Temperature and reaction time dependence of (Na,Ca,Cs)-zeolite formation.

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Long-term stability of engineered barrier system (EBS) materials in repository conditions is a primary concern for radioactive waste repository success. EBS designs generally include bentonite as barrier between the canister and host rock to provide: 1) a physical barrier to prevent natural fluid from interacting with the waste package and 2) a chemical barrier that attenuates radionuclide migration. Bentonite interaction with host rock and groundwater in water-saturated geologic repositories may result in the formation of secondary minerals that affect the sealing properties of the bentonite. Specifically, the formation of zeolites may negatively affect bentonite swelling properties but may also help attenuate radionuclide migration through sorption and incorporation into zeolite structures. Here we discuss the experimental formation of analcime-group zeolite minerals with Na-, Ca-, and Cs-endmembers (analcime, wairakite, and pollucite, respectively) through hydrothermal bentonite alteration in geologic repository conditions. Experimental work was conducted at elevated pressure and temperatures conditions relevant to underground repositories. High-pressure Cs-bentonite experiments were completed from 150-400 °C and 500-1000 bar for 14 to 62 days. Gold capsules were loaded with a 2:1 water:rock ratio of unprocessed bentonite from Colony, Wyoming and an aqueous fluid containing 2 molal CaCl + CsCl + NaCl with an initial pH of ~5.8. At 200 °C Cs was found to be entrained in different phases including the zeolite clinoptilolite. Clinoptilolite is an accessory mineral in the initial bentonite mineral assemblage and the results suggest incorporation into existing clinoptilolite rather than any Cs-clinoptilolite precipitation. At reaction temperatures ≥ 300 °C and < 450 °C we observed newly precipitated crystals of pollucite in the bentonite matrix. After reaction at 450 °C only trace amounts of Cs were identified in mineral phases: instead, we observed the formation of analcime-wairakite minerals with little or no pollucite formation. Comparing these results to analcime-wairakite-pollucite stability at a range of pressure-temperature conditions sheds light on conditions that promote Cs immobilization through incorporation in pollucite crystal structures. These results expand the range of experimentally observed zeolite formation and define pressure and temperature fields that promote Cs entrapment during hydrothermal bentonite alteration. The potential benefits of radionuclide immobilization through zeolite formation are contextualized by thermodynamic and kinetic geochemical modelling of Na-, Ca-, Cs-zeolite formation and the anticipated effects to bentonite and EBS properties over time.