A highly efficient solvent system containing functionalized diglycolamides and an ionic liquid for americium recovery from radioactive wastes

Article abstract of DOI:10.1039/c2dt12364a

Three room temperature ionic liquids (RTILs), viz. C₄mim ⁺·PF₆^-, C₆mim ⁺·PF₆^- and C₈mim ⁺·PF₆^-, were evaluated as diluents

Full text of DOI:10.1039/c2dt12364a

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PAPER
A highly efficient solvent system containing functionalized diglycolamides and
an ionic liquid for americium recovery from radioactive wastes†
Arijit Sengupta,a Prasanta K. Mohapatra,*a Mudassir Iqbal,b Jurriaan Huskensb and Willem Verboomb
Received 7th December 2011, Accepted 23rd March 2012
DOI: 10.1039/c2dt12364a
Three room temperature ionic liquids (RTILs), viz. C4mim+·PF6−, C6mim+·PF6− and C8mim+·PF6−, were
evaluated as diluents for the extraction of Am(III) by N,N,N′,N′-tetraoctyl diglycolamide (TODGA). At
−
3 M HNO3, the DAm-values by 0.01 M TODGAwere found to be 102, 34 and 74 for C4mim+·PF6
,
C6mim+·PF6− and C8mim+·PF6−, respectively. The extraction of Am(III) decreased with increasing feed
acidity for all three diluents, indicating an ion exchange mechanism for the extraction. The stoichiometry
of the extracted species suggested that two TODGA molecules were associated with Am(III) during the
extraction for all three RTILs and the conditional extraction constants have been determined. The
DM-values for different metal ions followed the order: 75 (Am(III)) > 30.7 (Pu(IV)) > 3.9 (Np(IV)) > 1.19
(Pu(VI)) > 0.52 (U(VI)) > 0.12 (Cs(I)) > 0.024 (Sr(II)). The distribution behaviour of Am(III) was also
studied with a recently synthesized calix[4]arene-4DGA (C4DGA) extractant dissolved in C8mim+·PF6
.
−
Using this extractant diluent combination, the DAm-value was 194 at 3 M HNO3 using 5 × 10−5
C4DGA, suggesting a very high distribution coefficient at very low extractant concentrations. The
M
stoichiometry of the extracted species containing Am was found to be 1 : 2 (M : L) in C8mim+·PF6−. The
thermodynamics of the extraction was also studied for both extractants in C8mim+·PF6−. The use of
RTILs gives rise to significantly improved extraction properties than the commonly used n-dodecane and
an unusual increase in separation factor values was seen for the first time which can lead to selective
separation of Am from wastes containing a mixture of U, Pu and Am.
third phase formation.6 In this context diglycolamide-based
1. Introduction
extractants are found to be environmentally benign and do not
form a third phase. Among the diglycolamide-based extractants
TODGA (N,N,N′,N′-tetraoctyl diglycolamide) was found to be
one of the most promising extractants for the separation of triva-
lent actinides from a moderately acidic aqueous feed.7,8 Further,
a recent study has indicated that a mixture of CyMe4BTBP and
TODGA can be used for direct extraction of the trivalent acti-
nides which is termed as 1-cycle SANEX (selective actinide
extraction) process.9
It has been reported that the extraction mechanism with
TODGA involves reverse micelle formation with four TODGA
molecules,10 which results in unusually high extraction coeffi-
cients for the trivalent actinide ions as compared to the tetra- and
hexavalent actinide ions.8 The usual extraction tendency for acti-
nide ions with the common ‘actinide partitioning’ reagents such
as CMPO, malonamides, etc., is that tetravalent ions are more
extracted than the hexavalent ions, which in turn are more
extracted than the trivalent actinide ions. As diluents have a
very important role in the formation of reverse micelles, the
extraction trends and also the species extracted vary significantly
with the nature of the diluent. To exclude diluent effects, the
Nuclear energy is gaining an important role in worldwide
increasing energy demand. During reprocessing of nuclear fuel,
the strategy followed is partitioning of minor actinide and long
lived fission products followed by transmutation. The exploration
of various separation routes has been driven by the need for tech-
nology to successfully process the large quantity of high level
nuclear wastes. Solvent extraction is a widely used technique for
selective separation in addition to pre-concentration of metal
ions in biphasic water–organic solvent systems. Extractants such
as CMPO (carbamoyl methyl phosphine oxide), malonamide,
TRPO (trialkyl phosphine oxide), DIDPA (diisodecyl phosphoric
acid) and DGA (diglycolamide) are well known for the extrac-
tion of trivalent actinides from a moderately acidic medium.1–5
Use of CMPO leads to the generation of large amounts of sec-
ondary waste, while malonamide-based extractants are prone to
aRadiochemistry Division, Bhabha Atomic Research Centre, Trombay,
Mumbai-400085, India. E-mail: mpatra@barc.gov.in;
Fax: +91-22-25505151
bLaboratory of Molecular Nanofabrication, MESA+ Institute for
Nanotechnology, University of Twente, P.O. Box 217, 7500 AE
Enschede, The Netherlands
ligating sites can be grouped together on
a molecular
platform. Since four diglycolamide molecules are required for
complexation, calix[4]arene is the ideal platform. It is known
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt12364a
6970 | Dalton Trans., 2012, 41, 6970–6979
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that calixarenes appended with actinide specific ligating sites
such as CMPO give rise to more efficient metal ion binding and
extraction based on a co-operative complexation mechanism.11
We have synthesized diglycolamide functions preorganized at
the C-pivot and trialkylphenyl platforms resulting in good
extraction efficiencies of Eu3+ and Am3+.12 In addition, the
performance of
a
tripodal diglycolamide in solvent
extraction and supported liquid membrane studies was evaluated
by us for actinide extraction.13 However, very little work has
been reported thus far dealing with diglycolamide-functionalized
calixarenes.
