Very high CO2 solubility in NaHCO3 and Na2CO3 solutions

[jalil]

I have a simple code that calculates CO2 solubility in a brine at a fixed pressure. For 1m of NaCl:

Code: [Select]

SOLUTION 1
units mol/kgw
pH 7 charge
Na 1
Cl 1

GAS_PHASE 1
    -fixed_pressure
    -pressure 100
    CO2(g) 99
    H2O(g) 1
This results in a reasonable CO2 solubility of 1.088m

Now using a NaHCO3 brine:

Code: [Select]

SOLUTION 1
units mol/kgw
pH 7 charge
Na 1
C 1 as HCO3-

GAS_PHASE 1
    -fixed_pressure
    -pressure 100
    CO2(g) 99
    H2O(g) 1
results in a very high CO2 solubility of 11.78m.
I am assuming this is an error with the simulation (also assuming my simple code is correct, using phreeqc.dat). I noticed the same behavior with Na2CO3 solutions. What can possibly cause this?

[dlparkhurst]

Maybe it is the database that you used. You should use phreeqc.dat or pitzer.dat, and the results between the two solutions are similar.

Note that the "as HCO3" has no effect when the units are mol/kgw.

[jalil]

I double checked, I am using phreeqc.dat, tried it on two different machines suspecting that I accidently modified the database but still giving me the same results. pitzer.dat on the other hand gives reasonable results.

Note that the "as HCO3" has no effect when the units are mol/kgw.

Why is that? I checked the output and there is actually HCO3- close to the concentration I specified (the sum of mHCO3- and mNaHCO3 is close to 1). Is there an alternative correct way to specify the molality of HCO3-?

[dlparkhurst]

Using pitzer.dat, I get this for the sodium chloride, where 1.2 mol/kgw of CO2 dissolves.

Code: [Select]

                                            ----------------------------------
Component         log P           P    phi     Initial       Final       Delta

CO2(g)             2.00   9.968e+01  0.464   1.798e+01   1.678e+01  -1.209e+00
H2O(g)            -0.49   3.210e-01  0.099   1.817e-01   5.402e-02  -1.276e-01

-----------------------------Solution composition------------------------------

Elements           Molality       Moles

C                 1.206e+00   1.209e+00
Cl                9.977e-01   1.000e+00
Na                9.977e-01   1.000e+00

And this for NaHCO3, where 1.195 mol/kgw of CO2 dissolves:

Code: [Select]


                                                        Moles in gas
                                            ----------------------------------
Component         log P           P    phi     Initial       Final       Delta

CO2(g)             2.00   9.968e+01  0.464   1.798e+01   1.679e+01  -1.195e+00
H2O(g)            -0.49   3.225e-01  0.099   1.817e-01   5.433e-02  -1.273e-01

-----------------------------Solution composition------------------------------

Elements           Molality       Moles

C                 2.191e+00   2.195e+00
Na                9.980e-01   1.000e+00

The "as HCO3" is used to convert units to mol/kgw. Because you are entering mol/kgw, there is no conversion necessary. You are simply defining a solution with 1 mole of carbon per kilogram of water. Your choice of pH will determine the predominant carbon aqueous species.