Journal of Nuclear and Radiochemical Sciences, Vol. 5, No. 2, pp. 21-26, 2004

ArticlesEffect of Alkyl Group in 1-Alkyl-3-methylimidazolium Hexafluorophosphate Ionic Liquids
on the Extraction of Uranium by Tri-n-butylphosphate Diluted with Ionic Liquids
P. Giridhar, K. A. Venkatesan, T. G. Srinivasan, and P. R. Vasudeva Rao*
Fuel Chemistry Division, Chemical Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India
Received: August 9, 2004; In Final Form: October 12, 2004
Extraction of uranium by 1.1 M TBP dissolved in 1-octyl-3-methylimidazolium hexafluorophosphate (omimPF6)
ionic liquid (IL) as diluent has been studied as a function of concentration of nitric acid and uranium and that oftemperature. The results are compared with those of extraction in 1.1 M TBP in 1-butyl-3-methylimidazolium
hexafluorophosphate (bmimPF6). Distribution ratios (D) for extraction of uranium in 1.1 M TBP/omimPF6 were
marginally less than those for extraction by 1.1 M TBP/bmimPF6 at all nitric acid concentrations. The D values
increased with increase in the concentration of nitric acid but decreased with increase in the concentration of
uranium and temperature. In contrast to the behavior of 1.1 M TBP/n-dodecane, D values for uranium continued
to increase even for nitric acid concentrations higher than 5 M, when IL acted as diluent. Solubility of omimPF6
in nitric acid was 2 – 3 times less as compared to bmimPF6. The overall enthalpy change (∆Htot) for the extractionof uranium in 1.1 M TBP/omimPF6 was found to be more exothermic than for extraction by 1.1 M TBP/bmimPF6.
This behavior can be attributed to the lower extent of protonation of TBP in omimPF6 phase. Viscosity of TBP/IL
increased when butyl group was substituted with octyl group in IL.

1. Introduction
Room temperature ionic liquids (RTILs) are gaining recognition in various industries including nuclear reprocessing
industry owing to their impressive physical, chemical, andenvironmentally benign properties.1−6 Rogers and co-workers
have published a number of reports pertaining to the possible
application of ionic liquids for separating fission products andactinides from nuclear wastes.7−11 The ionic liquids (ILs), as thename implies, consist entirely of ions existing in liquid stateand have negligible vapor pressure and low flammability ascompared to volatile molecular organic compounds such as
chloroform, dodecane (DD) etc., which are used as the diluentsin solvent extraction processes. Several workers have employedeither crown ether with ILs9, 12, 13 as diluents for extracting 90Sr
and 137Cs or actinide specific extractants diluted with ILs for
extracting f-block elements.11 The main difference observed
was that these extractants in conjunction with ILs exhibited
substantially higher extraction of target metals from aqueous
solutions under the conditions, which gave negligible or meager
extraction with customary diluents. Recently, Visser et al.
have introduced task-specific ionic liquids (TSILs) incorporating specific extracting moieties 14, 15 linked to the imidazolium part of the IL. It was reported that these TSILs exhibit
very high distribution ratios for Hg2+ and Cd2+ when functionalized with sulphur containing moieties,15 and for Pu4+, Am3+ andUO22+ when functionalized with carbamoylmethylphosphineoxide moieties.15 In addition, Chun et al.13 have studied thestructural variation in 1-alkyl-3-methylimidazolium hexafluorophosphate ILs for investigating the variation in selectivity
for alkali metal ion extraction. It was found that by increasing
the chain length of alkyl group attached to the imidazole, theefficiency of extraction decreased where as the selectivity
increased. Recently we have also studied the extraction of
uranium by tri-n-butylphosphate (TBP) in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6)16, 17 and reported
that bmimPF6 also extracts uranium. In contrast to the extrac*Corresponding author. E-mail: vasu@igcar.ernet.in. FAX: +914114-280065.

