Studies on Hydrolysis and Complexation Reactions of Transuranium Elements

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Migration behavior of transuranium elements such as neptunium is very important for the performance assessment of geologic disposal of high level radioactive wastes. Generally speaking, however, the elements show complex chemical behavior, and the behavior is not completely understood yet. Because of their chemical nature, transuranium elements are very sensitive to redox conditions, and are likely involved in hydrolysis and complexation reactions in aqueous solutions.In the present study, the hydrolysis and complexation reactions of transuranium elements were studied in order to know the chemical behavior of transuranium elements in aqueous solutions under the conditions of geologic disposal. Obtained results are as follows :(1) The effect of carbonate ions on the solubility of Np (VI) was measured, and a new result was obtained for the formation of hydrolysis and hydroxycarbonate complex species of Np (VI).(2) A simple hard sphere model was presented to study the systematics for the termodynamic constants of hydrolysis and carbonate complexation of transuranium ions. The model was found to be useful to check the experimental values and to predict the unknown ones.(3) With the use of commercial humic acids, humate complexation of Np (V) was measured. The results were found to be successfully interpreted by a multi-site model.(4) Adsorption behavior of Np (V) and Cs onto quartz was measured and analyzed by an electric double layr model. The analysis was successful, and the pH and ionic strength dependences of distribution coefficients were well interpreted by considering a number of adsorption reactions on the quartz surface.