Cesium and strontium incorporation into zeolite-type phases during homogeneous nucleation from caustic solutions

Abstract

Formation of faujasite-and sodalite/cancrinite-type phases associated with caustic waste reactions in the environment may structurally incorporate contaminant species such as radioactive Sr 2+ and Cs + , and thus provide a mechanism of attenuation. 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 days. Syntheses were done with solutions in equilibrium with atmospheric CO 2 and with gas-purged solutions. Experimental products were characterized by bulk chemical analyses, chemical extractions, XRD, SEM/TEM, TGA, solid-state 27 Al 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 days of aging in purged solutions, XRD showed that zeolite X (faujasite-type) was the only crystalline product. After aging 30 and 548 days in solutions equilibrated with atmospheric CO 2 , a mixture of sodalite, cancrinite, and minor zeolite X were produced. Surface areas of solids at 30 days 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 27 Al 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 days, 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.

Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite

“…Additionally, at the Hanford site intentional and unintentional discharges of alkaline tank solutions to the subsurface may have resulted in the formation of pertechnetate-bearing feldspathoid phases. Previous investigations demonstrated that when simulated tank waste reacts with Hanford sediments, a range of feldpathoid phases form including sodalite. , This study provides the data needed to identify the chemical conditions required to maximize TcO 4 – incorporation into sodalite as well as the ability to predict the long-term stability of this phase in environmental systems.…”

“…For example, aluminosilicate sodalites, both natural and synthetic, can vary widely in composition but have the general formula of M 8 (Al 6 Si 6 O 24 )­X 2 , where M is a monovalent cation (such as Cs + , K + , Na + , etc.) and X can vary between monovalent or divalent anions (such as OH – , Cl – , Br – , I – , MnO 4 2– , ReO 4 – , or theoretically TcO 4 – ). − …”

Quantifying Particulate and Colloidal Release of Radionuclides in Waste‐Weathered Hanford Sediments

“…Strontium extended X‐ray absorption fine structure and transmission electron microscopy (TEM)–energy‐dispersive X‐ray spectrometry (EDS) analysis showed that neophases contained Sr and Cs in the HIGH treatments. Strontium was also clearly detected in neo‐phases formed in the LOW treatment, whereas Cs was below EDS detection (Perdrial et al, 2011; Rivera et al, 2011).…”

Release of aged contaminants from weathered sediments: Effects of sorbate speciation on scaling of reactive transport

“…Cesium was quite readily desorbed (up to 40% of total Cs in solids), while only 6% of the Sr was released to solution. A reactive transport model (Crunchflow) was developed to describe Sr and Cs release from the previously characterized solid phases (Rivera et al, 2011). Parameter estimation was performed (using the program PEST) to derive a set of selectivity coefficients describing exchange sites on both zeolite and feldspathoid phases, in addition to dissolution of solid HN phases.…”

“…(yellow atoms) and typical distances betwwen cations and framework Al (light blue) or Si (dark blue)atoms for a) Sr-exchanged zeolite-X; b) Nasodalite; c) Na-cancrinite. (Rivera et al, 2011) Using a suite of methods (silver thiourea with Cs and Rb competition and flow-through technique with Rb), we measured the density of high affinity sites (HAS) in pristine and hyperalkaline-weathered Hanford sediments exposed to high or low concentrations of Cs, Sr and I for 6 months or 12 months under either CO 2 -free or atmospheric CO 2 conditions. New mineral formation affected HAS density, with hyperalkaline-weathering of these sediments increasing the density of high affinity Cs and Rb adsorption sites relative to pristine Hanford sediments.…”