Activated carbon is actually a form of graphite; however, it is disorganized. It is full of impurities due to the activation process. Crystals of carbon are formed in the shape of six atom hexagons, which bind horizontally making a graphite sheet. These sheets then layer themselves on top of one another to form activated granules. The bonds holding these layers together are known as Van der Waals forces. The estimated distance between three of these layers is approximately 0.67 nanometers.
Activated carbon has pores of three varying sizes: macropores, mesopores and micropores. It is through these pores that gases and liquids are absorbed. Macropores, being the largest in size at approximately 1,000 angstroms, provide a way for the gases and liquids to access the smaller pores, which are deeper inside the graphite structure. The large number and size of pores in activated carbon also contributes to a higher surface area of the granules, which contributes to its absorptive properties. Surface areas of activated carbon can reach 1500 m^2 per gram. A gram of activated carbon is equivalent to one tablespoon.
Various atoms within the graphite will serve different purposes within the structure. Most of the atoms are neutral, and it is the surface atoms that will allow other substances to adsorb to them. Any corner atoms in the structure are more reactive, potentially, with metals. For this reason, activated carbon is best used as a filtering agent within plastic containers.
Activated carbon is used most often in water filtration as a powder, known as powdered activated carbon. This allows for more surface area, which increases absorptive properties. Activated carbon can also be found in granular form known as granular activated carbon. For the purposes of filtering blood in a process known as hemoperfusion, activated carbon is sometimes covered in a polymer.