Activated carbon is a filter material, that has already been used in aquariology for a long time. As much as this material is well known, its correct use and the effects of using it to treat aquarium water are very unclear. The difference in quality between various types is so crucial that every aquarist should test his carbon before using it. It is known that fragments of coral inserted in a calcium reactor can release phosphates. Surely it is less known that also some types of carbon can increase the phosphates in aquarium water.
How does activated carbon work?
Because of molecular forces, substances (gas, liquid, and solid particles) have the characteristic of attracting each other. These attraction forces, called Adhesion, execute their influence on the so-called limit surfaces, such as the external parts of bodies found in the water (carbon filtering), or on the external walls of air bubbles in the water (foam fractionation)(Naser 1986, Kloppel & Kloppel 1980). Naturally these adhesion forces also have an effect on the surface of the aquarium glass or on the ornamental items inside it. The available surface is, however, very limited, and it cannot be replaced when it is saturated. The absorbing ability of carbon, meaning how much matter can attach itself onto its surface, and its speed of absorption depend on several factors:
The surface. The more space is available, the more substances can bond. The surface of activated carbon varies from 400 to over 2000 m2/g. It should also be said, to set a limit, that the smallest pores, called “Sub-micropores”, which have a radius of 0,4 nm ( (1nm = 1 Nanometre = 1 thousandth of a Micrometer = 1 millionth of a millimetre), are not available to bigger molecules. In the field of aquariology, however, activated carbon should be used in the absorption of larger molecules, such as yellowing substances and the residues of medicinal ones. The chemical characteristics of the surface, just as the type and quantity of the oxides on it (groups of Quinone, Carbonyl, Carboxyl, Phenols etc..), also play a crucial role in the absorption process.
The lower the temperature, the more substances can be stored on the surface. However, the higher the temperature, the faster it can be stored. In aquariology, these rules are not particularly important because the temperature usually stays fairly constant. The aquarist should, however, try to avoid introducing into the aquarium carbon that has been previously used in cold water (for example to filter tap water). In this way, bringing the carbon to higher temperatures, the substances that have bonded to the carbon in cold water can then be released in warmer water. How fast some substances can be absorbed and at which temperature depends mainly on thermal valence factors. (Isothermal absorption) (RAMSCH, B. 1992).
The shape of activated carbon does not have an important role in its performance. The regular-shaped one only brings a small benefit that the fragmented (pieces of irregular shape) one does not provide: it allows the water to flow better through the bag that contains it. Carbon in powder reacts a lot faster. However, its use in aquariology is more complicated (Diatomaceous earth filter) and the time factor is not normally critical.
The concentration of the substance to be eliminated has a role that is just as important in the rate of absorption. The lower the concentration, the more slowly the substance can be absorbed. Especially in the micro-elements range, long contact periods are necessary.
How is activated carbon made?
Activated carbon is made from the most diverse materials such as peat, wood, soot, bones, blood, bituminous coal, coal, coconut shells, fruit seeds and sugar. As for the quality of the product, the determining factor is the method of activation. Finished activated carbon is composed of 83-93% carbon, and the rest consists in 1-3% of hydrogen and 0,2-10% of oxygen. Nitrogen and sulphur are present only in traces. (Lurgi 1989). One of the characteristics of chemical activation is that the raw material is dry and mixed with chemicals such as: phosphoric acid, sulphuric acid, potassium hydroxide, and sodium carbonate. Through aeration, from about 400° to 600°C, the chemicals extract oxygen and hydrogen atoms from the raw carbon. After washing and then drying, the finished product is ready. The other method to obtain activation consists in steam treatment. At temperatures around 800 to 1000°C, a nitrogen based atmosphere is put in contact with the raw material (C+H2O-CO+H2). Perfect control of the reaction’s temperature and time, just as the concentration of oxygen, carbon dioxide, water, carbon monoxide and hydrogen, influences the elimination of undesired atoms and avoids the combustion of the carbon during this process, determining the final quality and structure. The activation of this material, for the reasons above, is therefore not within the aquarist’s reach. Unfortunately the expression “activation” is used in the wrong way by some sellers, who define it as the de-gasification of activated carbon before its introduction in the aquarium. In several aquariology books the regeneration of used carbon through treatment in the oven is described. These statements make no sense. Usually specialized companies are able to regenerate carbon using the techniques we mentioned. Every regeneration, however, entails an alteration of the pore structure, affecting the quality in a negative way.