Tuning the extraction mechanism of uranyl ion in bicyclooctanium, propylpyridinium, piperidinium and imidazolium based ionic liquids: First ever evidence of 'cation exchange', 'anion exchange' and 'solvation' mechanism

Article abstract of DOI:10.1016/j.molliq.2021.116435

A novel diamide ligand, 5-bromo-N1, N3-diisopropylisophthalamide has been synthesized and characterized by FTIR, ¹H NMR, and ¹³C NMR. The extraction mechanism of uranyl ion with the diamide ligand has been studied in unexplored ionic

Full text of DOI:10.1016/j.molliq.2021.116435

Journal of Molecular Liquids 337 (2021) 116435
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Tuning the extraction mechanism of uranyl ion in bicyclooctanium,
propylpyridinium, piperidinium and imidazolium based ionic liquids:
First ever evidence of ’cation exchange’, ’anion exchange’ and ’solvation’
mechanism
Amit Pandey a,1, S. Hashmi b,1, G. Salunkhe b, Velavan Kathirvelu a, Keisham S. Singh c, Rohit Singh Chauhan b,
Arijit Sengupta d,e,
⇑
a Department of Applied Sciences, National Institute of Technology Goa, Ponda, Goa 403401, India
b Department of Chemistry, K.J.Somaiya College of Science and Commerce, Vidya-vihar, Mumbai 400077, India
c Bioorganic Chemistry Laboratory, CSIR-National Institute of Oceanography, Dona Paula, Goa 403004, India
d Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
e Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel diamide ligand, 5-bromo-N1, N3-diisopropylisophthalamide has been synthesized and character-
ized by FTIR, 1H NMR, and 13C NMR. The extraction mechanism of uranyl ion with the diamide ligand has
been studied in unexplored ionic liquid families. For the same metal–ligand system; cation exchange,
anion exchange, and solvation mechanism can be predominately achieved by changing the ionic liquid
system. The experimental results suggest that the change in extraction mechanism is induced by the
cations of the ionic liquids. In bicyclooctanium ionic liquid, the extracted species was [UO2(NO3)3L2]-;
In piperidinium and pyridinium-based ionic liquid, the species were UO2(NO3)2L and UO2(NO3)2L2
respectively. In imidazolium-based ionic liquid, the species was [UO2(NO3)L]+. Though the extraction effi-
ciency in methylpyridinium and bicyclooctanium based ionic liquids are better, the pipiredinium and
imidazolium-based ionic liquids exhibited a high degree of radiological stability. The aqueous solution
of sodium carbonate is found to be better in the back extraction of uranyl ions from the ionic liquids rel-
ative to EDTA and oxalic acid. However, the efficacy of back extraction differs largely on the ionic liquid
system.
Received 4 April 2021
Revised 2 May 2021
Accepted 5 May 2021
Available online 8 May 2021
Keywords:
Cation exchange
Anion exchange
Solvation
Speciation
Ionic liquid
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
zolium cation, the extraction mechanism depends on the length
of the alkyl group. If the butyl group at the N3 position is replaced
Though ionic liquid based research evolved mainly as a poten-
tial ’green’ alternatives to the volatile organic diluents due to some
of the advantageous properties like lower vapor pressure, high
flash point, high degree of chemical, thermal and electrochemical
stability; the high ’anti microbial’ characteristic of some of the
ionic liquids put doubts on the ’greenness’ [1–6]. Some classes of
ionic liquids are neither biodegradable nor completely incinerable.
However, the most interesting property of ionic liquid is its
tunability due to which it is widely accepted as a ’designer solvent’.
It has been reported that for N3 alkyl-substituted methylimida-
with an octyl or decyl, the extraction mechanism changes from
cation exchange to solvation [7–9].This was attributed to the ease
of solubilizing ability of N3 alkyl-substituted methyl imidazolium
cation in water, which reduces as the alkyl chain length increases.
