N -Alkyl calix[4]azacrowns for the selective extraction of uranium

Article abstract of DOI:10.1039/c8dt03140a

The selective extraction of uranium by N-octylcalix[4]azacrown (NOCAC) and N-ethylhexylcalix[4]azacrown (NEHCAC) was investigated. The ligands were synthesised in three steps through the functionalisation of t-butyl calix[4]arene at the distal-1,3-positions of the lower rim with ethyl acetate groups followed by cyclisation with (imino)bis(ethane-2,1-diyl)diamide. A detailed investigation on the effect of various parameters, such as the aqueous phase acidity (sulfuric acid), the ionic strength, and ligand concentration, on the extraction of uranium(vi) has been conducted. The effect of the H₂SO₄ concentration has been studied from 0.02 to 3 M. Preliminary studies carried out on NOCAC in dodecane/octanol diluents showed that the uranium extraction from sulfuric acid is more efficient at a low H₂SO₄ concentrations. The stoichiometry of complexation was estimated from the slope method, NMR titration, and electrospray ionisation-mass spectrometry analysis. Both ligands were found to be highly selective for uranium(vi) over other competitive cations present in a simulated leach solution containing seven competitive cations. The successful recovery of the uranium from the organic phase has been performed thanks to stripping steps involving ammonium oxalate, ammonium carbonate, and sodium carbonate as stripping agents.

Full text of DOI:10.1039/c8dt03140a

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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N-Alkyl calix[4]azacrowns for the selective extraction of uranium
Moheddine Wehbie,a,b Guilhem Arrachart, a,* Xavier F. Le Goff,a Iyad Karamé,b and Stéphane
Pellet-Rostainga,*
Received 00th January 20xx,
Accepted 00th January 20xx
The selective extraction of uranium by N-octylcalix[4]azacrown (NOCAC) and N-ethylhexylcalix[4]azacrown (NEHCAC) was
investigated. The ligands were synthesised in three steps through the functionalisation of t-butyl calix[4]arene at the
distal-1,3-positions of the lower rim with ethyl acetate groups followed by cyclisation with (imino)bis(ethane-2,1-
diyl))diamide. A detailed investigation on the effect of various parameters, such as the aqueous phase acidity (sulfuric
acid), the ionic strength, and ligand concentration, on the extraction of uranium(VI) has been conducted. The effect of the
H2SO4 concentration has been studied from 0.02 to 3 M. Preliminary studies carried out on NOCAC in dodecane/octanol
diluents showed that the uranium extraction from sulfuric acid is more efficient at a low H2SO4 concentrations. The
stoichiometry of complexation was estimated from the slope method, NMR titration, and electrospray ionisation-mass
spectrometry analysis. Both ligands were found to be highly selective for uranium(VI) over other competitive cations
present in a simulated leach solution containing seven competitive cations. The successful recovery of the uranium from
the organic phase has been performed thanks to stripping steps involving ammonium oxalate, ammonium carbonate, and
DOI: 10.1039/x0xx00000x
www.rsc.org/
sodium
carbonate
as
stripping
agents.
Calixarenes as chelating systems are well known in the field of
separation science,11 and, over the past three decades, have
been used as preorganised structures for the extraction and
complexation of uranium and others through host-guest
Introduction
Uranium is one of the more common elements in the Earth’s
crust. It can be found, in association with many other
elements, in rocks, soil, rivers, and ocean water.1-4 Uranium
can be mined as a primary product, co-product, or important
by-product. The extraction of uranium from ores or its
recycling from spent nuclear fuel has been the subject of a
considerable amount of research effort since the inception of
nuclear power generation.
recognition.12-15 This chelating systems can been also can be
17
immobilized by impregnation on solid supports.16,
More
generally, interest in these structures has arisen because of
their unique molecular architectures, which makes them
suitable platforms for constructing host molecules that can
selectively bind a variety of guest substrates, ranging from
cations and anions to fullerenes. The special feature of these
ligands is the conformational rigidity gained from the
organisation of the chelating systems on both rims of the
calixarene platform, in which the coordinated atoms are
placed in positions that ensure the appropriate orientation of
the donor groups for complexation.
At the start of the fuel cycle, conventional ores are leached by
acid or carbonate reactants; subsequently, a liquid-liquid
extraction process can be performed to recover uranium with
high purity. Among many leaching reactants, sulfuric acid
typically combines high leaching performance and relatively
low cost.
The coordinating ability of calixarenes could be extended by
the introduction of O-donor and N-donor atoms, such as in the
calix[4]azacrowns, where azacrown bridges are attached to the
calixarene platform.18-20 These modified compounds show
attractive properties based on their structures, especially
when the azacrown bridges are fixed at the distal 1,3-
positions.21-24 The azacrown bridges can be designed to have
amides and amine functionalities, which are both known for
their capability to bind lanthanide and actinide ions via
interactions with the carbonyl oxygen atoms of the amide
groups,25-28 and the nitrogen atom of the amino groups.29-31
However, these compounds have been studied in regards to
their complexation behaviour with lanthanide ions.32-34
Liquid-liquid extraction is one of the most widely employed
and useful techniques for the extraction and separation of
uranium from impurities present in natural resources, as well
as in nuclear wastes. Many different extractants have been
developed for the extraction of uranium such as trialkyl
phosphine,5 carbamoylalkyl-phosphonates,6 carbamoylalkyl-
phosphonic acid ligands,7 dialkyl phosphonic acid,8 diamides,9
and trialkyl amines.10
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On the basis of these findings, it appeared interesting to tion studies
evaluate such calix[4]azacrowns containing both amine and
The extraction profile of NOCAC and NEHCAC was established
for U (VI), Mo(VI), Zr(IV), Ti(IV), La(III), Ce(III), and Fe(III), which
are metals potentially present in uranium deposits. Equimolar
concentrations of the target metals were chosen arbitrarily.
