Carbonate complexation of Np(IV) in alkaline solutions

Reliable thermodynamic data for actinide (IV) have been required. 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 from 0.5 to 2.9 M (M ≡ mol·dm-3), hydrogen-ion concentration exponent (pHc = -log[H+]) from 8 to 13 and with the total carbonate concentration (CT) from 5x10-3 M to 7x10-1 M by an oversaturation method. It was found that the solubility of Np(IV) decreased with increasing pHc and increased with increasing CT. On the basis of analysis of the solubility data, the dominant aqueous species of Np(IV) were expected to be Np(CO3)44-, Np(CO3)2(OH)22- and Np(CO3)2(OH)44- in the present study. The apparent equilibrium constants of the carbonate and carbonatohydroxo complexes were obtained in the respective ionic strength and 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. (author)