Comparison of Bis-Triazinyl-Pyridines substituted or not by an alkoxy
group on position 4 of pyridine for selective extraction of americium(III)
Carine Duponta,b*, Clément Hillb, Franck Suzeneta, Gérald Guillaumeta
Institut de Chimie Organique et Analytique, UMR CNRS 6005, Université d’Orléans, BP
6759, 45067 Orléans cedex 2, France
b
CEA Marcoule, Nuclear Energy Division, Radiochemistry & Processes Department, SCPS,
LEPS, F-30207, Bagnols sur Cèze, France
a
The extraction of americium(III), curium(III) and lanthanides(III) from nitric acid by 2,6-bis(5,6-dimethyl-[1,2,4]-triazin-3-yl)-pyridine and 2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)4methoxypyridine was studied. The physico-chemical properties of these ligands, such as the
their protonation and complexation constants were also determined to describe the influence
of different substituting groups. The selectivity of substituted-BTP was confirmed both in
complexation and in solvent extraction experiments. The presence of an alkoy-group in
position 4 of the pyridine decreases the BTP selectivity.
Keywords: liquid-liquid extraction, americium(III), curium(III), lanthanide(III), separation,
nitric acid, BTP-4Me, 4-OMeBTP-4Me, TODGA, back-extraction, nuclear reprocessing,
complexation, protonation
Introduction
In France, nuclear waste issued from the industrial reprocessing of spent nuclear fuels (by the
PUREX process) are currently vitrified at La Hague plant, pending their final disposal in a
deep geological repository. The long-term radiotoxicity of spent nuclear fuels is governed by
their contents of long-lived radionuclides, the most important of which being uranium and
plutonium that can nevertheless be separated by implementing the PUREX process, in order
to fabricate new MOX (Mixed OXide)1 fuels. After plutonium, the most radiotoxic elements
are the minor actinides: americium, curium and neptunium. If also separated from the spent
fuels, the remaining waste could decay within a few centuries to the level of natural uranium
instead of more than 100,000 years2 in the absence of any partitioning. After their removal
from the spent fuels, minor actinides could be transmuted into short lived radionuclides by
neutron bombardment3,4. The difficulty faced when developing partitioning processes is that
spent fuels also contain lanthanides, which have the same chemical properties as the trivalent
minor actinides, but are responsible for large neutron absorption, preventing americium(III)
and curium(III) from transmutation. Moreover, the extractants and diluents used in such a
process should consist only of C, H, O and N atoms, to make them combustible to gaseous
products.
The evaluation of new metal binding agents for actinides and lanthanides provides means to
probe and compare the respective chemistries of An(III) and Ln(III)5. Thus, the actinide(III)lanthanide(III) separation is not easily achievable with common O-donor extractants.
Nevertheless, the 5f (and 6d) atomic orbitals of actinides are more able to interact with the
molecular orbitals of potential ligands than the 4f orbitals of the lanthanides, especially in the
case of soft donor ligands, such as Nitrogen. Hence, a large number of heterocyclic N-donor
extractants have been tested for the past decades in the framework of Euratom research
projects6. Unfortunately, most of these ligands appeared to extract trivalent actinides only
from solutions of very low acidity, rendering them useless for further process development.
However, trivalent actinides proved to form more stable complexes than lanthanides with
molecules such as terpyridines or 2,6-bis([1,2,4]-triazin-3-yl)pyridines (BTPs)7. It is likely
that the smaller steric demands of the [1,2,4]-triazin-3-yl ring by comparison with the 2pyridinyl ring, which allows the nitrogen atoms to approach more closely to the metal atom,
contributes to the greater ability of the BTP ligands to adopt the geometry required for metal
binding.
2,6-bis(5,6-dialkyl-[1,2,4]-triazin-3-yl)pyridines (See Figure 1) were the first N-donor ligands
to perform the difficult separation of trivalent actinides from feed solutions of rather high
acidities (e.g., extracting americium(III) from 1M nitric acid into 0.04M n-Pr-BTP in
kerosene/octanol, the observed distribution ratio is 13 and the separation factor for
americium(III) over europium(III) is SFAm/Eu = 130)8,9. Their extraction behaviour has been
extensively studied and the structures of actinide(III)-BTP complexes and lanthanide(III)-BTP
complexes have been established10,11. It seems likely that the efficiency of the BTP ligands is
due to the formation of single species, as shown in the nine-coordination structure of the
lanthanide cationic complex of formula [Ln(BTP)3]3+,12 in which the metal is completely
enclosed by the organic ligands to the exclusion of nitrate anions and water molecules.
