Chemical filtration we'll define as the non-living removal of substances from seawater solution on a molecular level by adsorption (carbon, i.e. charcoal, activated charcoal) and ion exchange (mainly resins). These two types of media, used apart or together are valuable adjuncts to biological, mechanical and physical filtration methods we've discussed. While not absolutely necessary, they are often employed to remove color, odor, nutrients, and suspended solids; enhancing water quality, improving livestock health and reducing maintenance.

Factors Affecting Usefulness: are several, and interactive. Follow along with me here. Obviously the smaller the grain size of the media, the more surface area, ergo reaction sites. Ditto for the purity and volume of the chemical filtrants. Other considerations include pH, temperature and contact time, i.e. placement and flow in your scheme. Know that specific filtrants are engineered to operate in or out of water, at a peak pH, and more actively at elevated temperatures, with longer contact time.

Carbons: There are many types of semi-confusing labels applied to these materials. Strictly speaking, carbon is the "root" element of them all. Beware of "reagent, "technical" etc. grade claims.

Carbon (without the 's') will refer to as activated charcoal has a greater absorption rate and capacity than ordinary (shiny, irregularly-shaped) charcoal (from animal or wood sources). You only want activated carbon, synonymously activated charcoal; the other stuff is a placebo, not even suitable for freshwater use.

Useful Lifetimes: When carbon is fresh/new it's at it's zenith in adsorptive capacity. With use this drops off logarithmically. As far as chemically filtering your water, the best carbons effective-lives run about two to three months. But as they say on late night television pitches, "But wait, there's more". Carbons also act to some degree as biological filter beds and even mechanical filtration media; therefore, let it be resolved that you will rotate two (or more) sets of carbon units, along with the mechanical media as per the previous Section.

A proviso to the above useful life-span is that enough carbon is employed to do the job at hand. One paragon of excellence in this area is the German canister filter manufacturer Eheim (tm). They're filters come in two models, provided with media, including an applicable volume of high quality carbon.

Re-charging: Everyone dreams of the big money they could save by somehow reversing the adsorption processes of their carbons. Allow me to save you high energy bills, a stinky kitchen, laughter and scorn from your significant other(s); such practices are the stuff of myth. Not near enough noxious material can be driven off by household efforts. The manufacture of useful aquarium grade (activated) carbon is done under high pressure (or vacuum), extreme heat in the presence of formulated chemicals. Mix the old, used carbon in your mulch pile and buy some new.

Ion-Exchange Media:

Are electro-chemically charged resin beads that remove charged particles from solution by exchanging them for others. For most systems these materials are too expensive and limiting in their capacity. I am familiar with some of the research going on in the fields developing ion-exchange technology; the near-future looks bright for development of these media for our use.

Times & Places: to use chemical filtrants abound. Many, if not most reef-keepers employ chemical filtrants full-time. Aquarium service companies keep these media fresh to insure crystal clear water and optimum fish and invertebrate health.

Other folks are more random in chemical filtering use. They'll remember to change their carbon when the water starts to "yellow" or the colors are fading in their most sensitive specimen.

For function and ease's sake, it's best to place these media in Dacron polyester bags and arrange their placement in a pressurized water flow. My favorite is a hang-on power or canister filter. The worst is to just throw the bag and contents into a sump or bury it in the gravel; there is not much hope of adsorption where water is not forced through the medium.

A note here concerning establishment of nutrient cycling and chemical media use. Most sources say to wait off until full nitrification is going, others state that some carbon in use at all times will only slightly slow down the process of bacterial growth. I would wait until cycling is in full swing to use them, or better still, utilize an old unit from an established system to get the new, sterile one started.

Dangers Associated With Chemical Filtrants:

pH shift, in particular to much lower range, may result in release of adsorbed matter; with possibly disastrous result. Another reason for sufficient buffering, caution, regular carbon and water changes.

Carbon Shock: There is some danger, especially in full-blown reef and intensive invertebrate systems of removing too much dissolved organic carbon too quickly. Some of the adsorptive matter is biologically important (pheromonal...). For these systems that have had no steady history carbon filtration Wilkens and Birkholz (1986) suggested working in an additional 1 1/2 ounce per twenty six gallons per month. Other authors estimates are less restricted than this value. The best measure of how much is right is best gauged by close observation of your livestock.

Loss of Trace and other Essential Materials: Once again, not a major concern with adequately fed and maintained systems. If/when in doubt, effect a water change and/or become an additive user.

Other Chemical Filtrants:

At least one other chemical filtrant deserves mention and use; PolyBio Marine Inc.'s Polyfilter (tm) a specially treated pad incorporating exchange polymers for adsorbing polar organics and nitrogenous compounds. This hydrophilic polymeric material really works. I have used them personally and in business for removing medications, limiting algal growth by adsorbing phosphate, quarantine and hospital tanks...

Depending on what criteria you use to define the varying forms of filtration commonly in use, protein skimming, ozonation and ultraviolet sterilization could well be grouped as chemical versus physical means. Their methods are different, though goals and results are the same; reduction in concentration of organics through removal from the system's water.

Chemical filtration provides a first and last final polish to captive seawater, removing unwanted metabolites (and possibly treatment chemicals).

Filtration Summation: 

As we have seen, one good working definition of filtration is the sum total of all that we do to lessen the amounts of dissolved organic carbon imparted to the water by our livestock and feeding. The list of those materials is quite long. By a combination of biological, mechanical, possibly physical and chemical filtration, in concert with good maintenance practices, we are able to promote good water quality, preserving disease resistance and overall health. 

Bibliography/Further Reading:

Delbeek, Charles J. 1990. Reef aquariums: chemical filtration; this aspect of filtration is very important in a reef tank, but is often misunderstood. AFM 4/90.

Dewey, Don. 1978. FAMA Product Test: Poly-Bio-Marine, Inc. Poly-Filter. FAMA 1/78.

Rice, Rip & Michael Robson. 1982. Biological activated carbon. Ann Arbor Science, 1982.

Wilkens, P. & J. Birkholz. 1986. Niedere Tiere-Roehren, Leder und Hornkorallen. Engelbert Pfriem Verlag, Wuppertal.