Immobilization of cesium in fly ash-silica fume based geopolymers with different Si/Al molar ratios

Geopolymers are considered as promising matrixes for waste solidification. However, the effects of the Si/Al molar ratio of geopolymer on the immobilization efficiencies for metal ions have not been fully studied and understood. In the present study, geopolymers with different Si/Al ratios were synthesized from coal fly ash and silica fume. Adsorption tests were conducted to evaluate their immobilization efficiencies for Cs⁺. The results indicated that geopolymer with low Si/Al ratio could have a better immobilization performance for Cs⁺ than that with high Si/Al ratio. High Si/Al ratio could contribute to a more compact structure of geopolymer. Each sorption process fitted better with the pseudo-second-order model, and all of them were governed by film diffusion. However, the diffusion mode was gradually closed to particle diffusion with the increase in the Si/Al ratio. Both Langmuir and Freundlich models could well fit the sorption data, and the free energy of each sorption process decreased with the increase in the Si/Al ratio according to D-R equation. The distribution of AlO₄ tetrahedron in the geopolymer structure plays a significant role in the immobilization of Cs⁺. Low Si/Al ratio could result in that more AlO₄ tetrahedrons distribute in the small rings (<eight-member), which has stronger locking effects on Cs⁺. However, high Si/Al ratio leads to the distribution of AlO₄ tetrahedrons mainly in larger rings (≥eight-member), and this could contribute to the high leaching amount of Cs⁺. In addition, high-temperature treatment could contribute to the formation of nepheline or pollucite in geopolymer matrix. These minerals locked Cs⁺ in their structures, and the leaching amount of Cs⁺ was reduced correspondingly from high levels (26.36%, 27.26%, and 66.92%) to very low levels (0.67%, 0.53%, and 0.95%).