Currently, owing to their easy synthesis and favourable metal extraction chemistry, the BTP
ligands are the bench-mark for the selective extraction of trivalent actinides from nitric acid
solutions containing various fission products (SANEX process), such as output solutions of
up-hill partitioning processes (e.g., DIAMEX or TRUEX processes), aimed at separating
trivalent actinides and lanthanides from PUREX raffinates.
The first counter-current SANEX tests carried out from 1999 to 2001 with genuine highly
active feed solutions showed promising results but the BTPs presented major drawbacks: low
stabilities with respect to hydrolysis and radiolysis 13. Following the identification of the
mechanisms of hydrolytic degradation, modified BTPs were designed, more resistant towards
hydrolysis14. For example, side groups without benzylic-H-atoms were attached in order to
form
2,6-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]triazine-3-yl)pyridine
(CyMe4BTP, see Figure 1). Unexpectedly, the selectivity towards actinides(III) was also
improved (SFAm/Eu > 1000), but the actinide(III) distribution ratios were so high that their
back-extraction from the loaded solvent was impossible.
R
N
N
R
N
N
N
N
R
N
N
N
N
N
R
N
N
N
Figure 1
2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridines (BTPs with R = n-Pr or i-Pr, on the left) and 2,6-bis(5,5,8,8tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]triazine-3-yl)pyridine (CyMe4BTP, on the right).
Researches on lanthanides(III)-BTP complexes have shown that the geometry and
conformational properties are determining factors15. A lot of theoretical studies have been
attempted to understand the selectivity of BTPs towards actinides(III)16,17. Theoretical studies
on substituent effects on BTP triazines have been reported18. Substitutions on triazines have
little influence on An(III)-BTP and Ln(III)-BTP bond lengths. An electro-donor or electro-
attractor group on triazine has an effect only on nitrogenous atom number 1 of the triazine.
Recently, TRLFS studies19 on BTPs, 4-OMe-BTP and 4-Cl-BTP, showed that ligands
functionalized with electro-donating groups favoured the formation of the 1:3 complexes,
whereas for ligands functionalized with electron-withdrawing groups the complexation
equilibrium was shifted towards the 1:1 complex, as compared to the unsubstituted BTP.
Hence, electron-donating groups increase the stability of Eu(III) and Cm(III) complexes and
electron-attractor groups decrease the complex stability. Extraction experiments show that the
extractants functionalized with electron-donating groups show better distribution ratios and
separation factors than acceptor-substituted extractants20.
Pursuing the quest to better understand of the origin of BTPs selectivity, which is crucial for
the design of optimized extractants, the aim of this study is to assess the effect of a
substitution on the 4-position of the pyridine ring of BTPs. Therefore, two BTP molecules
were synthesized (See Figure 2), one of which carrying an electron-donating substituent (OMe) in the 4-position of the pyridine ring, altering the electronic structure of the aromatic
system. This study reports the results of the determination of the complexation constants of
BTP-4Me and 4-MeO-BTP-4Me with actinides(III) and lanthanides(III). As these two
molecules are soluble in nitric acid solutions, batch tests were also carried out to assess the
ability of these latter two ligands to strip diamide-based solvents loaded with americium(III),
curium(III) and lanthanides(III). Preliminary results show that these hydrophilic BTP ligands
could be used as selective stripping agents in r-SANEX process.
