Of the three types of filtration used in the reef aquarium hobby today, biological, mechanical, and chemical, it is chemical filtration that is possibly the least understood. This post is all about activated carbon, the most popular type of chemical filtration used in aquariums.
What is activated carbon?
Activated carbon is a chemical filtration media that is very well suited for removing dissolved organic compounds. The benefits of regular use are crystal clear water and removal of foul odors. An additional benefit of carbon is it binds up some of the toxins released by tank inhabitants that are used by those organisms to wage chemical warfare against their neighbors.
Activated carbon is essentially just charcoal that is treated with oxygen to open up millions of pores at the microscopic level. These pores make the carbon highly reactive by way of adsorption.
Adsorption differs from absorption in that adsorption is accomplished by a weak chemical bond that occurs only at the surface of the compound. It is for this reason that the surface area of carbon has such an effect on its performance.
Absorption is a filtering effect that occurs throughout the entire volume. Imagine how a sponge soaks up and traps water for example. To some degree, both occur when activated carbon is utilized in the home aquarium, but mainly it is the adsorption effect that makes it a form of chemical filtration.
How does it work?
As stated above, activated carbon removes organic compounds from aquaria by adsorption and absorption principles. Both processes involve the transfer of the pollutants from the water to the solid phase.
Adsorption relies on electrostatic Van der Walls forces. This attractive “force” forms relatively weak bonds between the carbon and dissolved organic compounds in the water. Activated carbon could release, or desorb, what it removed at some point, but practically speaking, this rarely occurs. 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 compounds into the porous network where physical entrapment takes place. Ozone, for example, is absorbed into activated carbon and is thus “detoxified” and made safe for the aquarium.
When placing it in the reef system, high flow is beneficial, so we recommend placing it in a strong flow area of the sump or in its own reactor. As a side note, activated carbon can get dusty, so it is a wise choice to rinse it thoroughly to prevent small particles from drifting around your reef.
Activated carbon myths
There are some common myths about activated carbon I need to dispel:
Myth #1: It can be recharged in an oven.
FACT: The process of activating the material means increasing its internal surface area and getting rid of impurities. This is done at nearly 2,000°F in a controlled environment. Technically, it is possible to recharge spent activated carbon, but it won’t be done at home.
Myth #2: It will mess up your water chemistry.
FACT: Carbon does not remove ammonia, nitrite, or nitrate from water. It also does not have an effect on water hardness or alkalinity.
Myth #3: It will remove trace elements.
FACT: Activated carbon does not have a major effect on trace elements. Trace elements such as cesium, chromium, cadmium, selenium, cobalt, silver, lead, tin, helium, lanthanum, and cerium are not really soluble in sea water (and possibly toxic in non-trace quantities). Thus, the TLDR answer is that in the marine aquarium, activated carbon will not remove trace elements.
In short, activated carbon is the most prevalent form of chemical filtration in the hobby for a reason. It is highly effective and easy to use. Best of all, it is one of the least expensive methods to filter an aquarium given its dramatic improvements on water quality. Please see the video below for more information.