Adsorption and migration of Cs and Na ions in geopolymers and zeolites

Geopolymers may provide a more sustainable alternative to Portland Cement for various possible applications. Geopolymers have attracted particular interest for the immobilization of pollutants, owing to their high adsorption capacity, high thermal and chemical resistance, and low leachability. However, practical implementation is currently hindered by a limited understanding of how adsorption processes occur in geopolymers, and how they can be engineered to optimize the incorporation of pollutants and avoid their release. In this work, Molecular Dynamics simulations provide insights into these processes at the atomic scale, studying the role of host material composition and structure in the immobilization of Na and Cs ions. The simulations reveal that the most stable configurations for these ions are near the center of 6- and 8-membered aluminosilicate rings, where the coordination with the geopolymer is maximum. Higher contents of Al and degrees of crystallinity are found to yield more stable configurations for Cs ions, with more favorable adsorption enthalpies and lower diffusion coefficients. The comparison of different crystalline zeolite structures reveals that the framework of sodalite, used as the baseline to develop model geopolymer structures, is the most suitable for the immobilization of Cs since there are no channels and it is formed by small 4- and 6-member, all preventing Cs ions from escaping the cavities.