Dirty water flows into a canister and down through a bed of dark carbon granules. Impurity molecules cling to the carbon instead of passing through, so much cleaner water leaves at the bottom. A magnified view on the right shows why: countless tiny pores give the carbon an enormous surface to catch them on.
An activated carbon filter is a canister packed with dark, gritty carbon. Water enters at one end and flows through the carbon before leaving the other end. Along the way, impurities that were dissolved in the water stick to the carbon instead of continuing on.
On this diagram dirty water enters through the pipe at the top left, flows down through the bed of carbon granules, and cleaner water leaves through the pipe at the bottom right.
The amber area is water carrying impurities, and the brown dots are the impurity molecules. The dark dots are the carbon granules. The magnified circle on the right opens up a single granule to show its maze of tiny pores, with impurity molecules trapped inside them.
Adsorption on carbon is one of the most common ways to clean water and air at home. It sits inside jug filters, under-sink filters and many water-treatment stages, and removes the tastes, odours and many organic chemicals that make water unpleasant — using nothing more than an enormous surface area.
Water carrying dissolved impurities flows in through the inlet pipe and into the top of the canister. Those impurities are what the filter is there to remove.
The water flows down and spreads out across the whole bed of carbon granules, so it comes into contact with as much carbon surface as possible rather than rushing down one channel.
As the water passes the granules, impurity molecules cling to the carbon's surface — this is adsorption. They are held on the surface instead of staying dissolved in the water, so they travel no further.
The magnified view shows why the carbon can catch so much. Activation fills it with countless tiny pores, so a small amount of carbon exposes an enormous surface area — every bit of it able to grab impurity molecules.
With much of its impurity load left behind on the carbon, the water continues down and out through the outlet pipe. Over time the pores fill up, and the spent carbon has to be replaced or regenerated.
Adsorption is when molecules stick onto the surface of a solid instead of dissolving into it. In an activated carbon filter, impurity molecules from the water cling to the carbon's surface and are held there as the water passes by.
Because of its surface area. Activation fills the carbon with a maze of tiny pores, so a single gram can expose a surface as large as a football pitch. Every bit of that surface can grab impurity molecules.
In adsorption the molecules stay on the outside surface of the solid, like dust on a wall. In absorption they soak into the bulk of the material. Activated carbon works mainly by adsorption, which is why its surface area is what matters.
Yes. The same principle removes odours and some vapours from air, because gas molecules also cling to the carbon's pores. That is why carbon is used in range hoods, masks and air purifiers.
Because the surface fills up. Once most of the pores are occupied by trapped molecules, there is little room left and the carbon must be replaced or regenerated. This is called breakthrough.
No. It is good at trapping many organic chemicals, tastes and odours, but it does not reliably remove dissolved salts, heavy metals or microbes. For those, other treatments are needed.