tion behavior exhibited by TBP/n-dodecane, the distribution
ratios of uranium in 1.1 M TBP/bmimPF 6 continued toincrease even above 5 M nitric acid. The extraction equilibrium was similar to that of uranium extraction by TBP/DD.
2+
−
UO aq
+ 2NO3aq+ 2TBP →← UO2 (NO3)2 (TBP)2

While the investigation of ILs as diluents for reprocessing has
yielded promising results, the possible chemical and radiolytic
degradation6 and mechanistic aspects of extraction9, 12, 16−18 are
also being simultaneously investigated owing to the importance of actinide recovery. Swatloski et al. have reported theformation of 1-butyl-3-methylimidazolium fluoride from thedecomposition of 1-butyl-3-methylimidazolium hexafluorophosphate.19 Carda-Broch et al.20 have studied the physicochemical properties of bmimPF6 and reported a solubility of
1.8 wt% in water. Alfassi et al.21 have investigated the solubility of various ILs in water by electrospray ionization mass
spectrometry and reported the solubility of bmimPF 6 as 1.8
wt%. Further, substitution of PF 6− anion with bulkier imide
anion was found to decrease the solubility.21 Gutowski et al.15
have studied the extraction of actinides and fission products
using imide based ILs to avoid the solubility of PF6− based ILs
in water. Visser et al.9 have reported the solubility of bmimPF6
as 1.2 M when equal volumes of bmimPF 6 or crown ether
dissolved in bmimPF 6 was contacted (2 – 4 min) with 8 M
nitric acid. Antony et al.22 have studied the thermodynamic
aspects of imidazolium based ILs in water and reported thesolubility of bmimPF6 as 2 wt% in water. It was reported thatthe solubility decreased to 0.7 wt% when a butyl group was
replaced by lengthier octyl groups in 1-alkyl-3-methylimidazolium hexafluorophosphate (amimPF6). It is, thus, obviousthat by changing the cation and anion combination in ILs thesolubility of ILs can be altered.
This paper deals with the extraction of uranium by TBP/
omimPF6 from nitric acid medium and compares the resultswith those obtained for TBP/bmimPF6. The effect of concentration of HNO3 and uranium and that of temperature on the
distribution coefficient of uranium has been studied. Solubilityof amimPF6 and TBP/amimPF6 in nitric acid medium and the© 2004 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 11/11/2004

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GiridharJ. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

influence of temperature on the viscosity of solvents are reported.
1.2

2. Experimental
Materials: All the chemicals and reagents used were of analytical grade. 1-Methylimidazole, 1-chlorobutane, and hexafluorophosphoric acid were procured from Lancaster, UK. 1Chlorooctane was obtained from E. Merck. 1-Methylimidazole
was distilled before use and other chemicals were used without
any purification. TBP and uranyl nitrate were obtained from
E.Merck, Mumbai.
Preparation of ionic liquids. The procedure adopted for
preparing the ILs is described elsewhere.7 Briefly it involves
refluxing a mixture of 1-methylimidazole with chloroalkane in
the mole ratio of 1:1.2. The resulting product was washed few
times with ethylacetate followed by acetonitrile and evaporated
under vacuum. Nearly quantitative yield was obtained. The
hexafluorophosphate IL was then prepared by adding a precooled liquid of hexafluorophosphoric acid (1.3 mol) to the
mixture containing 1-alkyl-3-methylimidazolium chloride (1mol) in water kept at –5˚C. The entire mixture was stirred for
2 days and the bottom IL layer was separated and washed several times with water until the aqueous solution was no longer
acidic. The IL was heated to remove moisture at 70˚C using
rotary evaporator. An yield of 60% for bmimPF6 and 82% for
omimPF 6 was obtained. Elemental analysis yielded for
bmimPF6: C 33.81 (33.80), H 5.31 (5.32), N 9.85 (9.86) P 10.9
(10.9) and for omimPF6: C 42.52 (42.35), H 6.79 (6.81), N
8.28 (8.23) P 9.1 (9.1). IR bands: 3175, 3122 cm−1 (C-Hstretch) imidazole ring, 2964, 2932, 2868 cm−1 (C-H stretch)aliphatic, 1564, 1465, 1168 cm−1 imidazole ring symmetricstretch, 1425, 1378 (MeC-H asymmetric stretch), 834 cm−1 (PF stretch).
+
N

