The application of novel hydrophobic ionic liquids to the extraction of uranium(vi) from nitric acid medium and a determination of the uranyl complexes formed

Article abstract of DOI:10.1039/c1dt10755k

Novel ammonium based hydrophobic ionic liquids (ILs) have been synthesised and characterised, and their use in the liquid-liquid extraction of uranium(vi) from an aqueous nitric acid solution using tri-n-butyl phosphate (TBP), studied. On varying the nitr

Full text of DOI:10.1039/c1dt10755k

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PAPER
The application of novel hydrophobic ionic liquids to the extraction of
uranium(VI) from nitric acid medium and a determination of the uranyl
complexes formed
Thomas James Bell* and Yasuhisa Ikeda
Received 26th April 2011, Accepted 4th August 2011
DOI: 10.1039/c1dt10755k
Novel ammonium based hydrophobic ionic liquids (ILs) have been synthesised and characterised, and
their use in the liquid–liquid extraction of uranium(VI) from an aqueous nitric acid solution using
tri-n-butyl phosphate (TBP), studied. On varying the nitric acid concentration, each IL was found to
give markedly different results. Relatively hydrophilic ILs showed high uranium(VI) extractability at
0.01 M nitric acid solution which progressively decreased from 0.01 to 2 M HNO3 and then increased
again as the nitric acid concentration was increased to 6 M. An analysis of the mechanisms involved for
one such IL, pointed to cationic-exchange being the predominant route at low nitric acid
concentrations whilst at high nitric acid concentrations, anionic-exchange predominated. Strongly
hydrophobic ILs showed low extractability for nitric acid concentrations below 0.1 M but increasing
extractability from 0.1 M to 6 M nitric acid. The predominant mechanism in this case involved the
2+
partitioning of a neutral uranyl complex. The uranyl complexes were found to be UO2 ·(TBP)3 for the
-
cationic exchange mechanism, UO2(NO3)2(TBP)2 for the neutral mechanism and UO2(NO3)3 ·(TBP)
for the anionic exchange mechanism.
an increase in the concentration of nitric acid from 0.01 M to
8 M and determined that the U(VI) complex extracted into the IL
was UO2(NO3)2(TBP)2. For the bmimTf2N system, they observed
that the D(U) values decreased from 15.3 to 0.7 with an increase
Introduction
The current commercial process (PUREX process) for recovering
actinides, such as uranium and plutonium, from spent nuclear
fuel consists of the dissolution of the spent fuels in an aqueous
nitric acid solution and the subsequent selective extraction of
in the nitric acid concentration from 0.01 to 0.1 M (M = mol
dm-3). Above 0.1 M, the D(U) values were found to increase
U(VI) and Pu(IV) using a 30% solution of tri-n-butyl phosphate
with increasing nitric acid concentration as was observed with
bmimPF6. However, they gave no explanation for the decrease in
D(U) at low nitric acid concentrations or any information on the
(TBP) in kerosene.1 There are, however, a number of drawbacks
with this process including the volatility and flammability of
kerosene and the risk of the system becoming critical if the
complexes formed.
concentration of fissile products in the aqueous phase becomes
Wang et al. also studied the extraction of U(VI) from a nitric
acid medium into bmimTf2N using TBP as extractant.6 They
obtained data at 0.1, 1 and 3 M nitric acid, and determined that
the complex extracted into the IL was UO2(NO3)2(TBP)2 for all
acid concentrations.
too large. Ionic liquids (ILs) have the potential to act as substitutes
for the replacement of kerosene because they have high stability
to radiolysis,2 can help to reduce the risk of criticality occurring,3
and are non-volatile and non-flammable.
