Activated Carbon

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ACTIVATED CARBON by Bob Goeman

INTRODUCTION

As a chemical filtration substance, activated carbon is one of the best. It should be considered a tool in our arsenal for helping maintain water quality. Not only is activated carbon readily available, it has been on the market for decades. It remains the choice of most freshwater and marine aquarists over other forms of chemical filtration media. It is frequently used in municipal water treatment facilities, home tap water filters, and air purification equipment. It has probably been utilized longer than any known filtration substance. Well, maybe resins have been used the longest as resin/ion exchange is discussed in the Bible, the Book of Exodus 15:22-25, and the Book of Numbers 19:5-17, where bitter water was made sweet by casting a tree into the water. The wood probably removed certain ions from the water and purified it to a certain degree. Activated carbon remains a media of choice because it's efficient and cost effective. It also remains to this very day somewhat misunderstood and improperly utilized by many aquarists. To overcome this misunderstanding it's necessary to look more closely at carbon and what "activated" and how to use and properly maintain it mean.

THE ACTIVATING PROCESS

To begin, there are quite a few materials that can be used to make activated carbon. They include bituminous coal, lignite, peat, bone, nutshells, and wood. The best carbon for use in the aquarium is thought to be bituminous coal, second best may be lignite, with coconut nutshells a strong third. I should add there still remain differences of opinion on that subject. To make any of these materials useful in aquariums it must first be "activated."

The processing of the base material is first started by heating it to an approximate temperature of 900°E This is accomplished in the absence of air so as to drive off hydrogen and oxygen. The process results in a "charred" substance. The char or "charcoal" is again heated, sometimes to approximately 1600°F in the presence of steam, air, or carbon dioxide. The second heating process removes the remaining hydrocarbons and "activates" the product by creating many tiny holes, passageways and crevices both inside and on the surface of the carbon particle. During the activation process the carbon, in either granule or powder form, can be given different adsorption characteristics by treating it with the addition of inorganic salts such as copper, zinc, phosphate, sulfate, or silicate.

Since phosphate is frequently used and one of the product's original components, it's one of the reasons why aquarists should soak their choice of activated carbon in purified water before use. See the "Maintenance" section below for further details. After the activation process, an acid or alkaline wash or rinse in some cases, will further change the carbon's adsorption characteristics.

Although this process somewhat reduces the carbons adsorption capacity, it does reduce soluble contaminants that could leach into solution, especially phosphate. And, in our aquariums, we are especially concerned about phosphate. Next, let's look at how carbon works and what it removes from solution.

METHODOLOGY

The carbon particle can be thought of as a sponge. It has large outside holes and crevices leading inward to passageways with ever decreasing smaller and smaller channels. See the enclosed Activated Carbon Particle sketch. This makes the internal structure of the carbon granule much more important than its outside surface area. In fact, when activated carbon "surface area" is mentioned, it is really referring to its "internal surfaces; not its outer surface. The more internal channels there are, the more "surface area" and the more places for molecules and matter (adsorbates) to flow into. When this happens they become lodged and attached by both a weak physical force and what is termed molecule sieving. Adsorbates are then retained for removal from solution when the carbon is eventually discarded. This process is called "adsorption; and should not be confused with "absorption," which means to be simply taken into the media. It should also be noted that besides adsorption, other dissolved compounds can combine chemically with the previously adsorbed molecules and also be removed from solution.

Even though surface area (again, internal area) is extremely important, pore size and pore volume are also important considerations. Pore size relates to the size of the opening leading into the carbon particle. Very small openings allow only small molecules to enter. Medium size pores allow more complex molecules to enter. Large pores not only allow various size or complex molecules to enter, but also suspended solid material. Small pore size is not efficient for aquarium use since the openings easily plug, thereby causing the internal areas to never fully become utilized. Pore volume pertains to the amount of open space inside the carbon particle.

Within limits this simply means the more emptiness there is inside each particle the more space there is to pack in those molecules. The limiting factor here is the fragility of the carbon walls. Basically it can be thought of as the more internal surface area the greater its capacity and the larger its pore volume the better its efficiency Activated carbon in the aquarium removes dissolved organic molecules including non-polar proteins such as the phenols that yellow the water.

