Desalination 229 (2008) 294–301
Extraction chromatography of lanthanides using
N,N,N′,N′-tetraoctyl diglycolamide (TODGA) as the stationary
phase
M. Husaina,b, S.A. Ansaric, P.K. Mohapatrac, R.K. Guptab, V.S. Parmara,
V.K. Manchandac*
a
Department of Chemistry, University of Delhi, Delhi-110 007, India
b
School of Biotechnology, GGSIP University, Delhi-110 006, India
c
Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India
Tel. +91 (22) 25593688; Fax +91 22 25505151; email: vkm@magnum.barc.ernet.in
Received 20 February 2006; accepted revised 21 October 2007
Abstract
Studies on the uptake of several lanthanides, viz. La, Pr, Pm, Eu, Tb, Ho, Er, Yb and Lu from nitric acid medium
were carried out using extraction chromatographic resin made by impregnating Chromosorb-W with N,N,N′,N′tetraoctyl diglycolamide (TODGA). In the batch studies, distribution coefficient (Kd) values increased with the
HNO3 concentration reaching a plateau at around 2.5 M HNO3. At a given acidity, the Kd values of Ln(III) ions
followed the order of their ionic potential. Separation of lanthanides was attempted using TODGA/Chromosorb-W
column. The breakthrough capacity of the column was determined for all the Ln(III) ions in the presence of 1 g/L
of their respective carrier salts. An attempt has been made to separate lanthanides, viz. La3+, Eu3+, Tb3+, Ho3+, Yb3+
and Lu3+ using the column.
Keywords: TODGA; Extraction chromatography; Separation; Lanthanides
1. Introduction
Lanthanides are usually considered chemically
similar elements since 4f electrons are inactive as
they are well shielded by the outer 5s and 5p orbitals. The ionic radius of lanthanide ions de-
creases smoothly from La to Lu as a result of progressive filling of 4f orbitals across the lanthanides
[1]. Thus, the ionic potential of lanthanide ions is
one of the important properties that can be employed for their group separation. Separation of
lanthanides (as a group) by neutral organophosphorus extractants such as tri-n-butyl phosphate
*Corresponding author.
0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.desal.2007.10.016
M. Husain et al. / Desalination 229 (2008) 294–301
(TBP) has been reported earlier [2,3]. However,
this extractant was found insignificant for the
mutual separation of lanthanides. In addition to
this, use of phosphorus-based extractants is also
avoided from the waste management point of view
as large volumes of solid waste are generated
which encounters several problems in the secondary waste management. Use of amide-based
extractants is, therefore, preferable due to their
innocuous nature and complete incinerability.
Musikas et al. have extensively studied the
substituted malonamides for the separation of lanthanides and actinides from the solution of different origin [4–6]. However, any detail study on
the extraction of the entire group of lanthanide
ions by malonamides is scarce. In comparison to
malonamides, recently developed diglycolamides
(DGA) have been reported to exhibit higher distribution ratio of lanthanides and actinides in solvent extraction studies in strong acid range (>3 M
HNO3). These type of diamides are open chain
tridentate ligands which form coordination structure similar to the macrocyclic ligands [7]. The
hydrophobicity of DGA extractants is controlled
by the length of carbon chain attached to the
amidic N-atoms. Amongst several derivatives of
DGA studied, N,N,N′,N′-tetraoctyl diglycolamide
(TODGA, Fig. 1) shows enough lipophilicity in
n-dodecane and has been identified as one of the
promising extractants for lanthanides and actinides [8–21]. Solvent extraction studies employing TODGA have shown an increased extraction
of lanthanides with their ionic potential, which
forms our basis to attempt the separation of interlanthanides with this reagent using extraction
chromatography technique.
O
O
C8H17
N
C8H17
C
O
C
CH2
CH2
C8H17
N
C8H17
Fig. 1. N,N,N′,N′-tetraoctyl diglycolamide (TODGA).
295
Though solvent extraction processes are extensively employed for plant scale operations for
the recovery of metal ions in very large (kg/ton)
quantities, the major problem associated with this
technique is the generation of large volume of
secondary waste and handling of large volume of
inflammable diluents, particularly when the separation involves metal ions in low concentration.