R

+

N

refluxCl

R = butyl/octyl

N

Cl

N

-

R

30 hrs1-methylimidazole

HPF6

+
N

PF6

N
R

1-alkyl-3-methylimidazolium hexafluorophosphate

Scheme 1. Preparation of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids.

Extraction studies. All the extraction studies were carriedout at 298 K with 1:1 organic to aqueous phase ratio, unlessotherwise mentioned. 1.1 M TBP in amimPF6 was preparedand pre-equilibrated with desired nitric acid concentration.
Extraction of uranium as a function of nitric acid concentration
was studied by equilibrating 2 mL of organic phase i.e. 1.1 M
TBP/amimPF6 with 2 mL of nitric acid solution containing
233
U tracer. The concentration of nitric acid in the test solution
was varied from 0.01 M to 8 M. After three hours of equilibration, the radioactivity of 233U distributed between organic andaqueous phases was measured by liquid scintillation counting.
The distribution ratio (D) of uranium and % extraction (E)were calculated using eqs 1 and 2.
D=[233U]org[233U]aq(1)

E=[233U]org
× 100[ U]org + [233U]aq

(2)

233

Absorbance

1.0
0.8
0.6
0.4

bmimCl; ε = 2.2 L mol -1 cm-1
omimCl; ε = 7.8 L mol -1 cm-1
measured against water

0.2
0.0
0.0

0.1

0.2

0.3

0.4

0.5

0.6

[amimCl] /M

Figure 1. Variation of absorbance with concentration of 1-alkyl-3methylimidazolium chloride in water.

Extraction of nitric acid was also studied by equilibrating
equal volumes of 1.1M TBP/amimPF6 with nitric acid for threehours. The concentration of nitric acid was varied from 0.01 M
to 8 M. The amount of nitric acid present in organic/aqueous
phases was determined by titrating a known volume of theorganic/aqueous phase with sodium hydroxide. Similar experiment was performed when bmimPF6 or omimPF6 acted asorganic phase.
Enthalpy change accompanied by the extraction of uranium
in 1.1 M TBP/amimPF6 was determined by measuring the D ofuranium as a function of temperature ranging from 298 K to
338 K. In this study 2 mL of organic phase was equilibrated
with 2 mL of nitric acid spiked with 233U and the concentrationof nitric acid was varied from 2.0 M to 4.0 M.
The solubility of amimPF6 in nitric acid phase was determinedby contacting neat amimPF6 as well as 1.1 M TBP solutions in
amimPF6 with nitric acid of desired concentration ranging from0.01 M to 8 M. The experiment involved vigorous equilibrationof 1 g of organic phase with 1 mL of desired aqueous phase. At
various intervals of time, the shaking was stopped. The concentration of IL in aqueous phase was determined by measuring
independently the concentrations of imidazolium cation (byspectrophotometry) and PF6− anion (by phosphorous analysis).
The absorbance of imidazolium cation in the aqueous phase wasmeasured at the λmax of 289 nm. The concentration of amim+
cation in the aqueous phase was calculated with the use of calibration plot, shown in Figure 1. The molar absorptivity for
bmimCl and omimCl was found to be 2.2 L mol−1 cm−1 and 7.8
L mol−1 cm−1 respectively. The concentration of phosphorous in
the aqueous phase was determined by amidol method23 afterdigesting an aqueous aliquot with perchloric acid.
The effect of aqueous uranium concentration on the extractionof uranium was studied by equilibrating the 1.1 M TBP/amimPF6
phase with aqueous phase spiked with 233U tracer containing
uranium in the concentration range of 4.3 × 10−5 M to 8.6 × 10−3
M (added in the form of uranyl nitrate) in 3 M nitric acid.
3. Results and Discussion
A comparison of distribution ratios of uranium in amimPF6
and 1.1 M TBP/amimPF6 as a function of nitric acid concentration is shown in Figure 2. Modest D values were obtained
when amimPF 6 alone acted as extractant and the D values
varied from 0.004 (~0.3% extraction) to 0.41 (~30% extraction) when the concentration of nitric acid was varied from0.01 to 8 M. This behavior could be attributed to the solubilityof uranyl nitrate species in ILs, which are known for dissolving
and stabilizing a wide variety of ionic salts. 7, 8, 24, 25 The
increase in the solubility with increase in [HNO3] could be dueto the increased formation of species such as, [UO2(NO3)2],[UO 2 (NO 3 ) 3 ] − which can be extracted by the solvation/ion

