Retention of Nickel and Cobalt in Boda Claystone Formation

The Boda Claystone Formation (BCF) is considered to serve as a natural barrier to the potential high-level radioactive waste repository in Hungary. In order to evaluate the radionuclide retention capacity of the albitic claystone of the BCF, the adsorption and diffusion properties of the rock for Ni²⁺ and Co²⁺ cations (activation products) were investigated separately and in competitive conditions when the two ions were simultaneously added. Batch sorption experiments were performed with powdered and conditioned albitic claystone samples in synthetic pore water to obtain adsorption isotherms. In addition, adsorption tests were performed on petrographic thin sections to check the transferability between dispersed and compact systems. Correlation analysis of microscopic X-ray fluorescence elemental maps recorded on thin sections suggested that nickel is primarily bound to clay minerals (mainly illite and chlorite), which was confirmed by (scanning) transmission electron microscopy measurements. Around illite particles, a newly formed nickel-rich few atomic layer thick phyllosilicate phase was identified. The discrepancy between the experimental and modeled adsorption isotherm at high concentrations could be explained with this nickel-rich new phase. Apart from C_in = 10⁻³ M and only Ni²⁺ or Co²⁺ in the source, the apparent diffusion coefficients of Ni²⁺ and Co²⁺ (C_in = 10^−3–10⁻² M) were found to be similar. Overall, the BCF shows promising capabilities to retain the studied radionuclides.