Room temperature ionic liquids (RTILs) have aroused increas-
ing interest for their promising role as alternative diluent
medium in synthesis,14–17 separation18–22 and electrochemis-
try23,24 as a result of their unique chemical and physical
properties.25–29 These solvents exhibit several properties that
make them attractive as a potential basis for ‘green’ separation
processes, among them negligible vapour pressure, a wide liquid
range, non flammable, tunable viscosity and miscibility and
good thermal and radiation stability. Even minor structural vari-
ation, either in cationic or anionic moieties, can produce signifi-
cant changes in their physicochemical properties.30 This
tenability is obviously offering vast opportunities for the design
of ionic liquid-based separation systems, and also poses formid-
able challenges to separation scientists. Significant studies have
been reported on the extraction of Sr2+ from acidic nitrate media
by dicyclohexano-18-crown-6,31 uranyl ion by a CMPO–TBP
(tri-n-butyl phosphate) mixture,32 Ag+ by calix[4]arenes33 and
Eu3+ by 2-thenoyltrifluoroacetone (TTA) from a perchlorate acid
medium34 by various water immiscible N,N′-dialkyl imidazo-
lium-based ionic liquids. Odinets et al. synthesized a novel class
of functionalized ionic liquids with grafted CMPO moieties for
actinides and rare earth elements recovery.35 Attempts were also
made to understand the mechanism of metal ion transfer and
complexation of metal ions in ionic liquids.36–38 Recent reports
deal with the use of TODGA for the extraction of alkali metal,
alkaline earth metal and lanthanide ions with room temperature
ionic liquids.39,40 However, to our knowledge, the extraction of
actinide ions using TODGA in room temperature ionic liquids is
unprecedented.
Fig. 1 Structural formula of TODGA, C4DGA and Cnmim+.
2. Experimental
2.1 Materials
TODGA (Fig. 1) was obtained from Thermax Ltd, Pune, India.
The extractant was characterized by NMR, HPLC and GC-MS.
The synthesis of calix[4]arene-based DGA extractant C4DGA is
summarized in Scheme 1. Room temperature ionic liquids were
purchased from Iolitec, Germany and were used as received. The
dynamic viscosity and density of the ionic liquids were measured
using an Anton Paar equipment (Model No. SVM 3000). All reagents
were of AR grade and were used without further purification.
2.1.2 Synthesis of p-nitrophenol activated DGA (2). A sol-
ution of N,N-dioctyldiglycolic acid (1) (2.00 g, 5.6 mmol),
p-nitrophenol (0.81 g, 5.7 mmol), and DCC (1.22 g, 5.8 mmol)
in pyridine (60 mL) was stirred overnight at room temperature.
The solvent was evaporated and the residue was dissolved in n-
hexane, filtered and the filtrate was washed with 4% NaHCO3
solution (2 × 50 mL). The organic layer was dried with anhy-
drous MgSO4 and concentrated under reduced pressure. The
residue was purified by column chromatography (CH2Cl2–
MeOH, 98 : 2) to afford p-nitrophenol activated DGA (2)
(2.16 g, 81%) as a light yellow oil. 1H NMR: δ 0.81–0.93
(m, 6H, CH3), 1.14–1.38 (m, 20H, CH3(CH2)5), 1.45–1.60
(m, 4H, NCH2CH2), 3.18 and 3.31 (t, 2H J = 7.5 Hz, NCH2),
4.37 (s, 2H, OCH2), 4.58 (s, 2H, OCH2), 7.33 (d, 2H, J = 9.0
Hz, ArH), 8.28 (d, 2H, J = 9.0 Hz, ArH); 13C NMR: δ 14.3,
22.8, 29.5, 31.9, 60.1, 68.2, 69.4, 122.5, 125.5, 126.3, 145.9,
169.1; HRMS: m/z 479.3130 (M + H)+, calculated: 479.3121.