It is, therefore, imperative to look for an alternate
technique particularly when the metals quantities
involved are in g/mg range. In this regard, extraction chromatography offers distinct advantages
over solvent extraction such as lower inventory
of solvents, reduced volume of secondary waste
and ease of operation when performed in column.
In the present work, an attempt has been made to
investigate the sorption behaviour of selected lanthanides by extraction chromatographic resin
material employing TODGA as the stationary
phase. The mutual separation of few lanthanides
has been attempted with TODGA resin column.
2. Experimental
2.1. Materials and radionuclides
N,N,N′,N′-tetraoctyl diglycolamide (TODGA)
was synthesised at Department of Chemistry,
University of Delhi, Delhi. The details of the synthesis procedure and characterization of the product are reported elsewhere [22]. Chromosorb-W
(dimethyl dichlorosilane treated acid washed
celite diatomaceous silica, mesh size 60–80) was
procured from Johns Manville, USA, washed with
distilled water and acetone followed by vacuum
drying before use. Lanthanides such as La, Pr, Eu,
Tb, Ho, Er, Yb and Lu (as metal oxide) were procured from Leico Industries Inc., USA. The purities of all the rare earth oxides were >99.99%.
The tracers 140La, 142Pr, 154Eu, 160Tb, 166Ho, 171Er,
175
Yb and 177Lu were obtained after irradiation of
their respective natural isotopes in APSARA Reactor, Bhabha Atomic Research Centre at a neutron flux ~1012 ncm–2s–1. 147Pm was employed from
the laboratory stock, which was recovered from
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M. Husain et al. / Desalination 229 (2008) 294–301
high level waste (HLW) employing established
procedure [23]. All the other reagents were of
analytical reagent grade and were used without
further purification.
2.2. Preparation of chromatographic resin material
The extraction chromatographic resin material
was prepared by impregnating TODGA on
chromosorb-W. A known amount of TODGA was
diluted in acetone (1:1) and was mixed with an
equal weight of Chromosorb-W. The slurry was
equilibrated for 24 h in a mechanical shaker followed by the solvent removal by flushing nitrogen gas with gentle stirring. The resultant material was vacuum dried to constant weight. The
weight percentage of the extractant loaded on the
resin was calculated from the difference in the
weight of the solid support before and after impregnation of the extractant and was found to be
47% w/w. Loading percentage of the extractant
was also confirmed by elemental (C, H, N) analysis.
2.3. Batch distribution studies
The distribution coefficients (Kd) for lanthanides were measured in the batch experiments
as follows: aqueous solutions (1 mL) of the lanthanide radiotracer in required nitric acid solutions were agitated with a known amount of resin
material (~25 mg) in stoppered glass test tubes.
The equilibration of the two phases was carried
out in a thermostated water bath maintained at
25±0.1°C for 45 min. This time was found to be
sufficient to attain the equilibrium condition as
reported in our earlier work [11]. Subsequently,
the tubes were centrifuged, the aqueous layers
were separated and centrifuged for the second
time. Suitable aliquots (0.1–0.5 mL) of the aqueous phase were removed before and after equilibration for assaying radiometrically. Assay of all
radionuclides, except 147Pm, were carried out by
gamma counting employing a well type NaI(Tl)
scintillation counter connected with a PC based
multi channel analyser. Assay of 147Pm was carried out by beta counting using liquid scintillation counter employing a toluene-based scintillator containing 0.7% (w/v) PPO (2,5-diphenyl
oxazole), 0.03% (w/v) POPOP (1,4-di-[2-(5phenyloxazoyl)]-benzene) and 30% (v/v) HDEHP
(bis(2-ethylhexyl) phosphoric acid). The distribution coefficient (Kd) was calculated employing the
following formula:
K d = ⎣⎡( C0 − C ) / C ⎦⎤ ⋅ V / W (mL/g)
(1)
where Co and C are the concentrations of metal
ions (in counts per unit time per unit volume) before and after equilibration, V is the volume of
aqueous phase used (mL) and W is the weight of
the resin material employed (g). The material balance was within the error limits (±5%).
