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Activated carbon is an amorphous form of carbon. This means that it has no regular atomic structure, unlike the other forms (allotropes) of elemental carbon: diamond, graphite. Activated carbon works in two different ways. Activated carbon can be made from a wide range of source materials, s
uch as coal, coconut shells, wood, peat, or bone. The material is often charred, and then activated chemically or in a high temperature steam environment to produce an extensive network of pores. It can remove organic substances by absorbing them inside its sponge-like structure and/or by adsorbing them with chemical bonds. Absorption is based on the vast number of pores. You can imagine it as a material with too many pockets (very tiny pockets, the smaller the better), which can be filled with small things (molecules). It could be referred to as a very fine mechanical filter, which can absorb things the size of a molecule. Thus, it is important to keep those pockets functional for as long as possible. In this aspect, a good mechanical filtration is essential before water reaches the activated carbon part of your filter, otherwise carbon will be clogged very quickly. The word adsorb is important here. When a material adsorbs something, it means that it attaches to it by chemical attraction. The huge surface area of activated charcoal gives it countless bonding sites. When certain chemicals pass next to the carbon surface they attach to the surface and are trapped. Activated charcoal is good at trapping other carbon-based impurities (organic chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all - sodium, nitrates, etc., so they pass right through it. This means that an activated carbon filter will remove certain impurities while ignoring others. It also means that, once all of the bonding sites are filled, an activated charcoal filter stops working. At that point you must replace the filter. 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 activated carbon, it is attached on it by its hydrophobic (lipophilic) side and thus is removed from the water solution that contained it. However, carbon will remove not only organic compounds but also inorganic ones. See Fig. 1 and Fig 2. |
