Noble Gas Adsorption onto Zeolitic Materials in Atmospheric Conditions

Detection of radioisotopes of noble gases produced by nuclear detonations is one of the methodologies employed by the Comprehensive Nuclear-Test-Ban Treaty. Whereas noble gases are chemically inert, adsorption of pure noble gases has been reported to exhibit non-conservative behavior in naturally occurring nanoporous minerals, albeit under idealized single-component laboratory conditions. Extrapolation of single-component gas adsorption measurements to the multi-component ambient environment that is predominately nitrogen and oxygen, but importantly water, requires numerous assumptions and introduces uncertainty.

This work aims to experimentally examine multicomponent adsorption of Ar, Kr, and Xe on zeolitic materials using an adaptation of the volumetric method. In the most generic sense, the volumetric method measures porosity and gas adsorption by expanding a reference volume of gas to a sample material and the change on the resulting pressure of the system. To apply this method to a multicomponent system where different species are different in the amount of adsorption and speed, it is necessary to monitor the composition of the gas phase in addition to total pressure. In this work, gas composition is monitored using a quadrupole mass spectrometer continuously, enabling Ar, Kr, and Xe to be measured concurrently.

Tests were first conducted on natural clinoptilolite samples which were vacuum-dried and then were exposed to dry air. Relatively little Ar is adsorbed under all conditions tested. However, the heavier noble gases Kr and Xe continue to exhibit significant adsorption effects in vacuum-dried clinoptilolite, despite the overwhelming abundance of nitrogen and oxygen. When the samples were additionally exposed to wet air with different humidity levels, the quantity of Kr and Xe adsorbed was significantly reduced. However, while the quantity of Xe adsorbing was most significantly reduced between 0% to 8 % relative humidity, non-negligible Kr adsorption persisted up to at least 55% relative humidity, the highest humidity level tested. As Kr continued to adsorb, albeit to a lesser degree, but Xe did not, this indicates the reduction in noble gas adsorption is not simply a function of surface coverage. To further explore this phenomenon, additional zeolitic materials, both pure mineral phases and heterogenous rock samples, will be examined.

As this work shows that water can not only to decrease the total adsorption significantly but also can potentially differentiate gas compositions. Consequently, the scenarios where radioactive noble gases can be modeled as being conservative tracer gases may vary with both environmental conditions as well as subsurface geology. In systems where there is appreciable noble gas adsorption occurring, the timing and magnitude of radioactive noble gas signatures may be altered as observed by the International Monitoring System or during a potential On-Site Inspection.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.