I think it is bound to the substrate.
A negatively charged material (the substrate) attracts positive particles, as you state. Then the outer surface is positively charged, which attracts negatively charged items (like the carbonates).
Some things are bound more strongly than others, so the carbonates happen to stick pretty well.
That does make sense to me. I wonder if this layering effects extends past the addition of HCO3- ? Perhaps after th HCO3 is bound some K+ goes on top of it? Then HCO3- again and so on.
As far as I understand.
CO2 + H2O <> H2CO3 <> HCO3 <> CO3.
These are the chemical species that CO3 can transform to, with the reaction capable of going in both directions. If you add 1 dKH of CO3 to the water, and all of this converts to HCO3 with the addition of H+ (acid), then the dKH would drop by half to 0.5 dKH, because HCO3 only contributes half to KH.
The reason why HCO3 only contributes half to KH, is because HCO3 is actually HCO3-, and CO3 is actually CO3--, meaning that CO3 can accept two H+ ions before being converted to an acid, whereas HCO3 can only accept one H+ ion before being converted to an acid.
When the transformation is complete to H2CO3, the acid has been formed, and thus concentration of CO3 in the water is no longer resistant to addition of acid, and no longer registers as KH.
Alkalinity vs. KH
So by this logic, as CO3-- becomes more hydrated (more H+), it gradually reduces it's buffering effect and becomes less strong at the same time. So if I add K2CO3 directly to the water, it would be able to accept two times the number of hydrogens than KHCO3. When it takes up a H+ and becomes HCO3- we are effectively lowering the pH, but the KH is now reduced by half.
I'm very confused about this. Because if we continue to add acid into the water, the pH would go down in proportion to the carbonate left. If no carbonate were present, the pH would go down according to the pKa value of the acid. However, he reaction is reversible, so it's possible for H2CO3 to become HCO3- again if the surrounding H+ ions lower in concentration.
What I'm trying to say is...no matter the amount of acid (tannins) you add to the water, the total carbonate content does not change, only the form that it is in changes. So with aquasoil, that adds alot of acid via tannins and such would lower the pH...yes I get that. But it lowers the alkalinity as well.
Alkalinity = total buffering capacity of ions in solution (CO3, PO4 ect ect)
KH = carbonate hardness, or carbonate buffering only.
In our aquariums usually the vast majority of the total buffering power comes from carbonate sources so KH works well for CO2 calculations. The problem is our test kits measure Alkalinity, or total buffering.
Anyway I'm still left wondering where the KH goes with AS. Even if it's as you say, Audio, and the CO3 becomes H2CO3, the KH titration test kit would still measure it. But it doesn't. I put in 2 dKH worth of carbonate hardness, and in 12 hours the KH test kit registers less than 1.
I'm still leaning towards Diana's answer as being the correct one, and only because KH is actually removed from the water, not exchanged with H2CO3 which would still be seen with test kits.