Nonionic metal-chelating surfactants mediated solvent-free thermo-induced separation of uranyl

Article abstract of DOI:10.1039/b707016k

Thermo-responsive metal-chelating surfactants permit the solvent-free, cloud point extraction of uranyl nitrate and afford a real molecular economy compared to conventional separation techniques. The Royal Society of Chemistry and the Centre National de l

Full text of DOI:10.1039/b707016k

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Nonionic metal-chelating surfactants mediated solvent-free
thermo-induced separation of uranylw
a
vost,ab Laurence Berthon, Thomas Zemb and
c
b
Chantal Larpent,* Sylvain Pre
´
b
Fabienne Testard
Received (in Montpellier, France) 9th May 2007, Accepted 20th June 2007
First published as an Advance Article on the web 6th July 2007
DOI: 10.1039/b707016k
Thermo-responsive metal-chelating surfactants permit the sol-
vent-free, cloud point extraction of uranyl nitrate and afford a
real molecular economy compared to conventional separation
techniques.
the main drawbacks of the CPE technique is its poor mole-
cular efficiency in terms of the molar ratio of surfactant and
chelating agent to substrate.
A possible approach to overcoming this drawback is to
design special functional surfactants capable of recognizing
the target. Considering the separation of metal ions, an out-
standing issue in the field of environmental remediation, the
design of metal-chelating thermo-responsive surfactants
(MCTS), which combine metal binding affinity with tempera-
ture-dependent behaviour might afford a real molecular econ-
omy provided that the metallo-surfactant formed after
complexation remains thermo-responsive. Functional surfac-
tants have attracted a great deal of research attention. Among
them various chelating surfactants have been described and
Despite major progress in the field of separation techniques
in recent decades, the removal of pollutants from aqueous
media remains an important challenge and increasing envir-
onmental awareness has created a growing need for new
solvent-free sustainable processes. In this context, consider-
able interest is devoted to stimuli-responsive systems based on
polymers or surfactants which undergo a phase separation
upon heating or addition of salts and thus provide environ-
mentally benign alternatives to conventional liquid–liquid
6
,7
1
–3
used as catalysts or in conventional separation techniques.
extractions.
Surfactant-based separation techniques that
take advantage of the solubilization capabilities of supra-
molecular assemblies and do not require the use of organic
solvents and extractants are still attracting widespread atten-
On the other hand, polyoxyethylene substituted phosphine
8
ligands have been used in thermo-regulated catalytic systems,
but, to our knowledge, there is no previously reported example
of thermo-induced separation processes based on thermo-
separating metal-chelating surfactants.
1
,3,4
tion.
Among them, cloud point extraction (CPE), based on
the temperature-dependent properties of nonionic surfactants,
has been widely used for separation of various organic com-
pounds as well as in the concentration steps of analytical
As an illustration of the molecular economy and the en-
hanced extraction efficiency brought by the use of functional
surfactants, we report here on new MCTS specially designed
for the solvent-free thermo-induced and salt-regulated separa-
tion of uranyl nitrate from aqueous solutions. The separation
and concentration of uranium(VI) is indeed a topic of sustained
interest in nuclear fuel processing, where the limitation of
wastes is of prime importance, as well as in analytical science
for the detection of traces of this element owing to its high
1
,3,4
methods.
Cloud point extraction technology is based on
the separation of an aqueous micellar solution of a polyoxy-
ethylene (POE) surfactant into a concentrated phase (co-
acervate), containing most of the surfactant, and a dilute
aqueous phase when the temperature is raised above a given
1
,3,4
temperature referred to as the cloud point (CP).
The
extraction relies on the micellar solubilization of the solute
and hence requires large amounts of surfactant compared to
solute, and applies to quite hydrophobic solutes. Owing to
their solubility in water, separation and concentration of metal
ions by CPE are most often performed in the presence of an
additional chelating agent that forms a lipophilic metal com-
9
toxicity. In order to control simultaneously the surfactant
6
properties and the binding affinity, we designed and synthe-
sized a series of MCTS, with a di-block molecular structure, by
tethering nonionic alkylpolyoxyethylene surfactants (C
with a uranyl chelating group (Fig. 1).
i j
E )
1
,3–5
The driving force for the inverse temperature-dependent
solubility of C E in water is the dehydration of the POE
plex and thus favors the partition in the micellar phase.
