New Crystalline Silicotitanate (CST) Waste Forms: Hydrothermal Synthesis and Characterization of CS-SI-TI-O Phases
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INTRODUCTION Crystalline silicotitanates (CSTs), selectively remove 100 ppm Cs and Sr cations from SM Nat solutions over a broad pH range (I - 14); and are therefore effective for 1 cleanup of radioactive 137 Cs and 9(Sr from the Hanford waste tanks.' 13] The method under investigation for immobilization of the Cs-loaded CSTs is combination with High Activity Waste (HAW) and melting with borosilicate glass to create a glass log waste form. However, titania induces crystallization and phase separation in borosilicate glass; therefore, the Cs-CST must be diluted to a few weight percent which increases the volume and cost of waste form production. Preliminary work at PNNL showed that a waste form which is more chemically durable than the borosilicate glass logs can be generated by direct thermal conversion of Cs-CST.[ 4 ] In order to determine the viability of this potential waste form and investigate other alternative waste forms, identification and characterization of phases formed from the Cs-CST components must be addressed. The CST components may include Cs 2 O-Na2 O-X-TiO2 -SiO2 (X = BaO, SrO or a proprietary component). We are currently investigating formation and stability of Na 2 O-Cs 2 O-Si0 2 -TiO 2 -H2 O phases via I) hydrothermal synthesis at Sandia National Lab; 2) solid-state, melt or flux synthesis at Pacific Northwest National Lab[5 - 7 ] and 3) drop solution calorimetry of these phases at University of California, Davis. In this work, investigation of the CS2 O-Si 2 2-H2 0 phase space under hydrothermal synthesis conditions is presented. Two new phases are identified; one Preliminary structural and "zeotype" phase and one anhydrous, condensed phase. compositional analyses are presented on these phases. Additionally, preliminary results on properties pertinent to waste forms such as phase relations, leachability, chemical alteration and ion exchange of the new phases are discussed. Preliminary phase searches using the proprietary component show the same new phases are formed in its presence. However, due to a proprietary agreement between UOP (the commercial producer of CST) and Sandia National Labs, we cannot report on the specifics of these results at this time.
71 Mat. Res. Soc. Symp. Proc. Vol. 556
© 1999
Materials Research Society
EXPERIFM ENTAL The following general procedure was executed for hydrothennal synthesis of CsOSiO,-TiO,-HO phases. Solutions of tetraisopropyltitanium (TIPT), tetraethylorthosilicatc (TEOS), cesiumn hydroxide solution (50 wt %) and water were prepared and cooked in 23 ml Teflon-lined Parr reaction vessels. Products were collected by filtration and characterized by various techniques including powder x-ray diffraction (XRD), thennogravimctry and differential themial analysis (DTA-TGA), scanning and transmission electron microscopics (SEM, TEM), solid-state ?"Si and '33Cs magic angle spinning nuclear magnetic resonance spectroscopies (MAS NMR), 'H-2"Si cross polarization NMR (CPMAS NMR), and energy dispersive, atomic adsorption and atomic emission spectroscopics (EDS,
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