Revisiting the speciation diagram of Pu(IV)-acetate system taking into account polynuclear species
Résumé
The speciation of actinides is fundamental to better understand the behavior of these radioelements in condensed phases. Recently, it has been admitted that the competition between hydrolysis and complexation can lead to the formation of polynuclear species (or clusters) with actinide cations connected through oxo (O), hydroxo (OH) or aquo (H2O) bridges [1]. These species, stabilized at the surface by organic or inorganic ligands, can appear for low acidities and therefore be formed during geological storage. Different stoichiometries are possible depending on the conditions ranging from compounds with 2 metal centers to extreme cases involving 38 actinide cations [2], but most of the clusters observed in the solid state are hexameric clusters with octahedral geometry and are complexed at the surface with carboxylate functional ligands [3, 4]. If the structures of the clusters are often described in the solid state, they are much less so in solution they are not always correctly identified in solution. Therefore they are not taken into account in the main thermodynamic/speciation models. While few data exist for the hexameric clusters of Th(IV), U(IV) and Np(IV) in solution with formate and acetate [5], no information is reported for Pu(IV). The objective of this study is the analysis of Pu(IV) clusters in solution in the presence of carboxylate ligand: acetate is used as a surrogate of this family. Moreover, available data on speciation in solution of “classical” monomeric complexes of plutonium(IV) acetate are rare, incomplete and sometimes contradictory.
To detect all potentially formed plutonium-acetate species, solutions were first characterized by Vis-NIR absorption spectroscopy. Large variations of pH and acetate concentrations were used and six different species were identified: the Pu(IV) aquo cation (Pu4+), and five plutonium complexes with acetate. Those five acetate complexes were characterized by coupling experimental (Vis-NIR and EXAFS spectroscopies and ESI-MS spectrometry) and quantum chemistry. As a result, Pu6O4(OH)4(AcO)12(H2O)6 hexameric cluster has been identified [6]: the missing block in the An(IV) series with formate and acetate ligands. The four other complexes are attributed to Pu(IV) acetate monomeric complexes. Species of Pu(IV) with acetate being described, their fractions in solution were evaluated and reported on a speciation diagram. The hexanuclear complex was not mentioned in the literature (despite chemical conditions equivalent to ours), this study allowed to define the new and more reliable Pu(IV)-acetate speciation diagram given in figure 1.