Does activated carbon leak toxins back into tank?

[MalawianDude]

We are having a discussion about this on another place and am trying to figure this out, i believe it does and so do others i have spoken with others that believe so too. The fact that carbon does leak toxins back into the tank after so long if the carbon is not replaced...Please tell me what you think.

[MalawianDude]

k will do

[Muttling]

The concept you're referring to is called competative adsorbtion, but the effect is not nearly as serious as you're thinking when you talk about an aquarium setting.

In occurs, when you have a solution that includes both weakly adsorbed organic compound and strongly adsorbed ones. As the carbon is loading up, it removes everything with great efficeincy.

After a large portion of the carbon's receptor sites have become taken, the carbon will start to release the weakly adsorbed compounds in favor of more strongly adsorbed ones. Basically, the stronger adsorbed compounds start knocking the weakly bonded ones off the receptor sites.

The end result is that concentrations of strongly adsorbed compounds continue to decrease, but the concentration of weakly adsorbed ones start increasing.

In an aquarium setting, most things are strongly adsorbed. Furthermore, the total concentration organics is the main concern as this is a nutrient for nuissance algaes. (The carbon can remove toxins if they are present in the water, but they only occur under special conditions such as an octopus inking.) In competative adsorbtion, the total concentration of organics remain virtually unchanged. Thus, it's really a non-issue for an aquarium setting.

A common example of where this is a real problem is water that's contaminated with pesticides and a petroleum product like gasoline. Most of the gasoline organics are very strongly bonded while the pesticides are very weakly bonded.

[Randy Holmes-Farley]

I agree that competitive absorption is the issue here. How important it may be for a reef aquarium is unclear to me, but I would not assume that all organics are necessarily irreversibly bound to carbon or any other media that aquarists use to remove it.

In particular, if you have a special toxin issue (like a fish that is known to release toxins), I'd change the carbon reasonably frequently. Not only will that reduce the likelihood of release of those toxins, but most importantly, it will ensure that there is always capacity available for further absorption if toxins are released.