The majority of studies on the extraction of U(VI) using ILs have
been performed using imidazolium based ILs.4–15 For example,
Giridhar et al. have studied the extraction of U(VI) from a
nitric acid medium into 1-butyl-3-methylimidazolium hexafluo-
rophosphate (bmimPF6) and into 1-butyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide (bmimTf2N) using TBP as
extractant.4,5 They found that for the bmimPF6 system, the
distribution ratio [D(U)] of U(VI) varied from 0.05 to 33.2 with
Dietz et al. studied the extraction of U(VI) from a nitric
acid medium into various 1-alkyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide using TBP as the extractant.7
They observed that the D(U) values varied considerably depending
on the alkyl chain length of the IL cation in the range of nitric
acid concentration from 0.01 to 8 M. For the ILs with long
chain cations (such as C10), it was found that the D(U) values
increased with increasing nitric acid concentration and that the
U(VI) complex extracted into the IL was UO2(NO3)2(TBP)2.
For the ILs with short chain cations (such as C5), it was found
that the D(U) values decreased as the nitric acid concentration
increased from 0.01 M to approximately 1 M and then increased
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technol-
ogy, 2-12-1-N1-34 O-okayama, Meguro-ku, Tokyo, 152-8550, Japan.
E-mail: bell.t.aa@m.titech.ac.jp; Fax: +81-3-5734-3061; Tel: +81-3-5734-
3061
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Table 1 Ionic liquids synthesised and characterised
Yield/Purity (Cl-) ID
was then cooled to room temperature overnight and the volatiles
were removed in vacuo to give methoxyethyl trimethylammonium
chloride (19.3 g, 100% yield). This product (3.43 g, 0.02 mol)
was then dissolved in the minimum amount of distilled water
and lithium bis(trifluoromethylsulfonyl)imide (6.40 g, 0.02 mol)
added. The mixture was stirred for 2 h at room temperature. The
ionic liquid phase (bottom) was then separated from the aqueous
phase and washed twice with copious amounts of distilled water to
remove any LiCl impurity. [(CH3)3NCH2CH2OMe][Tf2N] (7.1 g,
80% yield) was obtained as a clear liquid at room temperature.
The structures of the ILs synthesised were determined using
1H and 19F NMR (JEOL JNM-L400WB) and the purity was
confirmed by ion chromatography (Toso IC-2001).
R
R¢
R¢¢
CH3 CH3
CH3 CH3
CH3 CH3
CH3 CH3
CH3 CH3
CH3 CH2(CH2)14CH3
CH3 (CH2)2O(CH2)2OC6H4- CH2C6H5
C(CH3)2CH2C(CH3)3
CH2CH2OMe 80%a/<10 ppm
CH2CH2CH3 88%a/<10 ppm
IL1
IL2
IL3
IL4
IL5
IL6
IL7
CH2CH2Cl
CH2C6H5
74%/<10 ppm
64%a/<10 ppm
CH2C6H4NO2 86%/<10 ppm
CH2C6H5
89%/<10 ppm
84%/<30 ppm
a ILs reported previously in the literature.18,19
with nitric acid concentration up to 8 M. They determined that
the complex extracted into the IL phase from a solution of low
NMR data for novel ILs (IL3, IL5, IL6 and IL7). All chemical
shift values in ppm:
2+
nitric acid concentration was UO2(TBP)2 and deduced that in
IL3: 1H (400 MHz, d6-DMSO) d = 3.13 (s, 9 H, N(CH3)3), 3.70
(t, 2H, NCH2), 4.03 (t, 2H, CH2Cl). 19F (376 MHz, d6-DMSO)
nitric acid solutions of high concentrations the neutral complex
UO2(NO3)2(TBP)2 was the principal species. Furthermore,
they considered that the differences in D(U) values observed
for the different chain length cations was due to the relative
hydrophobicity of the respective ILs.