Different solutions of NOCAC were prepared in
dodecane/octanol (8.5/1.5, v/v). Octanol was used to avoid
the formation of a third phase and to enhance the solubility of
the ligand in the diluent. The effect of the ligand was studied
from a metal/ligand ratio of 1:10 up to 1:50. The stoichiometry
of the complex was estimated from the slope method, NMR
titration, and mass analysis. Recovery of the extracted metals
from the organic phase was successfully performed by
stripping with ammonium carbonate, ammonium oxalate, and
sodium carbonate.
diamides groups for uranium extraction.
Here, we report the synthesis of two N-substituted
calix[4]azacrowns, N-octylcalix[4]azacrown (NOCAC) and N-
ethylhexylcalix[4]azacrown (NEHCAC), cyclised at distal 1,3-
positions as efficient ligands for the extraction of uranium.
Indeed, these ligands contain two different chelating sites:
alkylamine with soft nitrogen donor atom, which should
induce uranium selectivity, and two amide groups with hard
oxygen donor atoms, which should enhance the complex
stability and ligand solubility in the diluents. The synthesis,
characterisation, and preliminary evaluations of the ligands for
uranium extraction from sulfuric acid media in comparison to
competitive ions have been carried out. Mechanistic
investigations (slope method and NMR titration) indicate a 1:1
complexing species. The ligands showed the ability to extract
uranium preferentially over other metal cations present in an
equimolar leaching mixture. Also, the recovery of uranium has
been established.
Effect of the feed acidity
The effect of varying the sulfuric acid concentration in the
range 0.02–5 M on the extraction of U(VI) cations was
investigated with NOCAC. Figure 2 shows the distribution
coefficient values obtained for the extraction of uranium by
0.02 M of NOCAC from different feed acidity. The results show
the clear dependence of the extraction efficiency of NOCAC on
the feed sulfuric acid concentration. The distribution values
decreased sharply with a slight increase in the sulfuric acid
concentration from 0.02 M (D = 3.8) to 1 M (D = 0.23), which is
in good agreement with the liquid-liquid extraction study
reported for the extraction of uranium with tertiary amine
ligands.31, 36, 37 The reduced extraction capacity of the ligand
with increasing sulfuric acid concentration is mainly due to the
Results and discussion
Synthesis
N-Substituted calix[4]azacrowns were synthesised via a well-
known synthetic route in three steps, as shown in Figure 1. 4-
tert-Butyl-25,27-ethoxycarbonylmethoxy-26,28-
dihydroxycalix[4]arene (2) was prepared by reacting the 4-tert-
butylcalixarene with ethylbromoacetate in the presence of
potassium carbonate, as described in the literature35. Then,
the calixazacrown (3) was quantitatively obtained by amide
-
2-
increase in competition from HSO4 and SO4 anions. These
ions, when present at high sulfuric acid concentrations, inhibit
the uranyl sulfate complexes from interacting with the
protonated amino calix[4]arene, resulting in a decrease in
extraction.
coupling of the diester groups of calix[4]arene (2) with
diethylenetriamine. The success of the cyclisation/amide
formation on the calixarene lower rim was confirmed by NMR.
The spectra exhibit a signal at 8.2 ppm, which corresponds to
the resonance signals of the amide protons. Moreover, C=O
amide group is characterised by a 13C-NMR resonance signal
located at 168.5 ppm, as well as a broad signal in the Fourier
transform (FT)-IR spectrum at 1685 cm-1. Finally, in order to
increase the lipophilicity of the ligand, fatty alkyl chains (octyl
and ethylhexyl) were introduced to the central amino group
Effect of the ligand concentration
The extraction of U(VI) with varying NOCAC concentrations
was also investigated, and the results are shown in Figure 3.
Increasing the concentration of the ligand from 10 to 50 mM,
at 0.1
M H2SO4 aqueous feed solution, resulted in a
progressive improvement in the distribution coefficient from
0.8 to 23, respectively.
The data presented in Figure 3 indicates that, at a first
approximation, the ligand at 30mM extracts uranium
efficiently from the aqueous solution, where about 90% of the
through the reaction of calix[4]arene
(
3
)
with the
corresponding alkyl iodide/bromide in the presence of K2CO3
to afford the targeted N-substituted calix[4]azacrowns (4a
NOCAC and 4b NEHCAC).
uranium
was
extracted.
Solven
t
extrac
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Figure 1: Synthetic scheme for NOCAC (4a) and NEHCAC (4b).
spectra
of
4
uranyl
nitrate,
the free
ligand
-1.4
-1.6
-1.8
3
DU
2
1
0
(LI),
and the
metal/li
gand
-2.0
comple
0
1
2
3
4
5
x (LI-U
)
-2.0
-1.8
-1.6
-1.4
[H2SO4] (mol/L)
comple
x (1:1).
