why does it not precipitate to CaCo3?

[JonasRoman]

Hello, maybe this question is mostly adressed to Randy, so I would be very happy for his explanation, but other as well who knows. I have an own theory but before I establish that I must get this checked by a PRO like our Randy.

If we take the effluent water from the calciumreactor, we have around 45dKH and 740 ppm Ca in that water. The reason of no spontaneous precipitation of that is the low pH, which due two the CO2 content of that effluent water is maybe around 6.5. Well, at least so low so there exist no CO3-ions, and then the saturation index (CaxCO3) is far below the point when it forces CaCO3 spontaneously. The saturation index due to phosphate/Mg etc have to be at least above 5 to precipitate as far as I have understood, and with a pH of 6.5 we have such low CO3 (close to 0) so the sat.index is too low to accomplish CaCO3-formation.

But: If I now take this water from the calcium-reactor and aerate it, so the CO2 is degassing and pH rises to around 8.2, well, then we all know that the total alkalinity is the same (around 45dKH) and of course also the Calcium (around 740ppm). Why does this solution not precipitate to CaCO3 with a pH of 8.2, dKH of 45 and Calcium of 720?. We know empirically that it does not.
My question here is if my theory is correct:
When degassing the excess of CO2, so we reach the pH 8.2 we of course have the same total alkalinity, but we got an alteration in the composition of the total alkalinity so there will be less carbonates and more borates(and other ions) which build up this 45dKH alkalinity. That is not my question, that I know is the case. But is that also the reason why it is not precipitating? That is my question, is the cause of no precipitation in this scenario due to that we loose so much Carbonate-alkalinity during the degassing of Co2, and proportional increase the more borate-alkalinity (and other ions) so we get no saturation index above around 5?. Then it should by the way be very contraproductive to degassing the water from the calciumreactor too much, at least to the pH of 8.2.

Thoughts and response please

Jonas Roman

 

[Randy Holmes-Farley Verified Community Expert]

It will precipitate if you give it enough time, or a seed crystal surface (such as rock or sand). Without a seed, it can take a good while to initiate the precipitation in this supersaturated state. Add some aragonite sand and see what happens.

It's basically the reason that the ocean itself can be supersaturated. The crystals cannot initiate fast enough to offset the chemicals poisoning them from further growth (magnesium, phosphate, organics, etc.).

I've done some experiments with supersaturated seawater where I pushed the alk higher and higher using B-ionic. My experiment was to see if I could stabilize very high alkalinity (say, 20-100 dKH) using polyacrylic acid (which gets onto growing calcium carbonate surface and prevents further precipitation). It was very successful. The treated solutions were stable and the untreated ones precipitated.

I thought it might be useful to coral farmers and such to grow corals very fast. The concern is that I'm not sure the tissue growth can keep up, just like 10 dKH in ULNS systems, and also that the polymer might end up being depleted relatively rapidly.

 

[JonasRoman]

It will precipitate if you give it enough time, or a seed crystal surface (such as rock or sand). Without a seed, it can take a good while to initiate the precipitation in this supersaturated state. Add some aragonite sand and see what happens.

It's basically the reason that the ocean itself can be supersaturated. The crystals cannot initiate fast enough to offset the chemicals poisoning them from further growth (magnesium, phosphate, organics, etc.).

I've done some experiments with supersaturated seawater where I pushed the alk higher and higher using B-ionic. My experiment was to see if I could stabilize very high alkalinity (say, 20-100 dKH) using polyacrylic acid (which gets onto growing calcium carbonate surface and prevents further precipitation). It was very successful. The treated solutions were stable and the untreated ones precipitated.

I thought it might be useful to coral farmers and such to grow corals very fast. The concern is that I'm not sure the tissue growth can keep up, just like 10 dKH in ULNS systems, and also that the polymer might end up being depleted relatively rapidly.

ok, so there is just a matter of time, after a while it will precipitate? The reason of why I still think it is strange is that I know that magnesium etc can hold the water´s carbonates and calcium in solution up to around saturation index of 5, but in this scenario we have a saturation index of around 30-40..or? is Mg capable of holding these in solution up to these extremely high saturation values?
As it is an extreme situation and no adding of something else(beside the mg that already exist) to prevent precipitation, I think it is a little strange. But ok, if it is only a matter of time I can realize this. So if I aerate it and let it be, there will be cloudy after a while? How long time do you think?

 

[JonasRoman]

But isn´t it true that there also will be a shift between the components that build up the total alkalinity in that direction so the carbonatehardness will decrease and borathardness will increase with aeration of the effluent water? Does this phenomena not have a part in this at all? Not so much of the total alkalinity will after aeration be composed by carbonates,as it was before aeration. Or?

 

[Randy Holmes-Farley Verified Community Expert]

ok, so there is just a matter of time, after a while it will precipitate? The reason of why I still think it is strange is that I know that magnesium etc can hold the water´s carbonates and calcium in solution up to around saturation index of 5, but in this scenario we have a saturation index of around 30-40..or? is Mg capable of holding these in solution up to these extremely high saturation values?
As it is an extreme situation and no adding of something else(beside the mg that already exist) to prevent precipitation, I think it is a little strange. But ok, if it is only a matter of time I can realize this. So if I aerate it and let it be, there will be cloudy after a while? How long time do you think?

