Complexation of novel diglycolamide functionalized calix[4]arenes: Unusual extraction behaviour, transport, and fluorescence studies

Article abstract of DOI:10.1039/c1dt11561h

Three diglycolamide functionalized calix[4]arenes (calix[4]-nDGA) were synthesized and evaluated for their extraction behaviour towards lanthanide/actinide ions. Exceptionally high D_Am and D_Pu values indicate these radiotoxic element

Full text of DOI:10.1039/c1dt11561h

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COMMUNICATION
Complexation of novel diglycolamide functionalized calix[4]arenes: Unusual
extraction behaviour, transport, and fluorescence studies†
a
b
a
b
b
Prasanta K. Mohapatra,* Mudassir Iqbal, Dhaval R. Raut, Willem Verboom, Jurriaan Huskens and
Shrikant V. Godbole
a
Received 19th August 2011, Accepted 19th October 2011
DOI: 10.1039/c1dt11561h
Three diglycolamide functionalized calix[4]arenes (calix[4]-
nDGA) were synthesized and evaluated for their extraction
behaviour towards lanthanide/actinide ions. Exceptionally
high DAm and DPu values indicate these radiotoxic elements
can be selectively removed from nuclear waste solutions.
Transport and laser induced fluorescence studies indicated
strong complexation of the trivalent metal ions with the
calix[4]-4DGA ligand.
mounted on calixarenes have shown significant enhancement in
the complexation ability. Moreover, if the functional groups
8
are appended onto a calixarene framework, the energy spent
on bringing the ligands together for complexation is saved and
9
the reaction becomes favourable. It was of interest, therefore,
to synthesize DGA ligands mounted on a framework such as a
calix[4]arene so that the diluent dependence can be discounted.
The present paper deals with studies on the extraction of
lanthanide and actinide ions with three novel calix[4]arene-DGA
extractants (L
I
, LII, LIII) synthesized for the first time (see Fig.
Diglycolamide (DGA) extractants have been reported to be the
most promising reagents for ‘actinide partitioning’, a key step in
high level waste remediation for the mitigation of the radiotoxic
10
1
). Flat sheet supported liquid membrane transport studies as
3
+
well as the fluorescence spectral behaviour of their Eu complexes
are also reported. The solvent extraction studies were carried out
using mM concentrations of the extractants and a reasonably high
extraction of Am , Pu , and Pu was observed as compared
to the distribution ratio (D) values expected for a diglycolamide
1
hazards of long–lived radionuclides. The extraction property of
diglycolamides was also reported to be unusual as compared to
the common extractants for ‘actinide partitioning’ such as CMPO,
3
+
3+
4+
2
DIDPA, and TRPO. While these extractants mentioned above
1,7,11
4
+
2+
3+
extractant (Table 1).
extract the actinide ions in the order Pu > UO
2
> Am , the
3
+
4+
2+
1
Exceptionally high distribution ratio values were obtained
with the tetravalent and trivalent actinide and lanthanide ions
compared to reagents such as TODGA (where D values of the
actinide ions were significantly lower, Table 1), while the hexavalent
uranyl ion and Cs and Sr ions were practically unextracted.
The higher extraction of the tetravalent as compared to the
trivalent actinide ions is unusual for the DGA-based extractants
and possibly implies a different complex formation mechanism.
