Carbonate complexation of neptunyl(VI) ion

The neptunyl(VI) ion in aqueous carbonate system has been investigated by s olubility measurements in 0.1 M NaClO4 at 25 +/- 0.1 degrees C in equilibri um with 80% CO2/O-2 atmosphere. Neptunium was oxidized and stabilized in he xavalent state in the presence of ozone during the experiment. The solubili ty limiting solid phase was characterized as NpO2CO3(s) by X-ray crystallog raphy, UV-Vis-NIR photoacoustic spectroscopy and FT-IR photoacoustic spectr oscopy. For this solid phase the solubility product IgK(sp)(NpO2CO3) = -14. 04 +/- 0.07 was obtained. The stability constants of Np(VI) carbonate compl exes, i.e., Ig beta(101)(NpO2CO30) = 9.02 +/- 0.10 and Ig beta(103)(NpO2(CO 3)(3)(4-)) = 20.41 +/- 0.09, were determined together with upper limits of the dicarbonate complex, Ig beta(102)(NpO2(CO3)(2)(2-)) < 14.2, and the mon ohydrolyzed species, Ig beta(110)(NpO2OH+) < 9.0. These results were compar ed with those of uranyl(VI) and plutonyl(VI) ions in the literature.