The present work deals with the extraction of Am(III) under
acidic feed conditions using TODGA in three commercially
−
available room temperature ionic liquids, viz. C4mim+·PF6
,
−
−
C6mim+·PF6 and C8mim+·PF6 (Cnmim+ = 1-alkyl-3-methyl-
imidazolium). Extraction of Am(III) was also carried out using
calix[4]arene-4-diglycolamide (abbreviated hence forth as
C4DGA) in C8mim+·PF6−, which displayed a reasonably good
solubility of the extractant, favourable kinetics of extraction and
reasonably high distribution ratio values. The stoichiometry of
the complexes was determined for both the TODGA–RTIL as
well as the C4DGA–RTIL extraction systems. The thermodyn-
amic parameters, such as the change in enthalpy, entropy and
Gibb’s free energy during the extraction of the complexes, were
also calculated for both ligands. Finally, a highly favourable sep-
aration behaviour of Am from U and Pu was observed when the
C4DGA–RTIL extraction system was used leading to possible
separation of Am from radioactive wastes containing U, Pu and
Am for possible applications in neutron sources, smoke detec-
tors, etc.
2.1.3. Synthesis of C4DGA. A mixture of cone tetrakis-
(aminopropoxy)calix[4]arene (3)41 (0.45 g, 0.5 mmol) and 2
(1.20 g, 2.5 mmol) and triethylamine (0.25 g, 2.5 mmol) in
chloroform (50 mL) was refluxed overnight. The crude reaction
mixture was successively washed with 2 M NaOH solution (3 ×
50 mL), 1 M HCl (3 × 50 mL), and water (2 × 50 mL). The
organic layer was concentrated under reduced pressure and the
crude product was purified by column chromatography
(CH2Cl2–MeOH, 96 : 4) to afford calix[4]arene 4-DGA (1.10 g,
71%) as a dense oil. 1H NMR: δ 0.81–0.93 (m, 24H, CH3), 1.07
(s, 36H, t-Bu), 1.17–1.35 (m, 80H, CH3(CH2)5), 1.43–1.60
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Scheme 1
2.3 Distribution studies
(m, 16H, NCH2CH2), 2.25 (pentet, 8H, J = 6.0 Hz, OCH2CH2),
3.04–3.16 (m, 12H, ArCH2Ar and NCH2), 3.27 (t, 8H, J = 7.5
Hz, NCH2), 3.46 (t, 8H, J = 6.0 Hz, NHCH2), 3.89 (t, 8H, J =
6.0 Hz, ArOCH2), 4.09 (s, 8H, OCH2CO), 4.27 (s, 8H,
OCH2CO), 4.34 (d, 4H, J = 12.0 Hz, ArCH2Ar), 6.75 (s, 8H,
ArH), 7.97–8.07 (m, 4H, NH); 13C NMR: δ 14.3, 22.9, 27.1,
29.5, 31.6, 32.0, 34.0, 36.7, 46.3, 47.0, 60.1, 133.9, 144.8,
153.5, 168.4, 170.0; MS: m/z 2234.7 (M + H)+, calculated:
2234.8.
The distribution studies were carried out by equilibrating the
organic phase, containing the required concentration of
the extractants in room temperature ionic liquids, as well as the
aqueous phase, with required feed acidity, spiked with 241Am
tracer, with a phase ratio of 1 : 1 in a thermostated water bath at
25 0.1 °C. The tubes were centrifuged for 5 min to enable
phase separation. After centrifugation, both phases were separ-
ated and assayed radiometrically. The distribution ratio, DM, was
defined as the ratio of the activity per unit volume in the organic
phase to that in the aqueous phase. Typically, the concentration
of americium was in the range of 10−6–10−7 M. The thermodyn-
amic parameters were calculated from temperature variation
studies carried out in the temperature range of 20–40 °C.
The experiments were carried out in duplicate with a precision
of 5%.
2.2 Radiotracers
241Am tracer was purified by ion exchange methods prior to use
and its purity was checked by both alpha as well as gamma ray
spectrometry. 241Am was assayed by a NaI(Tl) scintillation
detector using 60 keV gamma ray. 239Np (t1/2 = 2.3 d) tracer was
prepared by irradiation of natural U (as uranyl nitrate hexa
hydrate) in a reactor at a thermal neutron flux of 1 × 1013 neu-
trons cm−2 s−1 followed by the purification method reported by
us in an earlier publication.42 Plutonium tracer (mainly 239Pu)
was purified from 241Am and the 233U tracer was purified from
229Th using standard ion-exchange methods reported before.43
85,89Sr and 137Cs were obtained from BRIT (Board of Radiation
and Isotope Technology), India and were used after checking
their radiochemical purity. The valency of Np was adjusted to
the +4 state by using hydroxylamine hydrochloride as the
holding reductant. The valency of Pu was adjusted to the +4
state by using NaNO2 and ammonium metavanadate as the
holding oxidant, while the conversion of Pu to the +6 state was
achieved using AgO as the oxidizing agent.44 The radiometric
assay of 233U and Pu was done by alpha-liquid scintillation
counting using the Ultima Gold scintillator cocktail, while that
of 85,89Sr, 137Cs and 239Np was performed by gamma ray count-
ing using a NaI(Tl) scintillation counter. Alpha-spectrometry
was carried out using a silicon surface barrier detector.
