SELECTED_OUTPUT of precipitated salts (in g precipitate per kg)?

[Baur-S]

Dear PhreeqcUsers,

The basis of my solution 1 is a chemical analysis of thermal water directly coming from a borehole. I have left out some ions to keep it simple.
In the first calculation step I want to rebuild the original water coming from the depth. I assume that in the water at depth there is so much CO2 that calcite is in equilibrium.
In the following calculation step, the water is transported to the surface and pumped into a basin. There it is in contact with ambient air but it has not cooled down yet.

My modeling in PhreeqcI (Pitzer database) looks like this:

Code: [Select]

SOLUTION 1
    temp      20
    pH        6.5
    units     mmol/l
    C         5
    Ca        150
    Cl        2300
    Mg        10
    Na        2000
    S(6)      6
SAVE solution 1
END

USE solution 1
REACTION_TEMPERATURE 1
    70
REACTION_PRESSURE 1
    140
EQUILIBRIUM PHASES 1   
Calcite 0.0      CO2     
SAVE solution 2
END

USE solution 2
REACTION_TEMPERATURE 2
    70
REACTION_PRESSURE 2
    1
EQUILIBRIUM_PHASES 2     
  CO2(g) -3.4         
END

According to the first calculation (70 C°, 140 atm), all minerals are undersaturated (SI < 0), only calcite has a SI = 0.
In the second calculation (70°C, 1 atm, p(CO2) = 0,0004 atm), some salts precipitate (SI > 0). The SIs of these salts are listed in the section "saturation indices". I am interested in the respective amounts (in g precipitate per kg of water) that precipitate compared to calculation 1. Is there a way to have these quantities listed (e.g. as selected output?).

Thank you in advance for dealing with my problem.

Kind regards,

Sabine Baur

[Leo]

Hi Sabine,

here's my approach to your question:

Code: [Select]

SOLUTION 1
temp      70
pressure  140
pH        6.5
pe        4
redox     pe
units     mmol/l
density   1
C         5 Calcite    0
Ca        150
Cl        2300
Mg        10
Na        2000
S(6)      6
-water    1 # kg

REACTION_PRESSURE 1
1

EQUILIBRIUM_PHASES 1     
CO2(g) -3.4
Calcite 0 0

SELECTED_OUTPUT
-file out.tsv
-reset false
-solution true
-simulation true
-pH true
-totals Cl Na Ca C C(4) # if you want to print out any total element/redox state concentrations
#-equilibrium_phases Calcite # if you want to print out amounts and changes of any equilibrium phase (not possible to convert units)

USER_PUNCH
-headings pressure Calcite_d CO2(g)_d
10 PUNCH PRESSURE
20 PUNCH EQUI_DELTA("Calcite") * 100 # convert from mol to g
30 PUNCH EQUI_DELTA("CO2(g)") * 44 # convert from mol to g
First, I tried to combine the first few steps, which give different results for your initial solution. You have to decide, if this fits your assumptions or not.

Also I included options to write the results to an output file using SELECTED_OUTPUT and USER_PUNCH. With USER_PUNCH, you have the option to include calculations, e. g. if you want to convert units before writing results to the output file.

If you want to "see" how much would theoretically precipitate, you would have to manually include these phases under EQUILIBRIUM_PHASES and also add them to SELECTED_OUTPUT and/or USER_PUNCH. However, you'll have to decide on the phases you want to include based on assumptions which phases would actually precipitate, not only due to their positive SI but due to kinetics etc.

Hope that helps. Feel free to ask if something is unclear.

[Baur-S]

Dear Leo,

thank you very much for your quick reply. I will have a close look at your modeling and see if I have any further questions.

Kind regards,

Sabine