Potassium Permanganate In Air Filtration Systems
December 21st, 2023 | Posted in Air Cleaners
In an earlier blog we discussed the properties of granular activated carbon (GAC) as a widely used sorbent medium for removing gas phase contaminants from air to be recirculated in the workplace air. GAC works by physical adsorption and is generally effective in removing many gas phase contaminants. There are, however, a number of gas phase contaminants that are not effectively removed by GAC. In these instances, a sorbent bed that acts via chemical adsorption (or chemisorption) is employed.
Unlike physical adsorption, which is reversible allowing regeneration of the sorbate, chemisorption works by chemical reactions on the surface of the adsorbent. Chemisorption is actually a two-step process. In the first step the adsorbates are physically adsorbed onto the adsorbent very much like the process described earlier for GAC. Once adsorbed, the gas phase contaminants react chemically with the adsorbent and are retained in the sorbent bed.
One of the more broad-spectrum chemical agents in common use is potassium permanganate (KMnO4). Potassium permanganate is an odorless, purple black sand-like material. In the air cleaning industry, potassium permanganate’s primary use is as a powerful oxidizing agent for the purpose of deodorizing by chemisorption. KMnO4 also exhibits characteristics that make it useful as a disinfectant as well as for deodorizing.
KMnO4 is often used as an impregnant on activated alumina. Potassium permanganate-impregnated alumina (PIA) can be employed in conjunction with GAC to provide a very broad-spectrum gas-phase air filtration system[i].
An example of the benefits of using a dual bed (GAC and PIA) gas phase filtration system has been described by Muller and England (1995). In their research they compared the efficacy of GAC and PIA for removing the following gasses: chlorine, formaldehyde, hydrogen sulfide, nitric oxide, nitrogen dioxide, sulfur dioxide and toluene. They found that, for the gases tested:
- GAC performed better against chlorine, nitrogen dioxide, and toluene.
- The PIA was more effective against formaldehyde, hydrogen sulfide, nitric oxide, and sulfur dioxide.
These results were obtained whether the GAC and PIA filtration systems were set up as two separate sorbent beds (GAC followed by PIA) or in a mixed bed containing both GAC and PIA blended together. Muller and England conclude that:
“Based on the results of testing performed, it has been shown that a single dry-scrubbing air filtration media may not be sufficient to adequately control multiple contaminants. The types and numbers of gaseous contaminants one would expect to encounter in outdoor and indoor air make it likely that gas-phase air filtration systems, used as part of an overall contaminant control and/or energy conservation program, need to be equipped with at least two dry scrubbing media – granular activated carbon and potassium permanganate-impregnated alumina. The preferred system would contain these media in two discreet filter beds in the order GAC-PIA”.
They state that this dual-bed arrangement may be analogous to roughing and polishing filters for particulate contaminants. Of course, there may not always be room in the HVAC system or the stand-alone air cleaner for dual sorbent beds. In these cases, a mixed sorbent bed containing GAC and PIA would be an appropriate application.
It should be noted that there have been some concerns regarding the use of GAC due to its potential for flammability in certain applications. Recognizing this potential, some jurisdictions (e.g. Suffolk County NY) disallow activated carbon for some applications (restaurant kitchen exhaust being an example). In these cases, PIA is the preferred sorbent although it is not as effective for many VOCs. PIA is also somewhat more costly; about 30% more than GAC.
For optimum control of gas phase contaminants two dry scrubbing media beds containing granular activated carbon and potassium permanganate-impregnated alumina are recommended. Mixed media beds can be used where space constraints are severe.
Talk with your Air Quality Engineering Applications Specialist of guidance on the right sorbent bed solutions for your application.
[i] Muller, Christopher O., and William G. England. “Achieving your indoor air quality goals: Which filtration system works best?.” ASHRAE Journal 37.2 (1995): 8.