50

1

30

0.1

0.01
0

40
30

20

20

10

10

1.5

1.5

1.0

1.0

0.5

0.5

2

4

6

0.0

8

0

20

40

60

[HNO3 ] eq /M

20

100

120

140

160

180

0.0
200

Figure 4. Rate of acid extraction into organic phase and solubility ofPF6− in aqueous phase.

1.1 M TBP/bmimPF6
bmimPF6
1.1 M TBP/omimPF6
omimPF6
T = 298 K

1.2

1.1 M TBP/bmimPF6
bmimPF6
1.1 M TBP/omimPF6

25

80Time /minFigure 2. Comparison of distribution ratio for extraction of uranium
by ionic liquids and 1.1 M TBP/ionic liquids from HNO3 medium.

1.0

omimPF6[H ]org /M

15+

amimPF6 in aqueous phase /wt%

50+

Distribution ratio

10

Extraction of acid in bmimPF6
Solubility of P (mg/mL) in aqueous phase from bmimPF6
Extraction of acid in omimPF6
solubility of P (mg/mL) in aqueous phase from omimPF6[HNO3]ini = 6 M

40[H ]org /M

bmimPF6
1.1 M TBP/bmimPF6
omimPF6
1.1 M TBP/omimPF6
T = 298 K, 1:1 phase ratio
3 hr equilibration

23
Concn. of P in aqueous phase /mg/mL

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Effect of Alkyl Group

0.8
0.6

10
0.4

5

0.2
0.0

0
0

2

4

6

8

0

1

2

3

4

5

6

7

[HNO3]aq /M[HNO3] /M

Figure 3. Comparison of ionic liquid solubility in aqueous phase atvarious nitric acid concentrations.

Figure 5. Comparison of acid extraction behavior of various ionicliquids and 1.1 M TBP/IL.

exchange by IL. A similar observation was reported by Visser
et al.14 for the extraction of various complexing metal ions from
aqueous phase into IL and it was indicated that ILs prefer toextract hydrophobic and larger complex species than hydrated
and smaller complexes.
The extraction of uranium by both 1.1 M TBP/amimPF6 wassimilar at all nitric acid concentrations and D gradually
increased from 0.05 to 33 when the concentration of nitric acidwas increased from 0.01 M to 8 M. In contrast to TBP/DD,17
the distribution ratios continued to increase even when theconcentration of nitric acid was > 5 M. This indicates that ILs
play a significant role in extracting uranium from aqueous
phase to organic phase at high nitric acid concentrations.
Table 1 shows the aqueous solubility of IL as a function of
the concentration of nitric acid. The solubility values wereobtained both from phosphorous analysis using amidol method21
and by the measurement of absorbance of imidazolium cation
dissolved in nitric acid. It can be seen that both the values are

