Stabilization and Destabilization of Hydrolyzed Pentavalent Actinide Complexes by Alkali Metals

Until now, the specific medium effects, such as those of alkali metals in the medium, on the formation of negatively charged metal-ligand species have been poorly understood, particularly at low ionic strength. This work focused on such effect on the hydrolysis of neptunium and plutonium(V) at an ionic strength of 0.1 mol·L^-1. Their hydrolysis constants were determined in LiCl, KCl, RbCl, and CsCl media by capillary electrophoresis (CE) coupled to an inductively coupled plasma mass spectrometer (ICP-MS) and compared with the data obtained by a previous study in NaCl. Ab initio simulation studies using ABINIT were also carried out to gain a better understanding of the interactions between the alkali metals and the hydrolyzed species. The results show the formation of a new type of ternary complex in LiCl and NaCl media at the edge between ion pairing and the formation of a classical ternary complex. This new type of complex stabilizes (in LiCl medium) or destabilizes (in NaCl medium) the hydrolyzed species. The potassium and rubidium cations form ion pairs with the hydrolyzed species, whereas cesium does not interact with them. These different effects show that alkali metals (with the exception of cesium) interact with complexes and may have a major impact on the speciation of metals in industrial processes and environmental media.