Also removed are amino acids, iodine, carbohydrates, pheromones and peptides along with various heavy metals such as copper, chromium, cobalt, zinc, vanadium, iron, molybdenum, and mercury. Some dissolved gasses can also be removed. Some examples are hydrogen sulfide, methane, and ozone along with airborne fumes from paint, pesticides and cigarette fumes. Organic dyes like methylene blue and malachite green along with chlorine and chloramine are also removed. Activated carbon is very effective at removing substances used for treating aquariums for 3isease. It will remove agents like copper, sulfa drugs, and antibiotics. Of course, 1 should not be utilized until the treatment cycle is completed.

Unfortunately it also removes some beneficial trace elements as noted above. This should not stop aquarists from using this efficient and economical chemical filtration media in a well-managed system. It should also be noted that initially activated carbon does not remove ammonia, nitrite or nitrate. The nitrogen filtering capacity will occur with aged carbon after nitrifying bacteria and possibly some denitrifying bacteria colonize the media. It will then provide some additional biological filtration. However the system should not depend upon this additional biological filtration as it will be temporarily lost each time the media is replaced.

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SELECTION

There are many choices when it comes to selecting which carbon to use. Sometimes aquarists are faced with products that look like carbon, are carbon, but have never been activated. Products like anthracite coal are sometimes sold as carbon (which they are). They are many times located on a store shelf next to activated carbon products. Their lower price may be attractive. Yet, because they have not been activated they are unsuitable for use in the aquarium. Always be sure the product label says the carbon has been "activated." Most aquarists and many distributors are of the opinion that bituminous coal is the better material for making activated carbon because it has proven to be good at removing "gelbstoff." Gelbstoff is a term used to describe large complex organic compounds responsible for yellowing water. However some lignite based carbons are demonstrating a very good ability to out-perform bituminous products.

Nevertheless there is not yet enough firm data to prove this so bituminous coal remains my choice as to the best activated carbon material. Most activated carbons are sold as Granular Activated Carbon (GAC). This generally refers to carbon in a granular form, which is a particle larger than 0.1 mm. How much larger depends upon what the distributor or reseller wants to market. See "General Selection Attributes" for a recommendation as to ideal size. When it comes to selecting an activated carbon it's something like buying a used car. Unless you're an ace car mechanic, you really don't know what you're getting. In fact, if I have any complaints with purchasing activated carbon it's that most companies fail to define the product's technical attributes on its label. When this occurs the aquarist is left with only its general or visible attributes to make a judgment. What are the technical and general attributes of a good activated carbon product? Good questions!

TECHNICAL ATTRIBUTES

When it comes to technical data the total surface area (TSA), pore volume (PV) and ratio of TSA to PV are the most meaningful. TSA is expressed in square meters per cubic centimeters (m2/cc), and PV in milliliters per cubic centimeters (ml/cc). A study by the president of a large aquarium product company indicates that the better carbons have a TSA of 450 to 550 m2/cc and a PV of 0.45 to 0.60 ml/cc with a TSA/PV ratio of 700 to 1000. When it comes to TSA, one tablespoon of activated carbon may have enough surface area to equal that of a basketball court! If the TSA is given on the basis of weight, a quality carbon should have a TSA of at least 1000 m2/gram.

GENERAL ATTRIBUTES

Non-technical attributes are easier for most aquarists to use. These would be aspects like appearance, size, shape, does it float and make a hissing sound while becoming wet, and is the package labeled phosphate free. As for appearance, a dull black indicates a fairly porous particle and should be preferred over a shiny black particle, which is less porous. The particle's surface should be slightly irregular or rather rough. The particle itself should be more round than flat-sided, which would block water flow and reduce adsorption sites.

Size is very important with the most efficient size being the approximate size of a pinhead or slightly larger. Some may think powdered carbon would be better than granular carbon, yet this is not so. A smaller particle size only increases its "outside" surface area and actually impedes internal water flow. Much larger than optimum size produces non-uniform flow and retards thorough penetration. Another good point to remember when selecting an activated carbon is to purchase one that takes up the most volume for a given weight. Keep in mind that TSA and PV, not the weight, is what should be purchased.