diglycolamides extract in the order: Am > Pu > UO
2
. This is
rather unusual since the ionic potential of the trivalent actinide ion
is much lower than that of the hexavalent or the tetravalent actinide
ions and is explained on the basis of a reverse micelle mechanism,
+
2+
3
which has been supported by small angle neutron scattering and
4
dynamic light scattering data. The proposed mechanism involves
extraction of the metal ion into the core of the reverse micelle,
which contains four units of the diglycolamide at 3 M HNO and
3
Nevertheless, the results indicate L
extraction of Am and Pu from nuclear waste solutions at about a
00 times lower extractant inventory. It was reported that TODGA
extracts metal ions via a reverse micelle mechanism which may
I
can be used for the selective
the number of DGA molecules in the reverse micelle depended
5
on the feed acidity. Zhu et al. have studied the extraction of
1
about 75 elements with TODGA and reported a unique selectivity
based on the cation size. The reverse micelle formation is highly
dependent on the diluent properties and hence, the extraction
3
a,5
be responsible for the unusual extraction behaviour while the
higher extraction of Pu as compared to Am is as per the ionic
potential trend. Though Cs ion extraction was reported to be
insignificant with the much studied DGA extractants such as
TODGA and T2EHDGA, those reagents extract Sr ion to a
significant extent making it necessary to search for a suitable
decontamination step. Ligands LI–III appear unique as they no
longer show the preference for trivalent metal ions as displayed
with TODGA and T2EHDGA, while at the same time Cs and Sr
are not extracted to any significant extent making them potent for
minor actinide partitioning. Ligand L is particularly interesting
as it shows very high distribution ratio for the tetravalent and
trivalent actinide (lanthanide) ions. Another unique feature is the
4
+
3+
1
a,3
behaviour changes drastically with increasing diluent polarity.
Our earlier studies with DGA-based tripodal ligands have
+
6,7
shown exceptional extraction and transport properties. Ligands
2
+
12
a
Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mum-
13
bai, 400085, India. E-mail: mpatra@barc.gov.in; Fax: +91-22-25505151;
Tel: +91-22-25594576
+
2+
b
Laboratory of Molecular Nanofabrication, MESA+ Institute for Nan-
otechnology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The
Netherlands
I
†
Electronic supplementary information (ESI) available:
Solvent extraction data, transport data and fluorescence spectral data,
synthesis of the reagents, etc.
See DOI: 10.1039/c1dt11561h
2
+
relative non-extractability of the UO2 ion, which is extracted to a
3
60 | Dalton Trans., 2012, 41, 360–363
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I
Fig. 1 Structural formulae of the three novel calix[4]arene-DGA extractants L , LII, and LIII.
a
Table 1 Distribution data of some important actinide and fission product element ions relevant from the waste management point of view
Distribution coefficient
b
3+
3+,c
4+,d
2+
3+
+
2+
Ligand
Am
Pu
Pu
UO
2
Eu
Cs
Sr
C-4-DGA (L
I
)
26.5 ± 1.4
1.06 ± 0.01
0.04 ± 0.01
0.09 ± 0.01
30.4 ± 0.8
1.67 ± 0.00
0.29 ± 0.01
0.03 ± 0.01
67.9 ± 1.4
6.86 ± 0.25
1.12 ± 0.10
0.35 ± 0.03
0.17 ± 0.01
0.05 ± 0.01
0.04 ± 0.01
<0.01
155 ± 1.72
5.28 ± 0.02
0.28 ± 0.01
2.01 ± 0.02
<0.001
<0.001
<0.001
<0.001
0.023
C-2-DGA (LII
)
<0.001
<0.001
<0.001
C-2-DGA-2-Bu (LIII
TODGA
)
a
b
-3
c
Diluent: n-dodecane; Feed: 3 M HNO
3
.
2
Concentration of the extractant: 3 ¥ 10 M. A mixture of hydrazine hydrate and hydroxylammonium nitrate
was used as the holding oxidant.
d
was used as the reducing agent. NaNO
1
3+
relatively higher extent by the DGAs. The insignificant extraction
of the uranyl ion is probably due to the presence of the axial
‘
For all three ligands, Eu was extracted to a greater extent
3
+
than Am , which is in line with the observation for other DGA
extractants. However, the separation factor (DEu/DAm) value with
1
O’ atoms and the stereochemical restrictions associated with the
simultaneous complexation of the four DGA units. One of the
proposed steps prior to minor actinide partitioning is the recovery
of U, which can otherwise lead to significant hold up of the
extractant as well as cause serious problems such as ‘third phase
formation’, which can affect the hydrodynamics in plant scale
operations. The present results show insignificant U extraction,
making any prior U recovery step redundant. Moreover, extraction
n-dodecane as the diluent (5.85) was somewhat lower than that
1a
reported with TODGA (9). It was rather surprising that the more
organophilic LIII resulted in lower extraction of the metal ions as
compared to LII, containing hydrophilic OH groups. This may be
attributed to steric factors due to the presence of the n-butoxy
groups, while forming the ML type species.
