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 extraction technique. The data were interpreted using the thermodynamic formalism of Pitzer, which is valid to high electrolyte concentrations. A consistent model for the deprotonation constants of acetic acid in NaCl and NaClO4 media was developed. For the concentrations of acetate expected in a waste repository, only the neutral complex NpO2Ac(aq) was important in describing the interactions between the neptunyl ion and acetate. The thermodynamic stability constant log β0101 for the reaction NpO2+ + Ac- ↔ NpO2Ac was calculated to be 1.46±0.11. This weak complexing behavior between the neptunyl ion and acetate indicates that acetate will not significantly enhance dissolved Np(V) concentrations in ground waters associated with nuclear waste repositories that may contain acetate