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Something rather important is missing from these discussions of GAC.
Just what grades of GAC are you all referring to?.
The charcoal availiable in the NZ aquarium trade is pretty hopeless but
grades of GAC that will extract ions like Cu2+ and Fe2+ certainly exist.
To make activated carbon steam is injected into a dense vegetable
charcoal (often coconut husks) at very high temperatures. This makes it
very porous, but the porosity or surface area varies enormously. Hence
the sorbtion capacity can easily vary between grades by an order of
magnitude. The carbon is further treated by giving it an acid, base or
neutral wash giving those pores all sorts of polar functions. Some
manufacters even treat the substrate so it will act as an ion exchange
resin but this is very rare. Mostly GAC is just good at extracting large
organic compounds (EDTA's fairly marginal) as it can be made to do this
with very little expense.