1
d = -77.87 (2 ¥ CF3). IL5: H (400 MHz, d6-DMSO) d = 3.03
(s, 9 H, N(CH3)3), 4.61 (s, 2H, NCH2), 7.86 (dd, 2H, C6H4),
8.12 (dd, 2H, C6H4). 19F (376 MHz, d6-DMSO) d = -78.55 (2 ¥
CF3). IL6: 1H (400 MHz, d6-DMSO) d = 0.84 (t, 3 H, CH3), 1.22
(app s, 26H, (CH2)13), 1.75 (qu, 2H, CH2CH2(CH2)13), 2.91 (s, 6H,
N(CH3)2), 3.22 (t, 2H, CH2CH2(CH2)13), 4.46 (s, 2H, C6H5CH2),
7.49 (app s, 5H, C6H5). 19F (376 MHz, d6-DMSO) d = -78.64 (2
¥ CF3). IL7: 1H (400 MHz, d6-DMSO): 0.68 (s, 9H, (CH3)3), 1.26
(s, 6H, (CH3)2), 1.65 (s, 2H, CH2), 2.99 (s, 6H, N(CH3)2), 3.20
(app s, 2H, OCH2CH2N(CH3)2), 3.51 (t, 2H, OCH2CH2O), 3.80
(t, 2H, OCH2CH2O), 4.10 (t, 2H, OCH2CH2N(CH3)3), 4.58 (s, 2H,
N(CH3)2CH2C6H5), 6.81 (dd, 2H, C6H4), 7.22 (dd, 2H, C6H4), 7.45
(m, 5H, C6H5). 19F (376 MHz, d6-DMSO) d = -78.52 (2 ¥ CF3).
We are aware of only two studies in the literature that have
been performed with ammonium based ILs.16,17 Ouadi et al.
studied the extraction of U(VI) using ammonium based ILs
bearing phosphoryl groups.16 They obtained an impressive D(U)
of 170 by extracting U(VI) using a 30% v/v mixture of their
IL [(BuO)2OPNH(CH2)3N(CH3)3][Tf2N] and trimethylbutylam-
monium bis(trifluoromethanesulfonyl)imide [Me3NBu][Tf2N] in
order to lower its viscosity. Srncik et al. examined the extraction
of U(VI) using long chain quaternary ammonium ILs with selected
aliphatic and aromatic anions17 and found that U(VI) species were
successfully extracted from nitric acid of 2 M concentration. In
both studies however, no work was undertaken to determine the
extraction mechanisms and the chemical form of extracted species.
In this paper, we report the results of investigations to determine
the key factors for optimising the separation of U(VI) from nitric
acid medium using novel hydrophobic ammonium based ILs, and
a detailed examination of the extraction mechanisms and the
extracted U(VI) species.
Extraction experiments
All extraction studies were carried out with a 1 : 1 organic to
aqueous phase volume ratio at 298 K for room temperature ILs
and 348 K for non-room temperature ILs (IL3 and IL4).
The experiments for the extraction of U(VI) were performed
as follows: initially, a 1.2 M (30 wt%) solution of TBP in IL was
prepared and phase equilibrated with an aqueous solution of nitric
acid (0.01 M to 8 M) by shaking the two layers together at 2500 rpm
for 60 min. The TBP/IL layer was then separated. Uranium
nitrate (0.02 M) in an aqueous nitric acid solution (0.01 M to
8 M) was added to the TBP/IL layer. After shaking the aqueous
solution (3 mL) and the IL (3 mL) together at 2500 rpm for 60 min
followed by phase separation, the top layer contained the aqueous
solution and the bottom layer contained the IL. An aliquot from
the aqueous phase was taken together with an aliquot from the
starting uranium nitrate solution. Both were diluted 100 fold and
ICP-AES (PerkinElmer Optima 3000) analysis performed. The
distribution ratio (D) and the extractability (E, %) into the IL
phase were calculated by the standard method.4 An additional
experiment was also carried out using the same conditions as
above but with a 1 min extraction time at a concentration of 3 M
nitric acid.
Experimental
Synthesis of ionic liquids
The ionic liquids (ILs) used in the present study (see Fig. 1
and Table 1) were synthesised via a standard metathesis re-
action from their chloride salt, which was either commer-
cially available or synthesised as below. As a typical example,
[(CH3)3NCH2CH2OMe][Tf2N] was synthesised as follows: 2-
chloroethyl methyl ether (16.1 mL, 176.7 mmol) and trimethy-
lamine (45 wt% aqueous solution, 16.5 mL, 126.2 mmol) were
dissolved in acetonitrile (10 mL) in a sealed metal container.
