Hydrothermal conversion of analcime-pollucite solid solution from soil for immobilization of Cs in situand its characterization

Abstract

Pollucite, considered as an ideal host material for immobilization of Cs, rarely exists in pure form but analcime-pollucite solid solution in nature due to the coexistence of Cs and Na in fact. Thus, a novel hydrothermal method has been proposed to directly synthesis analcime-pollucite solid solution instead of pure pollucite so as to in-site immobilize Cs in its structure. The result showed that Cs/Na ratio of starting material, hydrothermal time and hydrothermal temperature were the main factors in the hydrothermal synthesis of the solid solution. Different Cs/Na ratio could result in different solid solution formations, and however, even with little polluted Cs in soil (small Cs/Na), the composition of the synthesized solid solution seemed to be closer to that of pollucite than analcime. This suggests that the hydrothermal synthesis of the analcime-pollucite solid solution can be used to immobilize the polluted Cs in soil. The solid solution could be formed readily even at a low curing temperature (150 °C) or for a short curing time (2 h), while a higher temperature and longer time could promote the solid solution formation. The extended X-ray absorption spectrum fine structure (EXAFS) and near-infrared spectrum (NIR) analyses proved that Cs⁺ entered into the structure of the formed solid solutions via replacement of water molecules, and it also led to a distortion of the framework of the solid solution with the change of the Cs–O bond length. This revealed that the Cs was indeed immobilized into the structure like that of pure pollucite. Because of the steady immobilization of Cs in structure, the concentration of Cs leached from the solid solutions was very low (4.3 נ10⁻⁶ g/(m²·d)), even lower than that of the pure pollucite (1.88 נ10⁻⁵ g/(m²·d)) and pollucite-based ceramic (2.67 נ10⁻³ g/(m²·d)). Therefore, the hydrothermal synthesis of the analcime-pollucite solid solution from soil (metakaolin) is available for effective treatment of huge amounts of Cs-polluted soil.