pH-dependent Cs leaching from nitrate bearing geopolymers: A combined experimental and model analysis

Geopolymers are considered as promising wasteforms for the immobilization of low-level radioactive waste streams containing ¹³⁷Cs. This work presents an experimental study and modelling of the pH-dependent leaching of Cs from nitrate bearing geopolymers. Longer curing periods and higher curing temperatures induce the formation of nitrate bearing zeolitic phases (NO₃-sodalite and NO₃-cancrinite), that exhibit higher Cs/Na selectivity relative to nitrate-free zeolitic phases (zeolite A and zeolite X). Such phase formation was found to have a significant effect on the pH-dependent leaching of Cs, as well as on the leaching of other major elements in the geopolymeric system. An innovative, semi-empirical geochemical thermodynamic model was developed for the sodium aluminum silicate hydrate (NASH) solid solution, based on the pH-dependent leaching experimental data. The model succeeded to describe most of the studied system, with emphasis on their interaction with Cs.