Interaction of tetravalent actinides and Np(V) with some hydroxy carboxylic acids

Abstract

Interaction of actinides(IV) with hydroxyisobutyric acid (HHIB) in aqueous solutions and in the course of crystallization of solid compounds was studied. The complexes MLn(4-n)+ (M = U, Np, Pu; L− is hydroxyisobutyrate anion; n = 1, 2, 3) exist in solution. Their apparent stepwise stability constants K′i were measured, and the overall concentration stability constants β3 of the complexes ML3+ were calculated. For U(IV) and Np(IV), logβ3 is close to 13.3–13.4, and for Pu(IV), logβ3 = 14.5 ± 0.9 (ionic strength I = 0.1–0.3). In the course of crystallization in air, complexes of U(IV) with hydroxyisobutyric acid, as well as those with citric acid, undergo oxidative degradation, which can be accompanied by complete oxidation of U(IV). The crystalline compounds formed in the process are oxalates of U(IV) or U(VI). The complexation of Np(V) with HHIB was studied. NpO2+ forms with HHIB the complexes NpO2L and NpO2L2−. Their concentration stability constants are logK1 = 2.04 ± 0.15 and logK2 = 0.71 ± 0.10 (I = 0.4), i.e., logβ2 = 2.75 ± 0.25.