Consequently, according to green chemistry concepts, one of
i j
groups on heating. Consequently, the CPs of C E surfactants
i
j
a
depend on their molecular structure with known relationships:
the CP increases as the number of ethoxy units is increased and
10
Institut Lavoisier, UMR-CNRS 8180, Universite´ de Versailles-St-
Quentin, 78035 Versailles, France. E-mail: larpent@chimie.uvsq.fr;
Fax: +33 1 39 25 44 52; Tel: +33 1 39 25 44 13
DSM/DRECAM/SCM/LIONS, CEA-Saclay, 91191 Gif/Yvette,
France
DEN/DRCP/SCPS/LCSE, CEA-ValRhoˆ, BP17171, 30207 Bagnols-
sur-Ce`ze, France
decreases with the length of the aliphatic tail. The choice of
b
the C
one to control the phase separation temperature. In this study,
we compare MCTSs 1 derived from C12 and 2 derived from
i j
E block in functional surfactants might therefore allow
c
E
5
w Electronic supplementary information (ESI) available: Synthesis
and characteristics of surfactant 1, NMR and ESI-MS spectra of
mixtures of surfactants and uranyl nitrate. See DOI: 10.1039/
b707016k.
11
8
the more hydrophilic C10E .
Since 1,n-diamides (with n = 3 to 11, e.g. malonamides,
s
uranyl ionophore ) are well-known ligands for the uranyl
1
424 | New J. Chem., 2007, 31, 1424–1428
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auxiliary ligands and shift the complexation equilibrium to-
ward the formation of a neutral nitrato complex with two
nitrate ions coordinated in a bidentate manner, as already
1
3
observed with classical diamides. ESI-MS spectra of 1 : 2
mixtures of MCTSs and uranyl nitrate in water display base
peaks at m/z 459.5 and 511.5, respectively for 1 and 2, which
2
+
are assigned to the [MCTS À UO
2
]
ions, thus indicating
that MCTSs form 1 : 1 complexes with uranyl at low uranium :
1
surfactant ratios.
7
MCTS 1 and 2 behave as classical nonionic surfactants and
exhibit a reversible temperature-dependent behaviour with
clouding and subsequent phase separation when a micellar
solution is heated above its CP. As expected, their CPs (for
1
wt% solutions in water), 53 1C for 1 and 74 1C for 2, increase
with the polarity of the surfactant unit. In the presence of
1
0
alkali nitrates, the changes in clouding temperatures are
1
,18
comparable to those observed with classical C E :
The
CPs of 1 and 2 are increased to 66 1C and 84 1C, respectively,
in water + LiNO 4 M and lowered to 20 1C and 26 1C,
respectively, in water + NaNO 4 M.
i
j
3
3
Interestingly, the clouding temperature of MCTS with a di-
block molecular structure is controlled by the surfactant unit
and can be further modulated by the addition of alkali
nitrates.
Temperature-induced separation of uranyl nitrate from
water without any added solvent is readily achieved with
MCTSs 1 and 2 (Table 1). The CPE procedure, shown in
Fig. 2, is remarkably simple and allows the concentration of
uranyl in the coacervate, which does not exceed 5% of the
overall volume. In agreement with the NMR analyses and as
1
2
previously observed in liquid–liquid extractions, the separa-
tion of uranyl is favoured in the presence of an excess of
nitrate anions (Table 1, entries 6, 8, 9). Considering the
decrease of CP towards ambient induced by sodium nitrate,
1
Fig. 1 (A) Structures and synthesis of MCTSs 1 and 2. (B) H NMR
titration curve for 1 in CDCl with increasing amounts of UO (NO
3
2
3 2
) .