This also indicates that the species involved can also be modified
from cationic to neutral. It was also reported in the literature that
the extraction kinetics are highly dependent on the length of the
alkyl groups of N3 alkyl-substituted methylimidazolium ion and
even the nature of the anionic moieties of the ionic liquid [10–
14]. The longer the alkyl chain length, viscous the ionic liquids
are, and hence slower is the mass transfer. The ionic liquid with
PFꢀ6 anion is more viscous compared to NTf2ꢀ anion and hence
resulting in slower extraction kinetics. The more branched ionic
liquids are more prone to radiation-induced fragmentation
[15,16]. Hence depending upon the requirement, the extraction
⇑
Corresponding author at: Radiochemistry Division, Bhabha Atomic Research
Centre, Trombay, Mumbai 400085, India.
E-mail address: arijita@barc.gov.in (A. Sengupta).
These authors contributed equally.
1
https://doi.org/10.1016/j.molliq.2021.116435
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
process can be modified by modification in ionic liquids [17]. The
extraction efficiency also depends on the nature of alkyl substitu-
tion in the imidazolium cations of ionic liquid. There are several
attempts to design the functionalized ionic liquid for task-
specific applications, where functionalities are covalently attached
to the ionic liquid moieties. Carbomyl methyl phosphine oxide
(CMPO), diglycolamide (DGA), trialkyl phosphine (TRPO) function-
alized ionic liquids have been extensively used for the extraction of
actinides [18,19]. The extraction behavior of neodymium and other
rare earth were investigated by trioctylmethylammonium dioctyl
diglycolamate functionalized ionic liquid [20]. Many amino-
functionalized ionic liquids have been exploited for CO2 capture
[21]. The homogeneous liquid–liquid extraction of metals by func-
tionalized ionic liquid is another new approach reported in the lit-
erature [22].
Application of ionic liquid not only improves the efficiency/
selectivity of metal extraction; but also the speciation may differ
[23,24]. Vast research on the application of ionic liquids in the sep-
aration of metal ions, mainly f-block elements is centered at
imidazolium-based ionic liquids only. Recently, it has been demon-
strated for the first time that a sulphoxide ligand in bicycloocta-
nium ionic liquid can be used for separation of f-block elements
[25]. However, different families of ionic liquids (pyridinium,
piperidinium, phosphonium, octanium, etc.) are still unexplored
and they may show some unique features. One such example is
the extraction of Sr2+ by Calix crown ether using pyridinium-
based ionic liquid. The extraction of Sr2+ predominantly follows
the least common ’anion exchange’ mechanism through a monoan-
ionic complex [26]. In view of that, pyridinium, piperidinium, and
bicyclooctanium families of ionic liquids are considered in the pre-
sent case and are compared with the most commonly used
methylimidazolium-based ionic liquid.
The success of nuclear establishment highly depends on the safe
management of radiotoxic material; efficient and selective separa-
tion scheme for actinides, lanthanides, and other fission products
[27–29]. The tri-n-butyl phosphate is the conventional workhorse
for efficient separation schemes in PUREX and THOREX processes
in nuclear establishment [30,31]. Phosphine oxides, phosphonates,
Cyanex 923, phosphinic acids, carbamoyl methyl phosphine oxide
are some of the ligands explored for uranyl extraction [18,32–35].
However, phosphate-based ligands are not completely incinerable,
hence there is always a quest for new nitrogen-based ligand func-
tionalities for the separation of uranyl ions. Secondary amides like
di-n-hexyl octanamide (DHOA), diamide, glycolamides are some of
the functionalities explored for efficient uranyl extraction [36–40].
In the present case, diisopropylisophthalamide in pyridinium,
piperidinium, imidazolium, and bicyclooctanium based ionic liq-
uids have been used for the extraction of UO22+ from the aqueous
nitric acid feed. The investigation deals with the understanding
of the extraction mechanism, species involved, etc. The radiation
and chemical stability of organic phases, stripping of metal ions
from organic phases have also been investigated.
used for the synthesis of ligand were freshly distilled before use.
Nitric acid (Suprapure grade) was purchased from Sigma Aldrich.