2.4. Preparation of column
The column was prepared by packing ~300 mg
of chromatographic resin material in a borosilicate glass column of about 4 mm diameter. Various column parameters were estimated by the
method reported in the literature [24]. The bed
volume and the bed density were calculated from
the column dimensions and the weight of the
packed chromatographic resin material. The volume of mobile phase (Vm), referred to as the free
column volume, was calculated from the difference in the bed volume and the volume of resin in
the bed, the latter quantity is being the weight of
resin material in bed divided by its density. The
volume of liquid stationary phase (Vs) was estimated from the weight of the extractant impregnated on the support material packed in the column divided by the density of the extractant. The
former quantity was obtained from the weight of
the resin packed in the column and the percentage of extractant loaded on the resin. The calculated parameters and specification of the packed
EXC column are listed in Table 1. The column
was preconditioned by passing excess of appro-
M. Husain et al. / Desalination 229 (2008) 294–301
Table 1
Characteristics of the TODGA chromatographic resin
material and the corresponding packed column
10
4
10
3
297
Stationary phase
Support material
Mesh size
Extractant loading, % w/w
Average density of resin, g/mL
TODGA
Chromosorb-W
60–80
47
1.08
Packed column
Bed volume, mL
Bed density, g/mL
Density of stationary phase, g/mL
Volume of stationary phase,
mL/mL of bed
Volume of mobile phase,
mL/mL of bed
1.257
0.629
0.891
0.252
Kd - Ln(III) ions
Resin material
10
2
10
1
10
0
La
Pr
Pm
Eu
Tb
Ho
Er
Yb
Lu
0.558
10
-1
0
1
2
3
4
5
6
[HNO3], M
priate nitric acid solutions, prior to the introduction of the sample solutions. All the column operations were carried out at ambient temperature
(25±0.2°C) at a flow rate of about 0.4 mL/min.
The breakthrough curves and elution profiles were
obtained by plotting radioactivity (in terms of
counts per unit time per unit volume) in the effluent versus volume of the solution passed.
3. Results and discussion
3.1. Batch studies
The extraction of Ln(III) ions from nitrate
medium with neutral TODGA molecules is usually represented by the following expression:
−
Ln 3+
(aq.) + 3NO 3 (aq.) + nTODGA (org.)
⎯⎯
→ [ Ln(TOGDA) n (NO3 )3 ]
←⎯
⎯
(org.)
(2)
The subscripts aq. and org. represent the species present in the aqueous and the organic phases,
respectively, while “n” is 3 for lanthanum and 4
for rest of the lanthanides as per the solvent extraction studies reported earlier [15]. Fig. 2 rep-
Fig. 2. Uptake of Ln(III) ions as a function of HNO3 concentration by TODGA/chromosorb-W resin; temperature:
25°C.
resents the Kd values for lanthanide (Ln(III)) ions
as a function of HNO3 concentration. The Kd values for Ln(III) ions were found to increase with
the nitric acid concentration up to 2.5 M HNO3,
beyond which a plateau was observed. Increase
in the Kd values was ascribed to increased nitrate
concentration with acidity which favours the formation of neutral extractable species as per the
law of mass action [Eq. (2)]. Moreover, an increase
in the HNO3 concentration corresponds to a reduction in the water activity, therefore, the energy required for dehydration of Ln(III) ions decreases and TODGA molecule easily form complex at higher HNO3 concentration. Saturation in
the Kd values of Ln(III) ions above 2.5 M HNO3
could be due the uptake of nitric acid by TODGA
caused by the high basicity of the TODGA molecule (KH = 4.1) [9]. The increase in the Kd value
was sharper for heavier lanthanides (Yb3+, Lu3+)
as compared to the lighter counterparts (La3+, Pr3+).
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M. Husain et al. / Desalination 229 (2008) 294–301
This behaviour was attributed to the increasing
ionic potential of Ln(III) where the complex formation with both, TODGA as well as nitrate ion,
increases sharply for the heavier lanthanides.
