The capacity of an activated carbon depends on several factors. The main ones are listed below.
Factors related to activated carbon
Average pore diameter
The diameter of the most suitable pores to adsorb a compound should be between two and five times the diameter of the molecule of said compound. If it is smaller, the molecule will not fit. If it is larger, the carbon will show a preference for molecules with greater mass. This is because the greater the mass of a molecule, the more strongly carbon adsorbs it.
Pore size distribution
In addition to the pores in which most of the adsorption phenomenon will occur, it requires transport pores. They are larger in diameter that facilitate diffusion.
Since adsorption is a surface phenomenon, the greater the surface area, the greater the capacity of an activated carbon.
An activated carbon can be produced with various degrees and types of oxidation. The oxidation of an activated carbon produces the formation of oxides at the edges of the graphitic plates. If the oxides are acidic in character, they will attract basic molecules and reject acidic molecules. And vice versa.
Activated carbon particle size distribution
The particle size of a carbon, be it powder, granular or pellet, affects the adsorption kinetics. The surface area of an activated carbon is practically not altered by the average particle size of the carbon. But while this size is smaller, the pores of the carbon are shortened, and the access routes to them increase. This substantially increases the adsorption kinetics.
Factors related to adsorbate that require retention
Ionic or covalent character
Activated carbon, being a covalent solid, preferentially adsorbs molecules with a higher covalent character.
Presence of acidic or alkaline groups
The more oxidized an activated carbon is, the more its surface chemistry influences its preference for molecules with acidic or basic groups to increase or decrease. Whether this preference increases or decreases will depend on the acidic or basic character of the oxides that are part of the carbon, and on the presence of acidic or basic groups in the adsorbate.
Mass and molecular size
Activated charcoal prefers the highest mass molecules that will fit in its pores.
Molecule shape (linear, branched, cyclic)
Rounded molecules, such as cyclic or branched ones, enter pores more easily. More linear molecules enter less easily when their size allows them to enter the pores of the carbon only when they impinge with the proper relative orientation. This phenomenon is known as steric effect or of orientation.
Related to the fluid to be treated
Presence of adsorbates that are not required to be retained and that compete with those that are required to be retained
The more competition from molecules that do not require separation from the fluid, the shorter the useful life of the activated carbon.
Temperature and viscosity
In the case of liquids, the higher the temperature, the lower the viscosity, which increases the diffusion rate of the adsorbates, both outside and inside the pores. This effect is almost always more beneficial than the decrease in adsorption capacity (which decreases the higher the temperature).
In the case of gases, the higher the temperature, the higher the viscosity. This is undesirable, because it hinders the diffusion of the adsorbates. It is also undesirable because it decreases the adsorption capacity of the carbon. Therefore, it is better that the temperature is low.
pH of aqueous solutions
The pH usually has an important influence on the adsorption capacity of an activated carbon. Adsorbates that are weak acids are best adsorbed in low pH solutions. Adsorbates that are weak bases are best adsorbed in high pH solutions.
The pH also affects the reaction rate between activated carbon and oxidant molecules (such as free chlorine, ozone, hydrogen peroxide, potassium permanganate…). Normally, at a lower pH, the reaction rate is higher.
Related to the dimensions of the granular or pellet activated carbon bed and the instantaneous volumetric flow of the fluid being treated
Cross-sectional area to flow and depth of bed
The thinner a carbon bed, the greater the turbulence, which increases the diffusion of adsorbates out of the pores of the carbon. In addition, it increases the relationship between the depth of the bed and the mass transfer zone, which leads to better use of the coal.
Relationship between the volume of the carbon bed and the volumetric flow of the fluid being treated
The higher this ratio, the longer the contact time between the carbon and the fluid, which is desirable.
Related to the turbulence with which the tank in which powdered activated carbon is applied to a liquid is agitated
With greater turbulence, the diffusion of adsorbates outside the carbon particles increases. This is very beneficial.
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