Charcoal (also activated charcoal) is known to adsorb a huge variety of substances including a variety of paints, dyes and many different kinds of ions. Moreover, the amount of adsorption at normal pressure and temperature is high. I know physisorption requires high capacity of Van-der-waal bonding with the adsorbate which is favoured by surface atoms with large, easily-dispersed electron clouds and chemisorption requires actual chemical bond formation which requires highly reactive surface atoms. Charged species get adsorbed to form a double layer when the surface is charged, either due to ionization or due to polarity.

But charcoal possesses none of the structural specialities required to be a great adsorbent. Neither it has charged centres (unlike colloidal proteins, or metal sols) nor it has a very high reactivity (Charcoal is almost the final product of combustion of wood, bone, sugar etc and is supposed to be immune to further oxidation even at considerable temperature). Why is it then an excellent adsorbent?


I see three reasons:

  • Activated carbon is commonly used to adsorb organics, that should bind well on coal.

  • For many purposes, activated carbon is treated with potassium or iodine to provide ions for charged centers.

  • Charring organic stuff is a cheap and easy way to create large surfaces for adsorption.

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    $\begingroup$ I am not asking "Why is charcoal used as an adsorbent so often?" but rather "What property of charcoal surface makes it a great adsorbent for most substances?". $\endgroup$ – Satwik Pasani Nov 25 '13 at 4:49
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    $\begingroup$ i believe what he meant is that charcoal has an amazingly big surface, and once activated it gain the property you are looking for on its large surface. and also its used because quite cheap. $\endgroup$ – Nicolas Manzini Dec 5 '13 at 20:07

Activated charcoal is a great adsorbent because of it's huge surface area. While it doesn't bind very many ions/atoms/molecules per surface area (which is the characteristic of a 'good' adsorbent), due to very big surface area per unit of mass it can adsorb a lot of particles. Actually, process of 'activating' charcoal is designed to maximize the surface area to mass ratio.


Charcoal isn't a particularly good adsorbent even though it is chemically very similar to activated carbon.

Activated carbon is usually made by more specialised processes that guarantee the final product will have a very large surface area (often >1,000m2/g). Manufacture usually involves pyrolysation with hot gasses, but many forms are also further activated by partially oxidising the surface (using gas containing oxygen) or bay adding chemicals such as phosphoric acid or potassium hydroxide before pyrolysis. These activation reactions create some functional groups on the surface that can enhance specific types of adsorption on the surface.

But the primary feature that matters is the high surface area, which normal methods for making charcoal do not achieve.


Charcoal is a good absorbent because its quite selective and cheap. Its high surface area means nothing if it doesn't bond to the components that you want absorbed and not a lot else- witness a household sponge which has high surface area and mercury- the mercury just slides off. Charcoal is used in the gold mining/processing industry. The gold(and other metals) is oxidised to form cyanide complexes. The charcoal mostly only absorbs the cyanide complexes not all the other rubbish and water. Charcoal wouldn't be much good for the water purification industry if it absorbed water. Charcoal absorbation processes are a multitudinous- its thousands of papers of primary metallurgy(gold).


Coal is more effective and efficient in adsorbing fumes due to several aspects:

  • We can increase the surface area so it can adsorb higher volume of fumes according to the low of mass and volume; we can use that to develop the efficiency of coal to adsorb a large amount of adsorbent matter
  • Low cost
  • Low transportation
  • Doesn't react with fumes

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