[en] Complexation of trivalent actinides with DTPA (diethylenetriamine pentaacetic acid) was studied as a function of pcH and temperature in (Na,H)Cl medium of 0.1 M ionic strength. Formation constants of both complexes AnHDTPA^- and AnDTPA^2- (where An stands for Am, Cm, and Cf) were determined by TRLFS, CE-ICP-MS, spectrophotometry, and solvent extraction. The values of formation constants obtained from the different techniques are coherent and consistent with reinterpreted literature data, showing a higher stability of Cf complexes than Am and Cm complexes. The effect of temperature indicates that formation constants of protonated and non protonated complexes are exothermic with a high positive entropic contribution. DFT calculations were also performed on the An/DTPA system. Geometry optimizations were conducted on AnDTPA^2- and AnHDTPA^- considering all possible protonation sites. For both complexes, one and two water molecules in the first coordination sphere of curium were also considered. DFT calculations indicate that the lowest energy structures correspond to protonation on oxygen that is not involved in An-DTPA bonds and that the structures with two water molecules are not stable. (authors)

[en] Because of the strong tendency of Pa(V) towards hydrolysis and polymerization, speciation studies have been conducted with the element at ultra-trace scale (CP_a < 10^-10 M). A systematic study using liquid-liquid extraction with a β-diketone as extractant has been performed at constant ionic strength and temperature. Under these experimental conditions, the variations of the distribution coefficient of ²³³Pa(V) as a function of nitrilotriacetic acid (NTA) and proton concentrations provides information on the stoichiometry of the complexes Pa-NTA in aqueous phase and also on their mean charge (slope method). Results indicate the formation of two successive complexes that are likely to be PaO(NTA) and PaO(NTA)₂^3-. The formation of complexes (1:1) and (1:2) are observed with actinides at oxidation states +3 and +4. In contrast, only (1:1) complexes are formed with actinides +5 and +6. In addition, capillary electrophoresis inductively coupled plasma mass spectrometry experiments have been performed with several actinides (²³⁹Pu(IV), ²⁴³Am(III), ²³¹Pa(V)) at tracer scale. Pu(IV) and Am(III) namely are known to form complexes of charge -2 ( Pu^IV(NTA)₂^2- and -3 (Am^III(NTA)₂^3-. Comparison of electrophoretic mobility for the complexes Am(III), Pu(IV) and Pa(V) with NTA, confirms the charge -3 for the maximum stoichiometry complex. The formation constants of PaO(NTA) and PaO(NTA) ₂^3- have been deduced from solvent extraction experiments. The value obtained for the (1:1) complex is similar to those relative to actinides + 6 whereas the formation constant of the (1:2) complex is close to the ones observed for the actinides at the oxidation state +3. Thus, these results emphasize the distinctive feature of protactinium chemistry as compared to the other actinides. To complete the results already obtained, a structural study by X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations is currently being processed to determine the coordination geometry and the interatomic distances of the formed complexes. Preliminary results on the structural study performed with the maximum stoichiometry complex will be also presented

[en] The complex formation of protactinium(V) with DTPA was studied at different temperatures (25-50 C) and ionic strengths (0.1-1 M) with the element at tracer scale. Irrespective of the temperature and ionic strength studied, only one neutral complex with (1:1) stoichiometry was identified from solvent extraction and capillary electrophoresis coupled to ICP-MS (CEICP-MS) experiments. Density Functional Theory (DFT) calculations revealed that two complexes can be considered: Pa(DTPA) and PaO(H₂DTPA). The associated formation constants were determined from solvent extraction data at different ionic strengths and temperatures and then extrapolated to zero ionic strength by SIT methodology. These constants are valid, regardless of complex form, Pa(DTPA) or PaO(H₂DTPA). The standard thermodynamic data determined with these extrapolated constants revealed a very stable complex formed energetically by an endothermic contribution which is counter balanced by a strong entropic contribution. Both, the positive enthalpy and entropy energy terms suggest the formation of an inner sphere complex. (authors)