Thermal desorption is a process of using heat to remove a material, usually organic, from a surface. In very simplistic terms, it could be thought of as boiling off a contaminant from a surface, although there is generally no liquid phase involved. For a more detailed explanation it’s necessary to start with an explanation of the opposite process, adsorption.
Adsorption is the term used to describe the adhesion of atoms or molecules to a surface. (The precise term for the surface is the “sorbent”.) It’s very important to stress that adsorption is not the same as absorption although the two terms are sometimes confused. Absorption involves a degree of diffusion between the surface and the contaminant. That is to say, in absorption the liquid or gas actually penetrates the surface whereas in adsorption the contaminant lies on the surface. Adsorption does however involve a degree of bonding between the surface and the contaminant, but this is due solely to the very weak electrical forces that exist between atoms rather than any actual bonding.
These bonds are broken quite easily, simply by increasing the kinetic energy of the contaminant, and that is achieved through heating. This is the process of thermal desorption.
Thermal desorption is a very effective way of removing contaminants from a surface, or sorbent, without the need for a solvent, and this makes it valuable as part of a process known as gas chromatography. Gas chromatography is a technique used by research scientists to investigate chemical compounds. It involves passing an inert gas over a hot surface to carry away the molecules released by heating. This provides the scientist with a clean, undiluted sample.
Thermal desorption is also used on a larger scale in several industrial processes such as the capture of carbon dioxide. While in these cases the sorbent may be liquid rather than solid, the principles are the same: heat is used to break weak bonds between the sorbent and a contaminant on its surface.
Another use of thermal desorption is in environmental cleanup operations, and in particular, to remove organic contaminants such as pesticides from soil. (Thermal desorption does not work on metallic contaminants.) Here the contaminated soil is first filtered to remove large clumps and then dried before being loaded into a large drum or “desorber.” This is then heated to release the contaminants, which are captured for further treatment and disposal. This kind of soil treatment is different to incineration because no burning or combustion takes place.
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