DIC concentrations under fixed pCO2 and increasing pressure

[thrbnmnn]

Ahoy forum!
After a lot of lurking around, I have to raise an issue myself. I tried to look it up, but I think there is somewhere a knot in my approach of thinking about the following problem:
I want to model how DIC concentrations develop under a fixed pCO2 of 15% (or in PhreeqC: -0.824) with increasing pressure, say 1-100 bar.
I understand DIC should increase but it doesn't in my model and I am sure I missed something but I can't really pinpoint the problem.
Here is my code

Code: [Select]

SOLUTION 1

EQUILIBRIUM_PHASES 1
    CO2(g)       -0.83 100

REACTION_PRESSURE 1
    1 100 in 50 steps

USER_GRAPH 1
    -axis_titles            "Pressure (bar)" "DIC (mM)"
    -initial_solutions      false
    -connect_simulations    true
  -start
10 plot_xy PRESSURE, TOT("C(4)")*10^3, color = Red, symbol = Square
  -end
    -active                 true
END
I know that one can use EQUILIBRIUM_PHASES or GAS_PHASE, both approaches didn't yield the proper result. Example 22 (https://water.usgs.gov/water-resources/software/PHREEQC/documentation/phreeqc3-html/phreeqc3-84.htm) uses the REACT approach to add moles of CO2 and increasing pressure thereby, but I want to do it with the fixed pCO2. Any idea where I went in the wrong direction? I am pretty sure it is a conceptual problem in my head. Happy for any advice!

[dlparkhurst]

You are fixing the partial pressure of CO2(g) at 0.15 atm, regardless of the total pressure of the system. The Henry's law constant actually decreases a little with pressure yielding a slight decrease in DIC as the pressure of the system increases. If you plot LK_PHASE("CO2(g)") you will see the decrease in the Henry's law constant.

Perhaps you wanted the CO2(g) partial pressure to be 15% of a gas mixture. Here is a script that simulates a large gas volume that is 0.15 atm CO2 and 0.85 atm inert nitrogen gas at 1 atm and 25 C. The simulation increases the pressure of the system to 100 atm incrementally.

Code: [Select]

SOLUTION 1
END
GAS_PHASE 1
-fixed_pressure
-volume 1e6
Ntg(g) 0.85
CO2(g) 0.15
END
USE solution 1
USE gas_phase 1
REACTION_PRESSURE 1
    1 100 in 50 steps

USER_GRAPH 1
    -headings               DIC P(CO2(g))
    -axis_titles            "Pressure (bar)" "DIC (mM)" "P(CO2(g))"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 plot_xy PRESSURE, TOT("C(4)")*10^3, color = Red, symbol = Square
20 plot_xy PRESSURE, PR_P("CO2(g)"), color = Blue, symbol = Square, y_axis=2
  -end
    -active                 true
END


[thrbnmnn]

Great, now I understand, thanks a lot!
The volume of 10^6 L is chosen to have enough CO2 available to achieve maximum possible dissolution in water under the given conditions, right?