Though the sorption of all lanthanides was almost identical at higher acidity (>2 M HNO3), a
significant difference in their Kd values was observed at lower acidities (<1 M HNO3). This typical behaviour of lanthanides at lower acidity can
be employed for their mutual separation. The Kd
values observed at 0.1 M HNO3 followed the order of ionic potential of Ln(III) ions. A plot of Kd
vs. ionic potential of Ln(III) ions is shown in
Fig. 3. Diglycolamides behave as hard donor
ligands due to their carbonyl oxygen atoms. In
general, the stability constant of Ln(III)-simple
hard ligands complexes should increase across the
lanthanide series due to decrease in the ionic radii, corresponding to an increase in the positive
charge density on the Ln(III) ions [25]. Our ob-
10
4
Yb
Ho
Lu
Er
Tb
Kd-Metals
Eu
10
3
Pm
Pr
10
2
La
2.8
Table 2
Separation factor (SF) of lanthanide ions by TODGA/
chromosorb-W resin at 0.1 M HNO3
Ln(III)
ions
Ionic
potential
Kd values
SF
La
Pr
Pm
Eu
Tb
Ho
Er
Yb
Lu
2.83
2.97
3.06
3.16
3.26
3.37
3.41
3.49
3.53
35
115
500
1535
3050
3800
4450
5950
6050
Pr/La = 3.29
Pm/Pr = 4.35
Eu/Pm = 3.07
Tb/Eu = 1.99
Ho/Tb = 1.25
Er/Ho = 1.17
Yb/Er = 1.34
Lu/Yb = 1.02
Lu/La = 173
servation followed the identical trend as shown
in Fig. 3. Similar observations have also been reported by solvent extraction studies employing
0.1 M TODGA in n-dodecane [8]. These observations clearly demonstrate that TODGA molecules sorbed on Chromosorb-W behaves similar
to that observed in liquid-liquid extraction systems. The separation factors between the lanthanide ions employing TODGA/Chromosorb-W
at 0.1 M HNO3 are given in Table 2. This table
suggests the possibility of the mutual separation
of lanthanides upto Eu(III) as the SF values for
adjacent elements are greater than 3. However,
mutual separation of lanthanides beyond Eu(III)
is seems to be difficult as the corresponding SF
values are very close to 1. Nevertheless, a lighter
lanthanide can be separated satisfactorily from a
heavier one such as La(III) from Lu(III), as their
SF value is 173.
3.2. Column studies
2.9
3.0
3.1
3.2
3.3
3.4
3.5
3.6
Ionic potential of Ln(III)
Fig. 3. Uptake of Ln(III) ions as a function of ionic potential by TODGA/chromosorb-W resin; aqueous phase:
0.1 M HNO3; temperature: 25°C.
The characteristics of the TODGA/chromosorb-W column are given in Table 1. The column
was preconditioned by passing excess (10 column
volumes) of appropriate nitric acid solutions, prior
to the introduction of the sample solutions. All
the column operations were carried out at ambient temperature (25±0.2°C) at a flow rate of
M. Husain et al. / Desalination 229 (2008) 294–301
0.4 mL/min. The performance of the column significantly depends upon the flow rate of the mobile phase. The effect of flow rate on the breakthrough of metal ions has been reported in our
earlier publication [26]. From our previous finding, a flow rate of ~0.4 mL/min was found satisfactory for the present 12 cm long column. The
breakthrough curves were obtained for all Ln(III)
ions in the presence of 1 g/L of their respective
salts at 0.1 M HNO3. Low acidic medium (0.1 M
HNO3) was selected for the column studies due
to the better separation factor observed in the batch
studies (Table 2). The breakthrough curve obtained for all Ln(III) ions under identical conditions is given in Fig. 4. It was observed that for
La(III) ions breakthrough appeared after passing
1.5 mL of the affluent solution. Whereas, in the
case of Pr(III) and Eu(III), breakthrough appeared
after passing 3 and 3.5 mL of the solution, re-
100
La
Pr
Eu
Tb
Ho
Er
Yb
Lu
% Breakthrough
80
60
40
20
0
1
2
3
4
5
6
7
8
Volume of solution passed (mL)
Fig. 4. Breakthrough curve for Ln(III) ions on TODGA/
chromosorb-W column; aqueous phase: 1 g/L of Ln at
0.1 M HNO3; temperature: 25°C.
299
spectively. Similarly, for the heavier lanthanides
such as Er(III), Yb(III) and Lu(III), breakthrough
could be detected at ~4.5mL of the affluent solution. It is evident from these investigations that
only few selected lanthanides can possibly be
separated from each other. The lighter lanthanides
such as La(III) and Pr(III) can be separated from
heavier ones like Er(III), Yb(III) and Lu(III).
