THERMODYNAMIC MODELING OF NEPTUNIUM(V)-ACETATE COMPLEXATION IN CONCENTRATED NACL MEDIA

The complexation of neptunium(V), Np(V), with the acetate anion, Ac-, was measured in sodium chloride media to high concentration using an e xtraction technique. The data were interpreted using the thermodynamic formalism of Pitzer, which is valid to high electrolyte concentration s. A consistent model for the deprotonation constants of acetic acid i n NaCl and NaClO4, media was developed. For the concentrations of acet ate expected in a waste repository, only the neutral complex NpO2Ac(aq ) was important in describing the interactions between the neptunyl io n and acetate. The thermodynamic stability constant beta(101)(0) for t he reaction NpO2+ + Ac- <----> NpO2Ac was calculated to be 1.46+/-0.22 . This weak complexing behavior between the neptunyl ion and acetate i ndicates that acetate will not significantly enhance dissolved Np(V) c oncentrations in ground waters associated with nuclear waste repositor ies that may contain acetate.