OMe
N
N
N
N
N
N
N
N
N
1
N
N
N
N
2
N
Figure 2
1 : 2,6-bis(5,6-dimethyl-1,2,4-triazin-3-yl)pyridine or BTP-4Me, 2 : 2,6-bis(5,6-dimethyl-1,2,4-triazin-3-yl)-4methoxypyridine or 4OMe-BTP-4Me
Experimental section
Organic Synthesis
General procedures and materials
Starting materials were purchased from Aldrich, Acros Organics and Alfa Aesar and used
without purification. Solvents were of HPLC quality. THF was distilled on
sodium/benzophenone. Thin-layer chromatography (TLC) was performed on 0.2mm precoated plates of silica gel 60F-254 (Merck). Purifications were realised with flash
chromatography on silica gel 40-70nm (230-400 mesh) under nitrogen pressure. Melting
points were determined on a Kofler melting point apparatus and were not corrected. Mass
spectra were obtained with a Perkin Elmer Sciex API 300 spectrometer. Samples were
dissolved in MeOH/H2O (95:5). IR spectra were recorded with ATR Nicolet iS10 equipped
with a diamond crystal (ATR-D). 1H NMR and 13C NMR were recorded on a Bruker Advance
II Ultrashield 400MHz or on a Bruker DPX-250MHz. Chemicals shifts are expressed in parts
per million downfield from tetramethylsilane used as an internal standard. Coupling constants
are given in Hertz.
R
R
R
NH2-NH2.H2O
overnight, rt
NC
N
2,3-butanedione
H2N
N
CN
3
R = H, OMe, OC12
commercial for R = H
NH2
N
H2N
THF/NEt3
refluxing, 5h
N
4
NH2
N
N
N
N
N
5
N
86% to quantitative
N
76-88%
Scheme 1 General procedure to synthesize 2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-pyridine.
General procedure for the synthesis of 2,6-biscarbamidrazone-pyridine
Under an atmosphere of Argon, 2,6-dicyanopyridine 3 (2g, 15.5mmol) was stirred over night
in monohydrated hydrazine (9.4mL, 0.22mol, 14.5eq) at room temperature. A white crude
product was filtrated on Büchner, washed twice with iced water and dried under reduced
pressure to yield 2,6-biscarbamidrazone-pyridine 4.
2,6-biscarbamidrazone-pyridine
White solid (quantitatively), mp 120°C. 1H NMR (DMSO-d6, 400MHz), δH 7.80 (2H, d,
8.0Hz), 7.65 (1H, dd, 8.0Hz), 6.03 (4H, s), 5.25 (4H, s). 13C NMR (DMSO-d6, 100MHz), δC
150.22, 143.47, 135.85, 117.83. IS-MS : 194.0. IR : 3455, 3142, 1668, 1585, 1569.
2,6-biscarbamidrazone-4-methoxypyridine
White solid (95%), mp 182-183°C. 1H NMR (DMSO-d6, 400MHz), δH 7.36 (2H, s), 6.09 (4H,
s), 5.30 (4H,s), 3.81 (3H, s). 13C NMR (DMS-d6, 100MHz), δC 165.2, 151.97, 143.61, 103.80,
55.11. IS-MS : 224.5. IR : 3278, 3172, 1660, 1590, 1118.
Synthesis of 2,6-dicyano-4-methoxypyridine is described by Wagenen21 from chelidamic acid
monohydrate commercial.
2,6-biscarbamidrazone-4-dodecyloxypyridine
White solid (86%), mp 116-118°C. 1H NMR (DMSO-d6, 250MHz), δH 7.32 (2H, s), 6.00 (4H,
sl), 5.17 (4H, sl), 4.02 (2H, J=6,3, t), 1.69 (2H, J=6,3, qt), 1.41-1.35 (2H, m),
1.24 (16H, sl), 0,85 (3H, t). 13C NMR (DMS-d6, 62.5MHz), δC 164.6, 152.2, 143.4, 104.1,
67.5, 31.3, 29.02, 29.00, 28,7. IS-MS : 378.5. IR : 2919, 2842, 1595, 1558, 1432, 1383, 1118,
841.
Synthesis of 2,6-dicyano-4-dodecyloxypyridine was realised from chelidamic acid
monohydrate commercial.
General procedure for the synthesis of 2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-pyridine
Under an atmosphere of Argon, 2,3-butanedione (1.5mL; 17mmol, 2.2eq) and trietylamine
p.a. (1.18mL, 8.5 mmol, 1.1eq) were added dropwise to a solution of 2,6-biscarbamidrazonepyrdine 4 (1.5g, 7.7mmol) in distillated THF (50mL) . Mixture was stirred at refluxing of
THF during 5h. After cooling, ether (15mL) was added and the precipate was filtrated on
Millipore filter, washed with ether to yield 2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-pyridine
5.