The reagents like NaNO3, N2CO3, n-dodecane, and oxalic acid, were
of analytical grade purchased from Sigma Aldrich. Deionized water
was used throughout the experiment. The U stock solutions were
prepared from U3O8 powder by dissolving in Conc. HNO3 followed
by repeated evaporation. The stock solutions were made in 1 M
HNO3.
NMR spectra were recorded on a Bruker Avance 400 MHz spec-
trometer at 400 MHz (1H) and 100 MHz (13C) with SiMe4 as inter-
nal references. The IR spectrum was recorded on IR Affinity-1,
Fourier Transform Infrared Spectrophotometer (Shimadzu). CHNS
analysis of the ligand was carried out in Elementar vario MICRO
cube CHNS Analyzer. The powder sample (3 mg) wrapped in a
small tin foil was directly dropped into the combustion tube where
it was fully combusted in the presence of oxygen. The MICRO cube
analyzed the CHN content (weight %) of the ligand in one single
run. Energy Dispersive X-ray Fluorescence (EDXRF) Spectroscopy
(Make: Xenemetrix, Model: EX3600SDD) was used for the estima-
tion of U [41–43].
2.2. Synthesis and characterization of ligands
2.2.1. 5-bromo-N1, N3-diisopropylisophthalamide (L)
Thionyl chloride (40 mmol, 2.90 mL) and 5-bromoisophthalic
acid (2 mmol, 0.490 g) were taken in a round bottom flask and
refluxed under nitrogen atmosphere for 16 hr ( see Scheme 1). A
clear transparent solution was formed. Excess thionyl chloride
was removed in a vacuum to give a colorless viscous liquid, 5-
bromoisophthaloyl dichloride [9,38]. The acid chloride was then
dissolved in dry dichloromethane (10 mL) and cooled to 0 °C. A
solution of isopropylamine (8.0 mmol, 0.655 mL) and triethy-
lamine (4.0 mmol, 0.56 mL) in dry dichloromethane (5 mL) was
added in the acid chloride solution drop by drop. The resulting
mixture was kept stirring at room temperature under a nitrogen
atmosphere for 16 hr [44]. The reaction mixture was then dried
over a vacuum. The residue was dissolved in dichloromethane
and washed with aqueous ammonium chloride solution followed
by water. The organic layer was dried with anhydrous Na2SO4
and concentrated to dryness yielded ligand (L). Purification by sil-
ica gel column chromatography with petroleum ether and ethyl
acetate (0 to 30%) mixture as eluent afforded 0.536 g (82%) of white
solid (L), Rf = 0.28, TLC (eluent: petroleum ether/ethyl acetate
30:70 v/v). The 1H NMR, 13C NMR, and IR spectra of the ligand
are shown in Figs. 2–4 respectively.
2.2.2. Analytical and spectroscopic data
CHNS analysis: Anal. Calcd: C 51.39; H 5.85; N 8.56. Found: C
51.62; H 5.85; N 8.50; FTIR (KBr, cmꢀ1): 3251 (s, NAH str. amide),
3078 (CAH str. aromatic), 2973–2874 (s, CAH str. alkyl), 1632 (s,
C@O str. amide), 1551 (s, NAH bend amide) 1591–1462 (C@C str.
aromatic), 894–714 (CAH bend aromatic); 1H NMR (400 MHz,
CDCl3, d): 8.027 (s, 1H, ArH), 7.964 (s, 2H, 2 ꢁ ArH), 7.26 (residual
solvent peak), 6.206–6.190 (d, 2H, 2 ꢁ NH amide), 4.300–4.249 (m,
2. Experimental
2H, 2 ꢁ CH isopropyl), 1.285–1.269 (d, 12H, 4 ꢁ CH3 isopropyl). 13
C
NMR (100 MHz. CDCl3, d): 164.52, 136.96, 132.63, 123.84, 122.97,
2.1. Materials and methods
42.40, 22.73.
