Complexation of protactinium(V) with poly(amino)carboxylic acids


Complexation of protactinium(V) with poly(amino)carboxylic acids

Mendes, M.; Le Naour, C.; Hamadi, S.; Den Auwer, C.; Moisy, P.; Di Giandomenico, V. M.; Hennig, C.

Protactinium is experiencing a renewal of interest in the frame of nuclear reactors based on thorium fuel. The isotopes 233Pa (intermediate in the production of the fissile 233U) and 231Pa (radiotoxic) are namely produced through nuclear reactions on 232Th: Aside from a possible use of thorium as nuclear fuel for energy production, studies on protactinium may provide additional information about the coordination chemistry of light actinides. The literature devoted to protactinium in aqueous solution is characterized by scarce and controversial thermodynamic data that originate from the strong tendency of Pa(V) towards hydrolysis and polymerization especially in non-complexing media. Modeling the behavior of this element in the reactor, in the reprocessing steps, in the geosphere and in physiological medium requires thermodynamic and structural data relevant to these various environments. The present work is the continuation of our previous studies devoted to hydrolysis and complexation of Pa(V) with sulfate ions. The aim is now to collect thermodynamic and structural data on Pa(V) in the presence of oxalic (H2C2O4) and diethylene-triamine-pentaacetic (H5DTPA) acids.
The apparent formation constants of Pa(V) complexes with oxalate and DTPA5- were deduced from tracer level experiments using the isotope 233Pa at ~10 12 M. At such low concentration of element, only partition or transport methods based on radiation detection can be used5. In this work, the technique of solvent extraction involving the chelating agent thenoyl-trifluoro-acetone (TTA) was chosen. The aqueous phase was a mixture of NaClO4, HClO4 and H2C2O4 or H5DTPA. The dissociation constants H2C2O4 or H5DTPA were taken from references. Firstly, extraction data, collected at constant ionic strength and temperature allowed to determine the maximum order of Pa(V) complexes, the mean charge of the predominant complex in aqueous phase and the number of TTA molecules per Pa atom in the extracted species. Secondly, a systematic study of the variations of the distribution coefficient D of Pa(V) as function of the free ligand concentrations performed at different temperature values, led to the determination of thermodynamic data relevant to complexation equilibria (formation constants, enthalpy and entropy variations). Figure 1 illustrates the variations of D as function of free DTPA5- concentration when temperature increases from 10 to 60°C. Whatever the temperature, these curves characterize the formation of a (1,1) complex that stability constant can be derived from the variations of (D0/D-1) as function of DTPA5-. At 25°C, the constant relative to the equilibrium PaO(OH)2+ + DTPA5- + H+ ↔ PaODTPA2- + H2O has been estimated to log1 = 29,0 ± 0,4 for an ionic strength equal to 3 M. In the oxalate system, the existence of the complexes (1,1) and (1,2) has been unambiguously demonstrated.
Since no structural information can be deduced from tracer level experiments, X-ray Absorption Spectroscopy measurements were performed on 231Pa samples in oxalic acid. As in concentrated sulfuric acid8, XANES spectra do not display the feature of the linear di-oxo bond that characterizes U, Np, Pu and Am at their higher oxidation states, whereas the analysis of EXAFS data has unambiguously demonstrated the presence of a short mono-oxo bond (1.73 Å).

Keywords: Protactinium; EXAFS

  • Lecture (Conference)
    Actinides 2009, 12.-17.07.2009, San Franzisco, USA

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Publ.-Id: 13451