in close agreement. Thus, it would appear that both the imidazolium and hexafluorophosphate ions distribute to the aqueous
phase and there is no preferential stripping/ion exchange of cationor anion, when IL is contacted with nitric acid. The solubility of
bmimPF6 and omimPF6 in 0.01 M nitric acid was found to be
1.9 wt% and 0.9 wt% respectively, which is in close agreementwith the values reported19−21 for these ILs in water. Solubility of
IL in nitric acid is one of the important parameters from the
PUREX process point of view. Visser et al.9 have reported thesolubility of bmimPF6 as 1.2 M when equal volumes of bmimPF6
or crown ether dissolved in bmimPF6 was contacted (2 – 4 min)with 8 M nitric acid. Figure 3 compares the aqueous phase solubility of amimPF6 and 1.1 M TBP/amimPF6 as a function of
nitric acid concentration. The solubilities of both the ILs increase
with increase in the concentration of nitric acid and the presenceof TBP in IL enhances the solubility. However, the solubility of
omimPF6 IL is 2 – 3 times lower depending upon [HNO3], ascompared to bmimPF6. The rate of distribution of IL to theaqueous phase and acid extraction by organic phase are shownin Figure 4. It can be seen that rapid extraction of acid takes
place and the equilibrium is established in five minutes. Similarly, the solubility of IL reaches the saturation value in less than
10 minutes of equilibration. The acid extraction isotherm of ILs
and 1.1 M TBP/ILs are shown in Figure 5. The concentration ofacid extracted by the organic phase increased with increase in
the concentration of nitric acid present in the aqueous phase inboth the cases. But the amount of acid extracted by omimPF6
was 2 – 3 times lower than the corresponding value for bmimPF6.
Figures 6a, 6b, and 6c show the IR spectra of 1.1 M TBP/
diluents (diluent is n-dodecane, bmimPF6, or omimPF6) equilibrated with various nitric acid concentrations. A doublet at
1281cm−1 and 1269 cm−1 can be assigned to the P = O stretchingof 1.1 M TBP/DD and it gets broadened and shifts to 1234 cm−1
and 1210 cm−1 respectively when 1.1 M TBP/DD is equilibrated

TABLE 1: Solubility of amimPF6 in Nitric Acid Medium[HNO3]ini Solubility of bmimPF6
M
P found, M bmim+ found, M
0.01
0.10
1.00
3.00
6.00
8.00

0.069
0.116
0.140
0.351
0.619
0.810

0.063
0.113
0.146
0.345
0.636
0.816

Solubility of omimPF6
P found, M omim+ found, M
0.026
0.033
0.080
0.121
0.214
0.310

0.025
0.030
0.076
0.116
0.201
0.309

Contact time: 3 hours. The values are based on phosphorous
analysis and spectrophotometric measurement of alkylimidazolium cation.

24

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Giridhar
3.2(B)

2 M HNO3;∆H = -23.6 kJ/mol
3 M HNO3;∆H = -21.6 kJ/mol
2.8
4 M HNO3;∆H = -18.2 kJ/mol
1.1 M TBP/omimPF6
2.6

(C)

2.4

(A)

ln D

%T

120

3.0

60

2.2
2.0
1.8

0

1.6

(C) 1.1 M TBP/DD + 8.0 M HNO3
(B) 1.1 M TBP/DD + 3 M HNO3
(A) 1.1 M TBP/DD

1600

1500

1400

1300

1.4
1.2
1200

1100

1000

Wave number /cm-1

2.95

Figure 6a. IR spectra of 1.1 M TBP/DD equilibrated with nitric acidof different concentrations.

200

1.0

(C)

%T

3.20

3.25

3.30

3.35

∂ ln D−∆Htot=∂ (1/T)R(A)(A) 1.1 M TBP/bmimPF6(B) 1.1 M TBP/bmimPF6 + 3 M HNO3
(C) 1.1 M TBP/bmimPF6 + 8 M HNO3
1500

1400

1300

1200

1100

1000

wave number /cm-1

Figure 6b. IR spectra of 1.1 M TBP/bmimPF6 equilibrated withdifferent concentrations of nitric acid.