3. Results and discussion
The room temperature ionic liquids used in the present study did
not extract Am(III) in the absence of TODGA (Table 1). Similar
behaviour was reported by Shimojo et al.39 in their studies on
lanthanide extraction using a different set of ionic liquids. In the
presence of TODGA as the extractant, however, a sharp increase
in the DM-values was observed. The distribution ratio values
obtained with 0.01 M TODGA in RTILs are significantly higher
than those obtained with TODGA in n-dodecane as the organic
phase (<0.1). Similar enhancements in the distribution ratio
values were reported with RTILs for Sr(II) extraction using
crown ethers,45,46 for Cs(I) extraction using calix-crown
ligands47,48 and for actinide extraction using a CMPO and TBP
mixture.22,49 The extractability of Am(III) in the ionic
liquids with TODGA as the extractant was found to be
−
−
C4mim+·PF6 > C8mim+·PF6 > C6mim+·PF6−. Shimojo et al.,
on the other hand, have shown that the extraction of Eu(III) with
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Table 1 DAm-values without and with 0.01 M TODGA, time to attain equilibrium DAm-values, and physical parameters of the ionic liquids
Equilibrium DAm
RTIL
No TODGA
0.01 M TODGA
Time to attain equilibrium (min)
Dynamic viscosity (mPa s)
Density (g cm−3
)
C4mim+·PF6−
C6mim+·PF6−
C8mim+·PF6
<0.01
<0.01
<0.01
102
34
74
60
60
180
250.86
308.48
694.19
1.366
1.290
1.235
−
Fig. 2 Extraction kinetics of americium from 3 M HNO3 feed into
0.01 M TODGA in Cnmim+·PF6− (n = 4, 6, 8).
Fig. 3 Extraction kinetics of americium from 3 M HNO3 feed into
−
0.001 M C4DGA in C8mim+·PF6
.
TODGA decreased with increase of the alkyl chain length.39
They have also reported that 10−4 M TODGA is capable of
extracting Eu(III), a trivalent lanthanide ion of comparable ionic
radius as Am(III), when the aqueous phase acidity was 0.01 M.
In the present case, a distribution coefficient value of ∼2500 was
obtained for Am(III) with 0.01 M TODGA in C4mim+·PF6− from
a feed containing 0.01 M HNO3, which is significantly lower
than that reported by Shimojo et al. for Eu(III). Higher extraction
of Eu(III) as compared to Am(III) has been reported previously
using TODGA as the extractant.7,8
consistent DAm-value of ∼35. On the other hand, for
C8mim+·PF6−, the DAm-value increased gradually up to 180 min,
followed by a plateau with a DAm-value of ∼70 showing the
slowest kinetics among the three RTILs. The observed slower
kinetics in all the cases as compared to the conventional diluents
like n-dodecane can be attributed to the high viscosity of the
ionic liquids. Table 1 shows that the dynamic viscosity of
−
C8mim+·PF6 is almost twice that of the other two RTILs
employed in the present work. As a consequence, C8mim+·PF6
−
showed the slowest kinetics among the three RTILs investigated,
though the butyl and the hexyl derivatives exhibited a nearly
similar extraction kinetics.
3.1 Extraction kinetics
The extraction kinetics was also studied using 0.001 M
−
C4DGA in C8mim+·PF6 (Fig. 3) under identical feed con-
In order to get information on the attainment of the extraction
equilibrium, the extraction kinetics was investigated. The kin-
etics of Am(III) extraction was reported to be fast when TODGA
in n-dodecane was used as the solvent system.8 In the present
study, the Am(III) extraction kinetics was studied from a 3 M
ditions. The concentration of C4DGA was kept 10 times lower
than that employed with TODGA as the extractant due to the
fact that four diglycolamide moieties are present in C4DGA to
make it a superior extractant than TODGA. The distribution ratio
of Am(III) increased sharply up to 180 min with a value of about
700 followed by a plateau with DAm ∼ 700. Though the vis-
cosity of the resulting organic phase is comparable with that
used with TODGA, the slower kinetics may also be due to the
conformational rigidity of the C4DGA ligand.
