Ion-exchange is a process where an ionized solute present in large numbers or with a strong charge takes the place of another ion that is attached to a matrix.

Adsorption is a process where polar solutes become attached to polar surfaces and non-polar solutes are pushed and held against non-polar surfaces by surrounding polar substances (water). Usually, polar and charged substances are hydrophilic (attracted to water) and non-polar and uncharged substances are hydrophobic (repelled by water). Since water is the solvent in all aquarium filtration, hydrophobic solutes are removed more easily by adsorption. As solutes become progressively hydrophilic, removal becomes more and more dependent on molecular sieving and opposite charge effects, including ion-exchange. Of the three processes, the most important for aquarium filtration is adsorption, followed by ion-exchange. Molecular sieve action is usually an integral aspect of both adsorption and ion-exchange, and is a limiting factor for both types of filtrants. Because the action of most chemical filters tends to be mixed rather than exclusively one process, it is best to look at specific filtrants rather than process types.

Carbon
Activated carbon is prepared by carbonizing coal, wood, bone, nut shells, or other organic material at 900°C, then activating with steam, air, or carbon dioxide at 800°-900°C. This treatment drives out hydrocarbons, increases surface area, and develops porosity. Differences in adsorption characteristics are due to these treatments and the addition of inorganic salts such as zinc, copper, phosphate, sulfate, and silicate before activation. Caustic and acid washes are also frequently used to both change adsorption characteristics and to remove soluble materials.
In the aquarium, activated carbon removes relatively non-polar or hydrophobic organic solutes from the polar solvent water. Charged solutes such as ionized salts are repelled by carbon and not adsorbed. Water is strongly polar and is poorly retained by carbon. The more non-polar or hydrophobic, or the less soluble in water, the solute is, the more strongly is it retained on carbon. Many metabolites, such as amino acids, are retained only at a certain pH, called the isoelectric point, where the solute has no charge. If the pH changes, the substance acquires a charge and is released by the carbon. Proteins and peptides tend to be strongly retained because they have many non-polar side chains. If the carbon is rich in zinc or copper it will retain ammonia and other amines through complex formation. Although ionizing salts are repelled by carbon, some heavy metals such as copper, mercury, and zinc are retained by carbon under alkaline conditions. Some carbons are rich in insoluble phosphates, carbonates, silicates, or oxides, and these carbons have a relatively high capacity for polar and positively charged groups. Carbons that have not been acid-washed have more of these polar groups than acid-washed carbons, but also contain more soluble contaminants, such as metals and carbonates, which can cause toxicity and pH problems, particularly in fresh-water aquaria.

Aside from the chemical nature of the carbon surface (non-polar), the major factor in carbon filtration is actually molecular sieving. Carbons can be looked upon as sponges or mazes with large openings that lead successively to smaller and smaller channels with smaller and smaller openings. The capacity and, to some extent, the adsorptive characteristics of a given carbon depend on its surface area and pore volume. Surface area refers to the internal surface of the carbon particles. The more channels inside the carbon, the greater the surface area. Pore volume refers to the amount of emptiness inside the carbon. The greater the surface area, the greater is the capacity; and the greater the pore volume, the greater is the efficiency. There is a working limit of about 0.7 ml/cc for pore volume,. since increasing pore volume also increases the fragility of the carbon. Increasing the surface area without increasing the pore volume results in diminished mean pore size (fewer large channels and more small channels), which, in turn, limits entrance to the carbon to progressively smaller solutes. The ratio of surface area to pore volume, then, is a valuable guide to the mean pore size: the greater the ratio, the smaller the pore size.

Several ways of grading carbon include surface area, iodine number, molasses index, and carbon tetrachloride activity, but none of these are, in themselves, meaningful for evaluating adsorbents for the specific use of aquarium water purification. The best measure of an adsorbent is the ratio of total surface area (TSA) to pore volume (PV). To facilitate interpretation, total surface area should be expressed in square meters per cubic centimeters (m2/cc) and pore volume in milliliters per cubic centimeters (mi/cc). If these are reported on the basis of weight (grams) instead of volume (cc), then the density in grams per cubic centimeters (g/cc) must also be reported. Unfortunately, with few exceptions, sources of aquarium carbon do not provide these valuable figures. While there are only five manufacturers of carbon in this country and all commercial aquarium carbons come from one of these, all these sources supply numerous grades of carbon from very economical water treatment grades to expensive pharmaceutical grades. Not all aquarium carbon vendors provide the best carbon for the application: many provide the carbon with the best profit margin.

My study of carbons for the purification of aquarium water indicates that the better carbons have a TSA of 450 to 550 m²/cc and a PY of 0.45 to 0.60 mI(cc with a TSA/PV ratio of 700 to 1000. Carbons that have not been acid-washed have better buffering ability for marine aquaria and greater retention of polar and charged solutes. Acid-washed carbons, however, are safer for fresh water and, with poor grades of carbon, the acid-washed versions are safer for marine aquaria as well. Given a choice, the acid-washed version of a particular carbon is usually preferable. Generally, the better carbons are prepared from bituminous coal. Acceptable carbons can be prepared from nut shells, wood and bone. Paper mills waste and other organic waste carbons are not acceptable. Some carbons are not truly activated carbons, but mere charcoal or, worse, just ground-up coal. If the TSA of a carbon is reported in units other than those used here, it is possible to convert the units for comparison by recognizing that

1 m2 = 1.2 yd² = 10.8 ft²;
1 cc = 0.06 in³ = 0.0338' fluid oz; and
1 g = 0.035 oz.

If the TSA is given on the basis of weight and the dry density is not given, then directly comparable figures are not possible, but, generally, a good carbon should have a TSA of at least 1000 m²/gram and the better carbons have a TSA of about 1500 mm²/gram. The PY should be at least 0.4 mI/cc.

Since an important consideration with carbon adsorption is surface area, it might be surmised that powdered carbon is better than granular carbon. This, however, is not the case, since the surface area that is important is the internal surface area, not the external. Diminished particle size only increases external surface area, and, by comparison to the total surface area, the gain in surface area from smaller particles is relatively slight. The choice of particle size, then, is not governed by surface area considerations. The particle size of choice should permit unimpeded and uniform flow through the carbon and allow rapid penetration of solute into the inner network of the carbon particle. The optimum size that satisfies these requirements is about the size of a pinhead, 0.5-1.5 mm or 1/32-1/16 in. (10-40 mesh). Smaller sizes impede flow while larger sizes produce non-uniform flow and retard penetration of solute into the carbon matrix. With a carbon about the size of a pinhead, nearly 90% of the available surface area will be utilized before exhaustion; but with a carbon about the size of a small pea (5-8 mm), only about 40% of the available surface area will be utilized before exhaustion, due to the inability of solutes to penetrate the carbon particle.

If specifications of a carbon are not available, or if you do not want to bother with all those numbers and calculations, what should you look for in a carbon? First, is it the right size? That should be pinhead size. Avoid the more common, convenient, and prevalent larger sizes. What is the appearance of a rinsed, but dry, particle? If it is dull, flat black, this indicates a fairly porous particle. If it is relatively shiny or glossy black, the carbon is relatively non-porous and should he avoided.