Log [NOCAC] free
Figure 2: Effect of feed sulfuric acid concentration (0.02–5 M) on the distribution
ratio of UO22+. Organic phase: 0.02 M NOCAC in dodecane/octanol (83/17 v/v);
aq. phase: 1 mM UO2(NO3)2 in H2SO4. Aqueous to organic ratio (A/O) = 1.
25
20
15
DU
10
5
0
0
10
20
30
40
50
Figure 4: Effect of the extractant concentration on the distribution ratios of
U(VI). Organic phase: NOCAC 10–50 mM in dodecane/octanol (83/17 v/v).
Aqueous phase: 1 mM UO2(NO3)3 in 0.1 M H2SO4. A/O = 1. LogDU = f(log
[NOCAC]): y = 0.968(±0.004)x - 0.025(±0.006), R2 = 0.999.
[NOCAC]ini (mM)
Figure 3 : Effect of the extractant concentration on the distribution ratios of
U(VI). Organic phase: NOCAC 10–50 mM in dodecane/octanol (83/17 v/v);
aqueous phase: 1 mM UO2(NO3)3 in 0.1 M H2SO4. A/O = 1.
The C=O stretching band of neat LI was observed at 1685 cm−1,
while the corresponding band after complexation was shifted
to 1628 cm−1. This redshift supports the results obtained in the
1H- and 13C-NMR analyses that the amidic carbonyl oxygen
atoms of LI molecules contribute to the coordination of LI to
U(VI). However, the asymmetric uranyl stretch, assigned to the
stretching band (υ3) of UO2 (O=U=O),38-40 found at 940 cm-1 did
not shift on complexation. The fact that the υ3 band is not
affected by complexation suggests the uranyl is not totally
dehydrated during its coordination with the amidic oxygens.41
Mass spectroscopy study
Slope method analysis
The plot of logDU versus the concentration of extractant
should result in a straight line with a slope corresponding to
the number of NOCAC molecules associated with the complex
formed during the extraction. The fundamental stoichiometry
of the U-NOCAC complex formed at 0.1 M H2SO4 feed was
determined from the logDu value with the NOCAC free
concentration (Figure 4). A straight line with a slope of
0.968 ± 0.004 was obtained, suggesting that the stoichiometry
of complexation between the extractant and uranium is 1:1,
which is consistent with the NMR and electrospray ionisation-
mass spectrometry (ESI-MS) studies.
Electrospray ionisation mass spectrometry (ESI-MS) was used
to confirm the stoichiometry of the complexes formed
between
(LI) and uranium. Species were identified by
comparison to the calculated isotopic patterns, as shown. The
spectrum in Figure 6 shows different peaks corresponding to
Determination of the complex structure
To determine the structure of the complex, NOCAC was
reacted with uranium nitrate (UO2)(NO3)·5H2O in a mixture of
CHCl3/MeOH (1:1) and analysed spectroscopically.
IR spectroscopic study
[(LIH)(UO2)(NO3)(OH)]+,
[(LIH)(UO2)(OH)2·3H2O]+,
[(LIH)(UO2)(OH)2·4H2O]+,
[(LIH)(UO2)(NO3)(OH)·H2O]+,
[(LIH)(UO2)(NO3)·2H2O]+, and [(LIH)(UO2)(NO3)2]+ located at
m/z 1294.6, 1304.7, 1311.6, 1321.7, 1330.0, and 1338.7,
The complexation of the uranyl cation via the amide carbonyl
groups was confirmed by FT-IR analysis. Figure 5 shows the IR
respectively. These obtained peaks indicate
stoichiometry in the gas phase.
a
1:1
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y of the complexation between the ligand NOCAC and uranium
estimated by the slope method. The result from 1H-NMR
analysis of the free ligand and the complex suggests a
dissymmetric structure of the calixarene42, which is indicated
by the shifts in the resonance signals (two singlets)
corresponding to the Ar-H protons into four singlets, in
addition to the splitting of the methylene bridging Ar-CH2-Ar,
which is observed in 2 AB systems, (Figure 7 and Table SI-1 see
supporting information).
The analysis of the 1H- and 13C-NMR spectra suggests a
complex in a partially flattened cone conformation, in which
two amidic oxygens and one phenolic OH are involved in the
complexation (Figure 8).
UO2(NO3)2.5H2O
940
NOCAC
1684
1629
NOCAC-U
940
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
The contribution of the carbonyl groups appears from the
downfield shift of the amide NH proton from 8.19 ppm in the
1H-NMR spectrum of the free ligand to 9.42 ppm in the 1H-
NMR spectrum of the complex. The signal of the phenolic -OH
groups initially located at 6.42 ppm was split into two
resonance signals, located at 7.3 and 7.61 ppm, respectively.
Thus, the appearance of two different signals corresponding to
the two phenolic -OH suggests that one of the OHs is involving
in a strong interaction with the uranium nucleus, which
explains the dissymmetry of the complex structure and the
resulting partial flattened cone conformation. Indeed, the OH
displaced at 7.61 ppm was not affected by increasing the
concentration of uranium, whereas the other OH shifts
Figure 5 : IR spectra of uranyl nitrate, free LI, and the LI-U complex.
1321.7
1338.7
1304.7
1330.0
1311.6
1
gradually from 6.42 to 7.3 ppm in the H-NMR spectra during
1294.6
complexation.
The appearance of two pairs of axial/equatorial AB patterns of
the characteristic bridging methylene groups (Ar-CH2-Ar) at
1
3.43, 3.46, 4.13, and 4.32 ppm in the H-NMR spectra during
complexation confirms this hypothesis, which is in agreement
with the data obtained from the 13C-NMR spectrum, which
showed only one signal located at 31 ppm corresponding to
Figure 6: Mass spectrum of LI complexes with UO22+ cations.