◦
The flask was heated with stirring at 140 C for 5.5 h. The flask
Analysis of chemical species in IL1
The concentrations of [cat]+ (cat = cationic component of ILs)
Fig. 1 Generic structure of ionic liquids synthesised.
10126 | Dalton Trans., 2011, 40, 10125–10130
and [Tf2N]- distributed into the aqueous phases were measured
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1
by using H and 19F NMR (JEOL JNM-L400WB) as follows.
Effect of nitric acid concentration and hydrophobicity of ILs on the
extraction of U(VI)
For 1H NMR measurements, a solution of sodium acetate in
D2O (0.05 wt%, 500 ml) was prepared and mixed with an aliquot
(100 ml) of the separated aqueous phase. The concentrations of
In order to examine the effect of nitric acid concentration on
the extraction of U(VI) in ILs, we performed extractions using an
aqueous phase containing uranyl(VI) nitrate (0.02 M) and an IL
phase containing TBP (1.2 M) at various nitric acid concentrations
(for IL1, 2, 6 and 7: 0.01 M to 8 M; for IL 3, 4 and 5: 0.01 M to
3 M) with a 60 min extraction time. The D(U) values calculated
were plotted as a function of nitric acid concentration as shown in
Fig. 2. Data for 1.2 M TBP/dodecane was also generated so that
the results in IL systems could be compared to those obtained
in the PUREX process (in line with the work of others in this
field, dodecane was used as a proxy for kerosene4,16). For two
of the ILs, IL6, and IL7, the extractability of U(VI) appeared to
resemble that observed with dodecane in that the extractability
increased steadily with nitric acid concentration to 6 M (D(U)
values: IL6 = 19, IL7 = 20 and dodecane = 24) after which it began
to decrease. This phenomenon contrasted with that for both IL1
and IL2 which showed high extractability of U(VI) at 0.01 M
nitric acid (D(U) values = 216 and 40 respectively), followed by a
gradual reduction as the nitric acid concentration was increased
to approximately 2 M. Above this concentration, the extractability
increased gradually up to 6 M nitric acid.
[cat]+ were evaluated from the area ratios of H NMR signals of
1
the cationic component of IL1 to the –CH3 signal of acetate. The
sample solutions for measuring concentrations of [Tf2N]- were
prepared by mixing the aqueous phase (100 ml) with a solution of
trifluoroacetic acid in D2O (0.03% v/v, 500 ml). The concentrations
of [Tf2N]- were obtained from the area ratio of 19F NMR signal
of [Tf2N]- to the –CF3 peak of trifluoroacetate.
Nitric acid and water determination in IL1
The amount of nitric acid present in IL1 was determined by
titration. The aqueous nitric acid solution (0.01 M) before (2 mL)
and after the phase equilibrium stage (2 mL) were titrated against
a 0.01 M aqueous sodium hydroxide solution using bromothymol
blue as indicator. The difference in volume of sodium hydroxide
required to neutralise the nitric acid (indicator colour change
from yellow to blue) was recorded. The procedure was repeated in
triplicate and an average reading obtained.
The amount of water present in IL1 was determined by Karl
Fischer titration (Metrohm 831 KF Coulometer) prior to and
following the pre-equilibrium stage at 0.1 M, 3 M and 6 M nitric
acid concentrations. Duplicate readings were obtained and an
average result calculated.
UV-visible analysis of complexes
UV-visible spectrophotometry of the suspected cationic and
anionic complexes in the IL1 phase after extraction, at 0.01 and
6 M nitric acid concentration respectively, was performed using a
SHIMADZU UV-2400 PC spectrophotometer. A spectrum of the
neutral complex in the dodecane phase after extraction was also
obtained.
