Hydrothermal Co-Processing of Coal Fly Ash Cenospheres and Soluble Sr(II) as Environmentally Sustainable Approach to Sr-90 Immobilization in a Mineral-like Form

Co-processing of radioactive effluents with coal fly ash-derived materials is recognized as a resource-saving approach for efficient stabilization/solidification of radioactive components of wastewater. In this context, the paper is focused on the hydrothermal synthesis of Sr2+-bearing aluminosilicate/silicate phases as analogs of a mineral-like 90Sr waste form using hollow glass-crystalline aluminosilicate microspheres from coal fly ash (cenospheres) as a glassy source of Si and Al (SiO2-Al2O3)glass) and Sr(NO3)2 solutions as 90Sr simulant wastewater. The direct conversion of cenosphere glass in the Sr(NO3)2-NaOH-H2O-(SiO2-Al2O3)glass system as well as Sr2+ sorption on cenosphere-derived analcime (ANA) in the Sr(NO3)2-H2O-ANA system were studied at 150–200 °C and autogenous pressure. The solid and liquid reaction products were characterized by SEM-EDS, PXRD, AAS and STA. In the Sr(NO3)2-NaOH-H2O-(SiO2-Al2O3)glass system, the hydrothermal processing at 150–200 °C removes 99.99% of the added Sr2+ from the solution by forming Sr-tobermorite and Sr-plagioclase phases. In the Sr(NO3)2-H2O-ANA system, Sr2+ sorption on analcime results in the formation of solid solutions (Na1−nSrn/2)AlSi2O6·H2O of the Na-analcime–Sr-wairakite series. The results can be considered as a basis for the development of environmentally sustainable technology for 90Sr removal from wastewater and immobilization in a mineral-like form by co-processing waste from coal-fired and nuclear power plants.