Synthesis of Composite Zirconosilicate Sorbents Based on Fly Ash Cenospheres and Their Sorption Properties for Cs+, Pb2+ and Cd2+ Cations for Immobilisation in Ceramic Form

Hollow aluminosilicate microspheres (cenospheres) of stabilised composition (glass phase - 95.4 wt%; (SiO2/Al2O3)glass = 3.1) isolated from coal fly ashes from coal combustion were used to prepare composite sorbents containing a sorption-active component based on zirconosilicates of framework structure. The synthesis products were characterised by X-ray diffraction analysis, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and their sorption properties for Cs+, Pb2+ and Cd2+ cations were studied. The zirconosilicate material shows a high distribution coefficient in the process of sorption of Cs+, Pb2+ and Cd2+ from aqueous solutions (103-105 mL/g). For Pb2+ as an example, it is shown that as a result of high-temperature phase transformation of the Pb2+-form of zirconosilicate, Pb2+ cations are stabilised in the crystalline phase of lead zirconate PbZrO3. The possibility to use spark plasma sintering for obtaining mineral-like ceramics based on zirconosilicate sorbent for immobilisation of radionuclide Cs-137 has been investigated. For ceramics sintered at different temperatures (800-1000 °C) the cesium leaching rate was studied, which is a criterion of hydrolytic stability of radioactive ceramics intended for disposal in rocks. The obtained values of leaching rate (~10-5g/(cm2•day)) meet the requirements of Federal Norms and Rules in the area of atomic energy use NP-019-2000 “Collection, processing, storage and conditioning of liquid radioactive wastes. Safety requirements”.