Complexation study of NpO+2 and UO2+2 ions with several organic ligands in aqueous solutions of high ionic strength

The acid dissociation constants, pKa, and the stability constants for NpO+2 and UO2+2 have been measured for certain organic ligands [acetate, α-hydroxyisobutyrate, lactate, ascorbate, oxalate, citrate, EDTA, 8-hydroxyquinoline, 1,10-phenanthroline, and thenoyltrifluoroacetone] in 5 m (NaCl) ionic strength solution. The pKa values were determined by potentiometry or spectrometry. These methods, as well as solvent extraction with 233U and 237Np radiotracers, were used to measure the stability constants of the 1:1 and 1:2 complexes of dioxo cations. These constants were used to estimate the concentrations required to result in 10% competition with hydrolysis in the 5 m NaCl solution. Such estimates are of value in assessing the solubility from radioactive waste of AnO+2 and AnO2+2 in brine solutions in contact with nuclear waste in a salt bed repository. (orig.)