Unclear calcite behaviour in CaCO3-H2O-CO2 system

[amargert]

Hello everyone!

I am trying to understand how properly model a CO2-H2O-CaCO3 system and interpret the output. Let's say there is a model:

SOLUTION 1
    temp      50.00
    pressure  99.67922
    pH        7 charge
    -water    1 # kg
REACTION 1
    Ca        0.2
    C         3.2
    O         6.6
    1
GAS_PHASE 1
    -fixed_pressure
    -pressure 99.67922
    CO2(g)    0.0
    H2O(g)    0.0
END

With such definition I basically mean that there is 0.2 moles of CaCO3 and 3 moles of CO2. Up to my knowledge, when dissolved CaCO3 splits into Ca(2+) and CO3(2-) and can further interact with the other dissolved species present in a solution. However, in the output there is CaCO3 as an aqueous species:

----------------------------Distribution of species----------------------------
                                                         Log       Log              Log        mole V
   Species          Molality    Activity  Molality  Activity     Gamma   cm³/mol

   CaCO3           3.713e-04   4.055e-04    -3.430    -3.392     0.038    -13.89
-------------------------------------------------------------------------------

This amount of CaCO3 is not negligible and it is not very clear whether it means solid or dissolved calcite. In general, I want to run a PHREEQC simulation to obtain a calcite SI and SR and then calculate the dissolution rate outside PHREEQC. With such results it seems like calcite phase equilibrium is already taken into account and has a significant impact on overall species distribution, which is not preferable for my study.
So, is CaCO3 in species distribution section from the output indicates precipitated or dissolved calcite? And in general, is it a good way to obtain a reasonable SI and SR values to calculate the dissolution rate?

P.S. I had an idea to delete Ca+2 + CO3-2 = CaCO3 reaction from phreeqc.dat to force PHREEQC not to account for calcite precipitation.

[dlparkhurst]

That is CaCO3(aq), which is part of the aqueous model by which the equilibrium constants for calcite were fit. If you remove it, you are changing the aqueous model and probably generating an inconsistent solubility or saturation index.

That said, it is just an aqueous model. Different databases may include or not include that aqueous species, or may have a different equilibrium constant. Pitzer.dat does not include any calcium carbonate ion pairs, and instead uses other parameters to account for calcite solubility. I think you have to go with each database, and not change the aqueous species unless you are very careful to be consistent.

[John Mahoney]

YOU Stated

P.S. I had an idea to delete Ca+2 + CO3-2 = CaCO3 reaction from phreeqc.dat to force PHREEQC not to account for calcite precipitation.

This is wrong on several levels and I suggest you start with some simple models to figure out what PHREEQC does. 

The  equation as written above  defines a solution_species, not a phase. SOLUTION_SPECIES do not have a name that is used by the program, The "names" is yo will are commented out in the database. PHREEQC identifies SOLUTION_SPECIES as the first term after the equals sign. 

#CaCO3   38
Ca+2 + CO3-2 = CaCO3
log_k 3.224 

SOLUTION_SPECIES generally have positive log_K values, but if they are hydroxides with a proton on the RHS,  then the log_k values are usually negative.
for example:
#CaOH
Ca+2  + H2O = CaOH+ + H+
log_k -12.78


Phases in the PHASES section of the database are always written as dissociations, and they do have names.

Calcite
CaCO3 = Ca+2 + CO3-2
log_k -8.48

Second issue is PHREEQC will not precipitate any phase unless it is included in the EQUILIBRIUM_PHASES block.  So you do not have to delete anything from the database.  There are a lot of options in using the EQUILIBRIUM_PHASES keyword.  You can easily maintain supersaturated conditions by not including the phase in EQUILIBRIUM_PHASES. 

HERE IS SIMPLE EXAMPLE I USE IN MY COURSE, parts of it go back to an older MINTEQA2 course from 1991. it also shows some simple save and use applications.    We add excess calcite and the pH goes to more than 10. SOLUTION is strongly oversaturated with respect to calcite.   we save that oversaturated solution and in the second step calcite is allowed to precipitate.  The pH goes to less than 10 and about 87% of the calcite "re-precipitates".  The the finals step we take the save calcite (87%) and dissolve it in and acidic solution. We can confirm the path be reviewing the Calcium concentrations.   

TITLE FIRST STEP add calcite in excess
SOLUTION 1 
units moles/L
pH 7.0

REACTION
Calcite
0.001
SAVE SOLUTION 1
END

TITLE SECOND STEP equilibrate solution
PHASES
Fix_H+
H+ = H+
Log k 0.0
USE SOLUTION 1
EQUILIBRIUM_PHASES 1
Calcite 0.0 0.0
SAVE EQUILIBRIUM_PHASES 1
END

TITLE THIRD STEP change solution
SOLUTION 2
pH 3.0
USE EQUILIBRIUM_PHASES 1
END

[amargert]

Dear David and John,

Thank you very much for your replies. I had an idea that CaCO3 is still an aqueous species. Since chemistry is far from my specialty the fact of CaCO3 presence in the output confused me, because I thought that such compound can only exist in solid state. According to your comments, my current model realization does exactly what I want, however it is indeed a good idea to investigate the other available databases and choose the best one for the purpose.

Appreciate your help!