Results and discussion
Hydrophobic ammonium based ionic liquids
A range of hydrophobic ammonium based ILs of the form
[R2NR¢R¢¢][Tf2N], (Tf2N = (CF3SO2)2N-) were produced (see
Fig. 1 and Table 1). A number of these were designed with
cations containing either an aromatic ring or a chloride ion.
An aromatic ring absorbs radiation energy and subsequently
relaxes non-dissociatively thereby bestowing extra stability on ILs
exposed to radiation. A chloride ion is known to be an efficient
neutron absorber and can therefore provide an extra measure
of safety in nuclear waste extraction systems. Several long-chain
cationic based ILs were also synthesised. Although determination
of an ILs hydrophobicity is known to be primarily controlled
by the anionic component, increasing the chain length of the
cation can also help to increase the relative hydrophobicity.20
Bis(trifluoromethylsulfonyl)imide was chosen as the anion because
it is known to be very effective in producing hydrophobic ILs with
low viscosity2 (Fig. 1). All ILs were liquid at room temperature,
except IL3 and IL4 (melting points: approximately 40 ◦C), and
synthesised in good yield with high purity.
Fig. 2 Dependency of D(U), on [HNO3] at constant TBP (1.2 M)
concentration in ionic liquids and dodecane.
Given these findings, it is suggested that the extraction of U(VI)
from the aqueous phase to the IL phase in our system could
proceed through either a cationic exchange, neutral partitioning
or anionic exchange mechanism depending on the nitric acid
concentration. In order to investigate these possible mechanisms,
IL1 and IL6 were selected for a more detailed study.
The cationic exchange mechanism would be expected to be
favoured at low nitric acid concentrations, given the lower
-
concentration of NO3 ions present. This suggests that at 0.01 M
nitric acid IL1 predominantly extracts via such a mechanism. This
is in contrast to IL6 which shows very little or no extraction ability
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at low nitric acid concentrations, suggesting that it is unable to
participate in this type of exchange. This may be explained by
the greater hydrophobicity of IL6 compared to IL1. The latter
consisting of much shorter chain cations.
As the nitric acid concentration increases, the neutral mecha-
nism or the anionic mechanism would be expected to predominate.
For the dodecane system, U(VI) extraction has been extensively
studied and is known to occur through the neutral mechanism:21,22
2+
-
UO2
+ 2NO3
+ 2TBP(org) ꢀ UO2(NO3)2(TBP)2(org)
(aq)
(aq)
The small decrease in D(U) values observed above 6 M nitric
acid concentration can be explained by competing extractions of
nitric acid and uranyl species by TBP.7 As IL6 shows a similar
trend to dodecane with varying nitric acid concentration, it is
reasonable to assume that it may follow a similar mechanism. To
examine this further, a 19F NMR spectrum (in D2O) was obtained
of the aqueous phase after extraction at 6 M [HNO3] in order
to determine if any IL anion (Tf2N-) was present, as would be
expected for the anionic exchange mechanism. No Tf2N- was
detected and therefore it is probable that IL6 is extracting U(VI)
through the neutral mechanism. For IL1, a measurement of the 19F
NMR spectrum was also performed and on this occasion, Tf2N-
was detected. However, although this could arise as a result of an
anionic exchange mechanism, it could also be explained by small
amounts of IL dissolving in the water layer during extraction as has
been observed for imidazolium based ILs.23,24 Therefore, further
studies on both the possible cationic and anionic mechanisms for
IL1 were undertaken, the results from which are presented in the
following sections of this paper.
As an additional experiment to confirm that the uranium
distribution equilibrium between the IL/dodecane phase and
the aqueous phase had been reached by the 60 min extraction
time, experiments were performed at shorter extraction times.
These results showed that a very short extraction time of 1 min
produced D(U) values equivalent to that for 60 min, and therefore
equilibrium had been fully reached in the experiments for both the
ILs and dodecane.