Ampika et al. studied the effect of chelating agent
diethylenetriamine pentaceticacid (DTPA) on the
distribution of Ln(III) by TODGA [16]. They reported a similar trend in the distribution ratio of
Ln(III) ions in the absence as well as in the presence of 0.01 M DTPA at pH 1.25 at the ionic
strength of 1 M NaNO3.
Extraction chromatographic separation of lanthanides has been attempted earlier using several
extractants, viz. HDEHP (bis-di-2-ethyl hexyl
phosphoric acid), HEHFP (2-ethyl hexyl phenyl
phosphoric acid), and Aliquat 336 (tri-octyl methyl ammonium chloride) [27,28]. Though
HDEHP gave the most promising results, their
application is limited due to metal loading limitations. A recent HPLC-based separation of lanthanides using reverse phase column have shown
encouraging results [28]. Comparative results of
intra-group separation of lanthanides have indicated that the separation efficiency follow the order: TODGA > Aliquat 336 > HDEHP. However,
in view of the superior extraction ability of
TODGA under acidic conditions, separation of
few selected lanthanide ions, viz. La(III), Eu(III),
Tb(III), Ho(III), Yb(III) and Lu(III) was attempted
during the present study. The elution profiles obtained using the above mentioned columns are
shown in Fig. 5. The feed solution containing all
the lanthanides was prepared at a pre-adjusted pH
value of 2.0, before loading on the column. When
elution was performed with 2-hydroxy isobutyric
acid, a well known complexing agent for chelating lanthanides, it was not possible to separate
the lanthanide ions as all the lanthanide ions were
eluted simultaneously. Since, 2-hydroxy isobutyric acid is a strong complexing agent, it forms
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M. Husain et al. / Desalination 229 (2008) 294–301
1.0x10
4
reflected strong sorption on TODGA resin at acidity >2 M HNO3. However, at 0.1 M HNO3 lanthanides reflected significant variation in their
distribution coefficient. The distribution coefficient of lanthanides increased with the ionic potential of the metal ions. The separation factor
between La(III) and Lu(III) was found to be 173
at 0.1 M HNO3. Under identical experimental
conditions, breakthrough for La(III) and Lu(III)
was observed after passing 1.5 mL and 4.5 mL of
the effluent solutions, respectively. Though the
mutual separation of all the lanthanides is not
possible with TODGA resin column, nevertheless,
lower lanthanide like La can be separated from
the higher one like Lu.
La
Ho
8.0x10
3
Counts per minute
Eu
6.0x10
3
4.0x10
3
2.0x10
3
Tb
Lu
Yb
0.0
0
20
40
60
80
Volume of eluent passed (mL)
Fig. 5. Chromatographic separation of lanthanides by
TODGA/chromosorb-W column; eluent: 0.01 M HNO3.
strong complex with all the lanthanides resulting
their elution from the column. On the other hand,
preferential elution of lanthanides was possible
when 0.01 M HNO3 solution was employed as
the eluent. Though >97% La(III) was eluted out
of the column in about 8 mL, for similar results
for other lanthanides much higher volumes of eluent were needed: Eu(III): 28 mL; Tb(III): 47 mL;
Ho(III): 72 mL. Only about 90% elution of Yb(III)
and Lu(III) was possible in 80mL of the eluent.
From the column studies it is concluded that only
preferential separation of lighter lanthanides from
the heavier one is possible.
4. Conclusions
The separation of mutual lanthanides was attempted by extraction chromatography technique
using N,N,N′,N′-tetraoctyl diglycolamide
(TODGA) as the stationary phase. Lanthanides
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