Final storage of radioactive cesium by pollucite hydrothermal synthesis

Abstract

The Fukushima nuclear accident has highlighted the importance of finding a better final storage method for radioactive cesium species. Cs is highly soluble in water, and can easily exchange with other alkali ions in zeolites or clays to form stable complexes. However, Cs+ is released from Cs+ complexes into water when surrounded by an excess of water. Pollucite may be the best final storage option for Cs+, but its typical synthesis requires heating to about 1200°C in air. Here, we show that the hydrothermal synthesis of pollucite can be completed at 300°C in three hours from any zeolite or clay. Furthermore, our procedure does not require ion exchange before synthesis. Radioactive Cs is usually found in complexes with clays. At that time, this method only requires calcium hydroxide, water, and three hours of hydrothermal synthesis, so the process is both inexpensive and practical for large-scale application. Pollucite is an analog of analcime zeolite, and contains a channel system 2.8 Å in diameter, which is formed by 6-oxygen rings. As the diameter of Cs+ is 3.34 Å and each Cs+ exists independently within a separate portion of the channel, Cs+ cannot exit the pollucite framework without breaking it.

Versatile chemical handling to confine radioactive cesium as stable inorganic crystal

“…In 2014 Yokomori et al . reported their pioneering research on hydrothermal storage of Cs as pollucite 15 , and other research group also studied the feasibility 16 . Whilst examining their work, it has been that clay, which is used as a source of aluminum and silicon, is much more easily dissolved in ethylene glycol than water, under similar temperature and pressure conditions.…”

“…During the present research, the following points have been found: cesium can be confined as a stable inorganic crystal, pollucite (CsAlSi 2 O 6 ), by the solvothermal treatment using MMLT and EG; (i) when heated to 350 °C, the maximum pressure reaches 14.8 MPa, and pollucite is easily formed within approximately 10 h, (ii) when heating is stopped at 320 °C, the pressure reaches 6.0 MPa and 8.7 MPa after 9.6 h and 20.6 h, respectively, and the latter condition is preferable, (iii) when heating is stopped at 300 °C, the maximum pressure remains as low as around 2 MPa, and synthesis is not very successful. In the pioneering work 15 by Yokomori et al ., the method is a hydrothermal procedure, and uses water, instead of EG. As different sources (such as bentonite) for Al and Si are used, they also needed other additives such as AlCl 3 .…”

Incorporation Behavior of Cesium into Pollucite and the Optimization of Synthesis Method

“…At present, conversion of various material such as natural zeolites, coal fly ash, smectite clay and chemical regents containing Si, Al and Na to POL or ANA by alkaline hydrothermal treatment method has been reported by many reseachers 3,[6][7][8][9] . However, the optimization of synthesis condition to improve the Cs + removal and the solution stability of obtained product have not been reported.…”

Mild hydrothermal synthesis of pollucite from soil for immobilization of Cs in situ and its characterization

“…For example, pollucite was hydrothermally synthesized in 240-300 °C for days or hours from clinoptilolite [23]. Therefore, the hydrothermal technology might also have a capability of converting Cs-polluted soil into pollucite which can be used to immobilize Cs into the structure of pollucite in-situ and thus save a huge of amount of solidification basis materials (cement, or glass etc.)…”