Calculation of the Henry’s Constant and the Thickness of the Equilibrium Adsorption Layer of Radon in the Layer-by-Layer Measurement of the Sorbent Activity

Abstract

The radioactive gas radon is ubiquitous in the environment and is a major contributor to the human inhalation dose. It is the second leading cause of lung cancer after smoking. Radon concentrations are particularly high in the air of radon-hazardous facilities—uranium mines and radioactive waste repositories containing radium. To reduce the dose load on the staff, air in these premises should be continuously or periodically purified of radon. Carbon adsorbers can be successfully used for this purpose. The design of sorption systems requires information on both equilibrium and kinetic parameters of radon dynamic adsorption. The traditional way of obtaining such characteristics of the sorbent is to analyze the breakthrough curves. The present paper proposes a simple alternative method for determining parameters of dynamic radon adsorption (Henry’s constant and equilibrium adsorption layer thickness) from the results of a layer-by-layer gamma-spectrometric measurement of the sorbent. The analytical equation for smooth distribution of radon activity in the sorbent layer is obtained based on equilibrium adsorption layer theory for elute chromatography (pulsed injection of radon into the column). Using the dynamic adsorption of 222Rn on AG-3 activated carbon as an example, both equilibrium (Henry’s constant) and kinetic (thickness of the equilibrium adsorption layer) parameters of the adsorption dynamics were calculated. It was shown that the exposure duration of the column bed in the air flow and superficial gas velocity do not affect the result of the Henry’s constant calculation. The dependence of the equilibrium adsorption layer thickness on the superficial gas velocity over a wide range of values (5–220 cm/min) is described by the van Deemter equation. It was shown that the optimum air flow velocity, which corresponds to the maximum effectiveness of the bed, is 15–30 cm/min. This corresponds to the minimum of the equilibrium adsorption layer thickness (about 0.6 cm). The developed mathematical model makes it easy to define both equilibrium and kinetic parameters of dynamic adsorption of radon based on discrete distribution of its activity over the sections of the adsorption column. These parameters can then be used to calculate and design gas delay systems. It can be useful for studying the sorption capacity of various materials relative to radon.