Re: Activated Charcoal

Several people have asked about the chemistry of activated charcoal. 
Activated charcoal is made by burning wood or bone. It is therefore a 
complex mixture of many different types of organic compounds and 
minerals. It is likely that polycyclic aromatic hydrocarbons make up a 
significant portion of the charcoal (benzene is an aromatic hydrocarbon 
... if you combine lots of them together they become polycyclic). The 
chacoal is 'activated' by being heated to a very high temperature in 
order to drive off the water molecules. This complex substance will bind 
anything that is hydrated in its natural state. This includes iron, 
chelated iron, other cations and organic compounds. The binding occurs 
through ionic interactions, hydrogen bonding, and also through Van der 
Waals forces (though Im told covalent bonding is unlikely). Substances 
which are bound by these forces are in equilibrium and can return to the 
'free' state so that a saturated carbon filter should still selectively 
remove substances which bind more tightly to the charcoal matrix.
In practical terms what this means is that carbon filters should not be 
used in a plant tank which receives regular additions of micronutrients 
(if you want to maximize the effectiveness of your micronutrients). 
I have recently removed my carbon filter and have noticed an increase in 
plant growth, though this anecdotal evidence is hardly conclusive. One 
concern is that the load of organic compounds (such as urea etc) in the tank 
could increase to toxic levels, though with lots of plants and few fish 
in the typical plant tank this may not be a problem ... something to 
think about.

Dr. dave.