Cesium immobilization in nitrate-bearing metakaolin-based geopolymers

E. Ofer-Rozovsky, M. Arbel Haddad, G. Bar-Nes, E. J.C. Borojovitz, A. Binyamini, A. Nikolski, A. Katz

Research output: Contribution to journalArticlepeer-review

22 Scopus citations

Abstract

Low-level nuclear wastes containing radioactive cesium (Cs) are often highly alkaline and rich in sodium nitrate. The goal of the present study was to examine immobilization of cesium in nitrate-bearing geopolymers with low Si/Al ratio (SiO2:Al2O3 < 2.0), which are known to have relatively high crystalline phase content. Geopolymers were prepared by activating metakaolin with a mixed NaOH-CsOH solution containing NaNO3 where Cs+ ions make up 7% of the total alkali content. Geopolymer samples were cured at 40±3 °C for up to 3 months. The structure of the cured geopolymers after varying curing times was characterized by XRD and FTIR, and the immobilization efficiency for Cs+, Na+ and NO3 was determined from leaching tests following the ANSI/ANS-16.1 standard procedure for low and intermediate level nuclear wasteforms. The first phase crystallized from the amorphous gel was zeolite A. This phase gradually undergoes further transformations to yield the nitrate-bearing feldspathoids, nitrate sodalite and nitrate cancrinite. The leaching curves for Cs+, Na+ and NO3 ions were clearly not linear with t1/2, suggesting a change in leaching mechanism after the initial 24 hours of the leaching experiment. The apparent diffusion constants, Da, as well as leaching indices for Cs+ and Na+ were therefore calculated separately for the 1st and 2nd time domains of the experiment (t1/2 < 1 days1/2 and t1/2 > 2.65 days1/2, respectively). The leachability indices for Cs+ were found to increase with increasing curing time (from 7.4 and 11.2 for the 1st and 2nd time domains after 1 month of curing to 9.6 and 12.6 for the 1st and 2nd time domains after 3 months of curing). The enhanced immobilization of Cs+ is correlated with the formation of the nitrate-bearing feldspathoid phases.

Original languageEnglish
Pages (from-to)247-254
Number of pages8
JournalJournal of Nuclear Materials
Volume514
DOIs
StatePublished - 1 Feb 2019
Externally publishedYes

Keywords

  • Cesium immobilization
  • Geopolymers
  • Nitrate
  • Nitrate-cancrinite
  • Nitrate-sodalite
  • Zeolite A

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