−
HNO3 feed solution using 0.01 M TODGA in Cnmim+·PF6
with an organic to aqueous volume ratio of 1. The ligand con-
centration was kept at 1 × 10−2 M, which is about 10 times less
than the usual concentration employed for actinide ion extraction
in the n-dodecane system. Fig. 2 represents the extraction kin-
etics of americium with TODGA into Cnmim+·PF6−. In case of
C4mim+·PF6−, a sharp increase in DAm-value was observed up to
60 min, while beyond that a plateau was observed with a distri-
bution ratio of ∼100. Similar observations were made in case of
3.2 Effect of the number of ligating sites and a substituent on
the calix[4]arene
−
C6mim+·PF6 where the DAm-value increased with the equili-
The effect of the number of ligating sites and a substituent on
the calix[4]arene extractant was also investigated using
bration time followed by a plateau beyond 60 min with a
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Table 2 Extraction of Am(III) with the diglycolamide functionalized
calix[4]arenes from 3 M HNO3 using 5 × 10−4 M extractant in
−
C8mim+·PF6
Extractant
DAm
C4DGA
194
0.692 0.021
0.233 0.065
2
C2DGA50
C2DGA-Bu50
−
C8mim+·PF6 as the diluent. For comparison purpose, the con-
centration of the calix-2DGA (abbreviated as C2DGA and
C2DGA-Bu50) ligands was taken as 5 × 10−4 M and the results
of Am(III) extraction from 3 M HNO3 feed solutions are listed in
Table 2. The equilibration time was fixed at 3 h, similar to that
used for the C4DGA as mentioned above. The results showed a
significantly lower extraction of Am(III) with the C2DGA com-
pounds, clearly indicating that the C4DGA is far more effective
for metal ion extraction. This is in line with our previous obser-
vations on the extraction behaviour of the three calix[4]arene
DGA compounds in n-dodecane as the organic diluent.50
3.3 Effect of the feed acidity
Fig. 4(a) shows the dependence of the distribution ratio of
Am(III) with the change in the feed nitric acid concentration. For
all the RTILs, an increase in the feed nitric acid concentration
led to a decrease in the distribution ratio values. Though a sharp
fall was observed in the lower acidity region, plateau like profiles
were noticed at higher acidities (beyond 3 M HNO3). Similar to
the trend mentioned above, the highest DAm-value (2502) was
−
obtained at 0.01 M HNO3 with C4mim+·PF6 as the diluent,
while the lowest DAm-value (1104) was obtained under identical
−
conditions with C6mim+·PF6 as the diluent. A drastic decrease
in the DAm-value was observed in the latter case by changing the
feed acidity from 0.01 M HNO3 (DAm = 1104) to 2 M HNO3
(DAm = 58) followed by a moderate decrease in the DAm-value
with a further increase of the feed acidity. A similar trend was
−
also noticed in case of C8mim+·PF6 as the diluent, where the
D
Am-value decreased from 2092 (0.01 M HNO3) to 241 (1 M
HNO3), followed by a moderate decrease upon further increase
of the feed acidity.
Fig. 4 Extraction behavior of americium from different feed acidities
The variation of the DAm-value with the feed acidity was
into (a) 0.01 M TODGA in Cnmim+·PF6− (n = 4, 6, 8) and (b) 7 × 10−4
−
studied using 7 × 10−4 M C4DGA in C8mim+·PF6 as the
−
M C4DGA in C8mim+·PF6
.
organic phase with phase ratio 1 : 1. The equilibration time was
kept at 3 h to assure complete equilibration. Also in this case,
the DAm-value decreased with increasing aqueous feed acidity
(Fig. 4(b)). The DAm-value was found to be 1847 at 0.01 M
HNO3, which decreased drastically up to 1 M HNO3 with a
DAm-value of 343 with much lower subsequent decrease upon
further increase in the aqueous phase acid concentration.
The decrease in the DAm-values with increasing feed acidity is
entirely opposite to the trend observed with the extraction of acti-
nides with diglycolamide extractants such as TODGA in diluents
like n-dodecane. The extraction profiles obtained in the present
study indicate that the extraction mechanism is not ion-pair type
as in case of the conventional diluents such as n-dodecane. On
the contrary, an ion-exchange mechanism is responsible for the
extraction of americium from nitric acid feed to the ionic liquid
phase. At higher acidity, due to the availability of large amounts
of H+, it competes with the metal ions in the ion-exchange
process resulting in substantially lower DAm-values. Extraction
of nitric acid was investigated by equilibrating the ionic liquid
solutions with 3 M HNO3 and the results suggested 7.3, 8.1 and
8.6% extraction of the acid using the butyl, hexyl and octyl
derivatives of the ionic liquids using 1.0 × 10−2 M TODGA as
the extractant.