NMR studies
the Ar-CH2-Ar in
a cone conformation (Figure SI-1 see
Figure 7 shows the NMR titration spectra of NOCAC with
variable amounts of uranyl nitrate. Significant shifts and
splittings of the resonance signals were observed with
increasing uranyl concentration until a 1:1 metal-ligand ratio.
The characteristic signals of the chelating sites of the free
ligand, including the phenolic OH groups initially present at
6.42 ppm, the amide NH groups at 8.19 ppm, and the
methylene groups linked to the amine nitrogen atom at 2.75
and 2.43 ppm, disappeared completely in the 1H-NMR
spectrum when [NOCAC]/[UO22+] = 1:1.
supporting information).43 Moreover, the AB system located at
4.88–4.68 ppm, which corresponds to the resonance signal of
the Ar-O-CH2-C(O) moiety, indicates the high rigidity of the
complex. These observations have been confirmed by 2D
analyses (Figure SI-2, SI-3 and SI-4 see supporting Information).
The dissymmetry, as well as the flattened cone conformation
of the structure, was finally confirmed using density functional
theory (DFT) calculations, as shown in Figure 9.
This observation from the 1H-NMR titration is in good
agreement
with
the
stoichiometr
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Figure 7: NMR titration spectra of the ligand NOCAC with UO2(NO3)2 at different ratios varied from ([NOCAC]/[UO22+] = 1:0 to 1:2), [NOCAC] = 30 mM in CDCl3.
structure by DFT allow to highlight the proton position and
movement observed by the NMR study.
The [UO2-NOCAC] structure was also obtained in a partially
flattened cone conformation, where one of the phenol groups,
as well as the amide carbonyls, interact with the uranyl cation.
The structure also shows the presence of a water molecule
bridged between the nitrogen atom of the amine and uranium
nucleus.
Proposed mechanism
Based on the preliminary study focused on the determination
of the structure of the uranyl-ligand complex, a mechanism
can be suggested for the steps following a preliminary
protonation of the amine site by sulfuric acid (whatever the
acid concentration), as represented by the following equation:
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
Figure 8: The proposed structure of the U-NOCAC complex, UO22+= red ball.
ꢁꢁꢁꢁ
2ꢀ ꢂ ꢃꢄꢅꢆꢇ
↔
ꢈLHꢉꢄꢅꢆꢇ.
ꢊꢋ ꢈ1ꢉ
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
Here, ꢈꢀꢃꢉꢄꢅꢆꢇ represents the dimer form of the protonated
extractant, and the overbar refers to species in the organic
phase. The absence of the overbar denotes aqueous species.
ꢄꢍ
Uranyl sulfate species can exist as ꢌꢆꢄꢅꢆꢇ
,
ꢌꢆꢄꢈꢅꢆꢇꢉꢄ
, and
ꢌꢆꢄꢈꢅꢆꢇꢉꢎꢇꢍ, depending on the concentration of sulfuric acid.
Referring to the literature,44, 45 at pH 1, the major species of
the aqueous uranyl sulfate is ꢌꢆꢄꢅꢆꢇ, with a minor quantity of
ꢌꢆꢄꢈꢅꢆꢇꢉꢄꢄꢍ. At a high concentration of ꢃꢄꢅꢆꢇ, bisulfate and
trisulfate anions exist as major species.
At low acidity, two possibilities can drive the extraction of
uranium based on:
A combination of solvation and anion exchange mechanism
ꢈ ꢉ
involving water molecules, as represented by ꢊꢋ 2
.
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
ꢈꢀꢃꢉꢄꢅꢆꢇ ꢂ 2ꢌꢆꢄꢅꢆꢇ ꢂ 2ꢃꢄꢆ
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
↔ 2ꢏꢈLHꢉꢈꢌꢆꢄꢉꢈꢅꢆꢇꢉꢈꢆꢃꢉꢄꢐ ꢂ ꢃꢄꢅꢆꢇ ꢊꢋ ꢈ2ꢉ
A solvation mechanism, as shown in ꢊꢋ ꢈ3ꢉ
.
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
ꢈꢀꢃꢉꢄꢅꢆꢇ ꢂ 2ꢌꢆꢄꢅꢆꢇ
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
ꢈLHꢉꢄꢈꢌꢆꢄꢉꢄꢈꢅꢆꢇꢉꢎ
↔
ꢊꢋ ꢈ3ꢉ
Figure 9: Structure obtained by DFT calculations on the U-NOCAC complex.
At high sulfuric acid concentrations, an anionic exchange
mechanism takes place according to ꢊꢋ ꢈ4ꢉ
.
ꢄꢍ
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
ꢈꢀꢃꢉꢄꢅꢆꢇ ꢂ 2ꢌꢆꢄꢈꢅꢆꢇꢉꢄ
The chelating behaviour of the macrocycle was also evaluated
through molecular modelling of its uranyl complexes (DFT
calculations) with the B3LYP functional and 6-31G basis set for
H, C, N, and O and the Stuttgart RSC 1997 effective core
potential (ECP) for U. The modelling is based on the structure
of the ligand that has been obtained by single X-ray diffraction
(Figure SI-5 see supporting information). The proposed
ꢄꢍ
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ
↔ ꢈLHꢉꢄꢈꢌꢆꢄꢉꢄꢈꢅꢆꢇꢉꢎ ꢂ 2ꢅꢆꢇ
ꢊꢋ ꢈ4ꢉ
Finally, whatever the acid concentration, if we consider that no
water molecules are involved in the extraction mechanism,
only one complex system can be suggested, as shown in Figure
10 below.
