Most gravity fed water filters, and many force fed filters, contain a charcoal filter. Charcoal is formed by the destructive distillation of wood. This yields several burnable gases, wood alcohol, acetic acid, and some other products. The solid residue resulting from this process is charcoal.
Charcoal is a black, brittle solid that is very porous. It is also odorless and tasteless. Though denser than water, charcoal can float! It does this because it is able to adsorb solids and gases so well. When charcoal has the opportunity to adsorb enough gases, they make it float.
Activated charcoal, coal, or carbon is charcoal that has been processed to make it extra porous. Because of this, just one gram of activated carbon has a surface area of approximately 500 m2 but this could be as much as 1500 m2! Considering that it takes 454 grams to make a pound and that a tennis court has 260 m2, it is easy to see just how porous it is! This increased surface area means that more impurities will touch the charcoal as they pass by.
The secret of charcoal's filtering ability is in the fact that it is so good at adsorbing (not absorbing). "Adsorption is the concentration of a gas, liquid, or solid on the surface of a liquid or solid with which it is in contact." Just one cubic centimeter of charcoal is able to adsorb 90 cc of ammonia gas though it adsorbs other substances far better.
Pollutants that are dissolved in the water as it passes through the filter come in contact with the activated charcoal. These substances are actually attracted to the charcoal by van der Waals forces. Wiki explains these forces this way. "In physical chemistry, the van der Waals force is the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules."
It gets more technical that that but, suffice it to say, molecular forces bind some compounds to the charcoal. Activated carbon does not bind well with some chemicals. These include alcohols, glycols, ammonia, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.
In one sense this is good. For example, our bodies need the minerals in water and we wouldn't want them filtered out. Some who live in cities where the water if fluoridated want it left in the water for family dental health. But other substances on the list are clearly unwanted and the filter must contain other adsorbers to remove these.
In conclusion, water and contaminants pass through the activated charcoal filter and, because of the filter?s porosity, the substances will likely come in contact with the carbon. The van der Waals forces will cause the substances to be attracted to the charcoal where they will remain until the filter is washed or replaced. For the contaminant, it is dead end road. For the person drinking the water, it is Water Purifiers Manufacturers refreshing and healthy.