil shale residue [10]. The separation
of individual REEs is particularly difficult due to their extremely similar
Ionic liquids (ILs), composed of tunable cations and anions, are crit-
ical environmentally friendly solvents with low melting points [18].
They have attracted extensive attention due to their unique physico-
chemical properties, such as negligible vapor pressure, nonflammability
and structural flexibility [19–21]. Recently, endowing ILs with carboxyl-
ate groups through the anion exchange method has been a promising
⁎
Corresponding authors at: Key Laboratory for Green Chemical Process of Ministry of
Education, Hubei Key Laboratory for Novel Reactor and Green Chemical Technology,
School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan
430074, China.
E-mail addresses: dygzhangli@163.com (Y. Ding), zhangyi_1208@whu.edu.cn
(Y. Zhang).
https://doi.org/10.1016/j.molliq.2021.115549
0167-7322/© 2021 Elsevier B.V. All rights reserved.

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
approach to efficiently extract REEs. In light of these findings,
deprotonated sec-octylphenoxy acetic acid (HSOPAA), N,N-dioctyl
diglycolamic acid (DODGAA) and oleic acid (OL) can be used as anions
[22–24]. To obtain hydrophobic carboxylate-based ILs, tri-n-
pHvalues,saltconcentration,initialREEsconcentrationandtemperature,
as well as the regeneration of REEs, have also been investigated, and the
results demonstrate the potential of using synthesized ILs for further ap-
plications related to industrial-scale REEs separation.
octylmethylammonium ([N1888
]
+), tetraoctylammonium ([N8888
]
+),
and trihexyl(tetradecyl)phosphonium ([P66614
]
+) are usually used as
2. Experimental
cations due to their long alkyl chains and commercial availability
[22–24]. In previous studies, better extractability and selectivity for
Y(III) have been observed in [P66614][SOPAA] than in conventional ex-
2.1. Chemicals and reagents
tractant counterparts, demonstrating the excellent superiority of ILs
Benzyltributylammonium chloride ([N444Bn][Cl]), myristic acid
(MA), dodecanedioic acid (DDA), yttrium and lanthanide chloride hexa-
hydrate (Y, La, Nd, Eu, Ho, Yb, 99.99%), kerosene, ethylenediaminetetra-
acetic acid disodium salt dihydrate (EDTA) and anion-exchange resin
(Amberlite-717) were purchased from Aladdin Co., Ltd. N-octanol,
methyl red and bromocresol green indicators were obtained from
Macklin Co., Ltd. Sodium hydroxide, sodium chloride and xylenol or-
ange indicators were supplied by Sinopharm Chemical Reagent Co.,
Ltd. Hydrochloric acid was obtained from Xilong Scientific Co., Ltd. All
chemicals were used without further purification.
+
over organic extractants [25]. After combining [N1888
]
with the
phenoxyacetic acid analogs n-octylphenoxyacetic acid (OCTPOA), the
generated [N1888][OCTPOA] also exhibits advanced extraction perfor-
mance towards Y(III), which is attributed to the optimized Y−O bond
distances, as confirmed by the extended X-ray absorption fine structure
(EXAFS) [26]. In terms of other REEs, different kinds of carboxylic
acid-based ILs have emerged as popular and efficient extractants. For
example, the newly fabricated tetraoctylammonium di(2-ethylhexyl)-
oxamate ([N8888][DEHOX]) exhibited better extraction efficiency in all
lanthanides than [N8888][DODGA] and can be reused without loss of ex-
traction performance after HNO3 regeneration [23]. More interestingly,
the most common vegetable oils, such as peanut oil, rapeseed oil,
sunflower seed oil and flaxseed oil, can be converted into carboxylic
acid-based ILs, resulting in the selectivity of mixed lanthanides during
extraction that is similar to the selectivity of conventional extractants
such as 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester
(P507) and di(2-ethylhexyl)phosphate (P204) [27]. Nevertheless, the
carboxylic acid based ILs still can be improved in their extractability
for REEs and the high viscosity for facile use. Therefore, we expect that
by integrating anions with dicarboxylate groups and cations with
shorter alkyl chains, ILs can be constructed with a hydrophobic nature,
lower viscosity and higher extraction efficiency.
