AM Activated Carbon
Activated carbon removes organic compounds from aquarium by adsorption and absorption principles. Both processes involve the transfer of the adsorbate (pollutant) from the liquid phase (water) to the solid phase (carbon). Adsorption is the primary sorption mode relying on electrostatic Van der Walls forces. This attractive “force” forms relatively weak bonds between the carbon and adsorbate. In theory activated carbon could release or desorb what it removed at some point. But practical experience with aquarium filtration and laboratory experiments show desorption rarely occurs or causes any type of “toxic release”. Bacteria readily colonize the outer surface of the activated carbon and consume some of the sorbed organics. The bacterial action reactivates a small portion of the carbon and perhaps prevents desorption.
Absorption refers to the diffusion of a gas or compound into the porous network where a chemical reaction or physical entrapment take place. Ozone for example is absorbed into activated carbon where it oxidizes a portion of the carbon’s surface. Ozone (O3) is reduced to oxygen (O2) thus “detoxified” and made safe for the aquarium. Ozone does not accumulate or build-up in the carbon structure. A third process called chemisorption forms an irreversible chemical bond between the carbon surface and the adsorbate. Pollutants are tightly bound to the sorbent.
All three sorption processes occur simultaneously in the aquarium. The sorption process takes place in three stages:
1) Organic laden water contacts the activated carbon particle.
2) The adsorbate diffuses into the porous network.
3) Sorption onto the carbon occurs.
The sorption process has been described as the activity observed in a parking lot. Vehicles (organics) are moving freely on the main highway (aquarium water). Gradually vehicles enter the lot (pore) in search of a parking space (sorption site). As the parking lot becomes filled the sorption rate slows down. Sorption of large organic compounds takes longer than smaller compounds. The sorption rate is also influenced by water temperature, pH, and salinity, but these factors will not be discussed since they are “constants” in the marine aquarium.