New Insights into Formation
of Trivalent Actinides
Complexes with DTPA
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Abstract
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<sup>–</sup> and AnDTPA<sup>2–</sup> (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<sup>2–</sup> and AnHDTPA<sup>–</sup> 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