2.2. Apparatuses and measurements
The 1H and 13C NMR spectra of the ILs were obtained in CDCl3 with
an AVIII-500 BRUKER spectrometer. The IL structures were further char-
acterized using FT-IR (Nicolet 6700). The concentration of Y(III) in the
aqueous phase was determined using EDTA titration with xylenol or-
ange used as the indicator and hexamethylene tetraamine used as the
buffer. The concentration of single REEs ions in the REEs mixture was
determined by inductively coupled plasma optical emission spectrome-
try (ICP-AES, Agilent 700). The pH values were determined by a pH-ISE-
EC Hanna. The chloride concentrations in [N444Bn][MA] and [N444Bn 2
]
[DDA] were measured by the AgNO3 test.
Herein, two novel carboxylic acid-based ILs with a hydrophobic na-
ture were successfully synthesized and investigated as extractants for
2.3. Synthesis and characterization of ionic liquids
+
REEs extraction. (seen in Scheme 1) The cationic structures ([N444Bn
]
)
of the synthesized ILs exhibit larger electronegativity after benzyl is
substitutedforalkylchains,resultingindecreasedviscosityandbetterex-
tractability. Encouraged by the advancement of incorporating
dicarboxylate-functionalized groups in anions, the electronegativity can
be further strengthened, and a higher extraction capacity for Y(III) is ob-
served in [N444Bn]2[DDA]. However, [N444Bn][MA] provides better selec-
tive separation of mixed REEs (e.g., separation factor is 16.21 vs. 3.30 for
La/Yb and 5.13 vs. 1.65 for La/Y). Based on slope analysis and FT-IR char-
acterization, an ion association mechanism has been proposed to eluci-
date the extraction process. Moreover, the influences of extraction time,
2.3.1. Preparation of benzyl(tributyl)azanium myristic acetate [N444Bn
[MA]
]
A solution of [N444Bn]OH in ethanol was converted from [N444Bn]Cl
(1.5597 g, 0.005 mol) using amberlite 717-type anion-exchange resin.
Then, 1.1419 g MA (0.005 mol) was added to the [N444Bn]OH solution.
The mixture was then stirred at room temperature for 12 h until the
neutral solution was obtained. Ethanol and water were removed by a
rotary evaporator. [N444Bn][MA] was obtained after drying at 70 °C in a
vacuum for 12 h with 84.57% yield. The density and viscosity were
0.952 g·cm−3 and 343.7 mPa·s, respectively. The structure was
Scheme 1. Structures of ionic liquids (a) [N444Bn][MA] an [N444Bn]2[DDA] used in this study
2

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
identified by FT-IR, 1H and 13C NMR (see Supplementary Information,
Figs. S1, S2 and S5). FT-IR (cm−1): 2956–2869 (CH in CH2), 1638 (CO
in COOH), 1379 (C−N in N+444Bn). 1H NMR (300 MHz, CDCl3, δ/ppm):
0.85–0.88 (CH3, t, 3H), 0.97–1.01 (3CH3, t, 9H), 1.21–1.23 (8CH2, m,
16H), 1.36–1.45 (4CH2, m, 8H), 1.51–1.55 (1CH2, t, 2H), 1.73–1.81
(4CH2, m, 8H), 2.07–2.11 (1CH2, t, 2H), 3.20–3.24, (3CH2, t, 6H), 4.79
(1CH2, s, 2H), 7.42–7.49 (5Ar−H, m, 5H). 13C NMR (75 MHz, CDCl3, δ
ppm): 13.50 (3CH3), 18.26 (1CH3), 19.50 (3CH2), 20.60 (1CH2), 24.01
(3CH2), 26.72 (1CH2), 29.53 (4CH2), 29.81 (4CH2), 34.18 (1CH2), 38.39
(1CH2), 57.02 (1CH2), 5 s8.08 (4CH2), 62.45 (1CH2), 65.84 (1CH2),
127.36 (1Ar-CH), 127.90 (1Ar-CH), 128.37 (1Ar-CH), 129.22 (1Ar-CH),
130.58 (1Ar-CH), 132.26 (1Ar-CH), 179.34 (C).
1 and 2 ions, [M]aq is the equilibrium concentration of REEs ions in strip-
ping acid, and [M]ILs is the initial concentration of loaded REEs ions in
the extracting phase, respectively.
