Solubility of mixed-valence U(IV-VI) and Np(IV-V) hydroxides in simulated groundwater and 0.1 M NaClO4 solutions

The kinetics of U(VI) accumulation in the phase of U(IV) hydroxide and of Np(V) in the phase of neptunium(IV) hydroxide, and also the solubility of the formed mixed-valence U(IV)-U(IV) and Np(IV)-Np(V) hydroxides in simulated groundwater (SGW, pH 8.5) and 0.1 M NaClO4 (pH 6.9) solutions was studied. It was found that the structure of the mixed U(IV-VI) hydroxide obtained by both oxidation of U(IV) hydroxide with atmospheric oxygen and alkaline precipitation from aqueous solution containing simultaneously U(IV) and U(VI) did not affect its solubility at the U(VI) content in the system exceeding 16%. The solubility of mixed-valence U(IV-VI) hydroxides in SGW and 0.1 M NaClO4 is (3.6±1.9) × 10-4 and (4.3 ± 1.7) × 10-4 M, respectively. The mixed Np(IV-V) hydroxide containing from 8 to 90% Np(V) has a peculiar structure controlling its properties. The solubility of the mixed-valence Np(IV-V) hydroxide in SGW [(6.5 ± 1.5) × 10-6 M] and 0.1 M NaClO4 [(6.1±2.4) × 10-6 M] is virtually equal. Its solubility is about three orders of magnitude as high as that of pure Np(OH)4 (10-9-10-8 M), but considerably smaller than that of NpO2(OH) (∼7 × 10-4 M). The solubility is independent of the preparation procedure [oxidation of Np(OH)4 with atmospheric oxygen or precipitation from Np(IV) + Np(V) solutions]. The solubility of the mixed-valence Np hydroxide does not increase and even somewhat decreases [to (1.4±0.7) × 10-6 M] in the course of prolonged storage (for more than a year). © 2006 Nauka/Interperiodica.