(C)

(B)
(A)

%T

3.15

3.40

by 1.1 M TBP/omimPF6 can be given by the van’t Hoff’s relation shown in eq 3

100

60

(A) 1.1 M TBP/omimPF6(B) 1.1 M TBP/omimPF6 + 3 M HNO3
(C) 1.1 M TBP/omimPF6 + 8 M HNO3
1500

1400

1300

1200

(3)

where R is gas constant. The plot of ln D against 1/T is shownin Figure 7. From the linear regression of the experimental data,∆Htot for the extraction of uranium by 1.1 M TBP/omimPF6 wascalculated and these values are tabulated in Table 2 along withthe values obtained17 for the extraction of uranium by 1.1 M
TBP/bmimPF 6 . It can be observed from the table that theexothermicity decreases with increase in nitric acid concentration in both the cases, which could be due to the combinationof a number of factors as indicated by Burger.26 However, it isinteresting to note that the exothermicity observed for the
extraction of uranium by 1.1 M TBP/omimPF6 is higher thanthat for 1.1 M TBP/bmimPF6. This could be attributed to alower extent of protonation of TBP in 1.1 M TBP/omimPF6
phase, which results in increase in the availability of free TBP
for complexing the uranium transported to the organic phase,resulting in higher exothermicity.
The distribution ratios of uranium extraction by 1.1 M
TBP/amimPF6 at various initial concentrations of uranium inthe aqueous phase are shown in Table 3. It is seen that D values
TABLE 2: Enthalpy Change Observed for the Extractionof Uranium by 1.1 M TBP/ILs at Various Nitric Acid Concentrations

0
1600

3.10

-1

(B)

120

3.05

1000/T /K
Figure 7. Variation of ln D with 1/T for the extraction of uranium by
1.1 M TBP/omimPF6.

150

1600

3.00

1100

1000

wave number /cm-1

Figure 6c. IR spectra of 1.1 M TBP/omimPF6 equilibrated with nitricacid of various concentrations.

with 3 M and 8 M nitric acid. It is interesting to observe from
Figures 6b and 6c that the P = O stretching band of TBP appears
at 1263 cm−1 instead of 1281 cm−1 – 1269cm−1, when TBP isdiluted with IL. Equilibration of 1.1 M TBP/bmimPF6 with 3
M nitric acid significantly broadens the P = O band and shifts
the peak to 1250 cm−1 (Figure 6b). This band disappears when
TBP/bmimPF6 is equilibrated with 8 M nitric acid, probably
merging with the bands of bmimPF6 at 1200 cm−1. However,the features in IR spectrum of 3 M acid equilibrated 1.1 M
TBP/omimPF6 are different from 1.1 M TBP/bmimPF6. Due
to lower extraction of nitric acid by 1.1 M TBP/omimPF6, thepeak position at 1263 cm−1 is not affected and the band is only
broadened. However, this band merges with other peaks of
omimPF6 when equilibrated with 8 M nitric acid.
The effect of temperature on the distribution ratio for extraction of uranium and enthalpy of extraction (∆Htot) of uranium[HNO3], M2
3
4-∆Htot, kJ/mol
1.1 M TBP/bmimPF6

1.1 M TBP/omimPF6

17.0
15.2
14.4

23.6
21.6
18.2

TABLE 3: Variation in the Distribution Ratio of Uranium(at 298 K) with Initial Concentration of Uranium in the 3 M
Nitric Acid[U] ×104, M
0.43
4.3
21.5
43.0
86.0

Distribution ratio of U
1.1 M TBP/bmimPF6
1.1 M TBP/omimPF6
23.0
21.4
20.0
17.1
16.6

