Activated Carbon

[backtotropical]

Every aquarist has heard of and probably used activated carbon at some stage, and everyone has heard the rumour that once it becomes saturated, it will start to release the chemicals which it has adsorbed back into the water.

Can anyone provide any evidence one way or other? Does it really leach those nasties back into the water column, or is it an old wives tale? I'm interested to find out if this rumour is true.

I'm afraid i have no evidence to bring to the table for either argument, so i'll let someone else start.

Please discuss!

 

[Bignose]

This is from an old thread (http/www.fishforums.net/content/forum/72100/Does-Carbon-Leak-toxins-back-into-water-/):

The chemicals that get adsorbed onto the actived carbon are in equilibrium with the tank water. The activated carbon will take up chemcials until that equilibrium is reached. What that equilibrium point is depends upon the concentration of the pollutant and how used up the carbon is already -- what percentage of the active spots on the carbon are still available.

Normally, when using carbon to take out medications for example, it will take up an overwhelming percentage of the chemicals. Say from 100% to 0.1%. What I mean is that 0.1% is the equilibrium. Now, you do a 25% water change. So, concentration of chemicals is now 0.075% in your tank. If you do not change the carbon at this time, the carbon will release some of the chemicals, again going towards equilibrium. Since, most of the chemicals are on the carbon now (99.9% of the original amount) given enough time (and this depends on temp, pH, etc.) the sysytem will tend toward equilibrium again. Meaning the carbon realeases 0.025% back into your water, to bring the concentration of chemcials back to 0.1% -- equilibrium.

Not a huge amount, but you see how a little bit can be released back. And long-term exposure, even at very low concetrations can lead to ill health.


Here is even a little more:

Lets call c the concentration of any chemical in the water that will be adsorbed.

If the adsorbtion at equilibrium follows the Langmuir isotherm (probably the most common -- there are others) the concentration of the chemical on the carbon, which I will call n is equal to:

n = c/(K+c)

K is the called the adsorption or equilibrium constant. It will vary depending upon what chemical we are talking about, and what quality the carbon is.

The total concentration of chemical is T

T=n+c

or using the definition of the adsorbed amount

T = c/(K+c) + c

Now, if we lower c a little (by doing a partial water change, for example) T does get lowered to T2, but so does n -- so call the new n2.

But again, T2=n2+c2 where n2=c2/(K+c2)

where does the difference adsorbed (n2-n) go? back into the water.

The carbon will uptake and release chemicals until equilibrium is reached, every time. So, yes, it is usually a small amount that will get released back -- carbon is very efficient uptaking chemicals when active sites are available -- but a small amount does get released.

 

[backtotropical]

Thank you Bignose. That is very interesting.

Would you consider that no chemicals will be released by the carbon if water changes are not performed?

 

[waterdrop]

Interesting thread BTT, thanks.

What did you think of Gator's comment in the "New to the Hobby" poohbear1 thread about a lot of carbon being able to deplete oxygen and also that some of it was known to leach phosphate?

Wouldn't phosphate obey the same equilibrium rule cited above?

~~waterdrop~~

 

[Bignose]

Everything tends toward equilibrium. The amount of oxygen dissolved in the water, the amount of carbon dioxide dissolved in the water. The amount of argon dissolved in the water. The amount of everything adsorbed on activated carbon. The amount of calcium leached from a rock. Everything.

Phosphates can be absorbed by activated carbon. For that matter, darn near everything can, it's one of the neat properties of carbon. Whether it will leach phosphates or not depends on whether it had previously adsorbed phosphates. One that that is true about activated carbon is that not all activated carbons are the same. The AC made from coal is different than the AC made from coconut shells is different that the AC made from bark. And, what is adsorbed on it will depend on how it was made/what it's been exposed to.

If water changes aren't performed, then dynamics of the equilibrium won't change, at least with respect to the adsorption equilibrium. If things remain the same, then the equilibrium point will remain the same. If things change, then the system will tend toward a new equilibrium.

 

[fry_lover]

Interesting as i was thinking about starting a thread like this.

What if someone just used carbon to polish their water a bit and add a little bit of extra "umph" to the general filtration of the tank but did not dose with medications or add any chemicals to their tanks other than water conditioner to in-comng water....

Would it safe to use some activated carbon and just renew it monthly?

 

[fishyfeet]

Yes i used to do that, as long as you remember to renew the carbon monthly, if like me you forget and you lose some major fish you might think that as its not necessary to use it its not worth the risk, glad to see you around alex, di

 

[Miss Wiggle]

Interesting as i was thinking about starting a thread like this.

What if someone just used carbon to polish their water a bit and add a little bit of extra "umph" to the general filtration of the tank but did not dose with medications or add any chemicals to their tanks other than water conditioner to in-comng water....

Would it safe to use some activated carbon and just renew it monthly?

it's definatley safe to do this, it won't cause any harm but you have to consider what level of good it will actually do.

carbon is a chemical filtration media, it's designed to remove chemicals from the water, if there are no chemicals then it's not performing it's function, it will simply act as biological media and become a surface for bacteria to grow on.

biological media is designed to have the biggest surface area possible in a relativley small space so loads of bacteria can grow, carbon doesn't have as big a surface area as an equivalent amount of a 'proper' biological media. So all you are doing by adding carbon to the filter in this situation is reducing the biological filtration capacity, then to top it all off you are then removing some of the bacteria each month when you replace the carbon.

In this situation there is no extra 'umph' added by the carbon.

If you want to polish the water use a layer of fine filter floss, if you want the filter to be able to cope with the largest bio load then use more bio media.

 

[fry_lover]

Hi Di, thanks

I am not sure about other's experiences Miss Wiggle but i find activated carbon polishes the water far better than any amount of filter wool does.

 

[Bignose]

It depends on your tank, but one of the big minuses of AC is that carbon will uptake dissolved organic compounds (DOCs) in the water. And those DOCs are performing a very valuable function! They are helping confer protection from metal toxicity. Metals readily bind to DOCs, and when the metals are bound to the organic molecule, they are not nearly as readily taken up by the fish. So, the DOCs from plants and fish that are being excreted are helping to protect them -- if you are using carbon you are taking away some of this protection!

Metal toxicity is often overlooked in water quality issues. For example, the toxic levels of copper and zinc in the water to be safe for fish are 0.02 ppm (Cu) and 0.1 ppm (Zn). By comparison, copper and zinc are not toxic to humans until in very high levels, and the regulations of the water companies are really based on taste. Normally, the water companies aim to be around 1.3 ppm Copper and 5.0 ppm Zinc. (All these numbers from Diana Walstad's Ecology of the Planted Aquarium.) But, if you have these DOCs in the water, they can help neutralize heavy metals by binding with the metals.

Yes, your favorite water conditions probably helps detoxify heavy metals, but the DOCs in your tank are at the very least extra insurance. I wouldn't take away that insurance policy with activated carbons, again, especially since most of the benefits or carbon are minor or debatable.

 

[pastabake]

Couldn't agree more Bignose, though it also depends upon pH. As at a high pH metal toxicity isn't really a problem.

MW - filter wool works just as well as a biological media ... the only real way to 'polish water' without compromising your biological balance is to use something that you change or clean on a daily basis so that bacteria never get a chance to establish. I change my filterfloss on a weekly basis and from the state of it I know its not often enough.