3. Results and discussion
3.1. The effect of extraction time and aqueous pH
In solvent extraction investigations, accelerating kinetics is impor-
tant for improving extraction performance [28,29]. To study the extrac-
tion kinetics, a feed solution of 0.007 M YCl3 with pH value of 4 was
contacted with 0.018 M organic phase at an organic/aqueous phase vol-
ume ratio (O/A) of 1. As shown in Fig. 1a, the extraction efficiencies
reached 45.15% and 87.18% after shaking for 2 min for [N444Bn][MA]
and [N444Bn]2[DDA], respectively. Then, the extraction efficiencies
slightly increased within 10 min and were maintained at 89.01% and
47.45% after 20 min, indicating that 20 min was enough to achieve ex-
traction equilibrium between the two phases. Notably, to ensure that
the extraction equilibrium is reached and keep the experimental condi-
tions consistent, the shaking time is prolonged. Therefore, all further ex-
traction experiments were performed within 30 min.
To investigate the effect of solution acidity on extraction behavior,
the extraction efficiency of Y(III) was measured at different pH values
using [N444Bn][MA] and [N444Bn]2[DDA]. The initial aqueous phase
contained 0.007 M YCl3 and 0.1 M NaCl. By adding a certain amount of
1 M HCl or 1 M NaOH solution, the pH values of the aqueous phase
were adjusted and varied between 1.5 and 5.0. As presented in Fig. 1b,
all the extraction efficiencies of Y(III) increased with increasing aqueous
pH value. For example, the extraction efficiencies were 1.12% and
1.11% at pH = 1.8 for [N444Bn][MA] and [N444Bn]2[DDA], respectively,
and then increased to 48.27% and 94.14% at pH = 5.0. The negative
effect of high acidity could be attributed to the competitive extrac-
tion between Y(III) and H+ [30]. More specifically, the amount of
ILs combined with protons obviously increased with increasing H+
concentration in the aqueous phase, resulting in a reduced probabil-
ity of coordination between the ILs and Y(III), accordingly leading to
a decay in the extraction efficiency of Y(III) [31]. More importantly,
2.3.2. Preparation of benzyl(tributyl)azanium dodecanedioic acetate
[N444Bn]2[DDA]
A solution of [N444Bn]OH in ethanol was prepared from [N444Bn]Cl
(1.5597 g, 0.005 mol) using amberlite 717-type anion-exchange resin.
Then, 0.5758 g DDA (0.0025 mol) was added to the [N444Bn]OH solution.
The mixture was then stirred at room temperature for 12 h until the so-
lution became neutral. After the ethanol and water were distilled in a
rotary evaporator, the product was dried in a vacuum at 70 °C to yield
a slightly yellowish viscous liquid (90.55% yield). The density and vis-
cosity were 0.967 g·cm−3 and 1445 mPa·s, respectively. The structure
was identified by FT-IR 1H and 13C NMR (see Supplementary Informa-
tion, Figs. S3-S5). FT-IR (cm−1): 2950–2848 (CH in CH2), 1637 (CO in
COOH), 1389 (C−N in N+444Bn). 1H NMR (300 MHz, CDCl3, δ ppm):
0.98–1.02 (6CH3, m, 18H), 1.20–1.25 (6CH2, m, 12H), 1.37–1.46(6CH2,
m, 12H), 1.53–1.60 (2CH2, m, 4H), 1.73–1.81 (6CH2, m, 12H),
2.14–2.18 (2CH2, t, 4H), 3.23–3.27 (6CH2, t, 12H), 4.85 (2CH2, s, 4H),
7.33–7.36 (2Ar−H, m, 2H), 7.45–7.49 (8Ar−H, m, 8H). 13C NMR
(75 MHz, CDCl3, δ/ppm): 13.44 (6CH3), 19.45 (6CH2), 22.46 (2CH2),
24.00 (4CH2), 26.90 (2CH2), 29.13 (2CH2), 29.53 (2CH2), 29.88 (2CH2),
38.86 (2CH2), 56.91 (2CH2), 58.10 (4CH2), 62.44 (2CH2), 127.37 (4Ar-
CH), 129.15 (2Ar-CH), 130.50 (2Ar-CH), 132.26 (4Ar-CH), 179.34 (2C).
