Does Carbon Leak toxins back into water?

[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. 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.

 

[jflowers]

I think it soaks it up for so long then stops, don't think it would then release it JMO though.

Jon

 

[SirMinion]

This is how it works to my understanding: Chemistry 101....

Carbon removes metals, toxins and impurities from the water by chemically bonding with them as it has a high valiance for these compounds.

Only by placing the used carbon with a substance that has a higher valiance for the toxins would the carbon release them.

ergo, used carbon does not leak because it is not a physical sponge, but a chemical reaction.

The only reason toxins suddenly increase in a tank with old carbon is that the carbon is no longer absorbing them when all the activated carbon has boded with other compounds to saturation.

 

[fishdudein]

Seems like I remember hearing a few years back that there was a product, I think it was for neutralizing ammonia, came in little mesh bags you could put into your filter's media compartment or just lay in a corner of the tank. I never used these things myself, so I couldn't tell you the name of them. Anyway, these supposedly had a limit on their capacity, after which point they would release (or as jflowers suggested, maybe not release, but just fail to absorb anymore). I have never heard this said about activated carbon before, though. Maybe the story has got confuzzled somewhere?

 

[Bignose]

I just copied this out of my reply in the 'myths' thread that is going concurrently:

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 adsorped (n2-n) go? back into the water.

The carbon will uptake and release chemicals until equilibrium is reached, everytime. 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.

 

[SirMinion]

Bignose, your argument is convincing.. but flawed.
It's based on osmosis where two unequal concentration will always strive to aquire equalibrium, at that is certainly true.. but only if the chemicals absorbed by the carbon were still in their original state.

But the carbon chemically bonds[/i] with the compounds and will not release them unless another substance with a stronger 'pull' (valience) is introduced.

Here's an example.

Haemoglobin (in blood) has a high valience for oxygen and will take oxygen from the air to form oxyhaemoglobin.

If you put that oxyhaemoglobin filled blood in an atmosphere without oxygen, the oxygen in the oxyhaemoglobin will not leak out even though the concentration in the haemoglobin is greater than the atmosphere because it's a chemical bond.

 

[Dorkhedeos]

carbon might acually bad your tank, depending on which carbon you acually recieve. carbon exists as 3 isotopes. carbon 12, carbon 13, and carbon 14. living things are made of all 3. carbon 13 and carbon 14 are radioisotopes. radioisotopes are unstable isotopes whos nuclei undergo changes to abtain stability. carbon 14 emit beta particles. when the carbon decays, it becomes nitrogen 14 and an electron. the electron is the beta particle. anyways, you will most likely get carbon 12 because its the most common

Edit: by the way, 98.89% fo carbon is carbon 12, but you dont know when you will get that 1.11%

 

[Bignose]

Physical or chemical bonding -- it does not matter. Everything is in equilibrium. You cannot predict how quickly it will go back to equilibirum, but the system will always try. Every chemical reaction is reversible, even if only too a very very very small amount. Some oxygen will be released from your haemoglobin example -- it may not be much, an extraordinarily small amount, but some will. It has too -- nature always tends toward equilibrium. The equilibrium in your case may be very heavily skewed toward fully adsorbed, but some tiny amount will always be released.

The only way the carbon would not release chemicals back into the water is if the curve was a constant. n=K meaning that the amount adsorbed on the carbon would be independent of the concentration of chemicals in the water. This just is not so!

Look at this .pdf file from IUPAC for example. It deals with a more complicated function than the Langmuir isotherm I presented above, but it still fits experimental data from benzene (along with a few others). Benzene is a decent example molecule for a lot of the chemicals we try to take out of our water -- the organic molecules that cause smells and are in the medications.

The isotherm is not a flat line, meaning that it is dependend upon the concentration of the adsorbant, meaning that carbon will uptake and release until equilibrium is reached.

And the question is will carbon release toxins/.adsorbed material back into the water? The answer is definately yes, but I cannot say how much. It is undoubtedly a small amount, small enough that it may not even be dangerous to our fish, but some small amount will be released. Nature tends toward equilibrium.

 

[Bignose]

Even reactions that are very strongly favored one way will tend toward equilibrium. There will be a finite rate at which haemoglobin will give up its oxygen. It may be quite small, but it is non-zero.

Consider the example of nitrogen gas and oxygen gas kept together in a canister. The two reacting together to give water is very favorable, in terms of equilibrium. But you can wait for millenia and your canister will still have only a negligible amount of water in it. Just because something is out of equilibrium, does not mean it will happen fast.

And just because something is not likely to happen, does not mean that its chance of happening is zero. There is a finite, calculable chance that every molecule of air that is in the room you are in right now will rush into one corner. It is an incredibly small chance. But it is non-zero.

Just like, the carbon may hold on to a large amount of chemicals and not give them up readily, but there is a non-zero chance of a molecule becoming unbound and re-entering the water. Probability says it is so, and physics says it is so as the system will tend toward equilibrium.

It is possible to predict the equilibrium points, but usually not how quickly the system will reach it.

 

[Malawiandude]

is there any chance to get this in "rich dummy terms"? just minus out the rich part..

 

[SirMinion]

Pff. Nit picking.

 

[Bignose]

OK, I got one more idea to show ya that carbon adsorption is reversible. The fact that carbon regeneration is a major part of many industrial applications. Usually this is just accomplished by heating the carbon up, usually exposing it to steam at the same time. The increase in temperature increases the rate chemicals come off the carbon, and the steam carries the chemicals away. The equilibrium is shifted toward taking chemicals off the carbon in this case, the opposite the equilibrium point in our tanks where uptaking chemicals is predominant.

If only a higher valience chemical can knock chemicals off of carbon, as you said, carbon would eventually become saturated with highly valient chemcials and would never be of any good ever again. But carbon regeneration occurs frequently in many industrial processes.

To dorkhedos: 98.89% of carbon may be C12, with the rest being radioactive, but that is all carbon, not just the activated carbon you buy in the store. But all wood, driftwood, real plants, food, anything with carbon in it would have that small % be radioactive. This is one of those things that is everywhere all around us and you cannot stop it.

 

[Dorkhedeos]

if somebody happens to get a small chunk of carbon that isnt C12, that would screw up your aquarium wouldnt it?

 

[Bignose]

No, if 1% of all carbon is radioactive (and I am not sure of that number, just using what you said) that means 1 out of every 100 atoms is radioactive. There are incredibly higher numbers like on the order 10^23 atoms in just a few grams of carbon. (10^9 = 1 trillion by comparison.) This radioactive carbon is in everything, everywhere -- and live plants use it just like regular carbon. This is how carbon dating is done -- live plants have a centain percentage of this radioactive carbon in it -- when you make a table out of a tree or cloth out of cotton the carbon is not being constantly replenished from the natural supply -- the natural supply that has this 1% radioactive. So, you compare the percentage remaining of radioactive carbon in the old item with the base or naturally occuring amount of radioactivity.

Now, if the activated carbon is dug up from coal (and a fair amount is) the radioactive % will be lower, but there is still a percentage. It is not 1 out of every 100 chunks of carbon is radioactive, it is 1 out of 100 atoms, and any chunk you pick up will have many atoms in it that are radioactive. There is nothing you can do about this -- unless you plan on building a seperator. I hope you have spare millions (maybe even billions) of dollars lying around.

This radioactivity is everywhere, in everything that has carbon in it. Wood, cloth, food, you and me. When a Geiger counter is turned on, there is normal background radiation, and this is some of that.