Formation of faujasite- and sodalite/cancrinite-type phases associated with caustic waste
reactions in the environment may structurally incorporate contaminant species such as
radioactive Sr2+ and Cs+, and thus provide a mechanism of attenuation. In order to investigate
mineral evolution and structural incorporation of cations in simplified experiments,
aluminosilicate solids were precipitated homogeneously at room temperature from batch
solutions containing a 1:1 molal ratio of Si to Al and 10-3 molal Sr and/or Cs, and aged for 30 or
548 d. Syntheses were done with solutions in equilibrium with atmospheric CO2 and with gaspurged
solutions. Experimental products were characterized by bulk chemical analyses,
chemical extractions, XRD, SEM/TEM, TGA, solid-state 27Al NMR, and Sr EXAFS. Chemical
analysis showed that solids had a 1:1 Al:Si molar ratio, and that Sr was sequestered at higher
amounts than Cs. After 30 d of aging in purged solutions, XRD showed that zeolite X (faujasitetype)
was the only crystalline product. After aging 30 and 548 d in solutions equilibrated with
atmospheric CO2, a mixture of sodalite, cancrinite, and minor zeolite X were produced. Surface
areas of solids at 30 d were much lower than published values for zeolite phases synthesized at
high temperature, although particle aging produced more crystalline and less aggregated phases
with higher bulk surface areas. Characterization of products by 27Al NMR indicated only
tetrahedrally coordinated Al. Measured isotropic shifts of primary resonances did not change
substantially with precipitate aging although the primary mineral phase changed from zeolite X
to sodalite/cancrinite, indicating local ordering of Al-Si tetrahedra. Analysis of reaction products
by Sr EXAFS suggested Sr bonding in hexagonal prisms and six-membered rings of the
supercages of zeolite X that may be more site-specific than those of monovalent cations. For
samples aged for 548 d, interatomic distances from Sr-EXAFS are consistent with partial Sr dehydration and bonding to framework oxygen atoms in sodalite cages or in large channels in
cancrinite. Incorporation of Sr into both faujasite and sodalite/cancrinite phases is favored over
Cs during room temperature synthesis, possibly because of increased cation site competition
between Cs+ and Na+. Results of this study help to constrain cation incorporation into
sodalite/cancrinite mineral assemblages that form at caustic waste-impacted field sites and may
aid in the predictive modeling of contaminant release.