Activating Carbon Particles

Different kinds of activated carbon can be produced which can then be used for removing different kinds of impurities. The secret of activated carbon lies in its elaboration. For example, when acids are used during the activation procedure, the result is a carbon with an extremely porous structure. Distinctive kinds of carbon can be made if carbon is subjected to high temperatures and pressure, with the size of its pores varying dependent on the gases used during activation and the inorganic salts that were added before the activation procedure.

Activated carbon can be manufactured from carbonaceous material, including coal (bituminous, subbituminous, and lignite), peat, wood, or nutshells (i.e., coconut). The manufacturing process consists of two phases, carbonization and activation. The carbonization process includes drying and then heating to separate byproducts, including tars and other hydrocarbons, from the raw material, as well as to drive off any gases generated. The carbonization process is completed by heating the material at 400–600°C in an oxygen-deficient atmosphere that cannot support combustion.

The carbonized particles are made “active” by exposing them to an activating agent, such as steam at high temperature. The steam burns off the decomposition products from the carbonization phase to develop a porous, three-dimensional graphite lattice structure. The size of the pores developed during the process is a function of the time that they are exposed to the steam. Longer exposure times result in larger pore sizes. The most popular aqueous phase carbons are bituminous based because of their hardness, abrasion resistance, pore size distribution, and low cost, but their effectiveness needs to be tested in each application to determine the optimal product.

Absorption vs Adsorption

Active charcoal is a similar alternative, and can be good at trapping other impurities ("organic" chemicals), as well as things like chlorine. However since it absorbs, not adsorbs, many other chemicals are not attracted to at all - sodium, nitrates, etc. - so they pass right through it. This means that an active charcoal filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an active charcoal filter stops working. At that point you must replace the filter. In comparison, adsorption relies on the polarity of the various molecules. Polar molecules have two "poles" that differ in their affinity for the water molecules, so one side of the molecule is hydrophilic (likes water) and the other one is hydrophobic (dislikes water - likes lipids). When the polar organic molecule approaches the polar surface of GAC, it is attached on it by its hydrophobic (lipophilic) side and thus is removed from the water solution that contained it. However, GAC will remove not only organic compounds but also inorganic ones.