"Chemical" filtration.

So: mechanical filtration filters particulates. Now, in chemical filtration, substances that are dissolved in the water, whether as molecules or as dissociated ions, are eliminated in a couple of possible ways. They can be adsorbed to the surfaces of filter media, such as activated carbon or zeolites. Or they can be chemically bound to an ion-exchanging resin. In almost every case, these are solutes that are being removed from circulation, not particulates.

Flocculation is the exception, where "chemical" filtration operates on particulates.

Certain water "clarifiers" work by coagulating fine particles and colloidal matter in flocs that are massive enough to settle out, once mutually repelling charges have been neutralized. It's a two-step affair. Coagulation is a chemical process. Then gentle circulation encourages particle collisions and the accumulation of a floc dense enough to precipitate out. Floc is a spongy adsorbent substance, which would attract bacteria and other members of the biofilm community, given time. No need to vacuum it all out.

Alkalinity aids in floc formation, so in municipal wastewater management some extremely caustic alkalis are used--— quicklime or caustic soda ash--— that are unsuitable for aquaria. But some aquarium flocculents use alum (aluminum sulfate), which acts as a coagulant for small negatively-charged particles whose mutual repellance has been helping keep them in circulation. Mardel's "Brite n' Clear," for one example, is a simple solution of aluminum sulfate. A variety of synthetic polymers are also widely used. Flocculants aren't wholeheartedly recommended in aquaria that have fish in them. The mucus membranes of fishes' gills also bear negative charges, so they can attract the flocculant, if it is overdosed. Not good.

One manufacturer directly addresses this problem: KentMarine's website description of their ProClear Freshwater Conditioner has some useful technical information about using their patented long-chain polymeric flocculants:

"Whereas competing products may clog the fishes gills, suffocating them, Kent Pro Clear uses a blend of high tech polymers that will safely flocculate any algae, dirt, bacteria or other cloudy water causing particles. "

"Note: Do not use this product at night or on very very soft water (i.e. below 40 ppm total alkalinity) without first adjusting alkalinity as directed above! Operate aquarium lights a minimum of 4 hours after use and provide good circulation and aeration during use!"

Adsorption. The more common kind of chemical filtration acts on dissolved substances and involves passing water through granular activated charcoal or Polyfilter pads in the filtration system.

This process of adsorption shouldn't be confused with absorption. In absorption, a liquid, along with any dissolved molecules it contains, is held within the minute spaces of a porous solid by actions of surface tension and other physical forces. In adsorption, the ions and molecules dissolved in the water are bonded chemically or by physical forces to the medium's solid surfaces. Absorbed material can be flushed out, as when you squeeze a sponge; adsorbed material is "bound" to surfaces and can't be flushed out. It must be displaced, commonly by an ion that has a stronger charge. De-sorption could also occur if the pH were altered to an extreme of acidity or alkalinity, conditions not found in the aquarium but very useful in certain industrial processes.

What gets adsorbed? Adsorptive media bind to their vast surfaces a long list of organic pollutants, like phenols (responsible for some "fishy" odors and discoloration), thiols (sulfurous and stinky), and humic polyphenols like tannins (sometimes desirable, but also responsible for yellowish to brown discoloration of the water). Such large complicated organic molecules are the resistant products that remain when bacteria partially break down organic substances. Other pollutants taken up by adsorptive media are dyes and perfumes, medications, benzene and other volatile chemicals, etc.

What about metals? There's unresolved and sometimes vehement controversy about whether heavy metals like chromium and mercury, but also copper, manganese and iron, are adsorbed to chemical filtration media, and to what extent. Part of the confusion may come from not separately considering chelated metals and free metal ions. Activated carbon and other adsorptive media strongly attract both natural and artificial chelating molecules, together with the metal ions associated with them.

