As Burf and RMS have pointed out, activated charcoal is ordinary charcoal, with the additional characteristics of purity, and porosity (large microscopic surface area), and it is these characteristics that give activated carbon the ability to adsorb various pollutants, and this is purpose for which it has been engineered. It's also called, "activated carbon", according to Wiki:http://en.wikipedia.org/wiki/Activated_carbon You might also be wondering about this word "activated". Is it real jargon used by chemists? Or is it just some sort of made-up marketing buzzword? I mean I don't blame you for being skeptical of what's being sold on the street. I also would be inclined to think, "charcoal is charcoal is charcoal", and the only difference is in the branding. However, "activated" is a real word, a real jargon in the field of chemistry. This word also an antonym, "passivated". Also there's such a thing as "re-activated". Usually these words are used to describe the surface of a solid. Speaking generally an activated or active surface is one where chemical reactions can easily occur. Usually this surface has a large microscopic surface area, many many sites where the pure solid is exposed and available to react. In contrast a passive, or passivated surface, is one where chemical reactions will not occur easily. Typically this is because there is a layer of unreactive gunk of some kind, a so called "passivation layer"http://en.wikipedia.org/wiki/Passivation This idea of a passivation layer is important to the study of corrosion. For example, aluminum metal forms a nice thick passivation layer of aluminum oxide, just from being exposed to air, and this layer protects the aluminum from further corrosion. This is good news for people who spend a lot of time on airplanes. However for anyone who wants to use aluminum as an in a battery, or as fuel, the passivation layer is kinda bad news. He or she will have to figure out some way to remove, or break through, the gunky passivation layer, and get to active aluminum beneath. e.g.https://www.instructables.com/id/SODA-CAN-HYDROGEN-GENERATOR/https://www.instructables.com/id/Run-Your-Car-on-Hydrogen-from-Aluminum-Soda-Cans-a/ The usual trick for "activating" a surface is to burn away, dissolve, or otherwise get rid of the gunky passivation layer. The wiki article on activated charcoal had some words on how this was achieved for charcoal in particular. The usual trick for "passivating" an active surface is to intentionally add some reactant(s) that will cause a passivation layer to "grow" on this surface, and sort of fill in all the little reactive nooks and crannies. Again the typical application for this trick is to make a metal surface resistant to corrosion. A final note on activation: it is sometimes possible/desirable to repeat the activation process; i.e. to effect "re-activation". As I said the before, the passivation layer is basically a layer of gunk that is inhibiting, or preventing reactants from reaching, the layer below. For example, it may be possible to re-activate used activated carbon, just by putting it back into the oven for some time, in order to burn off the layer of junk, or pollutants, it has adsorbed.
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An "artist's interpretation", erm... picture, of what these "activated" and "passivated" surfaces look like:
Jack, you are one smart boy . Gotta love the graphic. It says it all.
Use regular charcoal, not briquettes. L
Activated charcoal is specially prepared to enhance it absorbent properties and is usually made from coconut shells. Charcoal briquettes are charcoal dust and chips stuck together with a resin and pressed into a convenient size and shape. It is used for cooking and heating purposes.
Charcoal briquettes frequently contain fillers, chemicals and other additives that would make them unsuitable (and potentially lethal) to use in a water filter. Activated charcoal is regular (pure) charcoal that has been specially treated to have gazillions of tiny pores, which increases the surface area enormously, which makes it much better at adsorbing undesirable substances than regular charcoal.