Zeolite and KH reduction

[Darkbluesky]

Hello,

Some time ago I started to follow this forum, but not registered or participated before; I like to learn by myself before asking, and really I think I have learned a lot by reading it.

Now I have a question it seems I can't find answer for. This is about zeolite (not necessary only in marine aquariums, also in freshwater ones): I understand from others and my own experience, that zeolite reduces KH. According my experience it really reduces it, a lot, but until now I have not found any information about the chemical process behind this reduction.

I have read some opinions telling that the zeolite adsorves it, but I can't really understand/believe it. I can't see how the zeolite can perform ion exchange with carbonates or hydroxiles because of:

- First, because the zeolite, AFAIK, does cation exchange, giving to water Na+, or K+ (if regenerated with KCl, as I do), but carbonates or hydroxiles are anions.

- Second, because after regenerating it, the remaining water of regeneration has a enourmous GH (it's logic) but the KH is same or lower than before putting the zeolite inside for regeneration solution!. In particular, for the regeneration of 3 lt of zeolite I put it in 3 lt water with required the dosis of KCl (or NaCl) according instructions (150 gr I believe...). The water used is tap water (GH=15 ºdH and KH=9 ºdH). After 24/48 hours regenerating, the remaining water is GH>>>50 ºdH (got tired of putting drops) and KH=5... The increase of GH is logical, the zeolite has given away what it had adsorved and has exchanged it for Na+ (o K+ in my case), but if the KH has not increased (in fact, has decreased) is first of all, because the zeolita has not given away carbonates nor hydroxiles that may have been adsorved, instead even saturated it has managed to decrease still the KH of the regenerating water...

Because of that I think that the zeolite does remove/degrade/eliminates some component counted in alkalinity, but not necessarily adsorves it (maybe it makes it precipitate? and how?). Of course it should be some chemistry reaction (biological process?) of some kind. But which one?

Does someone know the process behind this phenomena? I can't find ANYTHING about that, at side of other people finding the same results. I would like to understand it to decide better what to do and what not to do, and why.

Note to avoid confussion: I am really referring to KH (alkalinity), not to GH (Ca, Mg) which is reduced normally by ion exchange.

Thank you very much for any help!

 

[Tron_954]

Thought of it perhaps being a burst in growth?

anyways, maybe this area can help:

Zeovit

 

[Randy Holmes-Farley]

This is about zeolite (not necessary only in marine aquariums, also in freshwater ones): I understand from others and my own experience, that zeolite reduces KH. According my experience it really reduces it, a lot, but until now I have not found any information about the chemical process behind this reduction.

Zeolites themselves won't reduce alkalinity unless they are used in the acid (H+) form. In that case, swapping H+ for Na+/K+/ etc. will reduce alkalinity.

H+ + HCO3- --> H2O + CO2

 

[Darkbluesky]

Yes, thanks Randy. As my zeolite is in Na+, or K+ form, that will not be happening, as it gives away K+ and not H+ (which if it was the case, I agree, would decrease alkalinity).

On the other hand even if the zeolite is giving away K+, and adsorving H+ (which, a priori, I doubt), then it would increase pH, but that would not reduce carbonates... In fact what you do suggest, is foreseen with cationic resin in H+ form which is waiting for the bypass filter system to be finished, but that's another story .

So I agree with you, but in any case, zeolite (or maybe better expressed: something linked to it) does really reduce it, as explained as well in the tank as in regenerating water (!), so something is happening there for sure.

Tron, I don't think is related to growth as all my life (plants, etc) is stable and slow paced growth, and if it was growth, I wouldn't find the same results in regenerating water... But I'll see in your link. Tx

Reading around, someone suggested it was the result of some bacteria work settled in the zeolite... but no more info. Is it possible? And how?

 

[Randy Holmes-Farley]

What do you mean by regenerating water reducing alkalinity? What is this solution and how did you test it?

If you bind H+ and release K+, then that will boost alkalinity, not reduce it.

How do you know that there is no H+ on your zeolites?

There is no other direct process that I know of whereby zeolites will reduce alkalinity.