2.4. Extraction experiments
All extraction experiments were performed by mixing equal vol-
umes of organic phase (the as-prepared ILs dissolved in a mixture of
sulfonated kerosene and n-octanol with a volume ratio of 7:3) and
aqueous phase containing REEs ions. The extractions were performed
in vials and shaken at 1700 rpm for 30 min at 303 K in a temperature-
controllable turbo thermoshaker (HUXI, HX-20). Sodium chloride was
used to maintain constant ionic strength during the extraction. After
the extraction, two-phase separation was achieved by a benchtop cen-
trifuge (CENCE, TGL-16 M) operating at 8000 rpm for 5 min. The Y(III)
ions were back-extracted from the loaded organic phase with dilute hy-
drochloric acid. The stripping efficiency was calculated by measuring
the concentration of Y(III) ions in the stripping acid. The extraction effi-
ciency (E), distribution ratio (D), stripping efficiency (S) and separation
factors (β) were obtained using the following equations:
the comparison of the extractability also revealed that [N444Bn
]
2
[DDA] was much better than [N444Bn][MA] at pH values ranging
from 2.5 to 5.0, showing that [DDA]− was dominant in improving
the REEs extraction capacity over [MA]−
.
3.2. The effect of salting-out agent and extractant concentration
The salting-out agent is essential to the extraction process due to the
elimination of emulsification and improved extractability with the addi-
tion of NaCl [32]. Therefore, the effect of the salting-out agent on extrac-
tion behaviors was investigated in 0.007 M YCl3 solution with different
amounts of NaCl (Fig. 2a). As expected, the extraction efficiencies of Y
(III) for both [N444Bn][MA] and [N444Bn]2[DDA] significantly increased
with increasing concentrations of NaCl, which efficiently facilitated the
generation of chloro-complexes containing REEs ions and thereby en-
hanced extractability. The main reason is assumed to be that NaCl
with a high water-binding capacity can reduce the hydration of Y(III)
and cause free water to continue to be lost in aqueous solution [33].
Salt anions also act as bridges to connect with the IL and REEs ions
and undergo strong complexation at the liquid-liquid interface [34].
Furthermore, the effect of the extractant concentration was also studied
in 0.007 M YCl3 aqueous solution associated with the addition of 0.1 M
NaCl. As depicted in Fig. 2b, the extractabilities of Y(III) exhibited an in-
creasing trend with increasing IL concentration for both fabricated ILs.
For [N444Bn][MA], the extraction efficiency of Y(III) reached approxi-
mately 95% when the extractant concentration reached 0.035 M. In
the case of [N444Bn]2[DDA], a higher extraction efficiency (i.e., 98%)
was realized with a much lower IL concentration (i.e., 0.018 M),
½Mꢀi−½Mꢀt
E ¼
D ¼
ꢁ 100%
ð1Þ
ð2Þ
½Mꢀi
½Mꢀi−½Mꢀt
½Mꢀt
½Mꢀaq
S ¼
β ¼
ꢁ 100%
ð3Þ
ð4Þ
½MꢀILs
D1
D2
where [M]i and [M]t represent the initial and final concentrations of
REEs in the aqueous phase, D1 and D2 are the distribution ratios of REE
3

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
Fig. 1. The effects of (a) time and aqueous (b) pH on the extraction efficiency of Y(III). Organic phase: 0.018 M [N444Bn]2[DDA] or [N444Bn][MA]; aqueous phase: 0.007 M YCl3, 0.1 M NaCl;
O/A = 1, T = 303 K; (a) time = 1− 20 min, pH = 4; (b) pH =1.5− 5, time = 30 min.
demonstrating the superior extraction performance of Y(III) with
[N444Bn]2[DDA] compared with [N444Bn][MA] at the same extractant
concentrations.