A good carbon product should also be quite buoyant and emit a hissing sound as water tries to find its way inside each carbon particle and fill its passageways and caverns. Any carbon that quickly sinks or emits little or no hissing sound can be considered less than ideal.

Always look for a statement on the package label that states the carbon product is phosphate free or at a minimum that tests have shown it not to leach phosphate. See the "Maintenance" section below for further comments on this important aspect.

Some companies refer to their carbons with terms like laboratory grade, pharmaceutical grade, research grade, or premium grade. This terminology has no value as to the actual efficiency of the product. Some distributors invent these terms to suggest or imply quality. Be cautious. 1f so labeled it may be a signal the product could be less than desired for aquarium applications. On the other hand, it may be the best that particular brand has to offer. Check it out!

Also, the package price of activated carbon does not relate to its effectiveness. Some less expensive brands have been found to outperform more expensive brands. The selection process can be confusing. It comes down to staying with either well-known brand names or word of mouth recommendations from fellow aquarists. In general, it's a buyer beware situation or actually applying what was learned in this booklet.

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MAINTENANCE

Activated carbon is usually confined inside a mesh bag and should be placed in a convenient place in the system where water will slowly flow through it. Not over it or too quickly through it. Depending upon the rate of water flow, high water flow shear forces may simply wash away contaminants before they can be adsorbed. Or, in the case of newly installed carbon, high flow rates may quickly strip the system's valuable elements causing stress to the aquarium inhabitants. Slower moving water allows more time, similar to dwell time in protein skimming, for the adsorbates to be in contact with the adsorbent, causing less initial shock to organisms and greater product efficiency.

Just laying a sack of carbon somewhere in the aquarium system, sometimes referred as the passive flow technique, with water flowing past it is in my opinion the least efficient way to use the product. Some elements will be physically attracted to the carbon particles. Yet thorough penetration of the carbon particle or the entire carbon bed may never happen. Not only can detritus block the carbon bed's outer surface areas in passive use situations, the water flows in the interior of the bed are stagnated and mechanical sieving is not achieved. Different strokes for different folks!

If the ideal place in the system would have water flow through the carbon bed too quickly, consider dividing the bed into smaller ones and placing them in different areas in the system where flow is slower. The second bed could adsorb what a single bed may miss. By having a couple of small carbon beds and changing them at different times, i.e., the first bed in thirty days, the second bed thirty days later and then back to the first bed in thirty days, it will result in less of an impact to the loss of bacteria that colonize them and less initial trace element loss in the system. There is probably better water penetration in smaller beds and therefore better utilization of its carbon particles.

Many carbons are coated with very fine pulverized carbon dust when first removed from their container. It should be briefly rinsed under tap water. The length of this rinsing should be short so the carbon carrying capacity is not reduced by any possible impurities in the tap water.

There are only about a half dozen manufacturers of activated carbon in the U.S. Most distributors simply repackage the product and sell it under their own label. If the aquarist is new to the hobby and decides to use activated carbon in his or her aquarium, the safest thing to do is stay with a product from a reputable company. These carbons are mostly labeled as aquarium grade granulated activated carbon. They are generally well suited for our purpose. However, that still does not guarantee the product to be perfect, as most companies don't state the technical data needed to judge their actual usefulness.

Even if the general selection tips mentioned above have been followed, some carbons contain small amounts of phosphate. I've personally tested some that had well over 1 mg/1. Most well known authors recommend keeping aquarium phosphate levels below .05 mg/1. Higher levels can quickly lead to unwanted algae problems. Look for brands that are advertised as phosphate-free. It is a good first step for algal control. However, I don't know of a brand that guarantees its product to be 100°/ phosphate-free. Some are very close to it with remaining amounts so low they present no problems.

To alleviate any phosphate problems I suggest a sample of the carbon first be soaked in purified water (RO, DI or distilled) for a few hours before its solution is tested for phosphate. If there is none, that could be the brand to stay with. However if the test solution does contain phosphate, and before there is any further use of that brand carbon, it should be soaked overnight in purified water. Even then I still advise caution, as this additional soaking may not remove enough of its phosphate to make it usable in the aquarium. The pre-soaking of new carbon will also minimize any pH impact (rises in pH) it may have caused if it were immediately placed in the aquarium. Yes, pH is affected somewhat by fresh carbon. To what extent depends upon the alkalinity of the system, although most systems have very little difficulty with it.

