Myth busters - activated carbon will leach back toxins in the water once saturated

[Florin Ilia]

Hi,

There is this myth that activated carbon will start de-sorbing toxins back into the water once it has adsorbed to full capacity.

There are articles on the net that point both ways:
1. articles that confirm the myth;
and
2. articles that explain how AC needs very specific processes to de-sorb toxins (extreme acidity swings, big temperature swings a.s.o. which do not exist in aquaria), and say that AC simply turns to biological media once saturated.

I find theory #2 more believable, also I know that Tom Barr and ADA say the same, but I got into a debate with someone and I would like to bring more proof than "me and 2 gurus believe that..."

I am no scientist, but I would like to devise an experiment that I could perform at home and that would be as conclusive as possible. The experiment should be relevant to the aquarium environment - I only care about what happens in a "typical" freshwater tank, not about the general properties of AC.

Here is what I thought:
Step 0. Prepare a small quantity (100 grams?) of aquarium grade activated carbon, e.g. this one
Step 1. Prepare Solution X as a known concentration of substance Y in DI water
Step 2. Measure the concentration from step 1 to verify method
Step 3. Filter Solution X through the AC
Step 4. Measure concentration of the filtered solution; if it is indistinguishable from the initial known concentration, then the AC has reached saturation - continue to Step 5; otherwise, repeat Steps 3-4
Step 5. Continue to filter Solution X through the AC
Step 6. Measure concentration of the filtered solution; if it is larger than the initial known concentration, then the AC has leached - MYTH PROVED!!!; if it is indistinguishable from the initial known concentration, then the AC has NOT leached anything back; repeat Steps 5 and 6 sufficient times to declare MYTH BUSTED!!!

Problems:
Problem 1. What is a practical set-up for this experiment?
I am thinking of a miniature canister filter (such as the Dennerle Nano External Skimfilter), with just the AC as media, filtering water from a small container (5-10 liters or so), periodically compensated for evaporation with DI water.
Problem 2. I should probably run the experiment a long time.
I could of course force a quicker saturation of AC with a very concentrated solution, but that is not typical of aquariums. Also, the "leach test" (Steps 5 and 6) should run for quite a while (a month? a year? ) in order to suggest that the environment is stable.
Problem 3. One big risk is the measurement method and my measurement technique.
I have a Hach DR/890 and just enough experience to understand how easily measurements can be skewed. Even if I press the Read button repeatedly on the exact same sample, without even removing it from the device, with some methods I get slightly different readings - let alone if I redo the measurement from scratch. This leads to some requirements on the experiment:
- the concentration of Solution X should be large enough so that small inherent errors become negligible;
- the measurement method should be as reliable/repeatable as possible;
- I should probably calibrate my measurement method by standard testing sufficient times to understand its accuracy and precision; my ignorance of statistics won't help much here
Problem 4. What can I use for Substance Y?
I don't have measurement methods for the things that are typically removed by AC - organic compounds, medicine. What I have on hand are methods for inorganic compounds that the AC does not remove. I can think of two alternatives:
- use a substance that contains tannin (such as Atison's Betta Spa) and measure the color of water with Hach method 8025.
- use a chelated metal. In a previous experiment I found that Fe chelated with EDTA and/or DTPA is removed from the water by a high-grade AC. So theoretically I could measure for Fe and extrapolate. However (1) that experiment was disputed by Biollante, whose opinion I value, and (2) I am not sure if the Fe stays chelated long enough for my experiment to be conclusive.

Sorry for the long post. I will appreciate thoughts on the proposed experiment and the problems.

Alternatively, if there is a reference text that I could understand and that describes such an experiment, that would also be helpful.

Thanks,

Florin

[Tom Barr]

Over time, bacteria degradation is very likely to occur in the activated old spent carbon.

Unless the test loads the carbon up fast and without interactions with bacteria.........it will be an issue.

A good question to ask : what will get out of your efforts? What will you learn from it?
Are all activate carbons going to behave the same way for this particular substance?


