Study into the applicability of laboratory data to natural conditions: Hydrolysis of Np(V) and Np(V) complexation by carbonate and humate

The hydrolysis of Np(V) has been studied by way of determining the pH dependence of the solubility in the pH range from 7 to 13.8, for the ionic strengths of 0.1, 1.0, and 3.0 M NaClO4, at a temperature of 25deg C. At high ionic strengths of > 1 M the amorphous phase of NpO2OH changes into a thermodynamic, more stable, modified phase with a solubility that is reduced by the factor of 5.1. The determined solubility data are used for deriving the first and second hydrolysis constants at the different ionic strengths, and for both phases the solubility product is determined. The results show good agreement with earlier results obtained by the authors for I = 1.0 M, but reveal clear deviations from data found in the literature. The investigations of the complex formation of Np(V) with carbonate have been done by way of solubility measurements in the pH range from 6.5 to 10, for the ionic strengths of 0.1, 1.0 and 3.0 M NaClO4, at a CO2 partial pressur of 0.03% and a temperature of 25deg C. With lower ionic strengths, the solid phase is NaNpO2CO3, whereas with higher concentrations of Na2CO3, Na3NpO2(CO3)2 as a stable, solid phase is observed. The complexes NpO2CO3-, NpO2(CO3)23-, and NpO2(CO3)35- develop as dissolved species. The solubility products and the complex formation constants of the dissolved species and of the solid phases are derived by numerical simulation of the solubility curves for the ionic strengths of 0,1, 1.0 and 3.0 M. The complexation constants thus obtained could be verified by absorption spectroscopy, for the complexes NpO2CO3- and NpO2(CO3)23-. (orig./BBR)