5-Bromoisophthalic acid, isopropylamine, triethylamine and
the ionic liquids (Fig. 1); 1-Hexyl-1,4-diaza[2,2,2]bicyclooctanium
bis(trifluoromethylsulfonyl)imide (IL1), 1-Propylpyridinium bis(tri
fluoromethylsulfonyl)imide (IL2), 1-Butyl-1methylpiperidinium
2.3. Extraction
The extraction profile of uranyl is obtained by measuring DU as
a function of aqueous feed acidity in various ionic liquid systems at
a constant ligand concentration (1 mM) and temperature (300 K).
An equivalent volume of the organic (IL) and aqueous solutions
were used in the extraction process. The equilibration time and
bis(trifluoromethylsulfonyl)imide
(IL3),
1-Hexyl-
3methylimidazolium bis(trifluoromethylsulfonyl)imide (IL4) were
procured from TCI Chemical Pvt Ltd., India. Dichloromethane and
thionyl chloride were purchased from SRL Chemicals. The solvents
2

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
Fig. 1. The structures of the ionic liquids used in the present investigation.
Scheme 1. (a) Synthesis of 5-bromoisophthloyl dichloride; (b) Synthesis of 5-bromo-N1, N3-diisopropylisophthalamide (L).
concentration was varied in water to investigate their involvement
the time of centrifugation were 180 min and 5 min, respectively.
The Distribution ratio was calculated as follows [45]
in the extraction of uranyl ion by the ligand. Stripping studies were
carried out in two steps. In the first step, the extraction process was
done. while in the subsequent step, the stripping was carried out
using sodium carbonate, oxalic acid, and ethylenediaminete-
traacetic acid (EDTA). Several consecutive contacts of the stripping
agent were made for achieving cumulative back extraction of locked
up uranyl from the ionic liquid phase. The % stripping was calcu-
lated as follows [46]
½MꢂIL
DU
¼
ð1Þ
½Mꢂaq
Where ½MꢂIL, and ½Mꢂaq are the analytical concentrations of uranyl in
ionic liquid and aqueous phase. The dependence of the DU as a func-
tion of ligand concentration was investigated by keeping the acidity
of the feed solution as a constant (1 M HNO3). For nitrate ion vari-
ation, aqueous phase NaNO3 concentration was varied keeping
other parameters constant. LiNTf2 salt and bromide salts of the cor-
responding ionic liquid cations are water-soluble and hence their
½Mꢂ
%Stripping ¼
aq x100%
ð2Þ
½MꢂIL
3

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
Fig. 2. 1H NMR Spectra of 5-bromo-N1, N3-diisopropylisophthalamide.
Fig. 3. 13C NMR Spectra of 5-bromo-N1, N3-diisopropylisophthalamide.
4

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
Fig. 4. FTIR Spectra of 5-bromo-N1, N3-diisopropylisophthalamide.
For the radiation stability study, the solvent systems were irradi-
ated with various gamma radiation doses using 60Co as a source.
With the irradiated solvent systems, the D values were calculated
and the deviation was monitored.
3. Results and discussion
3.1. Extraction profile
Extraction of uranyl ion has been carried out with a 1 mM con-
centration of ligand. The extraction profile as a function of aqueous
feed acidity is shown in Fig. 5. In IL1, the DU increases up to 2 M
HNO3 reaching a maximum value of ~12, and remains almost con-
stant between 2 M and 4 M HNO3 followed by a marginal decrease.
The decrease in DU value at higher acidity is not very significant. In
the case of IL2, the DU value increases rapidly up to 1 M HNO3 fol-
lowed by a marginal increase from 1 M to 2 M HNO3 and remained
unchanged beyond 2 M HNO3. In the plateau region, the DU value
was ~20. In IL3 based system, the trend was almost similar. The
enhancement in DU value was gradual up to 4 M HNO3, followed
by a plateau. The maximum DU value obtained in this case was
~13. Interestingly, using IL4 as a solvent system, the trend in DU
vs aqueous feed acidity is unique compared to other ionic liquid
systems, i.e. DU value was maximum (~8) at 1 M HNO3 and
decreases with an increase in feed acidity. This diluent-induced
difference in extraction profiles is not very common in molecular
diluents. However, in ionic liquid, structural modification can bring
changes in extraction properties. The above fact was verified and
well-reported for imidazolium-based ionic liquids [47,48]. In the
case of methylimidazolium-based ionic liquid, it was demon-
strated that if the alkyl group at the 3rd position is butyl, then
there is a chance of cation exchange mechanism, where, the butyl
imidazolium cation is getting exchanged with the cationic metal–
ligand complex. However, for higher homologue, i.e. C8mim+ or
Fig. 5. The variation in DU value as a function of aqueous feed acid concentration
using 1 mM ligand in different ionic liquid.