gradually increased with increasing Y(III) concentration. The saturation
loading capacities of Y(III) for [N444Bn]2[DDA] were much higher than
that those for [N444Bn][MA] (0.433 vs. 0.172 mol·mol−1, the mole ratio
of Y(III) to ILs), which could be attributed to the fact that there were di-
carboxylic acid groups in DDA and accordingly more carboxylates in
[DDA]− compared with [MA]−. More importantly, the large electroneg-
ativity derived from the carboxylate group obviously promotes the co-
ordination of the C_O bond with Y(III), ultimately giving rise to the
higher loading capacity towards Y(III) [35]. Benzyl groups could usually
act as weak electron donors due to the inductive effect. Therefore, the
3.3. Determination of loading capacity
The loading capacities of [N444Bn][MA] and [N444Bn]2[DDA] were
tested with different initial concentrations of Y(III) in feed solutions
ranging from 0.004 to 0.02 M. The concentration of the extractants
and the aqueous phase pH were 0.018 M and 4.0, respectively. As illus-
trated in Fig. 3a, the extraction efficiencies of Y(III) gradually decreased
when the initial concentrations of Y (III) in the feed solution were in-
creased. At an initial Y(III) concentration of 0.0046 M in the aqueous
electronegativity of [N444Bn]
+ in our as-prepared ILs became stronger
after the alkyl chains of the quaternary ammonium cation were re-
placed by benzyl groups, also contributing to proton dissociation and
enabling cations to extract REEs.
phase, approximately 51.15% of the Y(III) was extracted by [N444Bn
]
[MA], whereas ~99% of the Y(III) was extracted by [N444Bn]2[DDA]. Be-
cause the IL extractant was excessive, all the Y(III) in the aqueous
phase could be extracted from the organic phase with a low Y(III) con-
centration (0.0046–0.0067 M). In the high Y(III) concentration region,
the IL quantities were insufficient to coordinate with the Y(III) ions,
leading to a sharp decreasing trend. In addition, Fig. 3b showed that
the loading capacities of Y(III) in [N444Bn][MA] and [N444Bn]2[DDA]
3.4. Extraction mechanism
Understanding the extraction mechanism of IL is beneficial for de-
veloping more efficient IL extractants for the extraction and separation
of REEs. Two possible mechanisms for REEs extraction with ILs are
termed the ion exchange mechanism and the neutral mechanism
Fig. 2. The effects of (a) NaCl concentration and (b) extractant concentration on the extraction efficiency of Y(III). Organic phase: (a) 0.018 M [N444Bn]2[DDA] and [N444Bn][MA],
(b) 0.005− 0.025 M [N444Bn]2[DDA] and 0.005− 0.045 M [N444Bn][MA]; aqueous phase: 0.007 M YCl3, 0.1 M NaCl, pH = 4, O/A = 1; T = 303 K, time = 30 min.
4

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
Fig. 3. The effects of initial Y(III) concentration on the (a) extraction efficiency and (b) loading capacity of [N444Bn]2[DDA] and [N444Bn][MA]. Organic phase: 0.018 M [N444Bn]2[DDA] or
[N444Bn][MA]; aqueous phase: 0.004− 0.021 M YCl3, 0.1 M NaCl, pH = 4, O/A = 1; T = 303 K, time = 30 min.
The subscripts (org) and (aq) indicate the organic and aqueous
phases, respectively. The distribution ratio (D) and equilibrium constant
(Kex) are defined as Eqs. (6) and (7).
½ILsꢀxYCly
D ¼
ð6Þ
ð7Þ
3þ
Y
ðaqÞ
½ILsꢀxYCly
Kex
¼
Y
½ILsꢀðorgÞ½Cl−ꢀðyaqÞ
x
3þ
ðaqÞ
Applying Eq. (6) to Eq. (7) yields Eq. (8)
D
Kex
¼
ð8Þ
ð9Þ
½ILsꢀðorgÞ½Cl−ꢀðyaqÞ
x
Fig. 4. FTIR analysis of the organic phase. (a) [N444Bn][MA]; (b) [N444Bn][MA] loaded with Y
(III), (c) [N444Bn]2[DDA]; (d) [N444Bn]2[DDA] loaded with Y(III).