Dissolved chemical molecules get bonded to the filter medium with an electro-chemical bond. Normal rinsing doesn't break this bond. As the media surfaces get increasingly covered with a molecular monolayer of adsorbed substances, chemical equilibrium is eventually reached, where molecules are simultaneously adsorbing to the surfaces and desorbing from them, and the net adsorption decreases towards zero. The medium is spent.

Biofouling. Long before all the possible chemical-bonding sites can be occupied, the filtering medium has ordinarily become so clogged with biofilm that its chemical usefulness is impaired.

As soon as the medium you have selected for chemical filtration is set into the filter system, particulate matter may begin to be trapped in it, so you always want to set your chemical filtration medium downstream from your mechanical filter. Nevertheless, a bacterial film rapidly starts developing; within a very short time, the biological and mechanical effects of carbon in your filter have overtaken its initial chemical filtering.

Most chemical filter media do have a surprisingly brief effective life. The carbon in your filter does almost all its adsorbing work in the first 48 hours! I was amazed when I first saw the graphs printed with Tim Hovanec's Aquarium Fish articles about activated carbon. The administration and dosage foldout that is currently enclosed with Maracide states "If an activated carbon filter is over five days old, it may be left in place." Whether you do leave your filter carbon in place when medicating or not, this goes to show how brief is the effective life of activated carbon.

I think that you should generally consider chemical filtration media as disposable. Activated carbon could only be reactivated at high heat in a closed kiln that was rendered free of oxygen (a "reducing" atmosphere). Not in your kitchen stove, as some aquarists were attempting, with disappointing results that made them skeptical of carbon's usefulness.

Re-release of adsorbents. An intuitive question is: Can adsorbed substances be re-released into the aquarium water somehow? The brief answer is: No. Still, some manufacturers of filtration media offer dark hints that cast doubts on rival products. In general, to release substances from an ion-exchange medium, you'd have to flush it with salty brine, or with extremely acid (below pH 3.0) or alkaline baths. "Recharging" situations like these would never be found in normal aquarium conditions. You'll hear timid hedges like "possibly under certain circumstances" from many knowledgeable people. But the real plain answer is simply "No."

An exception. Though substances adsorbed to granular activated charcoal by electrochemical forces are not going to be flushed back into the aquarium water, substances with a stronger ionic charge may "bump" ions with a weaker charge from sites on a pretty thoroughly saturated carbon filter.

It's good policy to keep the three aspects of filtration separate in the filter, and separate in your understanding. In other words, though any filter medium tends to become part of the biological filter, your biological filtration should be sufficient on its own; you shouldn't even be considering the possible biological filtering capacity that a couple of tablespoons of exhausted charcoal in your filter might be offering.

Chemical filtration and planted aquaria. During the 1990s, activated carbon became discredited by some planted-tank aquarists who were dosing their water daily and sensed that the carbon was adsorbing chelated metals meant for plant fertilizer. Carbon adsorbs the chelating end of chelated iron and copper. Iron and manganese were adsorbed in experiments by Shawn Keslar that he presented to the Aquatic-Plants Digest. It was the chelating molecule that was grabbed, not the attached metal ion: carbon does also very slightly adsorb non-chelated metal ions, but the effect is too trivial to concern us. The question revolves around whether you want to consider chelated copper and iron as toxic heavy metals (they are) and eliminate them, or whether you want to consider chelated copper and iron as essential trace elements (they are) required by your plants.

In a planted aquarium, the need for any chemical filtration as a permanent feature of your filtration regime is currently controversial. Many keepers of planted aquaria are now eliminating all carbon filtration. Some fishkeepers are blindly eliminating carbon and other chemical filter media, even in unplanted tanks.

You'll find parts of the discussion archived at www.thekrib.com, something to be taken into account before you make up your mind about carbon.

Links. Robert T. Ricketts' article on chemical filtration, part of his "Filter Basics" series in Tom Griffin's e-zine AquaSource, Dec. 2000, is archived at www.tomgriffin.com.