complex. In the present case, it is quite interesting to study ionic
liquids that have diverse cationic motifs. There are only a few
reports that describe the extraction of metal ions in ionic liquids
through the anion-exchange mechanism. One such example is
the extraction of uranyl ion by tri-n-butyl phosphate in ionic liquid
from higher aqueous feed acidity [49]. Recently anion exchange
mechanism was also reported for Sr2+ extraction by Calix crown
ether using pyridinium-based ionic liquid [26]. The increasing
trend in DU with an increase in aqueous feed acidity is an indica-
tion of either the solvation mechanism or anion exchange mecha-
nism. The initial enhancement in DU with aqueous feed acidity is
ascribed to the participation of nitrate ions. However, at higher
feed acidity, plenty of H+ ions are available and they compete with
uranyl ion resulting in either plateau or marginal reduction in DU
C
10mim+ ion, water solubility is very less and hence, the ion
exchange mechanism is prohibited and the extraction proceeds
through a solvation mechanism involving neutral metal–ligand
5

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
values. In the case of ionic liquid IL4, the extraction profile indi-
cates the extraction might proceed through the cation exchange
mechanism.
Solvation mechanism:
UO22þ þ mLIL þ 2NOꢀ ! UO2ðNO3Þ :mL
ð3Þ
ð4Þ
ð5Þ
3
2
aq
aq
IL
Cation exchange mechanism:
UO22þ þ mLIL þ nNOꢀ þ ð2 ꢀ nÞILþIL
3
aq
aq
2ꢀn
! ½UO2ðNO3Þn:mLꢂIL þ ð2 ꢀ nÞILaþq
Anion exchange mechanism:
UO22þ þ mLIL þ nNOꢀ þ ðn ꢀ 2ÞILꢀIL
3
aq
aq
nꢀ2
! ½UO2ðNO3Þn:mLꢂIL þ ðn ꢀ 2ÞILIꢀL
Where, m and n are the number of ligand molecules and nitrate ions
associated with each uranyl ion in the formation of the complex. IL+
represents the cationic part of the ionic liquid, while IL- is the anio-
nic part of the ionic liquid.
3.2. Speciation and extraction mechanism
The equilibrium constant for the uranyl extraction can be
defined as follows for solvation mechanism
½UO2ðNO3Þ2:mLILꢂ
h
i
Kex
¼
ð6Þ
m
2
UO22þ ½LILꢂ ½NOꢀaqꢂ
3
aq
DU
Kex
¼
ð7Þ
m
2
½LILꢂ ½NOꢀaqꢂ
3
Taking logarithm on both sides and rearranging, the equation takes
the form
LogDU ¼ LogKex þ mLog½LILꢂ þ 2Log½NOꢀaqꢂ
ð8Þ
3
Similarly, for the cation exchange mechanism, the equilibrium con-
stant can be written as
Fig. 6. The variation of DU values as a function of (a) ligand concentration in ionic
liquid phase; (b) nitrate ion concentration in the aqueous phase.
