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

Quanzhi Tian, Shingo Nakama, Keiko Sasaki

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42 Citations (Scopus)


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%).

Original languageEnglish
Pages (from-to)1127-1137
Number of pages11
JournalScience of the Total Environment
Publication statusPublished - Oct 15 2019

All Science Journal Classification (ASJC) codes

  • Environmental Engineering
  • Environmental Chemistry
  • Waste Management and Disposal
  • Pollution


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