How does activated carbon work?

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Nov. 28 2005 7:04 PM

How Does Activated Carbon Work?

What peach pits and molasses have to do with safe drinking water.

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Battling the benzene
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Battling the benzene
Daniel Engber Daniel Engber

Daniel Engber is a columnist for Slate

An explosion at a Chinese chemical plant two weeks ago spilled a 50-mile slick of benzene into the Songhua River, which provides drinking water for the city of Harbin. After the accident, the Chinese government made plans to ship 1,000 tons of activated carbon to the city's water treatment facilities and to dump more carbon straight into the river. Russian authorities—who expect the pollution to reach the city of Khabarovsk within the next few weeks—are shipping 50 tons of activated carbon to local treatment plants. How can activated carbon clean up a spill?

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Through adsorption. Carbon has a natural affinity for organic pollutants like benzene, which bind to its surface. If you "activate" carbon—by steaming it at 1,800 degrees, for example—it forms little pores and pockets that increase its surface area. (It's said that a teaspoon of activated carbon has the area of a football field.) Pesticides, chloroform, and other contaminants slide into the holes of this honeycomb and hold fast.

Municipal water treatment plants generally use activated carbon in two ways. They either run water through carbon filters—a bit like the ones you might have on your faucet or in your Brita—or they pass it over a bed of carbon. Either way, no carbon remains in the water once it's been thoroughly treated. (The carbon that the Chinese government dumped directly in the river was likely intended to pick up benzene from the surface of the slick; in a case like this, carbon is usually skimmed off the surface once it's adsorbed as much as it can.) How much pollution gets sucked up by the carbon depends on a number of factors, including the temperature and acidity of the water, the type and amount of pollution, and the amount of time the water spends in contact with the carbon.

The kind of carbon you use also makes a difference. Some manufacturers produce as many as 150 types of activated carbon, which differ in characteristics like density and pore size. (It also comes in powdered, granulated, and pellet forms.) Which kind you use depends on what you're using it for: A carbon with large holes would be best at picking up heavy organic chemicals, while smaller pores would catch the lighter pollutants. Different types of carbon can be activated in different ways or they can come from different source materials. Some are made from coal, wood, or sawdust, while others are made from peach pits, olive pits, or coconut shells. Manufacturers rate their products according to how much they can adsorb. The "molasses number," for example, tells you how well they adsorb the dark color from a mixture of water and molasses.

The water treatment plants in China and Russia will need as much carbon as they can get to soak up all the benzene on the river. Activated carbon can only be used until its pores fill up—which is why you have to change the filter in your Brita from time to time. It could take 1,000 tons of carbon to adsorb the 100 tons of benzene that spilled two weeks ago.

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Explainer thanks Peter Censky of the Water Quality Association and Ed Habayeb of General Electric.

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