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Systematics of Hydrolysis and Carbonate Complexation Constants of Ions of Transuranium Elements

A simple hard sphere model was presented to study the systematics of hydrolysis and carbonate complexation constants of transuranium ions. In the model, an octahedral structure is assumed, and a central transuranium ion and its ligands of water molecule, hydroxide ion and carbonate ion are all treated as hard spheres. The stability constants are calculated from the Coulomb interactions of the hard spheres by taking a parameter set for the effective radii and electric charges of the hard spheres. The model is applied to the hydrolysis, carbonate and hydroxycarbonate species, and the systematic behavior of the stability constants is discussed. Such models may be useful to check the experimental data and to predict the unknown data from a systematic point of view. In order to reach a unified understanding, theoretical considerations may be helpful as well as experimental measurements. The unified theory of metal ion complexation [5] is one of the models which may be used for such a purpose. Based on this theory, stability constants have been predicted for a number of species containing selected elements and ligands that are important for the safety assessment. It may be pointed out, however, that this theory tends to give higher values than the experimental ones for higher coordination species. In this paper, a simple hard sphere model is presented to study the systematic behavior of hydrolysis and carbonate complexation of transuranium ions. The effective radii and electric charges of the hard spheres are given empirically. In spite of the simplifications, the observed systematic behavior of the stability constants is well reproduced by the present model.