2
Another interesting feature of the calix-DGA extractants was
noticed during the flat sheet supported liquid membrane studies.
2
+
of PuO
2
I
was also found to be much lower (D-value with L is 1.06)
3
+
4+
3+
as compared to both Pu and Pu , suggesting that prior oxidation
of Pu to the hexavalent state can be done to suppress Pu extraction.
The species extracted by these ligands were ascertained by a
series of solvent extraction studies using a variety of diluents and
n-dodecane resulted in fairly good D-values. The slopes of the
While the extraction and stripping of Am was satisfactory using
L
I
as the extractant and 3 M HNO
3
and 0.01 M HNO as the feed
3
and strip solutions, respectively the transport of the metal ion was
negligible. On the other hand, when 0.01 M EDTA (pH 2) was
used as the strip solution in the receiver phase, fast mass transfer
3
+
linear regression fit lines of the log D vs log[L
I
] plots are ~1 for
resulting in ~90% Am transport in 6 h was noticed (Fig. 2). The
3
+
Eu , while it changes to close to 2 when the ligands LII and LIII
were used for extraction.† The general extraction equilibrium is
presented as eqn(1):
transport rates were drastically lower than those reported with
7
tripodal DGA ligands with comparable extractant concentration.
Though the slower transport rates in the present case can be partly
3
+
I
attributed to the larger molar volume of the Am -L complex, it
3
+
-
can also be due to slow kinetics of the metal ion release with either
M
(a) + nL(o) + 3NO
3
(a) = M(NO
3
)
3
·L(o)
(1)
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3+
-3
Fig. 2 Transport profiles of Am using 1.13 ¥ 10 M L
I
in n-dodecane
as the carrier solvent. Feed: 3 M HNO . Receiver: As indicated.
3
3+
Fig. 3 Emission spectra of Eu with the calix-DGA extractants in
acetonitrile : water (5 : 1).
distilled water or 0.01 M HNO
phase. On the other hand, EDTA can facilitate the decomplexation
reaction due to its high stability constant with the actinide ion.
Faster mass transfer with EDTA was also reported in our earlier
3
as the strip solution in the receiver
14
3
+
17
Eu with about 6 water molecules and 3 nitrate ions and LII
being non-coordinating, that with a lifetime of 565 ms suggests the
formation of an intermediate containing about 2 water molecules
and 3 nitrate ions in the inner co-ordination sphere and a possible
interaction by four donor atoms of the extractant molecule. The
third species with a lifetime of 1500 ms assumes the absence of
water molecules; it is possibly coordinated by 9 donor atoms from
7
work containing tripodal DGA as the carrier extractant.
Laser induced fluorescence spectral studies were also carried
15
-5
out using 1 ¥ 10 M Eu nitrate solution in 3 M HNO diluted
3
3
+
5
with acetonitrile (1 : 5). Emission spectra of Eu for the D
0
→
7
5
7
F
2
(617 nm, electric dipole), D
0
→ F
1
(592 nm, magnetic dipole)
5
7
and D
0
→ F
4
(690 nm, electric dipole) indicated a significant
enhancement in the fluorescence intensity in the presence of a
LII with the ethereal ‘O’ atom also coordinating. With increasing
comparable concentration of L
I
(Fig. 3). On the other hand, while
ligand concentration, the fraction of the third species increased.
Finally, with LIII, only two types of species were formed with
lifetimes of 184 ms (7%) and 565 ms (93%) indicating that complete
removal of inner sphere water molecules was not possible. Though
a 10 times higher concentration of LII showed a higher fluorescence
3
+
efficiency than only the Eu ion, very little change was seen when
III was used as the complexing agent.‡ Moreover, lifetime data
indicated formation of 1 : 1 complex species with L , while the
formation of 1 : 2 species was observed with both LII as well as LIII
L
I
solvent extraction data indicated 1 : 2 species formation with LIII,
the results from laser induced fluorescence studies showed a weak
complexation with the presence of 1–2 water molecules, which may
be responsible for the poor partitioning of the complex towards
the organic phase.
