Activated charcoal (also known as activated carbon) consists of small, black beads or a solid black porous sponge. It is used in water filters, medicines that selectively remove toxins, and chemical purification processes.

Activated charcoal is carbon that has been treated with oxygen. The treatment results in highly porous charcoal. These tiny holes give the charcoal a surface area of 300-2,000 m2/g, allowing liquids or gases to pass through the charcoal and interact with the exposed carbon. The carbon adsorbs a wide range of impurities and contaminants, including chlorine, odors, and pigments. Other substances, like sodium, fluoride, and nitrates, are not as attracted to the carbon and are not filtered out. Since adsorption works by chemically binding the impurities to the carbon, the active sites in the charcoal eventually become filled. Activated charcoal filters become less effective with use and have to be recharged or replaced.

What Activated Charcoal Will and Won't Filter

The most common everyday use of activated charcoal is to filter water. It improves water clarity, diminishes unpleasant odors, and removes chlorine. It's not effective for removing certain toxic organic compounds, significant levels of metals, fluoride, or pathogens. Despite persistent urban legend, activated charcoal only weakly adsorbs alcohol and it not an effective means of removal.

It will filter:

Chlorine

Chloramine

Tannins

Phenol

Some drugs

Hydrogen sulfide and some other volatile compounds that cause odors

Small amounts of metals, such as iron, mercury, and chelated copper

It won't remove:

Ammonia

Nitrates

Nitrites

Fluoride

Sodium and most other cations

Significant amounts of heavy metals, iron, or copper

Significant amounts of hydrocarbons or petroleum distillates

Bacteria, protozoa, viruses, and other microorganisms

Activated Charcoal Effectiveness

Several factors influence the effectiveness of activated charcoal. The pore size and distribution varies depending on the source of the carbon and the manufacturing process. Large organic molecules are absorbed better than smaller ones. Adsorption tends to increase as pH and temperature decrease. Contaminants are also removed more effectively if they are in contact with the activated charcoal for a longer time, so flow rate through the charcoal affects filtration.

Activated Charcoal De-Adsorption

Some people worry that activated charcoal will de-adsorb when the pores become full. While the contaminants on a full filter aren't released back into the gas or water, used activated charcoal is not effective for further filtration. It is true that some compounds associated with certain types of activated charcoal may leach into the water. For example, some charcoal used in an aquarium might start to release phosphates into the water over time. Phosphate-free products are available.

Recharging Activated Charcoal

Whether or not you can or should recharge activated charcoal depends on its purpose. It's possible to extend the life of an activated charcoal sponge by cutting or sanding off the outer surface to expose the interior, which might not have fully lost its ability to filter media. Also, you can heat activated charcoal beads to 200 C for 30 minutes. This will degrade the organic matter in the charcoal, which can then be rinsed away, but it won't remove heavy metals.

For this reason, it's generally best to just replace the charcoal. You can't always heat a soft material that has been coated with activated charcoal because it might melt or release toxic chemicals of its own, basically contaminating the liquid or gas you want to purify. The bottom line here is that you possibly could extend the life of activated charcoal for an aquarium, but it's inadvisable to try to recharge a filter used for drinking water.