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

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

“…As demonstrated above, the direct hydrothermal co-processing of Sr(NO 3 ) 2 solutions and cenospheres in an alkaline medium resulted in the crystallization of Sr-bearing silicate phases, such as analcime, (Sr,Ca)-tobermorite and (Sr,Ca)-plagioclase, among which the analcime phase incorporates a negligible Sr amount in all range of Sr 2+ content in the systems. At the same time, pollucite–analcime solid solutions (Na n Cs 1−n )AlSi 2 O 6 ·nH 2 O were easily crystallized as the main host phases for cesium in the similar co-processing route [ 38 , 39 ]. Moreover, Komarneni et al [ 47 ] reported about the formation of analcime-based Sr-wairakite, SrAl 2 Si 4 O 12 ·2H 2 O, when Sr(OH) 2 interacts with clinoptilolite and clay under more prolonged thermobaric treatment at 200 °C and 30 MPa.…”

“…The cenosphere-based hydrothermal synthesis seems to be the promising co-processing route to 137 Cs and 90 Sr immobilization in mineral-like forms because it can be implemented via the one-step mild thermobaric treatment of the Cs + /Sr 2+ -bearing alkaline solution-cenospheres mixture. As reported [ 38 , 39 ], in the Cs + -containing system, the pollucite–analcime solid solutions have been easily synthesized in a wide range of cesium concentrations at 150 °C using narrow cenosphere fractions with a high content of a glass phase (≥90 wt.%). Under applied hydrothermal conditions, the efficiency of trapping Cs + from simulant solutions was up to 96%.…”

“…ratios in the range of 1.2–3.5 was a key factor to use them as precursors for the synthesis of 137 Cs/ 90 Sr mineral-like host compounds (pollucite, Sr-feldspar). To achieve this, different approaches were used, such as (i) impregnation of loose cenospheres or cenosphere-based open-cell porous materials with Cs + /Sr 2+ salts followed by calcination of blends at 800–1100 °C [ 35 , 36 ], (ii) hydrothermal conversion of cenospheres into NaP zeolite followed by Cs + /Sr 2+ sorption and heating of the Cs + /Sr 2+ loaded zeolites up to 1100 °C [ 37 ] and (iii) direct synthesis of mineral-like aluminosilicate phases under hydrothermal conditions using cenospheres as a glassy Si and Al source [ 38 , 39 ].…”