A family of uranyl oxide hydrate phases with bivalent cations [MII(H2O)4][(UO2)3O3(OH)2]·H2O (MII – Mn, Co, Ni, Zn): synthesis, characterization and chemical stability in aqueous solutions

Abstract

A series of new uranium compounds [M^II(H₂O)₄][(UO₂)₃O₃(OH)₂]·H₂O (M^II – Mn, Co, Ni, Zn) were synthesized by the reaction of the metal salts with schoepite under hydrothermal conditions. X-Ray powder diffraction (XRD) studies have been carried out, and the indexing of diffraction peaks shows that all the phases [M^II(H₂O)₄][(UO₂)₃O₃(OH)₂]·H₂O crystallize in the triclinic crystal system and have lattice parameters similar to those of the known Ni uranyl oxide hydrate. IR spectroscopic studies were carried out; processes of dehydration and thermal decomposition were investigated and reported. A detailed investigation of the heterogeneous equilibria in the system “[M^II(H₂O)₄][(UO₂)₃O₃(OH)₂]·H₂O(cr) – aqueous solution” was performed. The pH ranges of the stability of the compounds and their solubility were experimentally established. Hydrolysis products isolated at the end of the solubility experiments were characterized by powder XRD and quantitative chemical analysis. For [M^II(H₂O)₄][(UO₂)₃O₃(OH)₂]·H₂O, the solubility products, Gibbs energies of formation (298 K), and solubility curves were calculated. Speciation diagrams of U(VI) and M(II) in saturated solutions and solids were constructed. The present work is the first complex research of the family of uranyl oxide hydrates with some bivalent cations (Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) including the study of the structure and chemical stability in aqueous solutions. The obtained results are important for the estimation of the long-term behavior of uranium in the environment and in spent fuels under repository disposal conditions.