A novel hydrothermal method to convert incineration ash into pollucite for the immobilization of a simulant radioactive cesium

Abstract

The Fukushima nuclear accident in Japan on March 11, 2011 produced huge amounts of Cs-polluted incineration ashes; conventional solidification methods seem unsuitable for the treatment of large amounts of Cs-polluted ashes. A novel hydrothermal method was developed to directly convert Cs-polluted incineration ash (rice husk ash) into pollucite to immobilize Cs in its crystal structure in situ. Results revealed that pollucite could be synthesized readily over a wide range of added Cs (Cs/Si=0.2–0.6); the addition of more Cs (Cs/Si≥0.5) caused the formation of a small amount of cesium aluminosilicate (CsAlSiO4), which exhibits poor immobilization behavior for Cs. Pollucite could be formed even for a short curing time (1h) or at a low curing temperature (150°C). However, a high curing temperature or a long curing time favored the formation of a pure pollucite. With the added calcium hydroxide, a tough specimen with a flexural strength of approximately 22MPa could be obtained, which suggested that this technology may be applied directly to the solidification of Cs-polluted incineration ashes. Hydrogarnet and tobermorite formations enhanced the strength of the solidified specimens, and meanwhile the formed pollucite was present in a matrix steadily. Leaching test demonstrated that the amount of Cs that leached from the synthesized specimens was very low (0.49×10−5–2.31×10−5) and even lower than that from the reference hollandite-rich synroc (2.0×10−2), although a higher content of Cs was found in the synthesized pollucite specimens (6.0–31.7%) than in the reference (3.7%). Therefore, the hydrothermal conversion of Cs-polluted incineration ash into pollucite can be applied to immobilize Cs directly.