URANYL OXALATE SPECIES IN NATURAL ENVIRONMENTS: STABILITY CONSTANTS FOR AQUEOUS AND SOLID URANYL OXALATE COMPLEXES

摘要

Uranyl ion, UO_2~(2+), and its aqueous complexes with organic and inorganic ligands, are the dominant species for transport of natural occurring uranium at the Earth surface environments. In the nuclear waste management, uranyl ion and its aqueous complexes are expected to be responsible for uranium mobilization in the disposal concepts where spent fuel is disposed in oxidized environments such as unsaturated zones relative to the underground water table. In the natural environments, oxalate, in fully deprotonated form, C_2O_4~(2-), is ubiquitous, as oxalate is one of the most important degradation products of humic and fulvic acids. Oxalate is known to form aqueous complexes with uranyl ion to facilitate the transport of uranium. However, oxalate also forms solid phases with uranyl ion in certain environments, limiting the movement of uranium. Therefore, the knowledge of the stability constants of aqueous and solid uranyl oxalate complexes is important not only to the understanding of the mobility of uranium in natural environments, but also to the performance assessment of radionuclides in geological repositories for spent nuclear fuel. In this work, we present the stability constants for UO_2C_2O_4(aq) and UO_2(C_2O_4)_2~(2-) at infinite dilution based on our evaluation of the literature data over a wide range of ionic strengths up to 9.5 mol·kg~(-1). We also obtain the solubility constants at infinite dilution for the following solid uranyl oxalates, UO_2C_2O_4·3H_2O and UO_2C_2O_4·H_2O, based on the solubility data in a wide range of ionic strengths up to 11 mol·kg~(-1). In our evaluation, we use the computer code EQ3/6 Version 8.0a. The model developed by us is expected to enable researchers to accurately assess the role of oxalate in mobilization/immobilization of uranium under various conditions including those in geological repositories.