The Science of Carbon Filtering

Stephen Andrews
06 Dec 2021

Standard cannabis grow rooms are commonly packed with carbon filters among other things. Carbon filters improve air quality and prevent the spread of marijuana smell. However, have you ever wondered how carbon filtering works?

Carbon filtering uses a bed of activated carbon to remove contaminants and impurities, which are invisible to the human eye but can circulate in the air. Each carbon granule has a large enough surface area where contaminants are trapped. 

Activated carbon is charcoal treated with oxygen. When carbon is subjected to oxygen, the process opens up a myriad of tiny pores between the carbon atoms. Special manufacturing techniques result in highly porous crystals with 300-2,000 square meters per gram surface area. Activated charcoals are then used to "adsorb" organic particles such as odors, tastes, or colors. 

What is "adsorption"?

It's one letter different than absorption, and it's an entirely different chemical process. When a particular substance adsorbs something, such as active carbon, there's a specific chemical reaction going on.

Adsorption is the quality of a solid compound to attract and trap gas, liquid, or dissolved solid molecules to its surface. As liquid or air comes into contact with active charcoal, intermolecular forces draw molecules into the millions of pores dotting the surface of the activated carbon. 

Pollution masks would be a more common example of adsorption from our daily lives. Masks are made of two or more layers of fabric, and between the layers, there is activated carbon either in the form of granules or a filter sheet. Whatever its form, the carbon acts as a purifier, allowing clean air to penetrate the nostrils while dust or smoke particles remain captured in the filter. The activated carbon element is the adsorbent, while the dust and smoke particles are adsorbates. 

Adsorption is different from absorption because molecules that are being absorbed rather than adsorbed are taken inside the absorbent material. Cotton wool used in bandages would be an example of absorbent; its main quality is that it can absorb blood from an open wound. An adsorbent can't do that; it can just trap adsorbates onto its surface. 

It's easy to distinguish absorbents from adsorbents. Adsorption does not change its adsorbent visually. Odor molecules simply attach chemically to the surface without changing the volume, color, or other substance properties of the adsorbent. In contrast, after cotton wool is applied for wound treatment, its volume changes, it becomes red and wet, and feels different on the touch. 

Micrograph of activated charcoal.
A micrograph of activated charcoal in water. The fractal-like shape of the particles hints at their vast surface area. Each particle in this image has a surface area of several square feet. This image is at a scale of 6.236 pixels/μm; the entire image covers a region of approximately 1.13 by 0.69 mm. Photo credit: Zephyris on Wikimedia, CC BY-SA 3.0

How does "active carbon" works?

The surface of an activated charcoal piece has countless bonding sites. When certain particles then pass over the carbon surface, they attach to it and stay there. Activated charcoal achieves splendidly at trapping volatile organic compounds. It will only remove certain impurities found in the air or water; it's not affecting all particles. 

Activated carbon can filter odors, hydrocarbons, and oil vapors from the air. Another of its superpowers is that it can also be utilized to detect and absorb radon in the air, a naturally-occurring radioactive gas associated with causing lung cancer. 

The purification qualities of active carbon have found a wide range of use, from utilization in processing facilities to coffee machines, air conditioning units, aquariums, and essential exhaust fans that filter the air of unwanted odors

Charcoal filters are available in solid carbon, impregnated foam materials, powder, and cloth. Filters do need to be changed regularly, every few months or so, depending on the type of filter, the size of the grow room, or the plants that are being grown. Depending on the odor of the plant, the carbon filters of an odor control system are replaced between 6-12 months.

When was the last time you changed your carbon filter?

Stephen Andrews