1. A long column you can pack and slowly filter water through, those carbon water filters like Britta etc might work nicely.

2. Not sure. Loaded as much as they can hold, then measure over two post loading.

3. I'd do 4-5 replications to account for error

3. Repeat the test at least once.

4. Tannins they make test for, so peat etc could be used and the test could measure the base line(but no reference stand is available really). Fe would work and is easy to make a standard reference from.
A stronger Chleator will hold for several weeks (DTPA in DI water etc).

[feh]

Hmm... interesting experiment.

[Biollante]

Hi,

Theory number two sounds better because I agree with it (it is easily researched fact).

I bought into a bogus method to “recharge” activated charcoal, presented by a well-respected hobbyist as a way to justify some really expensive activated charcoal.

• It appeared to work,
• I guess in part because I wanted it to be so and in
• part because I was using it in a well-maintained aquarium,
  
  • so gravel would have appeared to work.

As it turns out from a practical point of view the only way a hobbyist is going to “recharge,” actually it is de-adsorb activated charcoal is by the use of lye and/or muriatic acid, or a kiln and/or torch.

• Ultimately, even with high-end activated charcoal, it does not seem to be worth the cost, hassle or risk to de-adsorb activated charcoal. If I ran a large-scale gold/metal recovery operation, using tons of the stuff and all the flocculants to remove heavy metal, probably, since de-adsorbing would be part of the recovery process.

As a side benefit of my search was that used activated charcoal makes a great biological media (as the pricey Japanese guy suggests) precisely because it is so hard to de-adsorb, the little bacteria miners really have to dig and since life is generally lazy (efficient, I guess) they prefer what comes floating past.

I also found that for the planted tank hobby cheap, large pore bituminous coal or granular activated carbon, GAC, works just fine for most of our purposes.

The other problem faced is the design of activated charcoals to remove, or favor removal certain substances.

From what my limited intellect can process, it appears that phosphate is most cited as being “leached” from activated charcoal, since activated charcoal does not adsorb phosphate it cannot be leached back.

My meager understanding on this point is that phosphates are sometimes part of the physical makeup of the activated charcoal, but more often, the phosphate is there as a residue from a phosphoric acid bath and since it is not held chemically or physically it is easily washed away. This would not seem to be much of an issue in the planted tank.

Biollante

[client]

The idea to be taken out is that the activated carbon is releasing stuff into the water column, where it is further oxidized if necessary, through desorption. Adsorption is a totally new event for each substance to be adsorbed and there is a limit of the uptake per nutrient alongside a competition for adsorption between different nutrients.

There is a lot to say about the material to be adsorbed and also about the brand of the activated carbon, about its type and strongly about the way it was activated. There are nutrients that are adsorbed through van der waals forces or electrostatic interactions and there are humic substances that are adsorbed to the carbon sink through covalent bonding, so it is hard to generalize that the conditions of desorption are hard to achieve into a well maintained setting.

Extreme temperature and pH values are likely to destroy any chemisorbed aggregate, therefore this doesn't say that the activated carbon isn't going through desorption. The bacteria on the activated carbon surface, being well known that there is no room for them inside the micro pores, forms a slime coat in at least two months, hence it (the population) forms slowly.

Bioregeneration is a frequently occurring process capable of reducing the nutrients of desorption to unmeasurable amounts, and when I type frequently occurring process I would like to state that across its utilization the activated carbon is leaching often, due to a hysteresis, concentration gradient, busting the filter capacity once more. So it is cleaning itself every now and then.

To test the theory, maybe there is a good idea to get as many activated carbons as possible if they are available and to test them first in ro water for leaching phosphate and the like. Every single one will have different data. How to test it? Well, continuous EC measurement. Several plastic bottles (2L) half filled at most with the media left for not more than five to seven days per water change and with no circulation.

You have tested yourself how a sand substrate holds the ingredients for very diverse life forms, the primordial soup, into some other thread. Maybe it is a good idea to actually stir the bottle content with a piece of stick every morning if not every twenty minutes (E. Coli doubling time) to six hours at most to prevent the formation of some films.