2ꢀn
½ILþaqꢂ
2ꢀn
½½UO2ðNO3Þn:mLꢂIL
ꢂ
n
h
i
Kex
¼
ð9Þ
2ꢀn
½ILþILꢂ
m
UO22þ ½LILꢂ ½NOꢀaqꢂ
3
ilarly, a plot of Log(DU) value as a function of Log([NOꢀ]) would be a
aq
3
straight line with a slope value, which determines the number of
nitrate ions associated with each metal ion in the complex. Fig. 6
(a) shows the variation in DU value as a function of ligand concen-
tration. An increase in DU value with ligand concentration indicates
the direct participation of ligand molecules. For ionic liquids IL1 and
IL2, the slope is evaluated as 2, while it is 1 for ionic liquids IL3 and
IL4. This revealed that UO22+ formed 1:2 complex in ionic liquid IL1
and IL2, whereas 1:1 metal–ligand complex with IL3 and IL4. Fig. 6
(b) represented the variation in DU value as a function of nitrate ion
concentration in the aqueous phase. The slope for IL1 and IL2 were
found to be 3 and 2, respectively. However, for IL3 and IL4, the slope
is evaluated as 2 and 1, respectively. These results indicate that in
IL1, the species is [UO2(NO3)3L2]-, in IL2 the species is UO2(NO3)2L2,
whereas in IL3 the species is UO2(NO3)2L and in IL4 the species
involved is [UO2(NO3)L]+. This implies that in IL4, the cation
exchange mechanism predominates following cationic species,
and the cationic part of the ionic liquid will be exchanged in place
of the metal–ligand complex during extraction to maintain the elec-
trical neutrality. On the other hand, in IL1, the anion exchange
mechanism predominates involving anionic metal–ligand species.
To maintain the charge neutrality in both phases, an equivalent
amount of ionic liquid anion would get exchanged. However, for
IL2 and IL3, the solvation mechanism is predominant involving a
neutral metal–ligand complex. However, this has to be confirmed
The ratio of the concentration of cationic part of ionic liquid in both
the phase is known as the partition coefficient for that species. The
partition coefficient is a constant for a given temperature and also
considering Kex as the conditional extraction, the equation can be
simplified as follows
2ꢀn
½½UO2ðNO3Þn:mLꢂIL
ꢂ
n
h
i
Kex
¼
ð10Þ
m
UO22þ ½LILꢂ ½NOꢀaqꢂ
3
aq
LogDU ¼ LogKꢀex þ mLog½LILꢂ þ nLog½NOꢀaqꢂ
ð11Þ
3
For anion exchange mechanism the Kex can be written as follows
½½UO2ðNO3Þ :mLꢂðnꢀ2Þ
ꢂ
n
nꢀ2
½ILꢀILꢂ
½ILꢀILꢂ
n
IL
h
i
Kex
¼
ð12Þ
nꢀ2
m
UO22þ ½LILꢂ ½NOꢀaqꢂ
3
aq
Similarly, the partition coefficient for the anionic part of the ionic
liquid is a constant for a given temperature, and hence equation
can be written as
LogDU ¼ LogKex þ mLog½LILꢂ þ nLog½NOꢀaqꢂ
ð13Þ
3
Therefore, a plot of Log(DU) as a function of Log([L]) will be a
straight line with a slope, which determines the metal–ligand stoi-
chiometry, keeping nitrate ion in the aqueous phase constant. Sim-
6

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
functionalized ligand in C8mimNTf2. On a similar note, anion
dependence experiments suggested that, for IL2, IL3, and IL4, the
DU remained unchanged with an increase in the NTfꢀ ion concen-
2
tration in the aqueous phase. However, for IL1, i.e. for bicycloocta-
nium ionic liquid, the DU decreases with an increase in NTfꢀ2 ion
concentration in the aqueous phase with a slope value of ꢀ1. This
result confirms that, for IL2 and IL3, neither cation exchange nor
anion exchange mechanism was predominant. Therefore, the
extraction proceeds through the neutral metal–ligand complex
with a ’solvation’ mechanism. However, for IL1, i.e. bicycloocta-
nium ionic liquid the extraction proceeded via mono negative U-
ligand complex. To maintain the charge neutrality, one NTfꢀ2 anion
would be transferred from the IL phase to the aqueous phase for
the transfer of each U-ligand complex. In the case of IL4, C8mim+
cation got exchanged in place of mono positive U-ligand complex
and the extraction proceeded via cation exchange mechanism. This
particular case is very interesting to demonstrate that by changing
the cations of the ionic liquids, the extraction species and mecha-
nism can be changed from solvation to cation exchange or anion
exchange.