Then, logD can be concluded by Eq. (9)
logD ¼ log Kex þ x log ½ILsꢀðorgÞ þ y log Clð−aqÞ
As shown in Fig. 5a, the linear relationship between logD and log
[N444Bn][MA]couldbeobtainedwithaslopeof1.6intheextractantconcen-
trationregionrangingfrom0.008to0.021M, indicatingthatthestoichiom-
etryof[N444Bn][MA]withY(III)wasassumedtobeapproximately1.5:1.For
[N444Bn]2[DDA] (Fig. 5b), the plot of logD as a function of log[N444Bn]2[DDA]
resulted in a straight line with a slope of 2.02 when the extractant concen-
trations were changed from 0.005 to 0.013 M, also confirming that the stoi-
chiometryof[N444Bn]2[DDA]withY(III)is2:1duringtheextractionprocess.
In addition, three weakly coordinating rare-earth chloride ions were in-
volved in the rare-earth-containing complex according to the electroneu-
trality principle. Therefore, the extraction mechanism of Y(III) by ILs
could be represented by the following equations:
[24,36]. In this work, the extraction mechanism for this kind of IL was
determined by FT-IR characterization and slope analysis. The FT-IR spec-
tra of [N444Bn][MA], [N444Bn][MA] loaded with Y(III), [N444Bn]2[DDA] and
[N444Bn]2[DDA] loaded with Y(III) were displayed in Fig. 4. The compar-
ison showed that the peak of the C_O stretching vibration at
1638 cm−1 obviously increased to 1664 cm−1 when [N444Bn][MA] was
loaded with Y(III), demonstrating the interactions between the Y(III)
and C_O groups of [N444Bn][MA] [32,37]. Similar results could also be
obtained with [N444Bn]2[DDA] because the C_O stretching vibration at
1637 cm−1 in [N444Bn]2[DDA] increased to 1650 cm−1 in loaded
[N444Bn]2[DDA] [38]. Furthermore, the C−N stretching vibration at
1389 cm−1 in pristine [N444Bn]2[DDA] downshifted to 1373 cm−1 after
loading with Y(III), indicating that more electronegative [N444Bn]
+ par-
Y
Y
þ 1:5½N444Bnꢀ½MAꢀðorgÞ þ 3Cl−ðaqÞ ¼ ½N444Bn
ꢀ
1:5Y½MAꢀ1:5Cl3ðorgÞ
ð10Þ
ð11Þ
3þ
ticipates in this extraction [39]. Therefore, the ion association mecha-
nism could be used to elucidate the extraction process of Y(III) with
these carboxylate-functionalized ILs.
To further investigate the extraction mechanism, the complex stoi-
chiometries were determined using the slope analysis method [25,38].
The extraction equilibrium between Y(III), chloride ions, and ILs may
be represented by Eq. (5).
ðaqÞ
þ 2½N444Bnꢀ2½DDAꢀðorgÞ þ 3Cl−ðaqÞ ¼ ½N444Bnꢀ4Y½DDAꢀ2Cl3ðorgÞ
3þ
ðaqÞ
3.5. Determination of thermodynamic parameters
The effects of temperature on the extraction efficiencies of [N444Bn
]
Kex
3þ
[MA] and [N444Bn]2[DDA] for Y(III) from the aqueous phase were evalu-
ated in the range of 303–343 K. As shown in Fig. 6a, the results revealed
Y
þ x½ILsꢀðorgÞ þ yCl−ðaqÞ ⇔ ½ILsꢀxYCly
ð5Þ
ðaqÞ
5

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
Fig. 5. Plot of logD as a function of log[N444Bn][MA] (a) and log[N444Bn]2[DDA] (b). Organic phase: (a) 0.005− 0.013 M [N444Bn]2[DDA], (b) 0.008− 0.021 M [N444Bn][MA]; aqueous phase:
0.007 M YCl3, 0.1 M NaCl, pH = 4, O/A = 1; T = 303 K, time = 30 min.
that both the extraction efficiencies of Y(III) for [N444Bn]2[DDA] and
[N444Bn][MA] decreased with increasing temperature, mainly be-
cause high temperatures increased the motion of molecules and
thereby decreased the stability of the formed complex involved in
Y(III) and ILs [31].
The enthalpy change (ΔH) during the extraction could be deter-
mined based on the slope of the plot of logD versus 1000/T using the
van't Hoff Eq. (12):
Table 1
Thermodynamic parameters of Y(III) extractions in chloride medium (T = 303 K,
time = 30 min, pH = 4).
