The basics of activated carbon adsorption

Activated carbon is used in over 2,500 commercial products. Most wastewater treatment plants employ carbon filters to filter the water and air that exit the facility. However, their traits and qualities are not taught in "formal" schooling. You find out about them on the job.

Activated carbon is a solid adsorbent that is inert and is often used to remove dissolved pollutants from water and industrial gas-phase streams. As many readers are aware, it is manufactured from nearly any carbon-containing feedstock, including coconut shells and coal family members.

Adsorption refers to the buildup of a gas or liquid on the surface of a liquid or solid substrate, as opposed to absorption, which occurs when the approaching material penetrates the bulk or volume of the substrate.

Activated carbon is porous, affordable, and widely accessible for use as an adsorbent, providing a vast surface area for pollutants to be removed. It has the highest usable surface area per gram of any material available for physical adsorption. A teaspoon of activated carbon, in fact, has more surface area than a football field.

Physical occurrences

Because of its unique properties, activated carbon has a remarkable potential to collect water-dissolved pollutants such as flavor-, odor-, color-, and toxic-promoting species. Adsorption processes based on surface interactions between pollutants and carbon graphitic platelet surfaces are used to remove impurities.

Van der Waal forces and induced dipole interactions drive these contaminant-carbon surface interactions. Neutral organic molecules are converted into intramolecular dipoles by activated carbon graphitic platelets. The generated dipoles attract and cling together the molecules, causing them to precipitate out of solution in the carbon's nano-sized pores or adsorption spaces. This is known as premature condensation, which is aided by the activated carbon.

Activated carbon producers utilize a variety of feedstocks and process settings to produce a wide range of pore size distributions. To tackle aqueous and gaseous-phase issues with activated carbon, proper pore structure selection is critical.

Activated carbon powder

When compared to carbons with larger particle sizes, powdered, micron-sized activated carbon particles are milled from millimeter-granular activated carbon and demonstrate quicker kinetics and a better capacity for contaminant removal.

Powdered activated carbon can be used to treat infrequent pollutant events that affect municipal influent waters, such as algal blooms and industrial spills. To eliminate these pollutants using activated carbon, powder can be introduced to the clarity process settling unit. It can also shield fixed activated granular carbon beds from unexpected influent pollution.

If a plant lacks the infrastructure to employ granular activated carbon or does not have enough granular carbon between the influent and the effluent to economically use it for removal in occasional contamination occurrences, it can use powder instead. Single-use powdered activated carbon is used in a batch process to remove pollutants to acceptable mandated maximum contamination levels (MCLs), but not always to zero or undetectable contamination.

Activated carbon that has been used

Activated carbon does not last indefinitely. It must be replaced on a regular basis with virgin or reactivated carbon. Pores or physical adsorption gaps, which are nanometer-sized compartments between graphitic platelets, ultimately fill and lose their ability to remove adsorbates. Carbon pores are heterogeneous, with adsorption energies ranging from strong to weak. Figure 1 shows the graphitic platelet spacing. Closely spaced carbon graphitic platelets have high adsorption potential energies, whereas broad platelet spacings have comparatively moderate adsorption energies.