3.4 Effect of the TODGA concentration
The extraction of Am(III) with varying TODGA concentration
was also investigated in the three RTILs and the results are pre-
sented in Fig. 5(a). An increase in Am(III) extraction with
increasing TODGA concentration in the ionic liquids was
6974 | Dalton Trans., 2012, 41, 6970–6979
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Am3þaq þ nTODGAIL þ 3Cnmimþ
IL
3þ
$ AmðTODGAÞn IL þ 3Cnmimþ
ð1Þ
aq
The species with the subscripts ‘aq’ and ‘IL’ refer to those in
the aqueous phase and in the room temperature ionic liquid
phase, respectively. A strong influence of hydrogen ion on
Am(III) extraction can be explained on the basis of a competing
equilibrium reaction as follows:
Hþaq þ TODGAIL þ Cnmimþ
IL
$ H ꢀ TODGAþIL þ Cnmimþ
ð2Þ
aq
Formation of adducts of TODGA with nitric acid is well
reported8 and could be attributed to be the reason for a sharp
decrease in the DAm values with increasing acidity. In case of
molecular diluents like n-dodecane, though HNO3·TODGA
adduct also gets extracted, the effect of decreasing DAm with
acidity leading to a plateau is seen only at higher acidities.8 On
the other hand, the increasing trend with increasing acidity seen
at lower acidities is due to increasing nitrate ion concentration
(nitrate ion is part of the extracted species). In case of room
temperature ionic liquids, the decrease in DAm with increasing
acid concentration is seen in the entire range of acidity (Fig. 4)
which corroborates the fact that the extracted species does not
contain nitrate ion and the ion-exchange mechanism as indicated
above is prevailing. A similar ion-exchange extraction mechan-
ism was reported for the extraction of lanthanide ions using
TODGA.39 If DAm is the distribution ratio, PC mim+ represents
n
the partition coefficient and KAm the equilibrium constant for the
extraction of americium, then it can be written that
3þ
3
3þ
aq
KAm ¼ ð½AmðTODGAÞn
ꢁ
½Cnmimþꢁaq Þ=ð½Am
ꢁ
IL
ð3Þ
n
3
½TODGAꢁIL ½CnmimþꢁIL
Þ
3+
Or, substituting DAm for the term [Am(TODGA)n ]IL/
[Am3+]aq one obtains
n
KAm ¼ DAm=ð½TODGAꢁ ꢀ PC mimþÞ
ð4Þ
n
Fig. 5 Distribution ratio of americium at (a) varying TODGA concen-
−
trations from 3 M HNO3 feed into Cnmim+·PF6 (n = 4, 6, 8) and (b)
Taking the logarithm and rearranging:
varying C4DGA concentrations from
3 M HNO3 feed into
C8mim+·PF6
.
−
log DAm ¼ log KAm þ log PC mimþ þ n log½TODGAꢁ
ð5Þ
n
A plot of logDAm vs. the concentration of TODGA or
C4DGA should result in straight lines with a slope ‘n’, i.e. the
number of TODGA or C4DGA molecules associated with the
complex formed during the extraction and the intercept rep-
observed. This indicates that TODGA is participating in the
extraction process by being part of the extracted species (vide
infra). A similar increase in Am(III) extraction was also observed
resents log KAm + log PC mim+
.
n
−
with C4DGA in C8mim+·PF6 as the diluent (Fig. 5(b)).
On the basis of eqn (5), a slope analysis was conducted as a
function of the equilibrium concentration of a ligand in
Cnmim+·PF6− (n = 4, 6, 8) and used to determine the fundamen-
tal stoichiometry of the Am3+–diglycolamide complex formed in
3 M HNO3 feed. Fig. 5(a) represents the variation in the log
DAm value with the TODGA concentration. In case of TODGA
in C4mim+·PF6−, the slope is 2.23 0.07, while in case of the
ionic liquid with n-hexyl and n-octyl derivatives, TODGA con-
centration dependencies of 2.29 0.06 and 2.02 0.07 were
obtained. Fractional slope values (Fig. 5(a)) indicated extraction
of mixed species of 1 : 2 as well as 1 : 3 M : L compositions.
Extraction of mixed species has previously been reported in case
Shimojo et al. reported a similar increase in the extraction of
lanthanide ions with TODGA in ionic liquids.39 It was of interest
to understand the nature of the extracted species and the number
of TODGA molecules involved in the metal ion extraction.
3.5 Extraction mechanism and extraction constants
In view of the above observations, the extraction of Am(III) using
TODGA in ionic liquids can be best represented by an ion-
exchange mechanism as per the following equation.
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of molecular diluents.7,8 However, Shimojo et al., reported
Table 3 Extraction (K′Am) constants of americium(III) from 3 M HNO3
at 298.15 K
extracted species with 1 : 3 stoichiometry for lanthanide ion
extraction using Cnmim+·NTf2 as the ionic liquid medium.39
−
Ligand
Diluent
Log K′Am
The extraction of americium with TODGA can thus be expressed
as
1 × 10−2 M TODGA
C4mim+·PF6−
C6mim+·PF6−
C8mim+·PF6−
C8mim+·PF6
6.45 0.18
6.24 0.13
5.92 0.17
8.26 0.15
−
Am3þaq þ 2–3TODGAIL þ 3Cnmimþ
5 × 10−4 M C4DGA
IL
3þ
$ AmðTODGAÞ2 IL þ 3Cnmimþ
ð6Þ
aq
Note: The log K′Am values were obtained from the log D vs. log[L]
plots.
A similar study was carried out to determine the stoichiometry
of the complex formed by americium with C4DGA in
C8mim+·PF6−. The log DAm vs. log[C4DGA] plot (Fig. 5(b))
yielded a slope value of 1.8 (χ2 = 0.9979) indicating the extrac-
tion both ML and ML2 type of species from 3 M HNO3 with the
latter being present predominantly in the ionic liquid phase.