2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-pyridine
Orange solid (1.87g, 83%), mp 215-220°C. 1H NMR (CDCl3, 400 MHz), δH 8.79 (2H, d,
7.9Hz), 8.73 (1H, t, 7.9Hz), 2.79 (2x6H, 2xs). 13C NMR (CDCl3, 100 MHz), δC 161.3, 159.7,
157.2, 153.5, 138.3, 125.3, 21.9. IS-MS : 294.0, 316.5, 332.5. IR : 3065, 1567, 1523, 1386,
1355, 1287.
2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-4methoxypyridine
Yellow solid (88%), mp 202-203°C. 1H NMR (CDCl3, 400MHz), δH 8.35 (2H, s), 4.08 (3H,
s), 2.78 (6H, s), 2.72 (6H, s). 13C NMR (CDCl3, 100MHz), δC 167.7, 161.2, 159.7, 157.2,
154.9, 111.4, 55.9, 21.9, 19.6. IS-MS : 324.0, 346.0. IR : 1684, 1598, 1134.
2,6-bis-(5,6-dimethyl-[1,2,4]-triazin-3-yl)-4dodecyloxypyridine
Yellow solid (76%), mp 83-84°C. 1H NMR (CDCl3, 250MHz), δH 8.33 (2H, s), 4.25 (2H, t,
J=6.5), 2.78 (6H, s), 2.72 (6H, s), 1.89 (2H, qt, J=6.5), 1.47-1.55 (2H, m), 1.28 (16H,sl), 0.88
(3H, t, J=6.8). 13C NMR (CDCl3, 62.5MHz), δC 167.4, 161.4, 159.8, 157.3, 155.0, 112.0, 69.0,
32.1, 29.81, 29.78, 29.74, 29.71, 29.49, 29.47, 29.1, 26.1, 22.8, 22.1, 19.8, 14.3. IS-MS :
478.5, 500.5. IR : 2920, 2842, 1591, 1525, 1436, 1387, 1122, 987.
Thermodynamic studies
Materials
Methanol, purchased from Aldrich, was used without any further purification. The following
metal salts, purchased from Aldrich or Acros, were used: LaCl3.7H2O, EuCl3.6H2O,
NdCl3.6H2O and GdCl3 anhydrous. Quartz cells of 1cm path length with two faces QS
HELLMA were used. Samples were analyzed at 21°C in two UV-Visible spectrophotometers.
For inactive experiments, a UV-Visible spectrophotometer VARIAN Cary 500 with double
beam was used. The data obtained were treated with the program Carywin UV scan
Application 3.00(182). For the experiments involving radionuclides, a UV-Visible
spectrophotometer SHIMADZU UV3101PC N2 with double beam, directly located in the
glove-box (without optic fibers) was used. The data obtained were treated with the program
UV-PC. Concentrations of LnCl3 solutions were measured with an ICP-AES apparatus
JOBIN YVON 2000S with ICP JY V5.1 like software treatment. Densities of prepared
solutions were determined with a densimeter DMA48 Anton Paar at 20°C. Mathematic
treatment was realized with the program Hyperquad22.
Preparation of AmCl3 solution
AmO2 (30mg, 0.124mmol) was dissolved in 5mL of nitric acid at 4M. Mixture was stirred
over night. Purification was realized on an exchange ion column A 6 50W x 4 (200,00 mesh)
purchased from Bio Rad. Resin was beforehand rinsed with 50mL of 0.1M nitric acid. The pH
of AmNO3 solution was adjusted at 1.02 by addition of about 3mL of 8M NaOH and the
solution was passed through the column. Resin was washed with 15mL of 0.1M nitric acid.
Americium was eluted three times with a very low flow using 5mL of 5M nitric acid. A
radiological verification with a babyline detected the end of the elution. Americium solution
was separated in 3 tubes of 15mL, and 5mL of 8M NaOH were added into each tube. After
centrifugation, an orange precipitate was obtained. The precipitate was washed 3 times with
water and dissolved in 600µL of 0.1M chlorydric acid or 100µL of 1M chlorydric acid. The
pH was adjusted at 3.5 with a few microlitres of 0.1M NH4OH. Concentration of AmCl3 was
measured by alpha and gamma spectrometry (see Extraction experiments).
