Chelation. The humic substances discussed in the previous pages act as chelators. An organic or synthetic molecule that "chelates" ionic metals or other inorganic ions that are free in solution, grabs onto the ions as if with a lobster claw— Greek "chela"— and binds them. That way they are taken out of circulation,  hidden away, thus "sequestered"— another term you might hear. When metal ions are bound to another inorganic molecule instead, they are more usually said to be "complexed."
Chelated metal ions are rendered more stable, less prone to precipitate out of solution, and they are also  rendered available to the biological processes of plants and animals, more "bioavailable". The heavy metals like cadmium, mercury and lead, that animal metabolisms don't actively draw upon, remain in their chelated forms and pass right through animal systems and are eliminated. Even some inadvertently ingested ions are likely to be chelated quickly to an organic molecule in passing through an animal's system.
There's a chelating molecule EDTA (see below) included in many water "conditioners" which is intended to bind toxic free copper especially, but lead, cadmium, iron, mercury etc. are bound as well. Besides EDTA, which can be synthesized, there are many other man-made chelators.
How do humic substances work as natural chelators? The complicated flexible molecular structures of humic acids create spongy voids of various dimensions that apparently trap and adsorb other organic compounds, such as carbohydrates, proteins, lipids. The many negatively-charged sites of humic acids, and some of the comparatively simple amino acids, give them a striking ability to interact with positively-charged inorganic ions, too: they bind with heavy metal ions, hydroxides and minerals, including toxic pollutants, to form all kinds of stabilized soluble and insoluble compounds. So humic acids in the aquarium keep heavy metals— some of which also count as micronutrients— chelated, both in the water and in humus floc in the substrate.
Since in addition to the heavy metals, chelatable cations include calcium and magnesium, the chelating action of humic substances underlies the familiar water-softening capacity of peat filtration.
"But." you ask, "if chelated metals are less toxic to animals, how is it that chelated iron is featured in plant fertilizers? I understood that chelation makes these elements more available to plants?" Chelation works to keep ions bioavailable to plants partly by preventing the free ions from precipitating out of solution. For example, when iron isn't chelated, it oxidizes in the presence of oxygen into ferric hydroxides, amorphous, non-crystalline insoluble forms that precipitate out of the water. In such hydroxides, iron is no longer available to plants. Another aspect of chelation that tends to make nutrients bio-available is that chelation is only semi-permanent. It operates a bit like time-release capsules. At any moment, some chelated metal ions are being released as free ions, while others are being grabbed. During the brief time they are free in solution, ions of metals like copper and iron can be taken up by plants: they are momentarily "bio-available." That means they may be toxic too. But metal ions don't last long in solution. If they aren't scavenged by plants or algae, or rechelated, they accumulate a cluster of complementary ions or neutral molecules, some transient, some stable, such as hydroxides. So chelation can be thought of as an averaged state, rather than as the particular chelated or un-chelated state of one ion at a certain moment.
Once these cations are locked to the chelator, whether the chelator is a natural humic substance or an artificial one like EDTA, carbon filtration can adsorb them, and you're able to export kit and kaboodle from your system.
My understanding is that until you do, even after they've been adsorbed to carbon in the filter medium, these chelators can continue to pull substances from the water. Why is that? When an organic molecule that is chelating ionic iron gets adsorbed to activated carbon, the chelating bond isn't thereby broken. I mean to say that the forces that operate in adsorbing the big organic chelating molecule to carbon don't separate it from the iron. The iron is effectively removed from the water column, however. No problem, because the iron won't be available to plants anyway until the chelating bond degrades. And the chelation is constantly letting go and rebinding, so a proportion of Fe ions are constantly available, even from a chelating molecule that is adsorbed to the filter carbon.
Virtually all the chemical processes in the aquarium are reversible, though it may take a change of pH or of the dissolved oxygen content of the water to effect a reversal. The only permanent elimination from the system is is through export: rinsing or replacing filter media, pruning plant growth, partial water changing.
Other chelators. Many other kinds of dissolved organic carbon (DOC) chelate metal ions. Proteins, for example, chelate metals, and so do the polypeptides they are made from, and so do the amino acids that form the polypeptides. Sugars like malate and citrate have some chelating power. But humic substances, much less biodegradable than these, are the most important chelators in the aquarium.
Artificial chelators. When you hear that you need a "water conditioner" to bind toxic metals like chromium and lead and mercury (and copper), the ingredient involved is often EDTA. This synthetic chelating agent is also used in fertilizers, and it can be injected to detoxify victims of lead poisoning or radiation, for it binds the radioactive metal ions. (Don't try this at home!). EDTA ("ethylene diamine tetracetic acid" if anyone asks) is an artificial amino acid. Currently there are less expensive substitutes. The most inexpensive substitutes of all, though, are still the naturally-occuring humic substances.     
If you bought it separately, from a source like Fishy Farmacy, EDTA would cost in the range of $4 an ounce. It's toxic if overdosed. In acidic water like mine, the acidity constantly dissolves iron out of the substrate (there's lots of iron-rich  Flourite in my substrates), making the iron briefly available to plants, before humic substances (such as peat filtrate) chelate it. Plants don't draw up chelated iron, apparently, unless they're desperate for some iron. Metal ions don't last long free in solution but get surrounded with a cluster of other ions or neutral molecules; this is the state in which plants take up metal nutrients.
Besides ionic iron, copper, lead, cadmium or mercury, artificial chelators like EDTA act like humic acids to bind more than what you ordinarily think of as metals: cations of potassium (K++), calcium (Ca++), magnesium (Mg++). Calcium and magnesium ions form the hardness of water. By binding them and taking them "out of circulation" artificial chelators act to soften the water, behaving like the natural humic acids in peat filtration.
Chelators have a stronger affinity for more highly-charged cations than to those with fewer positive charges. So, though EDTA binds calcium and magnesium, it is fickle; heavily-charged metals can switch places with calcium or magnesium on the EDTA molecule. Cu++ or Fe+++ for example will knock Ca or Mg off the chelator. The upshot is that, though EDTA can soften water— until a heavy metal molecule comes along— and though it does provide short-term protection from metal toxicity-- plants and natural humic substances provide the more stable, long-term protection, through the humic acids they provide, both while they're living and while they're being decomposed.
Biosorption. Fishkeepers have often noticed that poisoning due to heavy metals is rare in well-established planted aquaria, where in fact plenty of humic floc incorporated in the substrate is chelating them. There is a further biological component to this aspect of "chemical" filtration: heavy metal ions also adhere to the cell walls of biofilm microbes, which carry net negative (anionic) charges, and in the gummy biofilm itself.
In experimental wastewater technology, microbes are even being deliberately cultured, to "filter" heavy metals from industrial effluents, a process called "biosorption." As you see, chemical filtration can't always be separated from biofiltration.
Chelation links. There's an excellent set of 1996 posts by Craig Bingaman and others, "Humus, humic acid and natural chelating agents," which you can find archived at TheKrib. An article by A..J. Leslie, "Aquatic use of copper-based herbicides in Florida," 1990, at the FL Bureau of Aquatic Plant Management website gives a brief review of the chemistry of chelation and the environmental transformations metals (copper is the subject) undergo in natural aquatic systems (but also in aquaria).