Though this is rather unusual in view of the large coordination
number feasible with 24 coordinating atoms present around the
metal ion, a similar extracted species was reported in our earlier
studies involving a tripodal diglycolamide ligand.13 The extrac-
tion of Am(III) with C4DGA can be represented as
suggesting that the Kform values are approximately two orders of
magnitude higher as compared to the extraction constants.
3.6 Determination of thermodynamic parameters
The effect of temperature on the extraction of americium from
3 M HNO3 by 0.01 M TODGA or 5 × 10−4 M C4DGA into
−
C8mim+·PF6 was studied as this RTIL gave reasonably high
Am3þaq þ 2C4DGAIL þ 3Cnmimþ
IL
distribution coefficient values with faster extraction kinetics. The
distribution ratio of americium either with TODGA (Fig. 6(a)) or
with C4DGA (Fig. 6(b)) was found to decrease with increasing
temperature, indicating the extraction process to be exothermic.
The change in enthalpy (ΔH) during the complexation can be
calculated by using the Van’t Hoff equation:
3þ
$ AmðC4DGAÞ2 IL þ 3Cnmimþ
ð7Þ
aq
From eqn (3), [Cnmim+]aq and [Cnmim+]IL can be regarded as
−
constant (the aqueous phase is saturated with Cnmim+·PF6 salt)
and, therefore, the conditional extraction constant K′Am can be
given as:
ΔH ¼ ꢂ2:303 R Δlog D=Δð1=TÞ
A plot of log D vs. 1/T gives a straight line with a slope of
−ΔH/2.303 R, while the change in Gibb’s free energy (ΔG) can
be calculated from the following equation
ð15Þ
0
KAm ¼ ½Am ꢀ L2 ꢁIL=½Am3þaqꢁ ½LꢁIL
ð8Þ
3þ
2
where L represents the extractant (TODGA or C4DGA when the
two ligand molecules are assumed to be present in the extracted
3+
species). Substituting DAm for the term [Am·L2 ]IL/[Am3+]aq,
ΔG ¼ ꢂ2:303 RT log KA0 m
ð16Þ
one obtains
The change in entropy (ΔS) at a particular temperature can be
calculated using the equation
K0Am ¼ DAm=½LꢁIL
ð9Þ
2
log DAm ¼ log K0Am þ 2 log½LꢁIL
ð10Þ
ΔG ¼ ΔH ꢂ TΔS
ð17Þ
The thermodynamic parameters calculated for the two extrac-
tion systems are summarized in Table 4. The ΔH values, calcu-
lated from the slope of the log D vs. 1/T plot (vide supra), are
−71.61 kJ mol−1 and −86.93 kJ mol−1, respectively, for
TODGA and C4DGA. The overall enthalpy change during
extraction (ΔH) is a sum of the contribution due to dehydration
of the metal ion (ΔH1), formation of the complex (ΔH2), and dis-
solution of the metal complex into the organic phase (ΔH3). The
favourable ΔH in C4DGA over TODGA can be mainly attributed
to the contributions from ΔH2 and ΔH3. Due to the pre-organized
structure of C4DGA, ΔH2 (C4DGA) > ΔH2(TODGA) and on
account of the higher lipophilicity of C4DGA, ΔH3(C4DGA) >
ΔH3(TODGA). The ΔG calculated from eqn (17) shows that the
extraction of americium by C4DGA (ΔG = −46.53 kJ mol−1) is
energetically more favoured than that by TODGA (ΔG =
−34.63 kJ mol−1), which is also reflected in their DAm-values
(vide supra). On the other hand, the negative entropy change in
case of TODGA (ΔS = −123.24 J mol−1) as well as C4DGA (ΔS
= −134.67 J mol−1) can be attributed to the loss of rotational
entropy of the ligands during complexation.
The K′Am-value is calculated from the intercept of a plot of
log D vs. log[TODGA]IL at a constant nitric acid concentration.
K′Am was calculated for americium with TODGA in
Cnmim+·PF6 (n = 4, 6, 8) and C4DGA in C8mim+·PF6−. The
−
complex formation equilibrium in an ionic liquid can be
described by the following equations.
