Solubility of Neptunium(IV) in Carbonate Media

Abstract

The solubility of Np(IV) in carbonate media under reducing conditions was studied. The concentration of dissolved Np(IV) was measured with ionic strengths of 0.5, 1.0 and 2.0 M (M = mol.dm-3), hydrogen-ion concentration exponent (pHc= -log [H+]) from 8 to 13 and with the total carbonate concentration (CT) from 5 x10-3 M to 0.1 M by the oversaturation method. A reducing agent used was sodium dithionite. It was found that the solubility of Np(IV) decreased with increasing pH and increased with increasing CT and ionic strength. On the basis of analysis of the solubility data, the dominant aqueous species of Np(IY) were expected to be Np(C03)2(OH)22" and Np(C03)2(OH)44" in the present study. The apparent equilibrium constants of the carbonatohydroxo complexes were obtained in the respective ionic strength of 0.5, 1.0 and 2.0 M, and the thermodynamic equilibrium constants at the zero ionic strength, were extrapolated by a model called specific ion interaction theory (SIT) . Comparing with the literature data of thermodynamic equilibrium constants, the reasonableness of the present data was discussed.