Preparation of solutions for inactive experiments
10mL of solutions containing increasing quantities of hydrochloric acid or LnCl3 solutions,
with a fixed quantity of BTP in MeOH/H2O (75:25), were prepared like the following
protocol: 500µL of 5.10-4M BTP dissolved in MeOH, x µL of hydrochloric acid or LnCl3
solution, (2500-x)µL of distilled water and MeOH were mixed until 10mL. All additions were
weighed precisely and volumes were corrected with density measurements. Solutions were
analyzed directly with the spectrophotometer.
Preparation of solutions for active experiments
5mL of solutions containing increasing quantities of BTP and a fixed quantity of AmCl3 were
prepared like the following protocol: x µL of BTP, the concentration of which ranged from
10-2 to 5.10-4M, 250µL of distilled water, 1000µL of AmCl3 solution and (3750-x)µL of
MeOH were mixed in gloves-box. All additions, except MeOH, were weighed precisely and
volumes were corrected with density measurements. Solutions were analyzed directly with the
spectrophotometer located in the gloves-box.
Extraction Experiments
Materials
The alpha spectrometer used to determine 241Am(III) and 244Cm(III) activities was a
CANBERRA apparatus with a silicium detector. 10µL of samples were dropped on an inox
disc and burned off before alpha analyses. The gamma detector used to determine 241Am(III)
and 152Eu(III) activities was a CANBERRA apparatus with a pure germanium detector,
coaxial type P. Analyzed samples consisted of 500µL of solutions in 2mL NALGENE tubes.
Spiked aqueous feeds were prepared by diluting EuNO3, AmNO3 and CmNO3 stock solutions
in HNO3 to obtain activity concentrations of about 8000 kBq/L for each radionuclide.
Liquid-liquid Extraction
In EPPENDORF microtubes, 700µL of aqueous phases containing both the studied BTP and
the radionuclides in nitric acid, were contacted with 700µL of organic phases containing
either DMDOHEMA or TODGA extractants (see Figure 3) dissolved in hydrogenated
tetrapropene (TPH). After shaking the tube-rack for 1h with a vortex IKA type Vibrax VXR,
thermostated at 21°C, and centrifugation for 3min, 600µL of each phase were removed for
radiometric analyses. Acidity of the aqueous samples was checked by an acido-basic titration
with 0.1M NaOH.
O
H17C8
O
N
N
C8H17
O
H17C8
O
H13C6
N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide
O
O
C8H17
N
N
C8H17
C8H17
N,N,N',N'-tetraoctyl-3-oxapentanediamide
or DMDOHEMA
or TODGA
Figure 3
1: DMDOHEMA, 2: TODGA
Results and discussion
Thermodynamic Experiments
The thermodynamic studies were all realised in a chloride medium, although nuclear wastes
are industrially dissolved in nitric acid, first because chloride ions do not absorb UV
wavelengths and second because they weakly complex metallic cations. All experiments were
performed in MeOH/H2O 75:25, because BTP are not soluble in H2O, and because methanol
is transparent between 200 to 800 nm. All solutions were supposed ideal: they were dilute so
as to suppose their ionic force constant.
Protonation determination using UV-spectra
A preliminary study of the acid-base behaviour of BTP-4Me and 4OMe-BTP-4Me in
methanol using pH-metry provided the protonation constants (Figure 4). The protonation
spectra resulted from the π → π* transition at 235nm of electrons from aromatic cycles. The
presence of an isobestic point demonstrated that two species were present in the solution
samples: the free-BTP and the monoprotonated-BTP. Complexes type 1:1 Metal:Ligand were
hence formed.
0,9
1,2
a)
pH = -0,12
pH = 0,74
pH = 1,69
pH = 2,25
pH = 2,9
pH = 3,06
pH = 5,75
pH = 7,07
0,7
0,6
Absorbance
[H+] 
0,5
0,4
[H+] 
0,6
0,4
0,2
0,2
0,1
0
200
pH=0,59
pH=1,60
pH=2,00
pH=2,59
pH=2,69
pH=2,99
pH=4,64
pH=4,65
pH=4,69
pH=7
0,8
[H+] 
0,3
b)
1
Absorbance
0,8
[H ] 
+
220
240
260
280
300
320
Wavelength (nm)
0
200
340
360
380
400
220
240
260
280
300
320
340
360
380
400
Wavelength (nm)
Figure 4
Spectral variations corresponding to the protonation in methanol/water (75:25) at 20°C with a) BTP-4Me and b)
4OMe-BTP-4Me, [BTP] = 5.10-4mol.L-1.
