Heat of immersion of clinoptilolite

The enthalpy of hydration of natural clinoptilolite was determined by isothermal immersion calorimetry on Ca, Na-, and K-exchanged clinoptilolite (Fish Creek Mountains, Nevada). Heats of immersion of clinoptilolite were determined at initial water contents ranging from n=0.02 to 0.85 (where n is the ratio [H₂O content]/[maximum H₂O content]). The heat of immersion of Ca-clinoptilolite ranged from -7.5 (n=0.86) to -25.7 kJ/mol-H₂O (n=0.19); values for Na-clinoptilolite ranged from -6.3 (n=0.85) to -21.8 kJ/mol-H2O (n=0.11); and values for K-clinoptilolite ranged from -7.7 (n=0.80) to -24.6 kJ/mol-H₂O (n=0.02). Linear regression of the calorimetric data provided the following values for the complete heat of immersion (from n=0): Ca-clinoptilolite, -30.3 ± 2.0; Na-clinoptilolite, -23.4. ± 0.6; and K-clinoptilolite, -22.4 ± 0.8 kJ/mol-H₂O. The heat of immersion measurements were compared to the enthalpy of hydration determinations of Carey and Bish (1996) in a thermogravimetric study on the same samples. The heat of immersion data are similar but of smaller magnitude than the values of enthalpy of hydration and are believed to be more accurate because of the lower uncertainty in the measurements.

The effect of dehydration of clinoptilolite on the thermal evolution of the potential high-level nuclear waste repository at Yucca Mountain was considered by comparing the amount of energy consumed by dehydration with the energy necessary to heat rocks lacking hydrous minerals. The extra energy consumed on heating clinoptilolite from 25 to 200 ^oC ranges between 70 and 80% in excess of that required for non-dehydrating materials. These results indicate that accurate thermo-hydrologic modeling of rock units at Yucca Mountain should consider the thermal effect of dehydration/hydration processes in clinoptilolite and other hydrous minerals, in addition to the water produced/adsorbed during heating/cooling.

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