Cenosphere-sourced hydrothermal synthesis of pollucite-analcime solid solutions as a low-temperature method to immobilize 137Cs in a mineral-like form

Two narrow fractions of coal fly ash cenospheres with ∼90 wt % glass phase and different Si/Al ratio (2.1 and 2.9 in the glass phase), shell porosity and thickness, globule size, and content of a mullite phase were used as a glassy source of Si and Al in the hydrothermal synthesis of pollucite-analcime solid solutions at 150 °C and autogenous pressure, 1.0 M NaOH, Cs/Na ratios of 0–0.25. Hollow microsphere globules of about 60–70 and 140–170 μm in sizes are produced as Cs immobilized forms in this process. The microspheres have a composite mullite/ANA wall, where ANA is analcime-pollucite solid solutions. Composition and morphology of solid products as well as composition of ANA phases were characterized by PXRD, SEM-EDS and STA methods. The influence of the mullite content and shell thickness on the preservation of the product spherical shape has been established. The degree of Cs+ recovery from alkaline solutions due to preferential involvement of cesium in the synthesis of ANA phases was up to 96%. [Display omitted] •Microspheres with a composite mullite/ANA wall are produced as a Cs mineral-like form.•Analcime-pollucite phases with non-significant difference in Si/Al ratios were obtained.•Mullite provides the preservation of a spherical shape of crystallized microspheres.•The average Cs content in solid products is 23–32 wt %.