Fig. 8 depicts the most plausible uranyl-ligand complexes in dif-
ferent ionic liquids. In IL2 and IL3, neutral species were formed.
However, the speciation and mode of coordination are quite differ-
ent. In IL2, two diamides, and two nitrate ions were found to be
coordinating in a monodentate fashion, whereas in IL3, one dia-
mide was found to be coordinating through bidentate fashion,
while two nitrates are in monodentate mode. In IL1, the species
was found to be anionic with two ligands and three nitrates all
are in the monodentate mode of coordination. However, in IL4
the species was mono positive with one diamide and one nitrate
both in the bidentate mode of coordination with uranyl to satisfy
the 8 coordination number.
3.3. Radiolytic stability
For long time use of the ligands, the solvent system must be
radiologically stable, since this would experience lots of radiation
coming out of the isotopes processed using the solvent system
[50,51]. To understand the radiolytic stability, the solvent systems
were exposed to gamma rays of different dose rates, and the DU
values were evaluated. The change in DU values in irradiated sol-
vent compared to the unirradiated solvent would pertain to the
radiological damage faced by them. From Fig. 9, it is clear that
for IL3 and IL4, the DU values of unirradiated systems were ~8
and ~4, respectively. On irradiation with gamma rays, no signifi-
cant change in DU values observed. This implies that these two sol-
vent systems were highly radiologically stable. However, IL2
showed a maximum reduction in DU values. In the unirradiated
condition, the DU was 17, which was drastically reduced to 2 at a
dose of 500 kGy. IL1 also showed similar results, i.e. unirradiated
system showed DU of 11, which reduced to ~ 3.8 after a dose of
500 kGy. Therefore, high radiological stability along with the effi-
ciency of the solvent should be considered for processing high
radioactive isotopes. In view of that, IL4 might be the best solvent
among the four ionic liquids.
Fig. 7. The dependence of DU values on the concentration of (a) cations of ionic
liquid and (b) anions of ionic liquids in the aqueous phase.
by further investigation on the dependence of cations or anions of
ionic liquids on the extraction properties of uranyl.
To further confirm the speciation and the extraction mecha-
nism, cation and anion dependence of ionic liquid on DU values
were monitored using the water-soluble salts of the same
[Fig. 7]. For example, in cation dependence experiments, the chlo-
ride salts of the corresponding ionic liquid were taken, while anion
dependence was checked using Li salt of NTfꢀ2 anion. The chloride
salts used in the present case are: 1-Hexyl-1,4-diaza[2,2,2]bicy
clooctanium chloride, 1-Propylpyridinium chloride, 1-Butyl-
1methylpiperidinium chloride, 1-Hexyl-3methylimidazolium chlo-
ride. In cation dependence experiments, the DU values were inde-
pendent of the concentration of ionic liquid cations in the
aqueous phase for IL1, IL2, and IL3, suggesting no cation exchange
mechanism involved. However, the DU value decreases with an
increase in ionic liquid cation concentration in the aqueous phase
for IL4. The concentration dependence of the DU value in IL4 con-
firms the participation of IL cation in the extraction process. More
the initial concentration of cations in water, the equilibrium of the
extraction process shifted more towards the backward direction,
i.e. the extent of uranyl extraction would reduce. The slope of the
linear dependence of DU in IL4 is evaluated as ꢀ1. This implies that
only one C8mim+ ion gets exchanged during each metal–ligand
complex extraction. This also proves that a singly charged metal–
ligand complex formed during extraction of uranyl with amide
3.4. Back extraction
The reusability of the solvent system is another important
aspect to be looked into. The expectation is to have solvent systems
having high extraction efficiency as well as can be regenerated very
easily after extraction. In view of this, stripping behavior needs to
be investigated. It is well-known fact that only dilute acid might
not be sufficient to back extract the complexed uranium from ionic
liquid. Several methodologies including electro-deposition have
7

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
Fig. 8. The U complexes with the diamides in different ionic liquids involved in the separation.
