Activated Carbon

Activated carbon is one of the most effective and widely used water filtration mediums in the world. It purifies water not by acting like a physical sieve or strainer, but through two distinct surface-level phenomena: physical adsorption and catalytic reduction.

1. Organic molecule removal: physical adsorption

For organic compounds — such as volatile organic compounds (VOCs), pesticides, herbicides, and industrial solvents — the primary mechanism is adsorption, in which a substance adheres specifically to the surface of a material.

• Large surface area: Activated carbon is essentially charred organic matter (usually coconut shells, wood, or coal) that has been treated with steam or chemicals at extremely high temperatures. This "activation" process blows the carbon apart at a microscopic level, creating a massive labyrinth of microscopic pores (macropores, mesopores, and micropores). Just one gram of activated carbon has a surface area of over 1,000 square meters.
• Van der Waals forces: As water flows through this labyrinth, organic molecules are drawn to the carbon walls by weak intermolecular electrical attractions known as Van der Waals forces.
• The "like attracts like" rule: Carbon in activated carbon is highly non-polar. Most organic molecules also have non-polar characteristics and are physically attracted to the carbon surface, becoming tightly bound to it.

2. Specific inorganics: catalytic reduction

Standard physical adsorption is ineffective for most inorganic compounds (like dissolved minerals, salts, and heavy metals) because these molecules are highly polar or carry an ionic charge and will flow right through the pores.

However, activated carbon does effectively remove one major class of inorganic chemicals: disinfectants like chlorine and chloramine. It does this through a chemical reaction called catalytic reduction.

• Electron transfer: When free chlorine or bromine species contact the activated carbon, the carbon surface acts as a catalyst. The carbon donates electrons to the chlorine or bromine, chemically reducing it to harmless ions (Cl⁻, Br⁻).
• Surface consumption: Unlike physical adsorption (which just fills up the pores), catalytic reduction actually consumes a tiny amount of the carbon surface over time.

Therefore, activated carbon has a finite treatment life and has to be changed out to be effective in the long term.

What activated carbon cannot remove

It is important to note that standard activated carbon filters will not remove dissolved inorganic minerals like calcium, magnesium, sodium, or fluoride. Because these ions are heavily hydrated (surrounded by tightly bound water molecules) and highly polar, they ignore the Van der Waals forces of the non-polar carbon.

To remove heavy metals (like lead) or specific minerals, the activated carbon must be chemically altered (impregnated with specific binders) or paired with a different technology like reverse osmosis.