Study of the hydrolysis of protactinium (V), at tracer scale, by solvent extraction method with thenoyl-tri-fluoro-acetone (TTA) as chelating agent. Characterization of the partition of TTA in the system TTA / H2O / toluene / Na+ / H+ / ClO4-

Hydrolysis of protactinium (V) according to the reactions: PaO(OH)2+ +H2O ↔ PaO(OH)2+ + H+ (K2] PaO(OH)2+ +2H2O ↔ PaO(OH)5 + H+ (K3) has been studied, at tracer scale, by solvent extraction method, with thenoyl-tri-fluoro-acetone (TTA) as chelating agent. A previous study concerning the partition of TTA between two immiscible phases (corresponding to TTA/toluene/Na+/H+/ClO4- system) has allowed a complete characterization of this system (partition constants, standard thermodynamic values, TTA hydration degree in toluene). Owing to specific properties of protactinium (V) (sorption onto various materials, formation of colloids), an extremely rigorous protocol has been established, protocol which could be used for other hydrolysable elements. Hydrolysis constants were deduced from a systematic study of partition of Pa(V) as a function TTA and proton concentration, ionic strength and temperature. Extrapolations to zero ionic strength were performed using SIT model and the specific interaction coefficients ε(i,j) as well as the Pitzer parameters β(0) and β(1) were determined. Standard thermodynamic data relative to hydrolysis equilibriums of Pa(V) were also estimated. (author)