3
LiNO has been chosen as the nitrate source. Efficient separa-
tions, with up to 60–70% of U extracted in the concentrated
phase and concentration ratio ([U] in the coacervate/[U] in the
aqueous phase) ranging from about 30 to 85, are achieved in a
1
2–15
ion,
we choose to graft a diamide clip, derived from
acetyllysine, to the tip of POE group in order to provide an
affinity for the targeted ion. Lipophilic diamides have been
used for the liquid–liquid extraction of U(VI) and the extrac-
tion yields were found to depend on the concentration of
single step in the presence of 4 M LiNO . MCTSs 1 and 2 are
3
quite efficient even at low surfactant : uranyl ratio (4 : 1 to
1
2
nitrate anions; we therefore anticipated that cloud point
extractions with our diamide-based thermo-responsive surfac-
tants might be further regulated by the concentration of added
salts thus offering a simple means to recover the surfactant.
Compounds 1 and 2 were easily prepared, in 72 and 77%
yields, in two steps from C E and C E by alkylation with
Table 1 Thermo-regulated separations of uranyl nitrate
a
Ui
b
b
Entry
Surf.
R
CP/1C
Uext (%)
R
Ue
R
Uc
1
2
3
4
5
6
7
8
9
2
2
2
2
1
1
1
1
1
0.25
0.50
0.75
1
63
47
35
23
52
23
12
53
55
22
82
69
63
53
47
60
47
45
0
0.17
0.32
0.40
0.47
0.07
0.22
0.32
0
0.19
0.35
0.48
0.56
0.09
0.28
0.38
0
1
2
5
10 8
bromoacetic acid followed by coupling with N-acetyllysine
methyl ester (Fig. 1A).
0.12
0.48
0.71
0.49
0.49
0.5
The interactions of MCTSs with uranyl nitrate were studied
c
by NMR and mass spectrometry. Fig. 1B shows a typical
1
titration curve based on the H NMR shifts in CDCl
d
19
14
14
0.09
0.07
0.07
nd
3
with
10
11
C
C
E
E
0.11
0.13
12
5
8
1
6
increasing uranyl nitrate concentrations. The protons close
to the amide groups are the most shifted while protons of the
10
0.5
a
Unless otherwise stated [surfactant] = 0.04 M, [LiNO
3
] = 4 M,
C
i j
E
unit are not significantly affected. These observations
3 Ui 2 3 2
[HNO ] = 0.01 M, R = UO (NO ) : surfactant molar ratio in the
indicate that MCTSs coordinate to uranyl ion with the oxygen
starting solution, separations were performed at
b
T
=
CP
+
RUe = mol U extracted per surfactant initially introduced
1
3
1
and RUc = mol U per surfactant in the coacervate. Without
0 1C.
atoms of the two amide groups. In water, similar variations
of the chemical shifts are observed but only in the presence of
an excess of nitrate anions suggesting that nitrates act as
c
d
LiNO
3
.
3
[LiNO ] = 2 M.
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Fig. 2 Thermo-induced uranyl separation (a) and surfactant recovery
b). Typical example: (a) uranyl separation (entry 6, Table 1): a
solution of 1 (S, 40 mM), UO (NO (U, 20 mM), HNO (10 mM)
and 4 M LiNO separates above 23 1C into a dilute phase I (1.95 mL)
and a concentrated deep yellow phase II (0.05 mL), U partition: 53%
in I and 47% in II (concentration factor [U]II/[U] E 20), 1 partition:
% in I and 93% in II. (b) Surfactant recovery: coacervate II is diluted
(
2
3
)
2
3
3
Fig. 3 Variations of CP in extraction experiments with surfactants 1
and 2. (A): CP (1C) versus RUc (U : MCTS molar ratio in the
coacervate). (B): Relative variation of CP [(CP during CPE–CP with-
0
7
2
3
in 1.95 mL water–HNO (10 mM), where it separates above 52 1C into
a dilute phase III and a concentrated phase IV, U partition: 99% in III
out U)/CP without U] versus RUc. 1 (y = À2.22x, R = 0.996),
2
2
(y = À1.25x, R = 0.998).
and 1% in IV, 1 partition: 4% in III and 96% in IV.
2
4,25
1
0
: 1; Table 1 entries 1–7) and permit the separation of up to
.4–0.5 U-per-MCTS. Blank experiments with classical non-
when the molar fraction of complex increases.