Fig. 3 (upper) A plot of [cat]+ vs. U(VI) for the extraction of U(VI) species
from the aqueous phase to the ionic liquid (IL1) phase at 0.01 M nitric
acid concentration. (lower) A plot of [Tf2N]- vs. U(VI) for the extraction
of U(VI) species from the aqueous phase to the ionic liquid phase (IL1) at
6 M nitric acid concentration.
present, had transferred to the IL phase. This indicates that there
are likely to be sufficient H+ ions available in the IL phase to play
a significant role in our system too.
Distribution behaviour of cations and anions between IL1 and
aqueous phase
In order to determine whether the proposed cationic exchange
mechanism was reasonable, we carried out extraction experiments
at 0.01 M nitric acid concentration by mixing the aqueous
solutions of uranium nitrate ((1.50 to 3.00) ¥ 10-2 M) with IL1,
and then measuring the mole numbers of uranium transferred
from the aqueous to the IL phase, and those of [cat]+ transferred
from IL1 to the aqueous phase. These were plotted against each
other as shown in Fig. 3 (upper). A straight line with a slope of
1 was obtained. These results indicate that a cationic exchange
mechanism is occurring with one cation from the IL involved.
Additionally, it has been reported that with some metal extrac-
tion processes with imidazolium ILs, movement of H+ between the
aqueous and IL layers may also play a part in the cationic exchange
process.25 This is considered to be due to large amounts of nitric
acid dissolving in the IL during the phase equilibrium stage.
Therefore, we examined by titration whether this was happening
in our system. As a result, we found that approximately 7.9 ¥
10-3 M of nitric acid, which equates to 79% of the total amount
In order to confirm whether the proposed anionic exchange
mechanism was reasonable, we carried out extraction experiments
at 6 M nitric acid concentration by mixing the aqueous solutions
of uranium nitrate ((1.50 to 3.00) ¥ 10-2 M) with IL1. The mole
numbers of uranium transferred from the aqueous to the IL phase
and those of [Tf2N]- transferred from IL1 to the aqueous phase
were measured and plotted against each other as shown in Fig.
3 (lower). As with the cationic exchange mechanism, a straight
line with a slope of 1 was obtained. These results indicate that
an anionic exchange mechanism is occurring with one anion from
the IL involved. For both the mechanism studies shown in Fig. 3,
it is possible to deduce that even when no moles of uranium are
present in the IL phase, some IL is present in the aqueous phase.
This indicates that IL1 is slightly soluble in the aqueous phase. To
quantify this, the extraction was repeated with no U(VI) present
and the amount of IL present in the aqueous phase determined
by NMR as above. From these measurements, it was found that
approximately 0.3% of IL was dissolved in the aqueous phase.
10128 | Dalton Trans., 2011, 40, 10125–10130
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The transfer of water from the aqueous phase to the IL phase
(containing TBP) during the pre-equilibrium stage at various nitric
acid concentrations (0.1 M, 3 M and 6 M) was also examined
by Karl Fischer titration. Large amounts of water dissolving in
the IL phase are known to modify the density and viscosity of
the IL phase so these findings might be of industrial interest.25
Additionally, experimental data on the transfer of water are very
scarce at present and only available for a few imidazolium based
IL extraction systems. The results showed that around 3 to 5% of
the available water was transferred according to the proportion
of nitric acid present (0.1 M [HNO3]: 25 992 ppm, 3 M [HNO3]:
36 857 ppm and 6 M [HNO3]: 45 208 ppm).
