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;
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!!!
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.