Consideration on Thermodynamic Data for Am(III) in Aqueous Carbonate system

Yamaguchi Tetsuji, Nakayama Shinichi

doi:10.3327/jnuce.3.49

Thermodynamic data for Am(III) in carbonate media presented in seven publications from six organizations of five countries were reviewed and discussed. Fairly good agreement was found for the equilibrium constant for the mono carbonate complex of Am(III), AmCO₃⁺. Two different analyses have been proposed for the formation of higher coordinated complex species, one of which assumed formation of pure carbonate complexes, and another assumed formation of both carbonate and hydroxy-carbonate complexes. This review found a set of solubility data that requires unreasonably large stability constants for the carbonate complex species unless the hydroxy-carbonate complexes are taken into account. This review estimated upper limits of the stability constants of the hydroxy-carbonate complexes from another set of solubility data that was originally interpreted only by the carbonate complexes. These upper limits were consistent to the reported stability constants for the hydroxy-carbonate complexes. There is a set of solvent extraction data that requires no hydroxy-carbonate complexes in the data analysis although the hydroxy-carbonate complexes could be dominant under the employed conditions. This review considered the hydroxy-carbonate complexes as well as carbonate complexes to develop a database that is consistent to all the published solubility data because the main purpose of this database is to estimate the solubility of Am for the performance assessment of geological disposal. Stability constants extrapolated to I=0 by specific interaction model were selected.

Consideration on Thermodynamic Data for Am(III) in Aqueous Carbonate system

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