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result from the chemical complexation of Mo with N inside N-
alkyl calix[4]azacrowns which takes place outside the cavity.
For both Mo and U the chemical complexation is relatively
similar as proposed in the literature which describe the
extraction of Mo by diamide or trialkyl amine.50, 51
The effect of the concentration of the ligand on the
extractability and the selectivity of uranium from the same
leaching solution was studied by varying the concentration of
NOCAC from 0.01 to 0.05 M. The results presented in Figure 11
show that, at a low ligand concentration (6.6 mM), a significant
amount of U and Mo were extracted without any remarkable
extraction for other elements.
Figure 10: Suggested ligand-uranyl extracted species.
Ligand selectivity study
For comparison purposes, the performance of 0.05 M NEHCAC
toward the extraction of uranium from 0.1 M H2SO4 was
evaluated, and the distribution coefficient was found to be
about 56 (Table SI2, see supporting Information). The
performance is nearly three times higher after replacing the
linear alkyl chains of the amine by branched alkyl ones. The
60
40
20
0
100
10
1
U
Mo
Zr
Ti
Ce
Fe
La
S (U/Mo)
S (U/Zr)
extraction is enhanced by the presence of the 2-ethylhexyl
chains, as reported in the literature.46,
47
In addition, the
positive influence on the efficiency arising from the 2-
ethylhexyl chain at the central amino chelating site in NEHCAC
could be due to a different conformation of the ligands around
the guest atom. Solvent extraction studies of several other
cations were carried out to evaluate the selectivity of the
ligand toward uranium.
Extraction experiments
0.1
The potential (efficiency and selectivity) of NOCAC and
NEHCAC was studied with respect to the preferential
extraction of U over Mo, Zr, Ti, Ce, Fe, and La in a simulated
leaching solution. The ligands (20 mM) in dodecane/octanol
(8.3/1.7 v/v) were mixed with a mixture of the metal cations,
each having a concentration of 25 ppm. As for the extraction
with U(VI) alone, the extraction data presented in Table 1 and
Table SI4 show clearly that NEHCAC is more efficient than
NOCAC, in which the DU of NEHCAC is about three times higher
than that of NOCAC.
0
10
20
30
40
50
[NOCAC] (mM)
Figure 11: Effect of the concentration of NOCAC on the distribution values and
selectivities, S(U/M). Organic phase: NOCAC 0.01–0.05 M in dodecane/octanol
(83/17 v/v). Aqueous phase: 25 ppm of each metal in 0.1 M H2SO4.
The extraction of metal cations increased gradually on
increasing the concentration of NOCAC, and, at 50 mM, the
ligand extracts mostly U (DU = 46.75), a significant amount of
Mo (DMo = 6.76), and a small amount of Zr (DZr = 0.8). However,
no real extraction was observed for Ti, Ce, Fe, and La at any
Table 1: Distribution values and selectivity factors of NOCAC and NEHCAC toward
U, Mo, Zr, Ti, Ce, Fe, and La cations from 0.1 M H2SO4 aqueous solution at 25 °C.
concentration (D < 0.05). The selectivity of U over Mo, S(U/Mo)
,
increased on increasing the ligand concentration from 1 to 7,
while the selectivity of U over Zr, S(U/Zr) showed a slight
decrease on increasing the ligand concentration, having a
value around 60 over the concentration range of the ligand.
These results are encouraging and can be applied for the
U
Mo
2.9
2.4
5.3
3
Zr
0.2
31
Ti
La
Fe
Ce
Du
S(U/M)
Du
6.8
0.05
137
0.05
332
0.03
>200
0.01
>1600
0.03
>200
0.02
830
0.03
NOCAC
>200
0.01
17
0.6
30
NEHCAC
selective extraction of U(VI), even though
a significant
S(U/M)
>1600
extraction of molybdenum is observed. Indeed, the
concentration of molybdenum in uranium ores is smaller than
that of uranium by about 31 times.31
Organic phase: 20 mM NOCAC or NEHCACin 83% dodecane/17% octanol (v/v);
aqueous phase: 25 ppm of each metal in 0.1 M H2SO4.
For both ligands, similar behaviour was observed with respect
to the extraction of the cations. The ligands are selective for
U(VI) with respect to other cations with separation factors
higher than 10, except for molybdenum.
Stripping and recovery of U
After the extraction steps with NOCAC and NEHCAC systems,
the loaded organic solutions contain, respectively, about 85%
and 92%of the U(VI) initially present in the feed sulfuric acid..
