Cesium immobilization in nitrate-bearing metakaolin-based geopolymers

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 t^1/2, suggesting a change in leaching mechanism after the initial 24 hours of the leaching experiment. The apparent diffusion constants, D_a, as well as leaching indices for Cs⁺ and Na⁺ were therefore calculated separately for the 1^st and 2^nd time domains of the experiment (t^1/2 < 1 days^1/2 and t^1/2 > 2.65 days^1/2, respectively). The leachability indices for Cs⁺ were found to increase with increasing curing time (from 7.4 and 11.2 for the 1^st and 2^nd time domains after 1 month of curing to 9.6 and 12.6 for the 1^st and 2^nd time domains after 3 months of curing). The enhanced immobilization of Cs⁺ is correlated with the formation of the nitrate-bearing feldspathoid phases.