ILs
ΔH (kJ·mol−1
)
ΔG (kJ·mol−1
)
ΔS (J·mol−1·K−1
)
[N444Bn]2[DDA]
[N444Bn][MA]
−5.73
−2.84
−55.37
−45.67
163.83
141.35
K
ex istheextractionequilibriumconstantandiscalculatedbyplotting
logD versus log[ILs] (Fig. 5). The change in Gibbs free energy was
−55.37 kJ·mol−1 and -45.67 kJ·mol−1 for [N444Bn]2[DDA] and [N444Bn
ΔH
2:303RT
logD ¼ −
þ C
ð12Þ
]
[MA], respectively. The negative values of Gibbs free energy confirmed
the feasibility and favorable nature of the extraction reactions. The
change in entropy (ΔS) at a particular temperature was calculated
using Eq. (14) as follows:
where R is the universal gas constant, and C is the constant. As shown in
Fig. 6b, plotting logD versus 1000/T resulted in a straight line with a
slope of 0.2993 for [N444Bn]2[DDA] and of 0.1485 for [N444Bn][MA]. As
presented in Table 1, the enthalpy changes were − 5.73 kJ·mol−1 and
-2.84 kJ·mol−1 for [N444Bn]2[DDA] and [N444Bn][MA], respectively,
showing that the extraction process of Y(III) involved an exothermic re-
action. The change in Gibbs free energy (ΔG) was calculated using
Eq. (13) as follows:
ΔG ¼ ΔH−ΔST
ð14Þ
The values of ΔS were determined to be 163.83 J·mol−1·K−1 and
141.35 J·mol−1·K−1, indicating that the extraction system was disor-
dered in nature [31,40].
ΔG ¼ −RTlnKex
ð13Þ
Fig. 6. (a) Effect of temperature on the extraction of Y(III) and (b) Van't Hoff plot correlating the distribution ratio with the extraction temperature. Organic phase: 0.018 M [N444Bn]2[DDA]
and [N444Bn][MA]; aqueous phase: 0.007 M YCl3, 0.1 M NaCl, pH = 4, O/A = 1; T = 303 K, time = 30 min.
6

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
distribution coefficients of La(III), Nd(III), Eu(III), Ho(III), Yb(III), and Y
(III) were obtained. The initial concentration in the aqueous phase
was approximately 0.001 M for each metal ion, and the aqueous pH
value was fixed at 3. Considering that the loading capacity of [N444Bn 2
]
[DDA] was approximately twice that of [N444Bn][MA], the concentra-
tions of [N444Bn][MA] and [N444Bn]2[DDA] were 0.037 M and 0.018 M, re-
spectively, to discern and compare their extractability more easily. As
shown in Fig. 8a, the order of the extraction efficiency and distribution
coefficient was La(III) < Y(III) < Nd(III) < Ho(III) < Eu(III) < Yb(III)
for [N444Bn][MA], which is the inverse order of the ionic radius of lantha-
nides because lanthanides with decreasing radius coordinate more
strongly with [N444Bn][MA] [27,41]. In the case of [N444Bn]2[DDA], a sim-
ilar order of mixed REEs extractions could be obtained, except for Eu
(III). In addition, selectivity is another important factor for evaluating
the extraction performance and can be described as the separation fac-
tor (β). From Fig. 8b, the calculated β values of the REEs pairs in [N444Bn
]
Fig. 7. Stripping of the loaded [N444Bn]2[DDA] and [N444Bn][MA] with different
concentrations of hydrochloric acid. [HCl] = 0– 1.0 M; T = 303 K, time = 30 min.
[MA] were almost always higher than those in [N444Bn]2[DDA]. For ex-
ample, the β values of La/Y were 5.13 and 1.65 after separation by
[N444Bn][MA] and [N444Bn]2[DDA], respectively. Similar results could
also be obtained from the β values of La/Eu (14.67 for [N444Bn][MA] vs.
4.12 for [N444Bn]2[DDA]) and La/Yb (16.21 for [N444Bn][MA] vs. 3.30 for
[N444Bn]2[DDA]), indicating that [N444Bn][MA] exhibited better selectiv-
ity for REEs separation than [N444Bn]2[DDA] and thereby could be used
in further recycling rare-earth metals from spent magnets.
