In What Way Does Activated Charcoal Purify Water?
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 an odorless and tasteless solid, porous, black, and brittle. Charcoal is normally too dense to float in water, yet sometimes it does. How can this be? It is because charcoal has the remarkable ability to adsorb solids and gases. Charcoal does this so well that it can adsorb enough gases to make it float.
The filters usually use activated charcoal, coal, or carbon. This means the charcoal has been processed to make it extra porous. The result is that just one gram of activated carbon has a surface area of between 500 m and 1500 m! When you consider that it takes 454 grams to make one pound and that a tennis court has 260 m, you can see it is very porous! The fact is, the greater the surface area, the more likely it is that 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.
Unwanted substances dissolved in the water pass through the charcoal and, because it is so porous, come in contact with the charcoal. The pollutants then are attracted to the charcoal by a force called van der Waals forces. Wiki defines this by saying, “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.”
Though this is very technical, it can be summarized by saying molecular forces bind some compounds to the charcoal. Activated carbon does not bind all chemicals equally well. It does not do as well with ammonia, alcohols, strong acids and bases, glycols, metals and most inorganics, such as fluorine, lithium, iron, sodium, lead, arsenic, 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 refreshing and healthy.
The best line of activated charcoal filter we are aware of is the Berkey Filter. Whether you choose their Berkey Light Water Filter or another model, each set of filters they ship with can be re-cleaned to purify up to 6,000 gallons of water.
