This question has bothered me for some time. If radon is a non-reactive noble gas, then why does it so easily adsorb onto activated carbon? What is the reaction? There must be a simple explanation, but so far it eludes me.
A possible explanation is given in the refference: https://pdfs.semanticscholar.org/7d49/1acf482d0d03652846f422ec606a66fc3731.pdf ”Radon, an inert gas, adsorbs onto surfaces by means of physical non-chemical interaction. Atkins (Atkins, P.W. Physical Chemistry. 3rd Ed. New York: W.H. Freeman; 1986; 770-779) describes the process of physical adsorption, or "physisorption", as a long-range but weak interaction of a molecule with a surface. The energy released in the adsorption of a molecule is known as the enthalpy of adsorption, and is usually comparable in magnitude to the enthalpy of condensation for that molecule, which is around 17 kJ1mol for radon. When a radon molecule strikes a surface it may either bounce off the surface, or if it strikes an "adsorption siten, it may dissipate enough energy to become physically adsorbed to the surface. As molecules adsorb onto a particular surface, adsorption sites can become filled, and so become unavailable for further adsorption. In our experiments, the number of available adsorption sites always greatly exceeds the number of radon molecules present, so we do not expect the filling of sites with radon molecules to lead to a significantly reduced adsorption probability for other radon molecules. On the other hand, more plentiful gaseous molecules such as water vapor can definitely exclude radon from adsorption sites, so we have carefully controlled the presence of water vapor in our experiments. A physically adsorbed molecule will eventually desorb from the surface. Due to thermal agitation, the adsorbed molecule "vibrates" within the adsorption site, and with every vibration there is a chance that the weakly bound molecule will leave the site. The probability that desorption will occur within a certain time period greatly increases as temperature rises. The Arrhenius rate equation, an exponential function involving activation energy and temperature, characterizes this process. The interplay of adsorption and desorption processes determines an equilibrium condition whereby the amount of material being adsorbed compensates the amount of material being desorbed and there is no net change over time. This equilibrium condition is the subject of this study.”
The half life of radon is 3.8 days to become to another substance which can chemically interaction with any materials.
Radon (and very many other non-reactive or weakly reactive substances) effectively adsorbs onto activated carbon (and other adsorbents) due to physical interactions (van der Waals forces). During physiosorption chemical bonds are not formed. Adsorbed atoms or molecules remain neutral and form, as a rule, many adsorption layers on the adsorbent surface. Therefore, physical adsorption can be compared with condensation.
In contrast to physiosorption, the process of chemisorption is a chemical reaction that usually stops after the formation of a monolayer of adsorbed atoms or molecules on the adsorbent surface. Chemisorption usually proceeds at a low rate and sufficiently high temperatures. To desorb substances adsorbed chemically is usually difficult. In connection with all this, physiosorption is more attractive for practical applications.
Physical adsorption forces have an electrical nature. They arise due to Coulomb interactions of neutral adsorbed particles (atoms, molecules) with particles that form an adsorbent. Such forces are divided into electrostatic and dispersion ones. Electrostatic forces are associated with the presence of constant electric moments, dipole (polar molecules), quadrupole, octupole. The dispersion interactions, unlike electrostatic ones, act between particles of any nature, regardless of their structure. Dispersion forces arise because of the constant motion of electrons relative to atomic nuclei. Due to this, fluctuations in the density of electron clouds take place in particles (including polar and nonpolar molecules), and virtual (instantaneous) electric moments continuously appear and disappear. When particles approach each other, the motion of their electrons is coordinated. The instantaneous electric dipole of one particle, acting by its electric field, synchronously induces an instantaneous electric dipole in the other particle and interacts with it.
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