A charcoal filter is a common part of most gravity fed and many force-fed water filters. Charcoal is the product of the destructive distillation of wood. This process yields wood alcohol, acetic acid, several burnable gases, and a few other products. The solid residue that results from this process is what we know as 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.
Charcoal is used in filters because of its ability to adsorb. This is not absorption but adsorption. "Adsorption is the concentration of a gas, liquid, or solid on the surface of a liquid or solid with which it is in contact." One cubic centimeter of charcoal has the ability to adsorb 90 cc of ammonia gas. Charcoal adsorbs other substances even 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."
Putting it simply, molecular forces bind some compounds to the charcoal. Not all substances are equally attracted to the activated carbon. Substances such as alcohols, glycols, ammonia, strong acids and bases, metals and most inorganics, such as boric acid lithium, sodium, iron, arsenic, lead, and fluorine are not filtered as well.
his is good news and bad news. It is good news because our bodies need the minerals in water. We don't want them filtered out. Fluorine is said to be good for dental health so many don't want that filtered either. However, others on the list above are unwanted and the filter must have other substances 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 refreshing and healthy.
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