17.2
16.6
14.3
13.1
11.8

J. Nucl. Radiochem. Sci., Vol. 5, No. 2, 2004

Effect of Alkyl Groupdecrease with increase in uranium concentration. It is difficult
to explain this observed behavior, since the concentration of
TBP in organic phase is much higher than the concentration of
uranium in aqueous phase.
Viscosity measurements. Viscosity of the solvent is an
important process parameter determining the hydrodynamic
nature of the solvent in solvent extraction process. It is essential that the viscosity of the organic phase (~2 cP) is sufficiently low to permit, ready flow of solutions, low power agitators
for phase dispersion and efficient phase separation. ILs generally have high viscosity due to strong ionic interaction.27
However, the viscosity of IL can be lowered by operating theextraction process at higher temperature in order to increase
the hydrodynamic properties. Figure 8 shows the effect oftemperature on the viscosities (η, in cP) of various ILs and 1.1
M TBP/ILs. It can be seen that the viscosity of octyl based ILs
are much higher than that for bmimPF6, which could be due tohigher interaction of bigger octyl groups with the adjacent layers of IL. Since the viscous flow is the rate phenomenon,28 it
can be represented by eq 4, which relates the activation energy,
Ea, and viscosity.
η = ηoe Ea /RT(4)

where ηo and R are pre-exponential factor and gas constantrespectively. The plot of ln η against 1/T is shown in Figure 9.
From the slope of the straight line the activation energy (Ea)
was found to be 42.2 kJ/mol and 33.6 kJ/mol respectively for
omimPF6 and bmimPF6. For 1.1 M TBP/omimPF6 and 1.1 M
TBP/bmimPF 6 , E a was found to be 33.0 kJ/mol and 27.9
kJ/mol respectively and these values were much higher thanthe Ea reported (18.2 kJ/mol) for 1.1 M TBP/Shell Spray Base.29

400

omimPF6
1.1 M TBP/omimPF6
bmimPF6
1.1 M TBP/bmimPF6

350

Viscosity /cP

300
250
200
150
100
50
0
300

310

320

330

340

350

T /K

Figure 8. Variation in the viscosity of ionic liquids as a function oftemperature.

6.5
6.0
5.5

omimPF6; Ea = 42.2 kJ/mol
1.1 M TBP/omimPF6; Ea = 33.0 kJ/mol
bmimPF6; Ea = 33.6 kJ/mol
1.1 M TBP/bmimPF6; Ea = 27.9 kJ/mol

ln η

5.0
4.5
4.0
3.5
3.0
2.5
2.80 2.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

1000/T /K-1

Figure 9. Arrhenius plot for the variation of viscosity with temperature for various ionic liquids and 1.1 M TBP/IL.

25

4. Conclusions
The extraction of uranium by 1.1 M TBP present in a neotericdiluent namely IL, 1-alkyl-3-methylimidazolium hexafluorophosphate, has been studied and the results are comparedwith butyl and octyl substituents in ILs. The distribution ratio(D) for extraction of uranium by 1.1 M TBP/omimPF 6 was
marginally lower than that observed in 1.1 M TBP/bmimPF6
and the D values were found to increase with increase in nitricacid concentration. In contrast to the extraction behavior exhibited by 1.1 M TBP/DD system, D values for the extraction of
uranium by 1.1 M TBP/amimPF6 continued to increase when
the concentrations of nitric acid was above 5 M HNO3. The
solubility of IL in nitric acid medium increased with increasein the concentration of nitric acid in both the cases, but, thesolubility of omimPF6 was 2 – 3 times lower depending upon
HNO3 concentrations. The overall extraction of uranium by
TBP/omimPF6 is more exothermic as compared to extraction
by TBP/bmimPF6 perhaps due to lower extent of protonationof TBP in omimPF 6 phase. Substitution of butyl group in
amimPF6 by octyl group increases the viscosity of IL two fold.
Similarly, the viscosity, at 303 K, increases from 66 cP to 94
cP when 1.1 M TBP is diluted with omimPF 6 instead of
bmimPF6.
In summary, the substitution of butyl groups in amimPF6 byoctyl group does not alter the extraction behavior of uranium
in 1.1 M TBP/ILs significantly. The solubility of IL in
aqueous nitric acid phase reduces by a factor of 2 – 3, when
omimPF6 is used as diluent, but, the viscosity of the solvent
increases ~2 fold.
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