Why is activated carbon used in aquaria?
We are all aware that the aquarium livestock generates toxic ammonia, which is converted in steps to relatively harmless nitrate by bacteria living in the filter bed. There are, however, a number of other contaminants generated in small amounts by the life process which can, over a period of time build up to a high enough to affect the health of the inhabitants of the aquarium.
A slight yellow tinge to the water can indicate the presence of at least one such contaminant. The most important function of activated carbon is to remove these impurities before they reach levels high enough to cause distress or organ damage.
Activated carbon performs other important tasks in the aquarium as well, removing toxic dissolved gasses such as hydrogen sulphide and methane, which result from imperfect water circulation. It acts as a catalyst to quickly convert dissolved chlorine gas to relatively harmless hydrogen chloride. It provides crystal clear water by removing minute suspended solids and colloids - those particles too small to be trapped by filter floss. It helps defend aquarium life on some occasions of accidental introduction of toxic substances such as pesticides,paint fumes,cigarette fumes and the like.
What is Activated Carbon?
Activated carbon, also called activated charcoal, is more involved in your life than you might suspect. It is used in the preparation of many products we use every day; to remove those impurities that cause an objectionable colour, taste, odour or health hazard from drinking water, waste water, foods and beverages (i.e. making sugar white instead of brown); to control air pollution (gas masks); and to separate/purify products in the chemical and pharmaceutical industries. The term "activated carbon" covers a family of materials made from carbon which have the ability to attract and hold certain substances on the carbon surface. This ability is called "adsorption". The adsorption process is an example of what is termed "chemical filtration", where certain dissolved substances are removed from the water. Filter floss, on the other hand performs "mechanical filtration" by trapping solid particles suspended in the water.
The raw materials most commonly used to create the carbon are coal, peat, lignite, wood and nutshells. None of these raw materials is pure carbon, although anthracite coal comes close. They contain unwanted hydrogen and oxygen as well. Those components are driven off by heating the raw material in the absence of air to produce a "char" (wood produces charcoal). The char is then activated by exposure to an extremely hot gas, usually steam at about 1500 deg. F, which etches tiny passageways through each carbon particle. The raw material selected, plus the manner in which it is activated, determines the characteristics of the finished activated carbon product and its cost. There are dozens of different kinds of activated carbon produced. Of these, only a handful will be well suited to any particular application.
Conversely, no one carbon will do well in all applications. For this reason, it is impossible to grade carbons, and terms such as "laboratory grade", ”research grade", "premium grade" and the like do not exist in the activated carbon industry. Such terms are invented by the manufacturers in the aquarium trade to suggest, or imply quality, because the real clue to a carbon's suitability for aquarium use - the specification sheet - would be unintelligible to most aquarists. In discussing the following questions though we will cover many of the items that are found on a spec.sheet.
How does activated carbon work?
A particle of activated carbon is mostly air. It has thousands of tiny holes and crevices through which water can circulate. When water carries an organic molecule – a contaminant – into these narrow passages a short range attractive force between the molecule and the carbon will become effective and the molecule sticks. This particular method is called physical adsorption. The other kinds of adsorption also involved with activated carbon are of only minor importance to the aquarium.
What type of activated carbon is best for aquaria?
The effectiveness of an activated carbon for aquarium water purification depends on several factors which include total available surface area, pore size distribution, pore volume and particle size:
Surface area. Carbons having more surface area for impurities to stick to will obviously hold more impurities. Interestingly enough, virtually all the useful surface area in activated carbon is along the interior pores and not on what we think of as the outer surface has a negligible surface area because it is not porous, and has no measurable adsorptive capacity.
(Although it can provide mechanical filtration, ordinary coal does not remove dissolved contaminants.)
Wood charcoal is primarily a mechanical filtration medium with a slight adsorptive capability. Higher on the surface area scale are the bone chars made from animal bones, and other surface activated carbons. These, as the name implies, have an active outer surface covering an inactive carbon or phosphate core. Highest in surface area are the totally activated carbons. These are porous throughout the carbon particle and a total area can range upward to approx. 1,000 sq. m per gram (300,000 sq. ft per ounce). Surface area is a spec sheet item and is always expressed in square metres per gram. For aquarium use, more surface area is not necessarily better, because extremely high surface area implies a preponderance of extremely small holes or pores, too small for some of the larger molecule contaminants that we want to remove. Look for surface area figures in the 500-900 sq. m/gm range.
Pore size distribution. Pore sizes in activated carbon are classified as micropores, transitional pores and macropores (i.e. small, medium and large pores). Small size pores are limited to the adsorption of small molecules, medium pores concentrate the larger organic molecules typical of many aquarium contaminants. Larger pores provide easy water access to the interior of the carbon grain, resist clogging, and also perform a mechanical filtration function by trapping minute suspended solids to improve water clarity.
Most activated carbons have too much small pore and not enough medium pore capacity to be really effective for aquarium use. What happens is that the carbon becomes saturated with large molecule contaminants in a relatively short time while still retaining lots of unused space for small molecules. A good clue to favourable pore size distribution is a spec sheet figure called pore volume.
Pore volume. The combination of a relatively high percentage of medium and large pores plus a high surface area results in a carbon which has a high pore volume - lots of empty space inside the particle. A good pore volume figure would be 1 to 1.5 cc (or millilitres) per gram. High pore volume carbons appear to weigh less - to have a lower apparent density - than other carbons made from the same raw materials.
Particle size. The smaller the carbon particle, the shorter the water travel distance to the interior surfaces. There are several advantages to be obtained from using smaller particles. The activated carbon works faster, remaining effective in the modern outside filters, where the water flow rate is much faster than the rate considered ideal for carbon filtration. Useful lifetime is improved with small particles since they are Less likely to clog on the outside before all available adsorption sites are occupied on the interior surfaces. Particle size is indicated on a specification sheet by a notation such as "4 x 12 screen size", meaning that the product contains all those sizes which will fall through a screen having four wires per inch and not fall through a screen with 12 wires per inch, giving a size range of approx. 1/4 to 1/12 inch. (The gravel industry uses the same system, as in "6-20 silica".)
Economy. Totally activated carbon is the kind recommended for main aquaria, and it is (naturally) the most expensive form of aquarium carbon. On the other hand, it is by far the most economical kind to use when measured on a basis of cost per month of water purification service. A little goes a long way. It has 5-10 times more capacity than bone char and 50-60 times more than charcoal, yet it costs about 2-4 times more