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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 (CE-ICP-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.

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 nonprotonated 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.

The complexation of protactinium(V) by oxalate was studied by X-ray absorption spectroscopy (XAS), density functional theory (DFT) calculations, capillary electrophoresis coupled with inductively coupled plasma mass spectrometry (CE-ICP-MS) and solvent extraction. XAS measurements showed unambiguously the presence of a short single oxo-bond, and the deduced structure agrees with theoretical calculations. CE-ICP-MS results indicated the formation of a highly charged anionic complex. The formation constants of PaO(C(2)O(4))(+), PaO(C(2)O(4))(2)(-), and PaO(C(2)O(4))(3)(3-) were determined from solvent extraction data by using protactinium at tracer scale (C(Pa) < 10(-10) M). Complexation reactions of Pa(V) with oxalate were found to be exothermic with relatively high positive entropic variation.