Stripping or back-extraction experiments were implemented to
strip uranium quantitatively from the organic phases using
aqueous bases such as ammonium carbonate and ammonium
Mo and U have a similar behaviour in regards to their
extraction, this is due to an ionic radii very close and to their
coordination number.48, 49 As for U, the extraction of Mo may
6 | J. Name., 2012, 00, 1-3
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Preparation of 25,27-ethoxycarbonylmethoxy-26, 28-
oxalate with an aqueous to organic (A/O) ratio of 1 or 5. Both
basic solutions showed promising results which allow to
recover the uranium in its carbonate or oxalate form. As
shown in Table 2, uranium can be totally recovered after
stripping the organic phases with the basic solutions with an
A/O ratio of about 5. In this condition, the molybdenum is also
mainly stripped from the organic phase.
dihydroxycalix[4]arene (2)
A solution of the starting tert-butylcalixarene (4 g, 6.17 mmol)
and K2CO3 (0.93 g, 6.78 mmol) in dry acetonitrile was stirred
under nitrogen for 1 h; subsequently, bromoethylacetate (2.11
g, 12.64 mmol) was introduced. The reaction mixture was
stirred for 16 h, and the crude product was concentrated
under reduced pressure, followed by the addition of cold
methanol. The obtained precipitate was filtered and washed
with cold methanol and dried to obtain the afforded
Actually, other stripping solutions, washing steps, or methods,
such as precipitation, are still under investigation to selectively
recover the uranium. Indeed, usually, the molybdenum dose is
not co-precipitated with uranium but can be removed by
bleeding after the uranium precipitation.52
compound 2 as a white powder in 86% yield.
1H-NMR (400 MHz, CDCl3) δ [ppm]: 1.0 (s, 18 H, CCH3), 1.29 (s,
18 H, CCH3), 1.36 (t, J = 7.2 Hz, 6H, -CH2-CH3), 3.35 (d, 4 H, J =
14.2 Hz, inner of Ar-CH2-Ar), 4.32 (q, J = 7.2 Hz, 4H, O-CH2-CH3),
4.47 (d, 4 H, J = 14.2 Hz, outer of Ar-CH2-Ar), 4.74 (s, 4 H, Ar-O-
Table 2: Recovery of the extracted U from an extracting phase (20 mM ligand in
dodecane/octanol (8.3/1.7)) by an aqueous solution (A/O = 1 or 5).
C
H2-CO), 6.84 (s, 4H, Ar-H), 7.05 (s, 4H, Ar-H), 7.08 (s, 2H, Ar-
O
H
).
Metal recovery (%)
13C-NMR (100 MHz, CDCl3) δ [ppm]: 14.2, 31.1, 31.7, 31.8,
33.8, 34.0, 61.3, 72.4, 125.1, 125.8, 128.0, 132.5, 141.6, 147.2,
150.2, 150.7, 169.3.
Stripping
A/O
solution
U
82
95
76
100
22
Ti
11
-
Mo
77
Zr
68
90
52
93
4.2
1
5
1
5
[(NH4)2C2O4]
(sat)
87
NOCAC
Preparation of calix[4]azacrown (3)
-
76
[(NH4)2CO3]
(1 mol/L)
Cavitand
mmol) with an equivalent amount of diethylene triamine
(0.414 g, 4.02 mmol) in 50 mL of mixture of
3 was prepared by mixing calixarene 2 (3.3 g, 4.02
92
1.3
100
18.3
[M]org ppm
a
Metal recovery (%)
methanol/toluene (1:1) and refluxed overnight at 80 °C. The
solvent was evaporated under reduced pressure, and the
resulting white product was then dried under vacuum to give
the pure compound quantitatively, 3.34 g.
Stripping
solution
A/O
U
85
Ti
33
2
Mo
86
Zr
1
5
1
5
83
89
65
74
8.4
[(NH4)2C2O4]
(sat)
89
91
NEHCAC
1H-NMR (400 MHz, CDCl3) δ [ppm]: 0.9 (s, 18 H, CCH3), 1.35 (s,
18 H, CCH3), 1.36 (t, J = 7.2 Hz, 6H, -CH2-CH3), 2.97 (t, J = 5.2 Hz,
80
13
5
82
[(NH4)2CO3]
(1 mol/L)
91
97
23.8
1.2
20.8
[M]org ppm
4H, NH-CH2-CH2-CO), 3.38 (d, 4 H, J = 13.2 Hz, inner of Ar-CH2
-
Ar), 3.56 (t, J = 5.2 Hz, 4H, NH-CH2-CH2-CO), 4.19 (d, 4 H, J =
13.2 Hz, outer of Ar-CH2-Ar), 4.74 (s, 4 H, Ar-O-CH2-CO), 6.373
(s, 2H, Ar-OH), 6.74 (s, 4H, Ar-H), 7.15 (s, 4H, Ar-H),
Experimental
13C-NMR (100 MHz, CDCl3) δ [ppm]: 30.9, 31.3, 31.7, 33.9,
34.0, 40.1, 49.0, 74.8, 125.3, 128.3, 129.0, 131.6, 143.0, 148.0,
148.9, 149.8, 168.5.
Chemicals and analysis
Chemicals (analytically pure) were purchased from Sigma-
Aldrich, Eburon Organics, or Alfa Aesar and were used without
further purification. Anhydrous solvents were purchased from
Acros (AcroSeal®).
General procedure for the preparation of N-alkyl
calixazacrown (4a,b)
A mixture of the calixazacrown (3) (1 g, 1.2 mmol) and K2CO3
(0.116 g, 0.84 mmol) in dry acetonitrile (20 mL) was stirred
under nitrogen for 1 h; subsequently, iodooctane or 2-
ethylhexyl iodide (0.317 g, 1.32 mmol) were introduced. The
resulting mixture was refluxed overnight; then, the solvent
was evaporated under vacuum. Then, 30 mL of CH2Cl2 was
added to the crude product, and the resulting salts were
isolated by filtration. Finally, the dichloromethane was
evaporated to afford the desired product in quantitative yield
Reactions were monitored by thin-layer chromatography
(Merck TLC Silica Gel 60 F254). Flash chromatography was
performed using a Combiflash Agilent Intelliflash 971-FP.
