Hydrothermal Modification of Chabazite for the Fixation of Cesium Ions

Abstract

During the accident at the Fukushima Dai-ichi Nuclear Power Plant at March 2011, radioactive cesium (¹³⁷Cs), which has a long half-life of about 30 years, was released into the environment around the plant. Therefore, methods for the recovery and fixation of ¹³⁷Cs are required. The CHA-type zeolite, chabazite, has high Cs⁺ selectivity; Cs⁺ ions have a diameter of approximately 0.36 nm and can thus enter into and be adsorbed by chabazite, which contains pores with diameters of 0.38 nm. However, some Cs⁺ ions in the large pores may be released by cation exchange in solutions with a high cation concentration. The ANA-type zeolite (ANA) has pores with 0.28 nm diameters, which are smaller than the diameter of Cs⁺ ions. ANA is synthesized by an alkaline hydrothermal treatment of natural zeolites such as clinoptilolite, mordenite, and chabazite. This hydrothermal modification may be useful for the fixation of Cs⁺. In the present study, the modification of chabazite exchanged with Cs⁺ to produce ANA by hydrothermal treatment was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), and the solution stability of Cs⁺ in the ANA was also evaluated using inductively coupled plasma-mass spectrometry (ICP-MS). Modification of chabazite exchanged with Cs⁺ to produce single-phase ANA was confirmed in a 1.0 mol/L NaOH solution held at 200 °C for 3 to 144 h. The intensity of XRD peaks corresponding to (211) and (220) planes decreased with increasing Cs⁺ concentration. SEM observations showed that the ANA crystals had a trapezohedral shape and a size of about 5 µm. EDX analysis of ANA crystals confirmed the presence of Cs⁺. The amount of Cs⁺ eluted from ANA in a 0.6 mol/L NaCl solution was less than 1.7%.