How to make the condition of constant pCO2 in solution?

[Jeonghwan Hwang]

Hello, this is Jeonghwan Hwang.
Firstly, thank you for reading.

I developed a sorption model that performed under constant pCO2 condition with 10^-3.5 bar.

I wrote the following to put the pCO2 value into the solution.
The value of CO2, -3.5056, is the logarithm taken after converting 10^-3.5 bar to atm.

=========================
SOLUTION 1
        -units mol/L
        pH 8.0
        U(+6) 9E-8
        Na 0.1
        C(4) 1 CO2(g) -3.5056
        Cl(+7) 0.1. charge
=========================

I wonder if this method is correct or if there is an additional setting to fix the pCO2.

For example, when calculating the equilibrium of a solution to fix H+,
I know that a sentence such as "Fix_H+ -9.0 NaOH 10.0" is applied.

My whole PHREEQC code is written as below;
Thank you

Sincery

Jeonghwan Hwang

=========================================================
database c://phreeqc/database/LLNL.dat

Solution_Species

#Aqueous U(VI) hydrolysis reactions
UO2+2 + H2O = UO2(OH)+ + H+
log_K   -5.25

UO2+2 + 2H2O = UO2(OH)2 + 2H+
log_K   -12.15

UO2+2 + 3H2O = UO2(OH)3- + 3H+
log_K   -20.25

UO2+2 + 4H2O = UO2(OH)4-2 + 4H+
log_K   -32.40

#Aqueous U(VI) carbonate reactions
UO2+2 + CO3-2 = UO2(CO3)
log_K   9.94

UO2+2 + 2CO3-2 = UO2(CO3)2-2
log_K   16.61

UO2+2 + 3CO3-2 = UO2(CO3)3-4
log_K   21.84

2 UO2+2 + CO3-2 + 3 H2O = (UO2)2(CO3)(OH)3-1 + 3H+
log_K   -0.86

3 UO2+2 + CO3-2 + 3 H2O = (UO2)3(CO3)(OH)3+1 + 3H+
log_K   0.66

3 UO2+2 + 6 CO3-2 = (UO2)3(CO3)6-6
log_K   54.0

SURFACE_MASTER_SPECIES
        Bentonite_s     Bentonite_sOH #s
        Bentonitea_w     Bentonitea_wOH #w1
   Bentoniteb_w   Bentoniteb_wOH #w2
   
SURFACE_SPECIES
        Bentonite_sOH = Bentonite_sOH
        log_k  0.0
   
        Bentonitea_wOH = Bentonitea_wOH
        log_k  0.0   

   Bentoniteb_wOH = Bentoniteb_wOH
        log_k  0.0   

#bentonite properties with tirtation curve   
        Bentonite_sOH  + H+ = Bentonite_sOH2+
        log_k  4.5
 
        Bentonite_sOH = Bentonite_sO- + H+
        log_k  -7.9
   
   Bentonitea_wOH  + H+ = Bentonitea_wOH2+
        log_k  4.5
 
        Bentonitea_wOH = Bentonitea_wO- + H+
        log_k  -7.9
   
   Bentoniteb_wOH  + H+ = Bentoniteb_wOH2+
        log_k  6.0
 
        Bentoniteb_wOH = Bentoniteb_wO- + H+
        log_k  -10.5
   
#Strong site      
        Bentonite_sOH + UO2+2 = Bentonite_sOUO2+ + H+
        log_k  3.1
   
   Bentonite_sOH + UO2+2 + H2O = Bentonite_sOUO2OH + 2H+
        log_k  -4.6

   Bentonite_sOH + UO2+2 + 2H2O = Bentonite_sOUO2(OH)2- + 3H+
        log_k  -12.6

   Bentonite_sOH + UO2+2 + 3H2O = Bentonite_sOUO2(OH)3-2 + 4H+
        log_k  -20.9

#Weak site
        Bentonitea_wOH  + UO2+2 = Bentonitea_wOUO2+ + H+
        log_k  0.5
 
        Bentonitea_wOH  + UO2+2 + H2O = Bentonitea_wOUO2OH + 2H+
        log_k  -5.7

EXCHANGE_MASTER_SPECIES
   Bentonite_ex    Bentonite_ex-
   
EXCHANGE_SPECIES
   
   Bentonite_ex- = Bentonite_ex-
   log_K 0
   
   Na+ + Bentonite_ex- = NaBentonite_ex
   log_K 0
   
   2 NaBentonite_ex + UO2+2 = UO2(Bentonite_ex)2 + 2Na+
   log_K 0.45
   
SURFACE 1
   -sites_units absolute
        Bentonite_sOH           5.00E-06
        Bentonitea_wOH        1.00E-04
       Bentoniteb_wOH        1.00E-04
       -no_edl
   
Exchange 1
   NaBentonite_ex         0.002175
   
SOLUTION 1
        -units  mol/L
        pH      8.0
        U(+6)   9E-8
        Na      0.1
   C(4) 1 CO2(g) -3.5056
        Cl(+7)    0.1.    charge
   
   
PHASES
        Fix_H+
        H+ = H+
        log_k  0.0
END


SELECTED_OUTPUT
        -file Fig2a.txt
        -reset              false
   -pH                 true
        -molalities      Bentonite_sOUO2+  Bentonite_sOUO2OH  Bentonite_sOUO2(OH)2-  Bentonite_sOUO2(OH)3-2  Bentonitea_wOUO2+  Bentonitea_wOUO2OH  UO2(Bentonite_ex)2
   -totals          Bentonite_s Bentonitea_w Bentoniteb_w Bentonite_ex U(3) U(4) U(5) U(6)
   
   
USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.25  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.75  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.25   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.75  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -4.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -4.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -6.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -6.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -7.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -7.5  NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -8.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -8.5   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -9.0   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -9.5   NaOH    10.0
END

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -10.0   NaOH    10.0
END

[dlparkhurst]

The following will cause the reacted solution to maintain equilibrium with atmospheric CO2(g). Note that at high pH, the total carbon in equilibrium with atmospheric CO2 will be quite large and ultimately beyond the limits of a phreeqc calculation.

Code: [Select]

USE solution 1
USE surface 1
USE Exchange 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.0   NaOH    10.0
        CO2(g)   -3.4   10
END