Retention mechanism of cesium in chabazite embedded into metakaolin-based alkali activated materials

• Published in: Journal of hazardous materials 2022-10, Vol.440, p.129732, Article 129732
• Main Authors: Chaerun, Raudhatul Islam, Soonthornwiphat, Natatsawas, Toda, Kanako, Kuroda, Kazuma, Niu, Xiaobo, Kikuchi, Ryosuke, Otake, Tsubasa, Elakneswaran, Yogarajah, Provis, John L., Sato, Tsutomu
• Format: Article
• Language: English
• Subjects: adsorbents; aluminum silicates; cesium radioisotopes; Chabazite; encapsulation; FE-EPMA, Raman spectroscopy; Japan; K-based AAM; nuclear power; phase transition; Pollucite; potassium; power plants; Raman spectroscopy; TEM; transmission electron microscopy; X-ray diffraction; zeolites
• ISSN: 0304-3894; 1873-3336
• DOI: 10.1016/j.jhazmat.2022.129732

Disposal of cesium-137 (Cs-137)-loaded chabazite generated from decontaminating cooling water of the damaged reactor at the Fukushima Daiichi Nuclear Power Station (FDNPS) has become a crucial concern. The potassium aluminosilicate-based alkali activated material (K-AAM) matrix is one of the candidate encapsulation matrices proposed for encapsulating cesium-137. In this study, chabazite loaded with a low Cs concentration (1 mg/g of Cs), embedded into a K-AAM matrix (K-AAM-C), was analysed to determine its capability to immobilise Cs, which was investigated by batch leaching experiments, field emission–electron probe microscopy analysis (FE-EPMA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. The leaching experiments revealed that K-AAM-C efficiently immobilised Cs, with only 3 % of the Cs leached out after 360 days of leaching in deionised water. Characterisation using XRD, TEM, and Raman analysis confirmed that the alkali-activator was responsible for the phase transformation of chabazite. FE-EPMA demonstrated that K entered the chabazite structure. This phenomenon resulted in the breakdown and subsequent reconstruction of the chabazite structure. TEM observation showed that the Cs was concentrated into the aggregates of precipitates, heterogeneously forming a pollucite-like structure in the chabazite after the fabrication process. Thermodynamic calculations indicated that pollucite was preferably stable in an AAM environment. When immersed in water, the amount of nano-pollucite increased over time, leading to the structural re-arrangement of aluminosilicate rings of chabazite according to TEM and Raman analysis. Pollucite is well known as a Cs-bearing natural zeolite, which can encapsulate Cs in its structure. Therefore, Cs retention was achieved in the spent chabazite adsorbent embedded into the K-AAM due to the resultant pollucite structure formed during AAM fabrication. [Display omitted] •K was responsible for the breakdown and reconstruction of the chabazite structure.•Heterogeneous nano-pollucite was formed within the chabazite at 25 °C.•The structure of chabazite was labile in the K-AAM environment.•The chabazite ring was able to rebuild to form more pollucite.