Ask Dr. Knowledge

What is activated charcoal?

Activated charcoal removes chemicals from aquarium water. Activated charcoal removes chemicals from aquarium water. (John Tlumacki/Globe Staff/File 2005)
February 15, 2010

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What is activated charcoal? I’ve seen it in aquarium filters and in the health food store. How does it differ from regular charcoal?

Charcoal is essentially pure carbon, and the terms “activated carbon’’ and “activated charcoal’’ are used interchangeably.

While coal comes from plant matter that loses its noncarbon components over a long time because of pressure and heat deep in the earth, charcoal is made quickly, by heating wood in the absence of oxygen. This leads to a charring (without burning the carbon into carbon dioxide), which drives off most everything except carbon. The process is done on a scale of hours, rather than the millions of years for coal.

Activated charcoal has been made highly porous, with the individual grains riddled with pores as small as a few billionths of a meter - or in modern parlance, a few nanometers, making this a very early example of large-scale nanotechnology, before people got excited about the word. All these pores mean a mere gram of activated charcoal (about the size of a large pill) can have up to 1,500 square meters of surface area.

The pores form when you heat wood or charcoal to very high temperatures - usually more than 600 degrees Celsius - in the absence of oxygen. As the charcoal crumbles under the heat, more and more surface area is exposed - just as the two halves of a cut apple have more surface area than a whole apple.

Many noxious substances (but not water) will stick to carbon, and the huge surface area afforded by even a bit of activated charcoal makes it useful for filters - in aquariums, where it takes unpleasant smelling chemicals out of the water, for example, and to improve the taste of whiskeys and vodkas. Activated charcoal’s uses vary widely, depending on the size of the pores and whether it is treated with chemicals that make it especially good at grabbing toxins.

Crude charcoal has a folk use, treating digestive problems, and in modern medicine activated charcoal is used to absorb toxins in cases of poisoning. You can find it as an over-the-counter remedy, but be careful: In addition to binding toxins, it will bind many medicines, so it’s not the sort of thing to use without advice from a physician.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.