exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

FT Amplitude
D
Complexation of protactinium(V) with organic ligands:
thermodynamic and structural study
IPNO Participation: M. Mendes, S. Hamadi, C. Le Naour
Collaboration : CEA Marcoule, CEA Bruyères-le-Chatel, FZD Dresden
Dans le contexte des réacteurs à combustible thorium, la sûreté des opérations de retraitement et des sites
de stockage nécessite une connaissance approfondie de la chimie en solution du protactinium, produit
sous forme des isotopes 231 et 233 dans ce nouveau type de réacteur. Ce travail présente les résultats
établis lors d’études sur le comportement du protactinium au degré d’oxydation V en présence d’acide oxalique
(complexant susceptible d’être présent dans certaines eaux souterraines) et d’acide diéthylènetriaminopentacétique
(agent décorporant des actinides). L’utilisation de la spectroscopie d’absorption X, de l’électrophorèse
capillaire couplée ICP-MS, d’une part, et de la technique d’extraction liquide-liquide avec l’élément
à l’échelle des traces, d’autre part, a permis de déterminer la structure des complexes d’ordre maximum
et de déterminer les grandeurs thermodynamiques relatives aux équilibres de formation de l’ensemble
des complexes impliqués.
Structural study
X-ray Absorption Spectroscopy measurements
were performed at ESRF-Grenoble on samples of
231 Pa prepared at IPNO in oxalic acid 1M and in a
mixture HCl/DTPA. XANES spectra show that protactinium(V)
does not form the transdioxo bond
that characterizes the other light actinides at their
higher oxidation states. The analysis of EXAFS
spectra (Fig.1) allowed to characterize the complex
PaO(C 2 O 4 ) 3– that displays a short mono-oxo bond
(Pa-O at 1.75 Å) and 3 bidendate oxalate ligands,
whereas no oxo bond was observed in the complex
between Pa(V) and H 5 DTPA.
5
B
presence of PaO(C 2 O 4 ) 3– and Pa(DTPA).
Thermodynamic study
In order to avoid the polymerization of protactinium
at high concentrations, the thermodynamic study
was performed with the element at tracer scale
(C Pa ~ 10 -12 M) using solvent extraction with TTA
(thenoyltrifluoroacetone) as chelating agent. First,
the logarithmic variations of the distribution coefficient
D of Pa(V) as a function of the total concentration
of ligand as illustrated in Fig.2 allowed to
determine the mean number of ligand per Pa atom
for complexes that are predominant in aqueous
phase. In oxalic system, the species PaO(C 2 O 4 ) + ,
-
3–
PaO(C 2 O 4 ) 2 and PaO(C 2 O 4 ) 3 are formed successively
(slopes –1 to –3 on Fig.2). With DTPA,
only the complex(1,1) is formed.
4
3
A
.
2
1
C
D
0,1
0,01
slope -1
slope -2
0
1E-3
0 2 4 6
R+Phi
1E-4
slope -3
Fig.1: Experimental modulus of the Fourier transform
of L III EXAFS spectrum of Pa 1.25 10 -2 M in
1M H 2 C 2 O 4 and fitted curve (dashed line)
Experiments involving Capillary electrophoresis
coupled with an ICP-sector field Mass spectrometer
were performed at CEA-BIII on samples of Pa
~10 -8 M in the presence of oxalic acid, on one
hand, and H 5 DTPA on the other hand. The electrophoretic
mobility of the oxalate and DTPA complexes
were proved to agree with a highly charged
anionic species and a neutral one, respectively.
This structural study allows to conclude to the
1E-5
1E-8 1E-6 1E-5 1E-4 1E-3 0,01 0,1
Fig.2: Variations of the distribution coefficient D of
Pa(V) as function of oxalic acid concentration at
40°C and µ=3M
The extraction data ((D 0 /D)-1), collected at different
values of ionic strength and temperature, were
plotted as function of the total ligand concentration
and adjusted with a polynomial of third order in the
case of oxalic system. The coefficients of this polynomial
are the formation constants i of the oxalate
C ox
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