Protonation constants are collected in
Table 1. Unexpectedly, the basicity decreases with the addition of the alkoxy-group on the
pyridine, although electro-donating O-alkyl groups (by mesomeric effect) should normally
increase the BTP protonation and basicity. Hence, the electronic properties are not the only
effect. Steric effects with conformation of BTP are most probably also important. A study by
Wipff23 in the gas phase on the protonation energies of substituted BTP, in position 4 of the
pyridine, showed that R donor groups (OMe, NH2 or NMe2) enhanced their basicity. Stability
of protonated BTP should therefore be the following BTP-4Me < 4-OMe-BTP-4Me.
However, this study in the gas phase does not take into account the possible different
conformations taken by the molecules. Electronic effects of the substituting groups are
increased but not the conformation effects. Therefore, the observed stability order is opposite:
4-OMe-BTP-4Me < BTP-4Me.
L
H+
La3+
Nd3+
Eu3+
Gd3+
Am3+
BTP-4Me
1.3 ± 0.1
2.0 ± 0.2
3.6 ± 0.4
3.6 ± 0.4
2.1 ± 0.2
4.4 ± 0.4
4OMe-BTP-4Me
0.8 ± 0.1
2.4 ± 0.2
4.0 ± 0.4
3.1 ± 0.3
3.1 ± 0.3
3.4 ± 0.3
4-OC12-BTP-4Me
0.7 ± 0.1
2.5 ± 0.2
2.7 ± 0.3
1.4 ± 0.1
2.5 ± 0.3
1.6 ± 0.2
Table 1 Overall stability Constants (log β) for proton, lanthanide and actinide complexes with BTP-4Me, 4OMeBTP-4Me and 4OC12-BTP-4Me in methanol/water (7525) at 20°C.
Complexation determination using UV-spectra
Complexation in methanol/water (75:25) was followed by spectrophotometric titrations of the ligands
against the metal ions. The stability constants (log β) derived from the spectral changes using the program
Hyperquad are given in
Table 1.
1
1,2
0,9
a)
[Eu3+] 
0,8
0,6
M/L = 273
M/L = 189
M/L = 93
M/L = 18
M/L = 13
M/L = 9
M/L = 4
M/L = 3
M/L = 1
M/L = 0
[Eu3+] 
0,5
0,4
0,3
0,2
0,1
0
200
[Eu3+] 
250
300
Wavelength (nm)
350
Absorbance
Absorbance
0,7
b)
[Eu3+] 
1
M/L = 93
M/L = 45
M/L = 9
M/L = 7
M/L = 4
M/L = 3
M/L = 2
M/L = 1
M/L = 0
0,8
0,6
[Eu3+] 
0,4
0,2
400
0
200
250
300
350
400
Wavelength (nm)
Figure 5
Spectral variations corresponding to the complexation in methanol/water (75:25) at 20°C of Eu3+ a) with BTP4Me and b) 4OMe-BTP-4Me, [BTP] = 5.10-4mol.L-1.
Lanthanide complexation
The π → π* transition of electrons from aromatic cycles are sensible with lanthanides and
actinides(III). With increasing EuCl3 concentration, a Eu-BTP complex species was formed
showing emission bands at 260, 280 and 320 nm for BTP-4Me and 310 nm for O-alkylBTP-
4Me (Figure 5). The experimental conditions implemented induced the formation of
complexes of 1:1 Metal:BTP stoichiometry. The variation of the stability constants of these
1:1 complexes shows a bell-shape along the lanthanide serie, from lanthane to gadolinium (
Table 1). This variation has already been observed with other polyheterocyclic nitrogenous
aromatic molecules17. The methoxy group improves the stability of the complexes except
Am(III) and Eu(III) (
Table 1), but the length of the carbon chain reduces the stability of the complexes. The
substituent effect is hence opposite to that old protonation. Kolarik24 has demonstrated that
lanthanides(III) complex stabilities increased with the length of the alkyl substituent on the
lateral triazines. An opposite phenomenon is apparently observed with the substitution of
position 4 of the pyridine of the BTPs.