3.6. Stripping properties
The stripping performance is a significant indicator for assessing the
recovery of REEs ions from the organic phase in industrial-scale applica-
tions [28]. The stripping properties of Y(III) were studied using hydro-
chloric acid at 0–1.0 M as the regenerant because the extraction
efficiency was highly pH dependent, as indicated in the aforementioned
4. Conclusions
discussions. Before the stripping test, 0.018 M [N444Bn][MA] and [N444Bn
]
Insummary, newkindsofcarboxylic acidbasedILswithtailoredelec-
tronegativity were synthesized and tested as high-performance
extractants for REEs from a chloride medium. The results indicated that
[N444Bn]2[DDA] and [N444Bn][MA] both efficiently extract Y(III) at the
low acidityassociated with salting-out agentNaCl, avoidingtheenviron-
mentalissuesproducedbyconventionalacidic andneutralextractants.A
linear regression analysis of the plot of logD as a function of log[ILs] re-
vealed the formation of the ~1:1.5 or 1:2 metal:ligand complexes of
REEs and ILs. In addition, the ion association extraction mechanism was
proposedtodescribetheextractionbehavioraccordingtotheslopeanal-
ysis method and FT-IR characterizations. The enthalpy change value, ΔH,
2[DDA] were applied to extract 0.0031 and 0.007 M Y(III) from solution,
respectively. As shown in Fig. 7, the Y(III) ions were obviously stripped
from the loaded [N444Bn][MA] because of the increase in acidity and
were almost completely stripped off through the point where the HCl
concentration reached 0.04 M. However, the stripping percentage of Y
(III) for [N444Bn]2[DDA] was still lower than that for [N444Bn][MA],
mainly because more H+ was expended to satisfy the large number of
carboxylate-functionalized groups in the IL anions. Fortunately, approx-
imately 83.82%, 93.75% and 96.56% of Y(III) could be back-extracted
from the loaded [N444Bn]2[DDA] with HCl concentrations of 0.1, 0.6
and 1.0 M, respectively, exhibiting excellent regeneration performance.
over the temperature range 303–343 K for [N444Bn]2[DDA] and [N444Bn
]
[MA] were calculated to be −5.73 and − 2.84 kJ·mol−1, respectively,
demonstrating that the extraction of Y(III) with the as-prepared ILs was
an exothermic reaction. Furthermore, the back-extraction experiments
indicated that [N444Bn][MA] exhibited better stripping properties than
[N444Bn]2[DDA], and Y(III) extracted with [N444Bn][MA] can be almost
3.7. Separation of mixed REEs
To investigate the separation ability of [N444Bn][MA] and [N444Bn 2
]
[DDA] in mixed REEs solutions, the extraction efficiencies and
Fig. 8. (a) Distribution ratio, extraction efficiency and (b) separation factor of REEs extracted by [N444Bn][MA] [N444Bn]2[DDA] at pH 3. Organic phase: 0.018 M [N444Bn][MA] or 0.037 M
[N444Bn]2[DDA]; aqueous phase: mixed REEs solution of 0.0012 M LaCl3, 0.0015 M NdCl3, 0.0014 M HoCl3, 0.0012 M EuCl3, 0.0012 M YbCl3 and 0.0014 M YCl3, 0.1 M NaCl, pH = 3,
O/A = 1; T = 303 K, time = 30 min.
7

Y. Deng, Y. Ding, Z. Huang et al.
Journal of Molecular Liquids 329 (2021) 115549
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Author contribution statement
Yu Deng: Investigation, Data curation, Formal analysis, Writing -
Original Draft.
Yigang Ding:Conceptualization, Supervision.
Zhong Huang: Methodology.
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Ying Yu: Investigation.
Jun He: Project administration.
Yi Zhang:Conceptualization, Writing - Review & Editing, Funding
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
[25] Y. Dong, X. Sun, Y. Wang, C. Huang, Z. Zhao, The sustainable and efficient ionic
liquid-type saponification strategy for rare earth separation processing, ACS Sustain.
Chem. Eng. 4 (2016) 1573–1580.
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This work was finically supported by the National Natural Science
Foundation of China (Grants No. 51804223) and the Scientific Research
Foundation of Wuhan Institute of Technology (No. K201761).
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8