One of the most often asked questions is how much should be used. That's difficult to answer. It depends upon many variables such as bio-load, whether there is a protein skimmer or ozone in use, type of system filtration Qaubert/NNR plenum system, the Berlin method, trickle filter, algal scrubber, undergravel filter, fluid bed), and what type of material was used to make the carbon. The list of variables could probably go on and on as almost everyone has a different idea as to what impacts its usage. Because of all the possible variables, we probably will never be able to come up with anything more then a fairly well educated guess.

As for some of those "educated" guesses, I've seen recommendations from small amounts to those bordering on the ridiculous. I won't go into all the details of who and how much was recommended because there were so many But I will say that one of the least amounts suggested was about 4 grams per gallon. I even considered that to be. slightly more than needed. Since we're not talking rocket science here, I recommend one heaping tablespoon of activated carbon, which is about 8 grams, per 5 gallons for fish-only aquariums. For reef aquariums I recommend the same amount of carbon per 10 gallons. This is not based on any scientific tests that I have accomplished, but what has worked well for me during the past fifty years I have kept both simple and complex systems in the freshwater and marine hobby.

I should note I have always believed in low bio-load systems and my carbon usage could be considered quite minimum. The aquarist reading this will just have to experiment to see what's best for his or her aquarium. But start with a little bit rather than more and play it safe. I've actually seen stony corals beginning to bleach-out within a few hours of installing a new batch of activated carbon in one of my systems! A question that seems to have aquarists divided is whether activated carbon should be used on a continuous basis. My position is it's better to constantly use carbon than to only place it in the system on what may be thought of as an "as needed basis"

Using a couple of small carbon beds and rotating their replacement schedule prevents any major removal of elements and also helps to protect against any sudden release of detrimental chemicals from organisms in the system. A comforting feeling!

Another question frequently asked is what is the length of time carbon will remain useful? To answer that we first need to discuss some conditions that affect its life span. They would include the presence of biological slime coatings on the carbon particles, the speed of the water flow through the media, size of the carbon bed, system bio-load, quality of the carbon particle, the use of medications, pre- filtering the water before it passes through the carbon bed, system location (not near a lighted area where algae will form), maintaining cleanliness of the carbon bed floss pre filter, and the frequency of water changes.

Therefore it's difficult at best to judge the length of time a carbon bed will remain useful. However, it is possible to use a simple test to somewhat judge life span by using the "White Paper" test. Simply hold a sheet of waterproof white paper in the aquarium water where the light is very good and as far in the aquarium from your eye as possible. Compare it to another piece of similar paper being held outside the aquarium. If the "white" on the internally held paper now appears slightly yellow compared to the one outside the aquarium, it's time to change the carbon bed.

Keep in mind the holding capacity of carbon can become exhausted. Its physical adsorption bonds are weak, such as those formed by the Van der Waals forces. When carbon reaches exhaustion, desorption or leaching of the adsorbates can return them back into the system's bulk water. This is important. This means carbon can unload a portion of what has collected back into solution. Therefore, when carbon is being used in the aquarium I recommend not going too long without replacing it. A safe schedule for most aquariums would be changing it every four to eight weeks. It's better to replace it with fresh carbon on a regular basis, than have it become the root of a problem.

Finally, it's not practical for the aquarist to regenerate carbon as it would require being heated to over 900°E Something the spouse may not like the oven used for. I know because I tried it! Under ideal conditions, activated carbon can remove up to 50°/ of its own weight in dissolved organic compounds. It's simply better to discard it and replace with new carbon than messing with it. That leads to the sixty-four thousand dollar question - who should use activated carbon?

My answer is simple - almost everyone. I say "almost" because no matter how many good points the product has, there will be those that are always looking at a glass that is half full, yet refer to it as half empty! Even though the product has what I like to refer to as some minor controllable deficiencies, with good management of the system's various chemical parameters, GAC remains the most economical, practical and proven chemical filtration media for use in our aquariums.