Adsorption behaviour of simulant radionuclide cations and anions in metakaolin-based geopolymer

Geopolymers are a class of alkaline-activated materials that have been considered as promising materials for radioactive waste disposal. Currently, metakaolin-based geopolymers (MK-GPs) are attracting interest for the immobilisation of radionuclides in contaminated water from the Fukushima Daiichi Nuclear Power Station. However, the associated chemical interaction mechanisms and the theoretical prediction of the adsorption behaviour of MK-GP in response to cationic radionuclides have not been thoroughly studied or fully understood. In addition, there is a lack of studies on the adsorption capacity of MK-GP for anionic radionuclides. In this study, two types of metakaolin-based (Metastar501 and Sobueclay) geopolymers were synthesised at a K₂O:SiO₂:H₂O ratio of 1:1:13. The binding capacity and interaction mechanism of MK-GP with Cs⁺, Sr²⁺, Co²⁺, I^-, IO₃^-, SeO₃^2-, and SeO₄^2- were evaluated based on the zeta potential, radionuclide binding, and alkali leaching. The results showed that MK-GP does not have the ability to incorporate anionic radionuclides irrespective of the metakaolin source used, but both types of geopolymers have a high capacity to immobilise cationic radionuclides. The uptake of Cs⁺ was observed as a one-to-one exchange between Cs⁺ and K⁺ whereas both one-two and one-one ion exchanges are possible in the case of Sr²⁺ and Co²⁺ with K⁺. The formation of cobalt blue (CoAl₂O₄) also contributed to the binding of Co²⁺. Thermodynamic modelling was conducted according to the ion exchange mechanism which predicts the binding of Cs⁺ and Sr²⁺ at low concentrations.