To further analyse the complexes present in IL1 at 0.01 M and
6 M nitric acid concentration, UV-visible spectrophotometry was
performed (Fig. 5). A spectrum of the known neutral complex
in dodecane (UO2(NO3)2(TBP)2) was also obtained, which was
found to be the same as that reported elsewhere26 (Fig. 5). As can
be seen, the spectrum of the complex present at 0.01 M nitric acid
concentration was well resolved but differed significantly from that
of the neutral complex. This finding is consistent with it being the
cationic complex as previously determined. The spectrum of IL1
at 6 M nitric acid concentration was not however well resolved
suggesting that the IL solution in this case consists of a mixture
of complexes. We have already demonstrated in Fig. 3 lower that
the anionic complex is likely to be present at this acidity and, after
taking account of the 0.3% of IL which dissolved in the aqueous
phase, it is possible to estimate from this graph that the majority
of the uranium present is probably being extracted through such
a mechanism. On comparing the spectrum to that of the assigned
neutral and cationic species, it is also feasible that either, or both, of
these might also be present in the IL solution. However, others who
have carried out similar studies to ours with imidazolium based ILs
have reported that the U(VI) extraction mechanism changes from
cationic exchange at low acidities to either extraction of the neutral
species or to anionic exchange at high acidities.7,25 It is therefore
likely that the IL1 solution at 6 M nitric acid concentration consists
of a mixture of the anionic and neutral complexes with the former
being in the majority.
Stoichiometry determination for complexes in IL1
The TBP stoichiometry of the resulting complexes was deter-
mined. A log–log plot of the D(U) values versus [TBP] provides
an indication of the stoichiometry of the predominant species
present in the IL phase in the form of the slope of the resulting
linear relationship.25
A slope of approximately 3 was obtained at a nitric acid
concentration of 0.01 M (Fig. 4) indicating the presence of 3 TBP
groups involved in the cationic exchange mechanism for IL1. A
slope of close to 1 was obtained at nitric acid concentration of 6
M (Fig. 4) indicating the presence of 1 TBP group involved in the
anion exchange mechanism for IL1.
Fig. 4 Log–log plot of D(U) as a function of [TBP] for IL1 (᭜ = 0.01 M,
ꢀ = 6 M [HNO3]).
Fig. 5 UV-visible spectra of the complexes present in IL1 at 0.01 M and
6 M [HNO3] and the neutral complex.
Based on the distribution behaviour and stoichiometry determi-
nation studies, it is possible to suggest that the following complexes
are extracted through the cationic exchange and anionic exchange
mechanisms.
Conclusions
A variety of novel ammonium based hydrophobic ILs have been
synthesised and examined as solvents for uranium extraction
from nitric acid solutions. Distribution ratios, comparable to, or
better than dodecane were obtained in a number of cases. The
mechanism of extraction and the subsequent complexes formed
have been shown to depend on the IL concerned. For a relatively
hydrophilic IL, both cationic and anionic exchange mechanisms
occur depending on the concentration of the nitric acid, whilst for
Cationic exchange:
2+
2+
UO2
+ 3TBP(IL) + ILcat+ + H+ ꢀ UO2 ·(TBP)3(IL)
+
(aq)
(IL)
(IL)
(aq)
ILcat+ + H+
(aq)
Anionic exchange:
UO2
+ 3NO3
+ TBP(IL) + Tf2N-
ꢀ
2+
-
(aq)
(aq)
(IL)
-
UO2(NO3)3 ·(TBP)(IL) + Tf2N-
(aq)
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a strongly hydrophobic IL the mechanism, which is independent of
the nitric acid concentration, involves the partitioning of a neutral
complex. Strongly hydrophobic ILs of the type reported here
therefore could have considerable potential for use as replacements
for dodecane as they do not suffer from the drawback of the loss
of IL to the aqueous layer which has been observed with other
ILs.
9 A. E. Visser, R. P. Swatloski, M. Reichert, R. Mayton, S. Sheff, A.
Wierzbicki, J. H. Davis Jr and R. D. Rogers, Chem. Commun., 2001,
135.
10 S. Mekki, C. M. Wai, I. Billard, G. Moutiers, J. Burt, B. Yoon, J. S.
Wang, C. Gaillard, A. Ouadi and P. Hesemann, Chem.–Eur. J., 2006,
12, 1760.
11 N. Kozonoi and Y. Ikeda, Monatsh. Chem., 2007, 138, 1145.
12 S. H. Ha, R. N. Menchavez and Y. Koo, Korean J. Chem. Eng., 2010,
27, 1360.
