Carbonate complexation of neptunium(IV) in highly basic solutions

The solubility of Np(IV) in carbonate media was studied for determination of thermodynamic data. The concentration of dissolved Np(IV) was measured with ionic strengths of 2.0 and 2.9M (M ≡ mol dm-3), hydrogen-ion concentration exponent (pHc = - log[H+]) of 12.0, 12.9 and 13.0, and with the total carbonate concentration (CT) from 5 x 10-2 M to 7 x 10-1 M by both oversaturation and undersaturation methods. It was found that the solubility of Np(IV) increased with increasing CT. Due to the dependencies of the solubility on CT and pHc, the dominant aqueous species of Np(IV) in highly basic solutions was expected to be NpCO3(OH)44-. Together with a carbonatohydroxo complex of Np(CO3)2(OH)22- estimated in our previous study, the apparent equilibrium constants of the carbonatohydroxo complexes were obtained at the respective ionic strength while the equilibrium constants at the zero ionic strength were extrapolated. The obtained equilibrium constants supported the literature values strongly, and contributed to improve the reliability of the thermodynamic data. (orig.)