Sorption Behavior of Nickel and Palladium in the Presence of NH 3 (aq)/NH 4 +
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Sorption Behavior of Nickel and Palladium in the Presence of NH3(aq)/NH4+ Taishi Kobayashi1, Takayuki Sasaki1, Ken-you Ueda1 and Akira Kitamura2 1 Graduate School of Engineering, Nuclear Engineering, Kyoto University, Yoshida-honmahi, Sakyo-ku, Kyoto, Japan 2 Radionuclide Migration Research Group, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, Muramatsu, Tokai, Ibaraki, Japan ABSTRACT It is necessary to assess the impact of nitrate salts and their reduction products (e.g. NH3(aq)/NH4+) contained in low-level radioactive waste generated from nuclear reprocessing process for the safety assessment of geological disposal of the waste. In the present study, sorption behavior of Ni and Pd on pumice tuff was investigated in the presence of NH3(aq)/NH4+. Under various NH3(aq)/NH4+ concentration, pH and ionic strength conditions, distribution coefficient (Kd) of Ni and Pd on pumice tuff was determined by a batch experiment. For Ni system, the Kd values showed no significant dependence on initial NH4+ concentration ([NH4+]ini < 1 M) in neutral pH region, which agreed with the prediction from thermodynamic data. For Pd system, the Kd values decreased with an increase of [NH4+]ini, suggesting the formation of stable ammine complexes (Pd(NH3)m2+ (m: 1 – 4)). The obtained Kd values for Ni and Pd were analyzed using a surface complexation model. By taking complexes predicted by thermodynamic data into account, sorption behavior of Ni and Pd in the presence of NH3(aq)/NH4+ were well explained. INTRODUCTION Some of low-level waste generated from nuclear reprocessing process contain considerable amount of nitrate salts which may affect the migration behavior of several radionuclides. Nitrate ion (NO3) and its reduced components, ammonia (NH3(aq)) and ammonium ion (NH4+) may form stable complex with radionuclide. Since nickel (Ni-63) as radioactivated product and palladium (Pd-107) as fission product potentially form stable ammine complexes [1], it is important to evaluate the impact of NH3(aq)/NH4+ on the migration behavior of Ni and Pd under relevant conditions. The sorption behavior of Ni and Pd, thus, may be influenced by the formation of ammine complexes. In the present study, we focused on the sorption behavior of Ni and Pd on pumice tuff in the presence of NH3(aq)/NH4+. Distribution coefficient (Kd) value for Ni and Pd on pumice tuff was determined by a batch experiment at various pH, ionic strength, and initial NH4+ concentrations and the obtained Kd values were analyzed using a surface complex model and values based on the thermodynamic data in order to understand and predict the sorption behavior in the presence of NH3(aq)/NH4+ . EXPERIMENT
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A batch sorption experiment was carried out to determine the Kd (m3·kg1) value of Ni and Pd on pumice tuff. The pumice tuff was purchased from Nichika Inc. and ground into a powder using a stainless steel mortar and pestle. The powder with a particle size of 63 – 125 Pm was then obtained by sieving. The chemical and mineralogical characteri
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