Thermodynamic properties of Pu[sup 3+] and Pu[sup 4+] aquo ions

The purpose of this work is to evaluate thermodynamic properties of some actinide ions in aqueous solution, particularly plutonium, taking into account their real characteristics, such as their covalent character and effective charge. The aquo ions are characterized by the charge q, crystallographic radius Rc with coordination number N, radius of the water molecule Rw and number H of molecules in the second hydration sphere. For the understanding of lanthanide and actinide aquo ion properties, we have performed EXAFS experiments on different trivalent and tetravalent ions (with concentration 7.10-3 to 0.2 M) in acidic solutions (HCl 1 M) at LURE. Based on our data and different published investigations, we have defined the coordination number N and the interatomic distances d between the cation and the oxygen atoms of the primary hydration sphere for the complete series of trivalent lanthanides, for some trivalent actinides (U, Np, Pu, Am and Cf) and for the tetravalent plutonium ion.