3
+
(
Table 2). Though the lifetime of Eu in the absence of the ligands
was 173–184 ms, it increased to about 1.7 ms in the presence of
. Appearance of additional peaks at 617 and 700 nm indicates a
strong interaction with the DGA moieties similar to that observed
L
I
In conclusion, appending the calix[4]arene framework with
four DGA moieties enhances the complexation ability of the
ligand enormously by ‘co-operative phenomena’ resulting from
16
with TODGA in an earlier publication. Fluorimetric titration
studies indicated the formation of 1 : 1 species even in the presence
of a 20 times excess of the ligand.† On the other hand, fluorescence
studies with LII pointed to three different types of species, one
decaying with a lifetime of 183 ms, and the other two decaying at
7
entropic stabilization. In addition, the calix[4]-nDGA ligands
exhibit a different complexation behaviour than the TODGA
ligand. Very interestingly, by suitably changing the oxidation state
of Pu to the +6 state, trivalent Am can be preferentially extracted
from a mixture of Am, Pu, and U. The preparation of Am
565 and 1500 ms, respectively. While the species with the lifetime
of 183 ms represents a predominant nitrato and aquo complex of
3+
Table 2 Emission lifetimes of the complexes of Eu with the three calix[4]-nDGA ligands
Ligand
Decay time (in ms)
Inference
No ligand
173–184
6 water; 3 nitrate
Coordination by at least 9 donor atoms, no water, no nitrate
6 water; 3 nitrate
2 water, 3 nitrate, coordination by one ligand with 4 to 6 donor atoms
Coordination by at least 9 donor atoms, no water, no nitrate
6 water; 3 nitrate
L
L
I
T
T
T
T
T
T
1
1
2
3
1
2
= 1700 (100%)
= 183 (13%)
= 565 (58%)
= 1500 (29%)
= 184 (7%)
II
L
III
= 565 (93%)
2 water, 3 nitrate, coordination by one ligand with four 4 to 6 donor atoms
3
62 | Dalton Trans., 2012, 41, 360–363
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selective membranes may lead to the development of sensors for
Am.
(aminopropoxy)- calix[4]arene and bis(aminopropoxy)calix[4]arene, re-
spectively, with p-nitrophenol activated DGA in the presence of triethy-
lamine as a base in refluxing chloroform. 1,3-Dibutoxycalix[4]arene 2-
DGA (LIII) was synthesized by the reaction of 1,3-dibutoxycalix[4]arene
with N-(3-bromopropyl)phthalimide to produce 1,3-dibutoxy-2,4-
bis(phthalimidopropoxy)calix[4]arene which was subsequently con-
Notes and references
I
verted to LIII. The formation of the calix[4]arene DGA ligands L –LIII
‡
The figure includes data with a 1 : 1 M : L ratio for comparison purposes.
was confirmed by the NMR data, the glycolic methylenes show singlets
at d 4.09 and 4.27, 4.11 and 4.16, 4.08 and 4.24, respectively. The
ESI mass spectra exhibit distinct M + H peaks. The detailed synthesis
protocol will be published shortly.
1
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Exch., 2001, 19, 91; (b) S. A. Ansari, D. R. Prabhu, R. B. Gujar, A. S.
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1
18; (c) G. Modolo, G. Asp, C. Schreinemachers and V. Vijgen, Solvent
3+
Extr. Ion Exch., 2007, 25, 703.
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2
(a) D. Serrano-Purroy, B. Christiansen, R. Malmbeck, J. P. Glatz and
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prepare– by the addition of NaNO . In both cases, the stability
2
of the oxidation states was checked intermittently by spectropho-
tometry as well as by TTA extraction. The actinides were assayed
by alpha liquid scintillation counting using Ultima Gold scintillator
cocktail.
3
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supplied by Edinburgh Analytical Instruments, UK controlled by CD
920 controller and pumped by OPO lasers. The life time data analysis
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and V. K. Manchanda, Spectrochim. Acta, 2009, A73,
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(
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0 Calix[4]arene 4-DGA (L
prepared by reaction of the corresponding known tetrakis
9
3
1
I
) and Calix[4]arene-2-DGA (LII) were
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 360–363 | 363