Calcium carbonate saturation
 

Hi Randy,

I have another question on what seems to becoming a geeky interest in calcium reactors. In a calcium reactor we must lower the omega for aragonite to below 1 for the calcium carbonate to dissolve. Ignoring kinetics for now, but will dissoloution of the media stop when we get to omega = 1 (I presume stuff still dissolves but also precipitates at the same rate?).

For example at pH 6.8 I calculate that omega=1 when alkalinity=11mEq/L (30 dKH), would this be the theoretical saturation we could acheive or could alkalinity be driven higher within the reactor or second chamber? I know often it is talked of driving the pH to higher levels with a second chamber as while the pH is less than 7.6 there is still CO2 in solution. But, isn't the aragonite saturation more of a limit than the fact there is still CO2 in solution?

[B]Ignoring kinetics for now, but will dissoloution of the media stop when we get to omega = 1 (I presume stuff still dissolves but also precipitates at the same rate?). [/B]

Yes, you cannot dissolve anything more if omega is one or higher. Then allowing kinetics back into the discussion, you likely won't even be able to reach 1.

Yes, the amount of "CO2" in solution is irrelevant. It is the pH, alkalinity, calcium level, and a few other minor factors (temperature, magnesium, etc) that impact the dissolution.

I didn't do a detailed calculation, but pH = 6.8 sounds about right for omega = 1 for that situation.

I used the CO2sys program with the following S=35, t=25, Alk=11.5 mEq/L, pH(NIST)=6.8 and it gave me an omega aragonite as 0.99 and omega calcite as 1.5.

One thing with the program is the calcium concentration is fixed at 420ppm for S=35 so I would expect with the increased calcium level the omega saturation to be > 0.99?

Yes, the increased calcium will raise the supersaturation some, but not nearly as much as the alkalinity did (because the 420 ppm only goes up to about 600 ppm in such a situation). That would make the omega for pH 6.8 about 1.4. So the real omega = 1 point will be a bit lower in pH (6.5-6.6 probably).