Study on Relation Between Migration Behaviors and Chemical Forms of Cobalt
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STUDY ON RELATION BETWEEN MIGRATION BEHAVIORS AND CHEMICAL FORMS OF COBALT SHINZO UETA" AND NAOTAKE KATOH*" " Naka Nuclear Development Center, Mitsubishi Metal Corporation, Mukohyama 1002-14, Naka-machi, Ibaraki-ken 311-02, Japan. "-Chemical Engineering Division, Kogakuin University, 1-24-2 Nishishinjuku, Shinjuku-ku, Tokyo 160, Japan. ABSTRACT Migration experiments focused on the relationship between sorption mechanism and chemical form of cobalt were carried out by means of column method. Adsorbents packed in the column were natural sand and artificial materials, for instance, charcoal. The experimental results were compared with three different migration models based on ion exchange, filtration, and adsorption, respectively. Cobalt existed as cation in the low pH region, where the migration behavior of cobalt was described by the ion exchange model. Cationic and colloidal species of cobalt coexisted in the pH 8.5 solution. The migration behavior of cobalt in this solution was satisfactorily described by the coupled model of the ion exchange and the filtration. Chelated anionic cobalt was not retained on the sand but retained on the charcoal and the other artificial materials to considerable extent. Breakthrough curves of cobalt in the charcoal column were successfully described by the adsorption model. Chelated cobalt was retained on the charcoal better than cationic cobalt. Distribution coefficients of chelated cobalt and cationic one were 670 ml/g and 130 ml/g, respectively. INTRODUCTION The purpose of this study is to understand the relationship between the chemical form of nuclides and their migration behavior through natural and/or artificial barriers. The number of possible chemical forms is theoretically infinite. They can be classified, however, into three principal groups, cations, anions, and the other forms, for instance, colloidal or organic species. At first we developed the migration model for both cationic and colloidal species on the assumption that the main sorption mechanism in the natural barrier was ion exchange and filtration for cationic and colloidal species [1],[2]. On the other hand, anionic species can not be retained in the geosphere which has little anion exchange capacity. Such nuclide species require certain appropriate materials as an artificial barrier. This paper reports the development of migration model for various chemical species with some experiments which were focused on the cobalt. We hope that the approach reported here is informative to investigators who conduct similar experiments. EXPERIMENTAL Tracer solutions were prepared to present three different chemical species of cobalt, Co2 1 , Co(OH) 2 , and Co-EDTA, which simulated the cationic, colloidal, and anionic species, respectively. Co" and Co(OH) 2 were formed in the different pH solutions of cobalt nitrate, pH 2, 8, and 8.5. The standard reagent, CjoH, 2 N2 OsNa 2 Co.4H2O, was used as Co-EDTA. The formation of each chemical species were confirmed by means of ultraviolet spectrophotometry, ultrafiltration, and the ot
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