These
observations may be explained by the formation of neutral
nitrato uranyl–surfactant complexes with stoichiometries
higher than 1 : 1, which might be favoured by the close
5 8
ionic surfactants C12E and C10E confirm that the covalent
linkage of the chelating moieties to the surfactant block
improves the extraction efficiency (Table 1, entries 10–11).
Interestingly, MCTSs afford a real molecular economy
compared to conventional extraction systems. For instance,
in a classical liquid–liquid process, the extraction of uranyl
was performed with a large excess of diamide extractant
2
6
proximity of the chelating groups at the micellar surface.
Accordingly, modifications of the morphology of aggregates
formed by functional surfactants upon binding of metal ions
have already been observed and micelle-to-vesicle transitions
have been rationalised by the coordination of two polar
2
7,28
(
about 300 to 1500 mol per U, the latter molar ratio leading
headgroups to the metal.
Although further studies are
to an almost quantitative extraction with a distribution coeffi-
1
required to determine the structure of the complexes formed
during CPE, the appearance of 1 : 2 and 1 : 3 complex ions
together with the predominant 1 : 1 complex ion in the ESI-
MS spectra of the coacervates supports this hypothesis (the
observed relative abundances are 100%, 90% and 25% for
1 : 1, 1 : 2 and 1 : 3 complexes, respectively).
2
cient of about 100) and in previously reported cloud point
extractions of uranyl nitrate, large excesses of surfactant
(
Triton X114, 0.25 to 2 wt%, i.e. about 100 to 1000 mol per U)
and chelating agent (PAN or 8-HQ, 5 to 10 mol per U) were
5
used.
Moreover, profiting from the effect of the nitrate concentra-
tion on the complexation rate, back-extraction, which allows
surfactant recovery, is readily achieved as shown in Fig. 2:
dilution of the coacervate in water in the absence of additional
nitrate salt followed by heating above the CP leads to a
coacervate containing more than 90–95% of the surfactant
initially introduced while U remains almost quantitatively in
the diluted phase.
It is noteworthy that the relative decrease of CP is steeper
with compound 1, derived from C E , than with 2, derived
1
2 5
1
0 8
from C E (Fig. 3B). In the latter case, the bulky POE group
imposes a larger area per molecule, consequently the curvature
of the aggregates and hence the CP are less sensitive to the
complexation yield. From an experimental point of view,
MCTSs with large POE groups like 2 should be preferred
because they offer a wider range of operating uranyl concen-
trations and temperatures: the clouding temperatures range
from 23 1C to 63 1C for UO (NO ) : surfactant ratios ranging
It is worth noting that significant decreases of the clouding
temperatures are observed when the extraction yields increase.
As shown in Fig. 3, for a given MCTS the decrease of the CP
displays a reverse linear dependence on the uranium : surfac-
tant molar ratio in the coacervate (RUc) i.e. on the molar
fraction of uranyl complex. Remarkably, this figure reveals
that the complexation yield can be easily deduced from the CP
2
3 2
from 1 : 1 to 1 : 4.
The functional surfactants reported here, which combine
temperature-responsive behaviour with chelating properties,
are useful tools for the solvent-free thermo-induced separation
of uranyl associated with easy recovery of the surfactant and
afford a real molecular economy. Their di-block structure
permits an easy control of their solution properties, thereby
allowing optimization of the operating temperature ranges by
the proper choice of the surfactant unit. The building block
strategy depicted here is expected to be quite general and
should give access to libraries of functional surfactants with
tunable metal binding affinities and modulable clouding tem-
perature ranges. The design of thermo-responsive surfactants
that bind transition metal ions and their applications for the
removal of heavy metals as well as ligands in thermo-regulated
phase transfer catalysis are currently under investigation.
1
9,20
measurement.