NMR analyses were performed on a Bruker 400 Ultrashield VS
spectrometer. Displacements are reported in parts-per-million
1
using the solvent (CDCl3: 7.26 ppm for H; 77.16 ppm for 13C)
as an internal reference.
Metal concentrations were determined using a Spectro ARCOS
ICPAES spectrometer. Background acquisition was made
before measurement. ESI-MS was performed on a Flexar SQ
300 MS instrument. DFT calculations were carried out using
the B3LYP functional and 6-31G basis set for H, C, N, and O. For
U, the Stuttgart RSC 1997 ECP was used.
for both N-octyl calixazacrown
calixazacrown (4b).
(4a) and N-ethylhexyl
N-Octyl calixazacrown (4a
)
1H-NMR (400 MHz, CDCl3) δ [ppm]: 0.87 (t, J = 7.2 Hz, 3 H,
CH2CH3), 0.89 (s, 18 H, CCH3), 1.14 (m, 10H, CH2), 1.34 (s, 18 H,
CCH3), 1.45 (m, 2 H, N-CH2-CH2-hexyl), 2.43 (t, 2H, J=7.6Hz, N-
Synthesis
C
H2-heptyl), 2.75 (m, 4H, NH-CH2-CH2-NH), 3.36 (d, 4 H, J =
14.2 Hz, inner of Ar-CH2-Ar), 3.52 (m, 4H, NH-CH2-CH2-NH),
4.15 (d, 4 H, J = 14.2 Hz, outer of Ar-CH2-Ar), 4.52 (s, 4 H, Ar-O-
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Journal Name
C
H2-CO). 6.42 (s, 2H, Ar-O
H
), 6.73 (s, 4H, Ar-
H
), 7.13 (s, 4H, Ar- without the phase modifier were mixed with the sulfuric acid
aqueous phase.
H
). 8.2 (t, 2H, J = 5.2 Hz, NH-CO).
13C-NMR (100 MHz, CDCl3) δ [ppm]: 14.15, 22.63, 27.30, 27.42, Organic phases were pre-equilibrated with an aqueous phase
29.39, 29.65, 30.86, 31.33, 31.68, 31.89, 33.95, 38.51, 54.98, at the same acidity as the extraction step without metal
74.62, 125.31, 125.89, 127.79, 131.59, 142.83, 148.00, 148.69, cations. The pre-equilibrated organic phases were then mixed
149.87, 168.47.
ESI-MS m/z: 944.5 [(M+H)+ ].
N-Ethylhexyl calixazacrown (4b
with an equal volume of an aqueous acidic stock solution of
the cations in a thermostated shaker (Infor-ht® Ecotron) at
)
25 °C for 1 h at 400 rpm. The phases were separated after
1H-NMR (400 MHz, CDCl3) δ [ppm]: 0.81 (t, 6 H, CH2CH3), 0.84 centrifugation at 4000 rpm for 30 min (Sigma 3-16 PK).
(s, 18 H, CCH3), 1.23 (m, 6H, CH2), 1.34 (s, 18 H, CCH3), 1.49 (m, The metal distribution ratios were measured under batch
3 H, C
H
and CH2), 2.18 (dd, 1H, J = 4.4 Hz, N-CH2), 2.34 (dd, 1H, conditions. Equal volumes of aqueous and organic solutions
C to obtain a good emulsion by
J = 4.4 Hz, N-CH2), 2.56 (dd, 2 H, J = 3.6 Hz, N-CH2-CH2-NH), were vigorously shaken at 25
°
2.86 (m, 2H, N-CH2-CH2-NH), 3.35 (d, 2 H, J = 14.2 Hz, inner of means of an automatic vortex shaker equipped with a
Ar-CH2-Ar), 3.41 (d, 2 H, J = 14.2 Hz, inner of Ar-CH2-Ar), 3.6 (s, thermostated cell for 30 min for equilibrium distribution
2 H, m, 2H, N-CH2-CH2-NH). 4.12 (d, 2 H, J = 14.2 Hz, outer of measurements. After phase separation by centrifugation, the
Ar-CH2-Ar), 4.17 (d, 2 H, J = 14.2 Hz, outer of Ar-CH2-Ar), 4.45 aqueous phase was analysed by ICP-AES (Spectro ARCOS) to
(d, 2 H, J = 7.4 , Ar-O-CH2-CO), 4.57 (d, 2 H, J=7.4 , Ar-O-CH2
CO), 6.47 (s, 2H, Ar-O ), 6.71 (s, 1H, Ar- ), 6.74 (s, 1H, Ar-
), 7.14 (s, 1H, Ar- ), 8.19 (t, 2H, J = 5.2 Hz, N
-
measure the concentrations of cations. From the results
H
H
H
), obtained by ICP-AES, the distribution ratios (DM = [Mn+]org
7.12 (s, 1H, Ar-
H
H
H-
/[Mn+]aq) were determined at equilibrium. The experiments
were carried out in duplicate measurements with a precision
CO).
13C-NMR (100 MHz, CDCl3) δ [ppm]: 10.16, 14.15, 23.24, 23.79, of ± 5%.