Fig. 9. The radiolytic stability of the solvent systems under the influence of gamma-
ray exposure.
been employed for back extraction [52]. However, another well-
accepted approach is to make use of the complexing agent in water
[53]. The idea is to make a stronger complex of uranyl in water so
that the uranium-amide complex in ionic liquid breaks up. Based
on the literature, 1 mM aqueous solution of oxalic acid, sodium
carbonate, and ethylenediaminetetraacetic acid have been chosen
as aqueous complexing agents for back extraction. Fig. 10(a) shows
the % of stripping of uranium using the stripping solution in a sin-
gle contact. The results indicate that an aqueous solution of sodium
carbonate is the best out of these three stripping solutions. Using
aqueous sodium carbonate as a stripping solution, the effect of a
Fig. 10. (a) The stripping behavior of uranium from the uranyl-diamide complex in
ionic liquid using different aqueous complexing agents; (b) Cumulative
stripping of locked up uranium using sodium carbonate in multiple contacts.
% of
8

A. Pandey, S. Hashmi, G. Salunkhe et al.
Journal of Molecular Liquids 337 (2021) 116435
cies is predominating. However, for bicyclooctanium ionic liquid,
monoanionic complex [UO2(NO3)3L2]- was predominating. The
radiation stability of pyridinium and imidazolium-based ionic liq-
uids was more than that of the other two ionic liquid systems. An
aqueous solution of sodium carbonate (1 mM) back extracts the
uranyl ion effectively from the ionic liquid phase compared to oxa-
lic acid and EDTA. Four contacts of eluent are required for back
extraction of uranyl from bicyclooctanium ionic liquid, whereas 2
contacts were sufficient for imidazolium and pipyredinium based
ionic liquid. In pyridinium-based ionic liquid, the uranyl complex
was so strong, and hence can not be stripped quantitatively even
after five contacts.
CRediT authorship contribution statement
Amit Pandey: Formal analysis, Investigation S. Hashmi: Data
curation, Investigation G. Salunkhe: Validation. Velavan Kathi-
velu: Supervision, Writing, Review and Editing. K.S.Singh:
Methodology. A. Sengupta: Concceptualization, Supervision, Writ-
ing Original Draft, Writing, Review and Editing. R.S.Chauhan:
Supervision.
Fig. 11. The variation in DU as a function of contact time of organic phase with 1 M
HNO3.
different number of contact cycles on % stripping was investigated.
Four contacts of IL1 were required for about 99.5% back extraction,
while 2 contacts of IL2 and IL4 were sufficient for almost 99% back
extraction. Back extraction from IL3 was not as satisfactory as
observed in other cases. Even after five contacts, the cumulative
back extraction was slightly less than 99%. Fig. 10 (b) depicts the
cumulative % back extraction of uranyl ion from the uranyl-
diamide complex in ionic liquid.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgment
The variation of DU values for the ionic liquids having prior con-
tact (1 hr) with 1 M HNO3 as a function of time is shown in Fig. 11.
This investigation reveals the chemical stability of the solvent sys-
tems in the presence of 1 M HNO3. The D values were monitored up
to 72 h i.e. 3 days of contact time. DU value is almost independent
of time from 1 hr onwards, suggesting that all the ionic liquid
based solvent systems are chemically stable. Table 1 summarizes
the mechanism, speciation and other chanrecteristic of extraction
for the four ionic liquid system used in the study.
The author acknowledges Dr. P.K.Pujari, Director, RC & I Group,
Head, Radiochemistry Division, and Dr. R. Acharya, Head, Actinide
Spectroscopy section, Radiochemistry Division, Bhabha Atomic
Research Centre, Mumbai, India. Dr. Velavan Kathirvelu acknowl-
edges the financial support by the Science and Engineering
Research Board, India (SB/FT/CS-119/2014)
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