13 A. Mudring and S. Tang, Eur. J. Inorg. Chem., 2010,
2569.
Acknowledgements
14 K. Binnemans, Chem. Rev., 2007, 107, 2592.
15 H. Zhao, S. Xia and P. Ma, J. Chem. Technol. Biotechnol., 2005, 80,
1089.
Dr Thomas James Bell would like to thank the Japan Society for
the Promotion of Science (JSPS) for the award of a postdoctoral
fellowship (ID 10085) and the sponsorship of this work.
16 A. Ouadi, O. Klimchuk, C. Gaillard and I. Billard, Green Chem., 2007,
9, 1160.
17 M. Srncik, D. Kogelnig, A. Stojanovic, W. Koerner, R. Krachler and
G. Wallner, Appl. Radiat. Isot., 2009, 67, 2146.
18 S. Archambeau, J-P. Cano, C. Galley and F. Malbose, International
Patent App No. PCT/FR2009/050339.
Notes and references
1 J. E. Birkett, M. J. Carrott, O. D. Fox, C. J. Maher, C. V. Raube, R.
Taylor and D. A. Woodhead, Chimia, 2005, 59, 898.
2 D. Allen, G. Barton, A. E. Bradley, T. Gorman, A. Haile, I. Hamblett,
J. E. Hatter, M. J. F. Healey, B. Hodgson, R. Lewin, K. V. Lovell, B.
Newton, W. R. Pitner, D. W. Rooney, D. Sanders, K. R. Seddon, H. E.
Sims and R. C. Thied, Green Chem., 2002, 4, 152.
3 C. D. Harmon, W. H. Smith and D. A. Costa, Radiat. Phys. Chem.,
2001, 60, 157.
4 P. Giridhar, K. A. Venkatesan, T. G. Srinivasan and P. R. Vasudeva
Rao, J. Radioanal. Nucl. Chem., 2005, 1, 31.
5 P. Giridhar, K. A. Venkatesan, S. Subramaniam, T. G. Srinivasan and
P. R. Vasudeva Rao, J. Alloys Compd., 2008, 448, 104.
6 J. S. Wang, C. N. Sheaff, B. Yoon, R. S. Addleman and C. M. Wai,
Chem.–Eur. J., 2009, 15, 4458.
7 M. L. Dietz and D. C. Stepinski, Talanta, 2008, 75, 598.
8 M. S. Murali, N. Bonville and G. R. Choppin, Solvent Extr. Ion Exch.,
2010, 28, 495.
19 Z.-B. Zhou, H. Matsumoto and K. Tatsumi, Chem.–Eur. J., 2005, 11,
752.
20 M. G. Freire, C. M. S. S. Neves, P. J. Carvalho, R. L. Gardas, A. M.
Fernandes, I. M. Marrucho, L. M. N. B. F. Santos and J. A. P. Coutinho,
J. Phys. Chem. B, 2007, 111, 13082.
21 M. L. Dietz, S. Jakab, K. Yamato and A. R. Bartsch, Green Chem.,
2008, 10, 174.
22 V. A. Cocalia, M. P. Jensen, J. D. Holbrey, S. K. Spear, D. C. Stepinski
and R. D. Rogers, Dalton Trans., 2005, 1966.
23 K. Shimojo, K. Kurahashi and H. Naganawa, Dalton Trans., 2008,
5083.
24 L. S. I. Toh, J. McFarlane, C. Tsouris, D. W. DePaoli, H. Luo and S.
Dai, Solvent Extr. Ion Exch., 2006, 24, 33.
25 I. Billard, A. Ouadi and C. Gaillard, Anal. Bioanal. Chem., 2011,
1555.
26 J. S. Wang, C. N. Sheaff, B. Yoon, R. S. Addleman and C. M. Wai,
Chem.–Eur. J., 2009, 15, 4458.
10130 | Dalton Trans., 2011, 40, 10125–10130
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