Clouding and subsequent phase separation of solutions of
nonionic surfactants are known to involve a micelle growth,
with a sphere to rod transition, driven by the reduction in
2
1
headgroup area upon dehydration. As already observed for
2
2,23
mixtures of nonionic surfactants,
the variations of CP are
consistent with the formation of mixed micelles containing free
MCTS molecules and uranyl–surfactant complexes during
extraction. A reduction of the area per surfactant molecule
upon complexation, which induces the formation of larger
aggregates, accounts for the observed linear decrease of the CP
1
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Experimental
surfactant- and U-poor upper diluted phase was separated
from the coacervate (highly viscous) and the concentrations of
uranium and surfactant were measured respectively by X-ray
Synthesis of surfactants 1 and 2. Typical procedure for 2
A solution decyloxyoctaethylene glycol C E (20.10 g,
3
0
fluorescence spectroscopy and total organic carbon titration
DC-190 Rosemount Dohrmann carbon analyser). XRF ana-
1
0 8
(
3
9.4 mmol, high purity monodisperse obtained from Nikko)
in 50 mL anhydrous THF was added under N upon stirring
to a suspension of NaH (60% in mineral oil, 5.50 g,
38.0 mmol) in 40 mL anhydrous THF. After 30 min, a
lyses (11.8 to 14.8 keV) were performed with a X-MET 920
METOREX apparatus after dilution of the samples in 1 M
2
nitric acid solution (reference solutions of U(VI) in nitric acid
À1
1
1
M in the 23.6 to 2364 mg L concentration range were used
solution of bromoacetic acid (8.30 g, 59.0 mmol) in 20 mL
anhydrous THF was added dropwise over 0.5 h. After stirring
for 12 h at 60 1C, the reaction mixture was quenched with 1 N
HCl and THF was removed under vacuum. The aqueous
for calibration).
Acknowledgements
phase was extracted with Et O and the organic phase was
2
washed with 1 N HCl, brine and then dried over MgSO . The
4
We thank Dr C. Madic and Dr H. Desvaux for helpful
discussions. Dr K. Baczko, Dr S. Lassiaz and N. Zorz are
gratefully acknowledged for their assistance in the surfactants
syntheses.
intermediate acid was isolated after removal of the solvent
(
22.40 g, 100%) as a yellow oil and used without further
purification for the following step. 1-hydroxybenzotriazole
HOBt (3.3 g, 24.3 mmol) and Ac-Lys-OMe (2.7 g, 14.4 mmol)
were added under N
mmol) in 25 mL dry DMF. After stirring for 15 min, EDC
3.4 g, 17.8 mmol) was added and the reaction mixture was
2
to a solution of intermediate acid (4 g,
7
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O–MeOH–iPrNH
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). R (AcOEt–MeOH 3 : 1) =
). H NMR (CDCl ): d (ppm) =
); 6.41 (d, J = 7.4 Hz, 1H, NH );
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.16 (t, J = 6.5 Hz, 1H, NH
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1
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(
2
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72.93, 170.19, 170.08 (CQO); 71.53 (CH CH O); 70.97,
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2
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À1
2.31 (CH
306 (NH), 2923, 2850 (C–H), 1745 (CQO ester), 1677 (CQO
CN–H O), m/z: 753.6
MH , 72%); 775.6 (MNa , 100%); 791.6 (MK , 13%).
Anal. Calcd. for C37 O: C, 58.32; H, 9.66; N,
Á 0.5 H
.67; O, 28.35. Found: C, 58.15; H, 9.80; N, 3.43; O, 28.41%.
Surfactant 1 has been obtained from C E using the same
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General procedure for CPE
10
1
1
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5 8
The appropriate amount of uranyl nitrate was added to 2 mL
À1
of a solution of surfactant (0.04 mol L ), lithium nitrate
À1 À1
(
4 mol L ) and nitric acid (0.01 mol L ) in water. After
homogenization, the cloud point (CP) was measured and the
solution allowed to stand at T = CP + 10 1C in a thermo-
stated bath until phase separation was obtained (1 to 4 h). The
1
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1
1
5 (a) M. Lerchi, E. Reitter, W. Simon and J. Fresenius, Anal. Chem.,
994, 348, 272–276; (b) J. Senkyr, D. Amman, P. C. Meier, W. E.
Morf and W. Simon, Anal. Chem., 1979, 51, 788–790; (c) M.
Shamsipur, A. Soleymanpour, M. Akhond, H. Sharghi and A.
