Conceptual Models > Equilibrium assumptions

Carbonate system

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docla:
I am working on simulation of a neutralization process. The process neutralizes an alkaline effluent (pH = 11.5 and over and Ca = 120 mg/L) using aeration with atmospheric CO2. I am using a common diffusion equation for transfer of CO2 into water:

rCO2 = kla (H pCO2 - CO2) with kla = transfer coefficient, H = Henry constant, pCO2 = partial pressure and CO2 = CO2 activity in water, and I am using the PWP model for calcite precipitation.

Carbonate system is generally assumed to include CO2, H2CO3, H2CO3* (standing for CO2 + H2CO3), HCO3- and CO32-. I was surprise to see that H2CO3 is not included in the PHREEQC database. The carbonate system equilibrium equations that I found in the database are:

CO3-2 + 2 H+ = CO2 + H2O
CO3-2 + H+ = HCO3-
Ca+2 + CO3-2 + H+ = CaHCO3+
Ca+2 + CO3-2 = CaCO3

Why is H2CO3 not present in the database? I am asking because I found in Stumm & Morgan (Aquatic Chemistry Chemical Equilibriua and Rates in Natural Waters, 1996) that hydration of CO2 into H2CO3 is not so fast compared to other dissociation equations (section 4.8). I feel that considering this would improve my model calibration. Is H2CO3 considered somewhere in the database that I did not see?

Thanks a lot,

Dominique

dlparkhurst:
CO2 in the PHREEQC database represents H2CO3*. We chose CO2 because it is my understanding that at equilibrium, CO2 predominates over H2CO3.  However, because the two species are not distinguished the choice of CO2* versus H2CO3* only negligibly affects the amount of H2O in the system.

Including both CO2 and H2CO3 may be more physically reasonable, but it would still be an equilibrium distribution, and your point is that CO2 -> H2CO3 is kinetic. So, it sounds like you want to have a KINETICS reaction for the dissolution of CO2 in your system.

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