Americium complexation
The absorption band of Am(III) at 503nm was used for the determination of the americium
complexation. This band corresponds to a 5f → 5f transition, not allowed by symmetric rules,
not blazing but with a sufficient molar extinction coefficient (between 350 and 400L.mol1
.cm-1 depending on the medium)25. The addition of BTP progressively removed the spectral
band of Am(III) and showed the occurrence of another band at 523nm, corresponding to AmBTP complex. The nature of the alkoxy group decreased the stability of the complex (
Table 1).
BTPs undergo more stable complexes with Am3+ than with Eu3+: americium complexation
constants are all higher than those of europium. Nitrogenous polyaromatic ligands show a
higher affinity for Am(III) than Eu(III) cations. It is consistent with the hypothesis on a
greater covalent nature of the N-Am(III) bond as compared to the N-Eu(III) bond. Table 2
shows that BTP-4Me is the most selective ligand in MeOH/H2O (75:25) medium. However,
the selectivity is decreased by an O-alkyl group and by a long alkyl chain.
L
Log KAm – Log KEu
BTP-4Me
4-OMe-BTP-4Me
4-OC12-BTP-4Me
0.8 ± 0.1
0.3 ± 0.1
0.2 ± 0.1
Table 2 Comparaison of Eu3+ and Am3+ complexation constants for BTP-4Me, 4-MeO-BTP-4Me and
4OC12BTP-4Me.
Extraction Experiments
These extraction experiments aimed at assessing the potentiality of BTP ligands to be used as
hydrophilic stripping agents, either in very acidic conditions, such as those of a DIAMEX
process (PUREX raffinates exceeding 3M nitric acid), or in more reduced acidic conditions,
such as those encountered in reverse-SANEX processes (feeds around 1M nitric acid).
In the implemented experiments, two organic molecules competed: in the organic phase, the
extracting Oxygen-donor molecule (either DMDOHEMA or TODGA, see Figure 3), a
lipophilic hard base presenting a high affinity for both trivalent actinides and lanthanides, and,
in the aqueous phase, the complexing Nitrogen-donor BTP, a hydrophilic soft base, selective
towards Am(III).
In the conditions mimicking the DIAMEX process, the initial concentrations of
DMDOHEMA and of nitric acid were kept constant. Addition of 4-OMe-BTP-4Me at various
concentrations, ranging from 0 to 0.1M did not modify the distribution ratios of Am(III) (DAm
~ 5.5) nor Eu(III) (DEu ~ 2.5), due to DMDOHEMA extraction properties. The same
observation was made with BTP-4Me. Hence, none of the tested BTP ligands allowed the
selective stripping of Am(III) from a DMDOHEMA loaded solvent, probably because the
BTPs were protonated at 3M nitric acid.
DM
100
10
1
0,1
0
0,05
Am
Eu
0,1
[BTP]
mol/L
D M = 1 or 50% extract
Aqueous phase: 0 ≤ [BTP-4Me] ≤ 0.1M, [HNO3] = 3M, spiked with 241Am(III) and in 152Eu(III)
Organic phase: [DMDOHEMA] = 0.65M in TPH
Graph 1 Extraction of Am(III) and Eu(III) by DMDOHEMA in the presence of 4-OMe-BTP-4Me.
The use of the diglycolamide TODGA in the organic phase, a much stronger extractant than
DMDOHEMA allowed to work at lower acidity. TODGA concentration was set at 0.05M in
TPH and nitric acid concentration at 1 M. Extraction experiments were realised with BTP4Me (Graph 2) and with 4-OMeBTP-4Me.
In the absence of BTP, TODGA extracted better Eu(III) than Am(III) (DEu = 57 and DAm = 6).
Addition of BTP-4Me led to a sharp decrease of DAm value. For [BTP-4Me] = 0.1M, the
distribution ratio of Am(III) was smaller than 1 (DAm = 0.3), whereas the distribution ratio of
Eu(III) was still superior to 10 (DEu = 31.5, hence SFEu/Am = 30), meaning that more than 50%
of Am(III) remained in the aqueous phase, whereas more than 90% of Eu(III) were extracted
by TODGA in the organic phase. Hence, BTP-4Me proved to complex Am(III) selectively,
and should be usable in a reverse-SANEX process, if implemented as a hydrophilic selective
An(III) stripping agent to unload TODGA solvents (since extraction and back-extraction
reactions are assumed totally reversible).
