Its capacity of adsorption! The term adsorb has a different meaning than absorb, and while a real sponge absorbs water by mopping it up through capillary action and suction, activation carbon adsorbs substances through a form of chemical attraction. You get rid of water from the sponge by squeezing it, but that doesn’t work with activated charcoal, since the substances are bound to it, not just physically constrained.
Activated charcoal is a 100% natural product, obtained from the carbonization of organic matter like hardwood, coconut, bamboo, peat moss, olive pits, coconut shells, sawdust etc. It begins as regular charcoal, as wood burns there is often not enough oxygen to allow for complete combustion. The water evaporates off, and the carbon in the wood turns into the black charred coals or crust we see when we put the fire out. As the fire smoulders for a period of time, the wood slowly dries and eventually changes into charcoal.
Activated charcoal begins as regular charcoal and is then “activated” with oxidizing gases, such as steam or air, at high temperatures. This oxidative process further erodes the charcoal’s internal surfaces. This increases its adsorption capacity by creating an internal network of even smaller pores or we can say the heat opens up millions of small spaces between the carbon atoms and turn it literally into an atomic sponge that adsorbs both organic and organic impurities. Activated charcoal has a massive surface area, and just ten grams has the same surface area as nine American football pitches or 77 tennis courts. Around a factor ten of regular charcoal. This huge surface area provides activated charcoal with innumerable bonding sites, and when chemicals that are attracted to carbon pass by in the water they are attached to the surface. They cannot get free again, as water in a sponge can, but are bound to the surface of the carbon. Because the digestive system has no effect on charcoal then whatever is bound to it passes naturally through the body. Therefore it is very effective in cleaning poisoning chemicals out of the water.
Tina Brandt Jensen, biochemist