The measurement will be difficult, but if the probe will be somewhat blander than the redox one, it would be nice to see that there will be nothing leaching from at least one brand of tested AC. Once the method is learned, a strong solution of ammonia or nitrate can be created to soak the subjects in it. After another week or so, let the media dry and then do another ro fill. And don't forget to stir, if you find it appropriate.

Activated Carbon for Water and Wastewater Treatment, Integration of Adsorption and Biological Treatment -Ferhan Cecen and Ozgur Aktas
Recent Progress in Slow Sand and Alternative Biofiltration Processes -Rolf Gimbel, Nigel J. D. Graham, M. Robin Collins
Activated Carbon Adsorption -Roop Chand Bansal and Meenakshi Goyal
Activated Carbon -Harry Marsh and Francisco Rodriguez Reinoso

[Florin Ilia]

Tom, feh, Bio, client, hi and thanks for your comments.

A good question to ask : what will get out of your efforts? What will you learn from it?

It's just an opportunity to play scientist in my living room, and also to answer a challenge in the most honest way that I can.

Are all activate carbons going to behave the same way for this particular substance?

I have no clue, but since you ask, probably not My question is, will most ACs used by aquarists in freshwater aquariums behave in a similar way? If the answer is still NO, then there's no point in even starting the experiment, and I would appreciate your advice here.

Over time, bacteria degradation is very likely to occur in the activated old spent carbon.

Unless the test loads the carbon up fast and without interactions with bacteria.........it will be an issue.

Because bacteria degradation will create substances in the water, that will look like the AC leaching?

1. A long column you can pack and slowly filter water through, those carbon water filters like Britta etc might work nicely.

Like this one?

I can't find a Brita that's "long".

2. Not sure. Loaded as much as they can hold, then measure over two post loading.

Two what? I hope the missing word is "weeks"

phosphate is most cited as being “leached” from activated charcoal

Something I'll have to watch for - whether phosphate interferes with my test method - whatever that will be.

The idea to be taken out is that the activated carbon is releasing stuff into the water column, where it is further oxidized if necessary, through desorption.

You're saying that, in a typical freshwater aquarium, AC DOES leach back adsorbed substances?

To test the theory, maybe there is a good idea to get as many activated carbons as possible if they are available and to test them first in ro water for leaching phosphate and the like.

Well, I think I'll start with the 2 kinds I have at the moment and go from there

a strong solution of ammonia or nitrate can be created to soak the subjects in it. After another week or so, let the media dry and then do another ro fill.

What's the role of the ammonia/nitrate?

Also client, thanks for the book pointers. Someone serious about the topic would probably have to read those as a first step. I am not that serious - it is too large a time investment for me to go that in depth. I just want to draw a conclusion that is relevant to "most" freshwater aquarists and that's it. If this goal can't be achieved with primitive living-room "science", so be it, I will simply not do it and fall back to trusting the scientists (and Biollante ).

[Florin Ilia]

Anyway, I am overwhelmed by work and by the high maintenance requirements of my low maintenance tank, this experiment will have to take a seat and wait.

[Snivi]

From what my limited intellect can process, it appears that phosphate is most cited as being “leached” from activated charcoal, since activated charcoal does not adsorb phosphate it cannot be leached back.

My meager understanding on this point is that phosphates are sometimes part of the physical makeup of the activated charcoal, but more often, the phosphate is there as a residue from a phosphoric acid bath and since it is not held chemically or physically it is easily washed away. This would not seem to be much of an issue in the planted tank.

Biollante

you are true on activated characoal phosphorique acide is used during production for increse surface per grammes.
Introducing new characoal on a phosphate color test is funny as you see the color change.
I used this test before to use a bag of characoal in a reef tank and in the same brand the result is not consistent.

[Tom Barr]

Anyway, I am overwhelmed by work and by the high maintenance requirements of my low maintenance tank, this experiment will have to take a seat and wait.

haha, that's the way these things sometime go ironically.