Bacteria generally cannot reduce alkalinity unless they precipitate calcium carbonate or if they generate organic acids (which usually is a temporary process and the alk comes back when those acids are metabolized) or function in the nitrogen cycle (or sulfate cycle in unusual circumstances). Continuously rising nitrate levels will reduce alkalinity.

I agree with Tron that if you do anything that increases growth of corals, that will, obviously, reduce alkalinity. Most people using zeolites are doing all kinds of other things that impact corals.

 

[Darkbluesky]

Thank you very much for your comments. First of all my excuses, I thought I have said it but checking my posts I see I missed it: I talk about a freshwater tank, not a reef one. I realize the differences between reef and freshwater ones, but anyway this forum and only other one, are the only places in internet where I have seen people talking about zeolite reducing kh. And as this forum has been of great help before I dared to ask you about this phenomena. I think that if I could understand it for reef tanks I may adapt that logic to freshwater...

What do you mean by regenerating water reducing alkalinity? What is this solution and how did you test it?

I mean that when I soaked the saturated zeolite in water with KCl to regenerate it (the zeolite, I mean), to prepare the regeneration solution I used tap water which had GH=15 ºdH and KH=9 ºdH, and after the regeneration time (48h), the remaining water had GH>>50º dH and KH=5 ºdH.

If you bind H+ and release K+, then that will boost alkalinity, not reduce it.

Yep, so that could not be happening.

How do you know that there is no H+ on your zeolites?

Who knows if there a 'few', but the zeolite comes from manufacturer in Na+ form, confirmed by manufacturer, and moreover I have regenerated it a couple of times already, first with NaCl, now with KCl. Maybe the zeolite can retain partially (very few) H+, but I strongly doubt it, as for regeneration it is soaked in a very high concentration of salt (NaCl or KCl) solution. Still, even if the zeolite could adsorv some H+ when regenerated, I guess that when I saw it was exhausted and put it to regenerate, how a exhausted / saturated zeolite could still drop KH from 9 to 5 ºdH ?

There is no other direct process that I know of whereby zeolites will reduce alkalinity.

I agree, and it is because of this, that I begin to suspect some indirect (biological?) process of some kind...

Bacteria generally cannot reduce alkalinity unless they precipitate calcium carbonate or if they generate organic acids (which usually is a temporary process and the alk comes back when those acids are metabolized) or function in the nitrogen cycle (or sulfate cycle in unusual circumstances). Continuously rising nitrate levels will reduce alkalinity.

And I have few source of ammonia (so nitrates). Nitrates were 0 (or undetectable), I had even to add K2NO3, to increase a little (to 5 ppm) the NO3 concentration for my plants... So then I discard the rising nitrate...

I agree with Tron that if you do anything that increases growth of corals, that will, obviously, reduce alkalinity. Most people using zeolites are doing all kinds of other things that impact corals.

I repeat my excuses, my mistake; I thought I have explained I am in freshwater, so no corals.

 

[Randy Holmes-Farley]

Oh, fresh water. I'm not sure that changes things significantly, but good to know.

Do you rinse the regenerated zeolite after regeneration to get rid of excess fluid, or just drain it?

What is the pH of your tap water? The KCl solution?

Either may have acidic impurities that could deplete alkalinity in fresh water. Have you tried the measure the alkalinity in that KCl solution?

 

[ahmet_cakir20]

good information. thanks

 

[Darkbluesky]

Well, I have to accept that I forgot to rinse it, I only drained it.

The characteristics of the regenerating water is:

Before regeneration (only tap water, no KCl): KH=9 º dH, pH=7.6-7.8

After regeneration: KH=5º dH, pH=? (I still keep the water of regeneration, so I'll try to measure it today, it is sitting there for 2-3 weeks past the regeneration of zeolite, though)

I missed to measure the pH & KH of the regeneration solution (tap water + KCl) before putting in the zeolite... In the next regeneration I'll measure it.

The pH of my tap water is 7.6 to 7.8 (after 1 hour of rest).

Still, I don't see how could I add some acidic element with just tap water and KCl (the container used is food grade plastic), specially in so much concentration as to reduce 4º degrees of KH in 48h of regeneration.

Note: For water measurements I use drop tests (API in particular), never paper strips.

 

[Randy Holmes-Farley]

FWIW, I don't see a clear explanation either.