Why the pore size distribution of activated carbon is more important than the specific or internal surface area
Highlights
- A large internal surface area (m²/g) is often seen as the most important indicator of high adsorption capacity.
- However, the pore size distribution of the activated carbon is at least equally important for product selection.
- For an effective purification solution the activated carbon must be matched to the dimensions of the target molecule(s).
A common misconception: activated carbon with a large internal surface area (expressed in m²/g) automatically has a high adsorption capacity and is therefore the best choice. While the internal surface area is certainly an important parameter, it does not tell the whole story.
The adsorption capacity of activated carbon is mainly determined by three factors:
- Pore size distribution
- Internal surface area
- Surface chemistry
Below, surface chemistry is disregarded and the focus is solely on pore size distribution and internal surface area.
Internal surface area versus pore size distribution
The internal surface area refers to the total surface area available within the pore structure of activated carbon. This is typically expressed in m²/g.
The pore size distribution, on the other hand, indicates which types of pores are present (micropores < 2 nm, mesopores 2–50 nm, macropores > 50 nm) and in what relative proportion. This distribution is essential, as adsorption can only occur if the molecules to be adsorbed can physically penetrate ‘suitable’ pores. A molecule with a dimension of 3 nm simply cannot adsorb in micropores smaller than 2 nm, regardless of the size of the internal surface area of the activated carbon.
By way of illustration – a fictitious example: A customer asks to remove a molecule with a dimension of 3 nm from an aqueous solution. Two types of carbon are available:
- Activated carbon A: internal surface area of 1500 m²/g, but exclusively microporous (< 2 nm)
- Activated carbon B: internal surface area of 800 m²/g, but fully mesoporous (> 2 nm)
Although the internal surface area of activated carbon A is almost double in size, this carbon will hardly or not be able to adsorb the target molecule. Activated carbon B, with a much smaller internal surface area but the correct pore size distribution, will be far more effective for this purpose.
The above example illustrates why the internal surface area as such is insufficient as a selection criterion for finding the right type of activated carbon. For commercial purposes emphasis is often placed on the (high) internal surface area as a quality indicator. Which is not illogical, since this is an easy parameter to communicate and especially compare. But in practice this may lead to incorrect product selection, especially in complex applications such as air and water purification or industrial emission control.
In practice
At Cargen, we look beyond the internal surface area of activated carbon. We analyse the pore size distribution in relation to the target molecule(s), so that specifically the activated carbon that actually works in the intended application is recommended.
Conclusion
A large internal surface area is not always a guarantee of good adsorption by activated carbon. The correct pore size distribution is at least equally important, if not more so, because the molecules to be adsorbed must have access to the pores based on their dimensions. In order to achieve an effective purification solution, the type of activated carbon must be matched to the dimensions of the target molecule(s).