I don't know how long. Depends strongly on the pH.

The tendency toward precipitation will depend on the actual carbonate concentration. That concentration is directly related to the alkalinity (double the alkalinity and you double carbonate at fixed pH) and very strongly on the pH. pH 7 has ~10X as carbonate as pH 6 when alkalinity is fixed.

I don't see borate alkalinity being very important. It is only 3% of the total alk in seawater and will be less in reactor effluent.

 

[JonasRoman]

I don't know how long. Depends strongly on the pH.

The tendency toward precipitation will depend on the actual carbonate concentration. That concentration is directly related to the alkalinity (double the alkalinity and you double carbonate at fixed pH) and very strongly on the pH. pH 7 has ~10X as carbonate as pH 6 when alkalinity is fixed.

I don't see borate alkalinity being very important. It is only 3% of the total alk in seawater and will be less in reactor effluent.

yes I know this things and maths, and that is why I am confused. With an alk of 45 and pH of 8.2 you will have a CO3 conc which is 6 times higher than i natural sea water. Natural sea water has a saturation index of 3-5, and if you take the Ca in account it should the be a saturation index of around 35!. i can not understand how the water can hold these ions in solution despite the presence of mg. I know that Mg can hold these in solution so we can have a supersaturation around 3-5...but 35??...thta is why I thought it was curious why we do not see immediate precipitation. If you for instance take the fluid from a the 2 balling canisters, the KH and Ca canister, and mix them, you have an immediate cloudiness.(well of course even higher than dKH 45 etc I know) ..This should happen even with the water from the ca -reactor when rising the pH, but it does not, at least not so fast...and therefore I was wondered if there was another mechanism. But ok, i understand you, it will happen but take time. I think I have to do some experimental then because when I tried this last it was no precipitation after 24 hours...

 

[Randy Holmes-Farley Verified Community Expert]

You might find this interesting:

http://science.sciencemag.org/content/322/5909/1819.full

and

http://www.sciencedirect.com/science/article/pii/0021979771903638

"Stable supersaturated solutions of calcium carbonate have been prepared at 10, 25, and 40°C by careful control of experimental conditions. Upon the addition of seed crystals of calcite, the crystallization kinetics have been followed by monitoring the calcium and hydrogen ion concentrations."

 

[JonasRoman]

You might find this interesting:

http://science.sciencemag.org/content/322/5909/1819.full

and

http://www.sciencedirect.com/science/article/pii/0021979771903638

"Stable supersaturated solutions of calcium carbonate have been prepared at 10, 25, and 40°C by careful control of experimental conditions. Upon the addition of seed crystals of calcite, the crystallization kinetics have been followed by monitoring the calcium and hydrogen ion concentrations."

Thanks I Will read them later and maybe they explain My wonderings

 

[JonasRoman]

I have now looked at the first article and that even more give my initial question a value.
In this article it already in prenucleation stage eats up calcium (and carbonates I guess)! And already in not supersaturated levels.
And it is of course even more noticeable when ph is rised.
The time scale is within 30-60 minutes.

So I still wonder why we do not see alkalinity depletion when aerate the effluent water from the calciumreactor. For instance in 30 mintutes. Shouldn't some precipitation occurs? The pH is 8.2 and we have an extremely supersaturated situation.
As I do not see this depletion after aeration for hours I thought it could be because you degassing CO2 and thus alter the ions that build up the alkalinity and then not any more have that supersaturated solution to achieve precipitation.

 

[Randy Holmes-Farley Verified Community Expert]

I think you just are making a supersaturated solution that lacks seed crystals for nucleation.

 

[JonasRoman]

I think you just are making a supersaturated solution that lacks seed crystals for nucleation.

oki. I was mostly standing on the first article when they do not talk about seeding particles and even consumption of Ca before regulary nucleation, the stage they call prenucleation stage, and thus already in that stage see consumptions of Ca and CO3. As I did not see that in the water from the reactor when degassing, BUT see it when adding NaOH, I thougt it was dependent of someting else. Well, thanks for the links and comments, this is not an easy subject.

 

[Randy Holmes-Farley Verified Community Expert]

If there is such a thing as prenucleation in this situation, it will easily come back apart in the low pH of alkalinity and calcium testing.

 

[JonasRoman]

Alk testing I agree and understand but for calciumtesting there is no acid adding(or??)...and my test was only by checking ca.
I thought like you that this could be that way so I started with testing the pH of the both reagents fluid of salifert ca-test with bromocresoolgreen and get no colour shift at all so I concluded that the ca testing procedure did not lower ph.
Comments?

 

[Randy Holmes-Farley Verified Community Expert]

You are right, not acid in the calcium test, but the calcium chelator may strip it away from the prenucleated complexes.

 

[JonasRoman]

Ok

 

[revhtree Verified Admin]

I'm just here looking in on this conversation feeling dumb!

 

[Randy Holmes-Farley Verified Community Expert]

lol

 

[BluewaterLa]

I'm just here looking in on this conversation feeling dumb!

Your not alone Rev
I'm following along and can keep up like a fat guy in a marathon but some of this conversation is above my pay grade haha
Interesting to the point where I am reading the links and looking up terms to Better understand just what the REEF is going on here.

Please proceed, my brain cells haven't had this much stimulation since I got popped while welding in the rain