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 NH 3(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&lt
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 [NH 4+]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 NH 3(aq)/NH4+ were well explained. © 2013 Materials Research Society.