R. Massah, Talanta, 2002, 58, 237–246.
6 During these NMR titration experiments, the added solid uranyl
nitrate solubilizes in the chloroform solutions (with molar ratio U/
MCTS of about 1.2–1.5). These observations further indicate that
MCTSs form coordination complexes with U(VI).
correlations have been reported between the size of the micelles
(aggregation number) and the cloud point (macroscopic prop-
a decrease of the cloud-point corresponds to an
increase of the micelle aggregation number (correlated with a
reduction of the area per head group).
1
21,23,25
erty):
25 (a) S. Puvvada and D. Blankschtein, J. Phys. Chem., 1992, 96,
5579–5592; (b) J.-L. Li and B.-H. Chen, Chem. Eng. Sci., 2002, 57,
2825–2835.
26 N. A. J. M. Sommerdijk, K. J. Booy, A. M. A. Pistorius, M. C.
Feiters, R. J. M. Nolte and B. Zwanenburg, Langmuir, 1999, 15,
7008–7013.
27 (a) X. Huang, M. Cao, J. Wang and Y. Wang, J. Phys. Chem. B,
2006, 110, 19479–19486; (b) X. Luo, S. Wu and Y. Liang, Chem.
Commun., 2002, 492–493.
1
7 Small peaks, with relative abundances of about 10%, correspond-
ing to 1 : 2 uranyl–surfactant complexes were also observed.
8 (a) K. S. Sharma, S. R. Patil and A. K. Rakshit, Colloids Surf., A,
1
2
2
003, 219, 67–74; (b) H. Shott, J. Colloid Interface Sci., 2003, 260,
19–224; (c) H. Shott, Colloids Surf., A., 2001, 186, 129–136; (d) T.
Iwanaga, M. Suzuki and H. Kunieda, Langmuir, 1998, 14,
775–5781; (e) P. Firman, D. Haase, J. J. M. Kahlweit and R.
Strey, Langmuir, 1985, 1, 718–724.
28 An additional way of explaining the results is through considera-
5
tion of the packing parameter, p = v/a
length and volume of the aliphatic chain and a
o
l
c
(where l
c
and v are the
o
the area occupied
29
1
9 Similar correlations between the stability constants of crown ether
surfactants for alkali cations and the CP variations in the presence
by a surfactant headgroup). This factor is commonly used to
predict the morphology of surfactant aggregates: the prediction of
p is p r 1/3 for small spherical micelles, 1/3 o p r 1/2 for
cylindrical micelles and 1/2 o p r 1 for vesicles or bilayers. The
coordination of two surfactants to a same uranyl (i.e. formation of
1 : 2 complexes) will allow two separated headgroups to get close to
20
these cations have already been observed .
0 M. Okahara, P.-L. Kuo, S. Yamamura and I. Ikeda, J. Chem. Soc.,
Chem. Commun., 1980, 586–587.
2
2
2
1 A. Blom and G. G. Warr, Langmuir, 2005, 21, 11850–11855, and
references therein.
2 The CPs of mixtures of nonionic surfactants depend on the
o
each other: the average area per molecule (a ) will therefore be
reduced resulting in an increase of the value of p and, hence, in a
micellar growth.
composition as CP = a
the CPs of surfactants A and B, a
3 G. Briganti, S. Puvvada and D. Blankschtein, J. Phys. Chem.,
991, 95, 8989–8995.
a
CP
a
+ a
b
CP
and a
b
(where CP
a b
and CP are
23
a
b
their molar fractions)
.
29 (a) J. N. Israelachvili, D. J. Mitchell and B. W. Ninham, J. Chem.
Soc., Faraday Trans. 2, 1976, 72, 1525–1568; (b) D. J. Mitchell and
B. W. Ninham, J. Chem. Soc., Faraday Trans. 2, 1981, 77,
601–629.
30 M. K. Tiwari, A. K. Singh and J. S. Sawhney, Anal. Sci., 2005, 21,
143–147, and references therein.
2
2
1
4 Controversy still persists regarding the origin and microstructure
in solutions near their cloud point because both micellar growth
and intermicellar interactions may be important. However, clear
21
1
428 | New J. Chem., 2007, 31, 1424–1428
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