DM
100
10
1
0,1
0
0,05
Am
Eu
0,1
DM = 1 or 50% extract
[BTP]
mol/L
Aqueous phase: 0 ≤ [BTP-4Me] ≤ 0.1M, [HNO3] = 1M, spiked with 241Am(III) and in 152Eu(III)
Organic phase: [TODGA] = 0.05M in TPH
Graph 2 Extraction of Am(III) and Eu(III) by TODGA in the presence of BTP-4Me.
Like with BTP-4Me, the distribution ratios of Am(III) and Eu(III) decreased as the
concentration of 4-OMeBTP-4Me grew in the aqueous phase, but the decrease was less
significant for Am(III) than it was observed with BTP-4Me. Besides, the solubility limit of 4OMeBTP-4Me was reached for 0.8M.
Two concurring trends could hypothetically explain these results: 1) the 4-OMeBTP-4Me is
less selective than the BTP-4Me as shown by the complexation constants reported in Table 2 ;
2) the methoxy group increases the lipophilicity of the OMeBTP-4Me, and hence part of the
latter distributes from the aqueous to the organic phase, thus favouring the formation of
lipophilic mixed complexes with TODGA in the case of Am(III). The first hypothesis seems
however more straightforward.
The selectivity of BTP-4Me towards actinides(III) (Am and Cm) was studied (Graph 3). For
[BTP-4Me] = 0.1M, Cm(III) shows the same behaviour as Am(III). The distribution ratios are
DAm = 0.8, DCm = 0.64 and DEu = 29. Hence, the separation factor Eu/Am is 32. A chemical
separation of An(III) from Ln(III) might therefore be possible by selectively stripping the
An(III) from a TODGA loaded solvent (reverse-SANEX process).
DM
100
10
Am
Eu
Cm
1
D M = 1 or
50% extract
0,1
[BTP] mol/L
0
0,05
0,1
Aqueous phase: [BTP-4Me] = 0 or 0.1M, [HNO3] = 1M, spiked with 241Am(III), 244Cm and in 152Eu(III)
Organic phase: [TODGA] = 0.05M in TPH
Graph 3 Extraction of Am(III), Cm(III) and Eu(III) by TODGA in the presence of BTP-4Me.
The selectivity of BTP-4Me was also assessed when the aqueous phase contained macroconcentrations of lanthanides, and Am(III) and Eu(III) in tracer conditions (Graph 4). A
solution containing macro-concentrations of lanthanides was prepared to represent an UOX1
type solution (combustion rate of 33 GWd/t, after 3 years of cooling). The distribution ratios
of Eu and Am decreased slightly as compared to the spiked aqueous phase (Graph 3).
DM
100
Am with multiLn
Eu with multiLn
10
Am without multiLn
Eu without multiiLn
1
D M = 1 or 50%
extract
0,1
[BTP] mol/L
0
0,05
0,1
Aqueous phase: [BTP] = 0 or 0.1M, [HNO3] = 1M, spiked with 241Am(III) and in 152Eu(III), with or without
multiLn solution simulating an UOX1 PUREX raffinate
Organic phase: [TODGA] = 0.05M in TPH
Graph 4 Extraction of Am(III) and Eu(III) in the presence or not of various lanthanides and BTP-4Me.
Elements
Eu
Gd
Sm
Y
Nd
Pr
La
Ce
Concentration in g.L-1
0,028
0,017
0,107
0,078
0,780
0,217
0,239
0,464
Table 3 : Constitution of multiLn solution simulating an UOX1 PUREX raffinate
Conclusion
The influence of O-alkyl groups on the chemical properties of BTPs was studied by
determining protonation and complexation constants as well as by implementing liquid-liquid
extraction. As regard protonation, the mesomeric electro-attractor effect of an O-alkyl group
(attached on the position 4 of the pyridine) leads to the decrease of the BTP basicity. Methoxy
group raises the stability of BTP-Ln complexs and lowers the selectivity of the BTP towards
Am(III). Electronic effects are surprising but other parameters like the conformation of the
molecule impacts the stability of the complexes. In liquid-liquid extraction, the lipophilic
nature of the –OMe group decreases the separation of Am-Eu. Good results can be obtained
with BTP-4Me, used as a hydrophilic selective An(III) stripping agent, and an industrial
reverse-SANEX process should be conceivable. However other studies would be interesting,
such as the behaviour of the molecule with temperature, kinetic studies, process development
studies, management of the reagents in process, etc.
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