Calcite dissolution under low (fixed) pH and varying pressure

[mr85woly]

Hi everybody,

I'm rather new to modelling with PhreeqC and I'm not sure if I have to right solution for my problem:
In the model I dissolve a solid calcite phase in a regime with a fixed pH value of 3 and various pressure conditions (1 to 221 atm in 20 steps). In the end I'd like to see how much CO2 is created during the process and how much of it is dissolved in the water (saturation index /fugacity of CO2) for each pressure. I'd like to have a graph showing pressure vs saturation index of CO2. The problems are: 1) the pH slighty changes to about 3.119 due to "charge balance". It's not crucial to fix the pH to exactly 3, but I'm still confused why the buffering is not working. And 2) I get a saturation index of CO2 of about 3.3, rising with pressure to about 3.5. This seems rather high and I'm not sure if the values are correct or if I missed something.
So this is what I did, I appreciate every help from you :-)

TITLE Calcite_dissolution carbfix.dat
PHASES

#new phase to fix pH value
Fix_H+
     H+ = H+
     log_k  0.0

#new phase: use NaOH or HNO3 to fix pH
NaNO3
   NaNO3 = Na+ + NO3-
   log_K -20

SOLUTION 1 pure Water
    temp      20
    pH        3
    pe        4
    redox     pe
    units     mg/l
    density   1
    -water    1 # kg
REACTION_PRESSURE 1
    1 221 in 20 steps
EQUILIBRIUM_PHASES 1
    Calcite   0 10
    Fix_H+    -3 HNO3      10
    NaNO3     0 10
SELECTED_OUTPUT 1
    -file                 calcite.sel
    -saturation_indices   CO2(g)

USER_GRAPH 1 Example 2
    -headings               Pressure vs SI (CO2)
    -axis_titles            "P (atm)" "saturation index CO2" ""
    -axis_scale x_axis      0 230 20 20
    -axis_scale y_axis      3 4 0.2 0.2
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x PRESSURE
20 graph_y SI("CO2(g)")
  -end
    -active                 true
END

[dlparkhurst]

I ran with phreeqc.dat and did not see any pH other than 3.0. It also produces SI values around 2.8. The results seem to be dependent on your database.

If you use GAS_PHASE, phreeqc.dat will use a Peng-Robinson non-ideal approach, which is probably needed at the higher pressures.

[mr85woly]

Thank you for your really quick answer! I used the "carbfix.dat" (like indicated in the title), but forgot to mention in my explanation. With the phreeqc.dat it looks much better!