3þ
Am3þIL þ 2LIL ¼ ½Am ꢀ L2ꢁ
ð11Þ
ð12Þ
ð13Þ
IL
3þ
3þ
2
Kform ¼ ½Am L2 ꢁIL=½Am
ꢁ
½LꢁIL
IL
Kform ¼ K0 ½Am3þꢁ =½Am
ꢁ
3þ
IL
Am
aq
Kform ¼ K0 =PAm
ð14Þ
Am
where PAm is the partition coefficient of Am3+ defined as the
ratio of [Am3+]IL to [Am3+]aq. Table 3 lists the extraction con-
stants (K′Am) of americium for TODGA as well as C4DGA in
Cnmim+·PF6−. As indicated in Table 1, the PAm values are <0.01
6976 | Dalton Trans., 2012, 41, 6970–6979
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Table 4 Thermodynamic parameters (ΔG, ΔH and ΔS) of americ−ium
extraction from 3 M HNO3 using TODGA/C4DGA in C8mim+·PF6 at
300 K
ΔG
ΔH
ΔS
Ligand
(kJ mol−1
)
(kJ mol−1
)
(J K−1 mol−1
)
1.0 × 10−2
TODGA
M
−34.64
−46.53
−71.61
−86.93
−123.24
−134.67
5.0 × 10−4 M
C4DGA
Table 5 Distribution data of actinides and fission products from 3 M
HNO3 feed solutions using TODGA and C4DGA as the extractant in
C8mim+·PF6− as the diluent
Distribution coefficient values using
DGA extractants
Metal ion
TODGA
C4DGA
Am(III)
Pu(IV)
75
194
30.7
1.19
0.52
3.92
0.024
0.12
20.6
0.64
0.19
7.46
0.017
0.15
2+
PuO2
2+
UO2
Np(IV)
Sr2+
Cs+
Table 6 Comparative separation behaviour in ionic liquid and
conventional diluent with TODGA and C4DGA
Solvent system
SFAmU (DAm/DU)
SFAmPu (DAm/DPu)
TODGA–n-dodecanea
TODGA–C8mim+·PF6
C4DGA–n-dodecaneb
132
144
45
25
63
173
303
−a
−b
C4DGA–C8mim+·PF6
1021
a 1.0 × 10−2 M TODGAwas used. b 5.0 × 10−4 M C4DGAwas used.
Fig. 6 Variation in distribution ratio of americium at (a) different temp-
eratures from 3 M HNO3 feed using 1.0 × 10−2 M TODGA in
Interestingly, the DAm-values with C4DGA are much higher than
those obtained with TODGA, while the D-values for U(VI) and
Pu(VI) are significantly lower with C4DGA than with TODGA.
The low extraction of U and Pu in the hexavalent oxidation state
is rather surprising and may be based on the unusual coordi-
nation geometry of the actinyl ion and the strain associated with
complex formation using the ligand. The separation factor values
(defined as the ratio of the distribution ratio values of the con-
cerned metal ions) were determined for the metal ions Am–U
and Am–Pu in both extractants, viz. TODGA and C4DGA, in
C8mim+·PF6− and (b) different temperatures from 3 M HNO3 feed using
5 × 10−4 M C4DGA in C8mim+·PF6
.
−
3.7 Extraction of actinides and selective Am extraction from
wastes
Solvent extraction studies of several other actinide ions such as
UO22+, Np4+, Pu4+, and PuO2 were carried out and the results
2+
−
are listed in Table 5. As most radioactive wastes consist of long
lived fission product nuclides such as 90Sr (t1/2: 28.5 y) and
137Cs (t1/2: 30.1 y), distribution data of Sr(II) and Cs(I) are also
included using both TODGA as well as C4DGA in
C8mim+·PF6 as well as n-dodecane (the conventional diluent
used for metal ion separations, especially actinide separations)
and the results are listed in Table 6. It is interesting to note that
the ionic liquid medium shows an unusually high enhancement
in the separation factor values of the metal ions as compared to
n-dodecane. Moreover, C4DGA was found to be a superior
extractant from the actinide separation point of view as well.
−
C8mim+·PF6 at concentrations of 1 × 10−2 and 5 × 10−5 M,
respectively, from 3 M HNO3 feed solutions (Table 5). It is inter-
esting to note that the D-values for Am(III), Np(IV), and Pu(IV)
are significantly higher compared to those obtained with other
metal ions, viz. PuO22+, UO22+, Cs+ and Sr2+, indicating the
effectiveness of TODGA in RTIL for minor actinide partitioning.
−
And finally, the combination C4DGA in C8mim+·PF6 results in
a highly efficient separation system involving Am extraction
(Table 6).
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in room temperature ionic liquids instead of the conventional
n-dodecane as the diluent showing improved separation factors.
The unusually high extraction of Am(III) can be attributed to the
unique ion-exchange extraction mechanism in the RTIL
medium, while the high viscosity of the RTILs is responsible for
the slower extraction kinetics. Am(III) can be selectively
extracted from a mixture containing U(VI), Pu(IV) and Pu(VI), and
Am(III) by adjusting the oxidation state of Pu to +6. The large
negative entropy changes observed during the thermodynamic
studies, point to the formation of ordered extracted complexes
with both TODGA as well as C4DGA, possibly without the lib-
eration of the inner-sphere water molecules, indicating outer-
sphere interactions.
The analytical significance of this work lies in the exception-
ally high separation factors obtained and this can be applied for
the selective extraction of Am(III) from acidic waste samples.
Moreover, Am(III) can be selectively recovered from high level
waste, which is of utmost importance for possible applications in
industry.
Acknowledgements
The authors (A.S., P.K.M.) thank Dr A. Goswami, Head Radio-
chemistry Division, BARC for his keen interest in this work.
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