Processes > Dissolution and precipitation

Modeling mineral dissolution in equilibrium with atmospheric CO2 at high pH

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Darren_Chevis:
I conducted a series of batch reactions to look into the solubility of a Pb phosphate mineral over a pH range of 2 to 10 and am using PHREEQC to simulate these dissolution experiments.  I set CO2(g) as equilibrium phase with a log PCO2 of -3.5 and fixed the pH using KOH.  As a check on the model output, I calculated the total CO2 at each pH assuming an open system using the equations outline in Appello and Postma (Chapter 5.2).  My calculations and the model output for total CO2 agreed well up to pH 8; however, it began to diverge somewhat at pH 9 and a good bit at pH 10.  There was a large increase ionic strength and total aqueous Pb compared to the experimental results.  For the simulations at pH 10, I redid it by setting the C(4) concentration to the calculated total CO2.  The output seems to be in a much more reasonable but the log fco2 isn't -3.5(-3.74 in my case). 

I am rather new to using PHREEQC so I wanted to reach out to the forum to ask if defining the total CO2 from a separate calculation for the higher pH values would be considered a valid way of going about the modeling?       

dlparkhurst:
I doubt you can attain complete atmospheric equilibrium with CO2 above 9. The concentrations of dissolved inorganic carbon become prohibitively large. So, I think you are correct to try to use experimental values rather than atmospheric equilibrium

See this post for more discussion https://phreeqcusers.org/index.php/topic,2615.msg9902.html#msg9902.

Darren_Chevis:
Thanks David! I appreciate the insight!

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