28.86, 30.93, 31.23, 31.38, 33.93, 33.96, 36.92, 38.45, 55.4,
Back-extraction experiments
58.53, 74.68, 125.21, 125.42, 126.14, 127.55, 128.04, 131.33,
131.81, 142.88, 148.01, 148.75, 149.87, 168.54.
ESI-MS m/z: 944.5 [(M+H)+ ].
The loaded organic phase was stripped with solutions of
ammonium carbonate, ammonium oxalate (typically 0.5 M),
and sodium carbonate. Back-extraction was performed at
room temperature (22–25 °C) by mixing equal volumes of
Preparation of the LI-U complex
Solutions of uranyl nitrate in methanol (0.5 mL) and NOCAC in organic and aqueous phases for 1 h (A/O ratio of 1). After
chloroform (0.5 mL) were mixed in an open 2-mL vial at separation by centrifugation (4000 rpm for 10 min), the metal
various UO22+/NOCAC ratios, ranging from 0.25:1 to 2:1.
The obtained solutions were heated for 1 h at 60
concentrations were measured in the aqueous phase by
°
C and then ICP/OES.
left to stand overnight at room temperature; subsequently,
they were dried under vacuum to obtain the complex as
brown powder.
Conclusions
1H-NMR (400 MHz, CDCl3) δ [ppm]: 0.87 (t, J =7.0 , 3 H,
CH2CH3), 0.89 (s, 18 H, CCH3), 1.23 (m, 10H, CH2), 1.34 (s, 18 H,
CCH3), 1.75 (m, 2 H, N-CH2-CH2-hexyl), 3.43 (d, 2H, J = 8 Hz,
inner of Ar-CH2-Ar ), 3.47 (d, 2H, J = 8 Hz, inner of Ar-CH2-Ar ),
3.59 (m, 4H, N-CH2-CH2-NH and N-CH2-heptyl), 3.81 (m, 4H, N-
Two N-substituted calix[4]azacrown extractants in 1,3-
alternate conformation were synthesised in yield (>80%) and
characterised using 1H, 13C, DEPT, COSY, and HSQC-NMR, as
well as ESI-MS and DFT studies. The two ligands were
investigated for the extraction of uranium from sulfuric acid
solution. Preliminary extraction studies and mechanistic
investigations have been performed for the N-
octylcalix[4]azacrown (NOCAC) ligand. The effect of various
parameters such as the feed acidity and NOCAC concentration
were studied. The variation of the concentration of NOCAC at
a constant uranium concentration and specific acidity has been
studied to determine the stoichiometry of the extracted
complex. The slope analysis method indicates a 1:1 molar ratio
for U:NOCAC in the extracted complex. This was also
supported by NMR titration experiments. Selectivity studies
using NOCAC and NEHCAC have been performed on a leaching
solution containing seven metals representative of uranium
ores. The results highlight that the NOCAC and NEHCAC
molecules are selective extractants for uranium over other
metal ions present in the simulated leach solution. The
extraction of uranium was enhanced on replacing the linear
alkyl chains of the amine by branched alkyl chains. Successful
stripping and recovery of the extracted metals by both ligands
CH2-CH2-NH and N-CH2-CH2-NH), 4.14 (d, 2 H, J = 16 Hz, outer
Ar-CH2-Ar), 4.32 (d, 2 H, J = 13.2 Hz, outer Ar-CH2-Ar), 4.37 (m,
2H, N-CH2-CH2-NH), 4.66 (d, 2H, J = 15.2 Hz, Ar-O-CH2-CO), 4.88
(d, 2H, J = 15.2 Hz, Ar-O-CH2-CO), 6.87 (s, 2H, Ar-
Ar- ), 7.09 (s, 2H, Ar- ), 7.13, (s, 2H, Ar- ), 7.30 (s, 2H, Ar-OH-
U), 7.16 (s, 2H, Ar-O ), 9.40 (m, 2H, N -CO).
H), 6.90 (s, 2H,
H
H
H
H
H
+
ESI-MS m/z: 1338.66 [(M+H+UO2(NO3)2 ].
Extraction experiments
The metal stock solutions were prepared at the desired acidity
from 10000 mg L-1 ICP standards (in 1% HNO3). The desired
concentrations were prepared by dilution using ultrapure
water (MilliQ, Millipore, >18 MΩ cm−1) and the acidity was
adjusted with sulfuric acid or nitric acid.
Different organic solutions were prepared from NOCAC or
NEHCAC at different specific concentrations in a mixture of
dodecane/octanol (83/17 v/v). Octanol was used as a phase
modifier. Turbid solutions were obtained when solutions
8 | J. Name., 2012, 00, 1-3
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have been recovered from the organic phase using ammonium 17. C. Xiao, A. Zhang and Z. Chai, Journal of Hazardous Materials,
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Conflicts of interest
There are no conflicts to declare.
20. A. Hamdi, S. H. Kim, R. Abidi, P. Thuéry, J. S. Kim and J.
Vicens, Tetrahedron, 2009, 65, 2818-2823.
21. R. Abidi, I. Oueslati, H. Amri, P. Thuéry, M. Nierlich, Z. Asfari
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The authors acknowledge B. Baus Lagarde for her technical
support regarding the ICP/OES analyses.
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We gratefully acknowledge the financial support for this
project by the Lebanese University, CEDRE program, CNRSL
(the National Council for Scientific Research, Lebanon), French
CNRS, Montpellier University, and the Labex Project CheMISyst
(ANR-10-LABX-05-01).
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DOI: 10.1039/C8DT03140A
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