Calculation of amount of precipitated minerals

[mukherjeeindrani09]

I have some physicochemical data of a borehole including cations, anions, EC/TDS, pH and temperature = 12°C and I want to calculate how much amount of the supersaturated minerals would be precipitated in mg/L when the borehole temperature changes from 12°C to 25°C.

Can someone help me here please.

I have below data -

Code: [Select]

TITLE Borehole groundwater sample collected on 24/10/2023 at 10:30
SOLUTION 1
    temp      12.0
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 125
    Cl        94
    S(6)      228
    Ca        106
    Mg        54
    Ba        0
    Na        32
    K         8
    Mn        0.8
    B         0
    N(-3)     0.3
    Al        0.07
    Si        0
    Fe        6.41
    -water    1 # kg
END

[dlparkhurst]

You will need to use EQUILIBRIUM_PHASES, which will react each mineral to equiilbrium.

You can then use the Basic function EQUI_DELTA in USER_PUNCH (SELECTED_OUTPUT must also be defined) to get the moles of mineral that reacted. GFW("Al(OH)3"), for example will calculate the molecular weight of Gibbsite.

REACTION_TEMPERATURE can be used to change the temperature.

Note that some minerals are supersaturated at 12 C, and in most cases, minerals become more soluble at higher temperature.

[mukherjeeindrani09]

I have used the below model

Code: [Select]

USER_PUNCH 1
    -headings ABCD
    -start
10 PUNCH EQUI_DELTA("Calcite")*GFW("CaCO3")*1000
    -end
SELECTED_OUTPUT 1
    -file                 selected_output_2.sel
    -high_precision       true
    -simulation           true
    -step                 true
    -active               true
    -user_punch           true
EQUILIBRIUM_PHASES 1
REACTION_TEMPERATURE 1
    17  22  27  32  37  42  47 
    52  55
TITLE

SOLUTION 1
    temp      12.6
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        6.4
    Alkalinity 125
    Cl        94
    S(6)      228
    Ca        106
    Mg        54
    Na        32
    K         8
    Mn        0.8
    N(-3)     0.3
    Al        0.07
    -water    1 # kg

END
The output of this -

Code: [Select]

EQUI_DELTA   SI
0.0000e+00 -1.15
0.0000e+00 -1.12
0.0000e+00 -1.08
0.0000e+00 -1.03
0.0000e+00 -0.979
0.0000e+00 -0.918
0.0000e+00 -0.850
0.0000e+00 -0.777
0.0000e+00 -0.698
0.0000e+00 -0.648

It means the saturation index of calcite is increasing with temperature increase and the mineral is in an undersaturated condition but 0 mg of calcite in the equilibrium-phase assemblage that reacted during the current calculation. In other words, calcite did not react at all, which seems unreasonable.

[dlparkhurst]

Check the description of EQUILIBRIUM_PHASES in the manual.

[mukherjeeindrani09]

It seems I need to define the minerals in the equilibrium phase. I have added calcite -

Code: [Select]

USER_PUNCH 1
    -headings EQUI_DELTA SI
    -start
10 PUNCH EQUI_DELTA("Calcite")*GFW("CaCO3")*1000, SI("Calcite")
    -end
SELECTED_OUTPUT 1
    -file                 selected_output_2.sel
    -high_precision       true
    -simulation           true
    -step                 true
    -active               true
    -user_punch           true
EQUILIBRIUM_PHASES 1
    Calcite   0 10
REACTION_TEMPERATURE 1
    17  22  27  32  37  42  47 
    52  55
TITLE Sample

SOLUTION 1 Sample
    temp      12.6
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        6.4
    Alkalinity 125
    Cl        94
    S(6)      228
    Ca        106
    Mg        54
    Na        32
    K         8
    Mn        0.8
    N(-3)     0.3
    Al        0.07
    -water    1 # kg

END
Which resulted in below output - which is still unreasonable. I am not sure what I am missing. Could you please help me here.

Code: [Select]

         EQUI_DELTA                   SI
 0.000000000000e+00 -1.151107278269e+00
-1.323817211872e+02   0.000000000000e+00
-1.277038058017e+02   0.000000000000e+00
-1.220108976500e+02   0.000000000000e+00
-1.153465859325e+02   0.000000000000e+00
-1.077660370659e+02   0.000000000000e+00
-9.933889065589e+01   0.000000000000e+00
-9.015048906007e+01   0.000000000000e+00
-8.030141521264e+01   0.000000000000e+00
-7.412173627486e+01   0.000000000000e+00



[dlparkhurst]

Consider the results of this script.

Code: [Select]

SOLUTION 1 Sample
    temp      12.6
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Fe        6.4
    Alkalinity 125
    Cl        94
    S(6)      228
    Ca        106
    Mg        54
    Na        32
    K         8
    Mn        0.8
    N(-3)     0.3
    Al        0.07
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES 1
    Calcite   0 10
SAVE solution 2
END
USE solution 2
USE equilibrium_phases 1
REACTION_TEMPERATURE 1
17
END

[mukherjeeindrani09]

Thank you @dlparkhurst

I have tried with below code

Code: [Select]

SOLUTION 1
    temp      12.6
    pH        6.4
    pe        4
    redox     pe
    units     mg/l
    density   1
    Al        0.07
    Alkalinity 125
    Ca        106
    Cl        94
    Fe        6.4
    K         8
    Mg        54
    Mn        0.8
    N(-3)     0.3
    Na        32
    S(6)      228
    -water    1 # kg

END
USE solution 1
EQUILIBRIUM_PHASES 1
    Calcite   0 10
    Anhydrite 0 10
    Al(OH)3(a) 0 10
    Goethite  0 10
    Gibbsite  0 10
    Hematite  0 10
SAVE solution 2
END
USE solution 2
USE equilibrium_phases 1
REACTION_TEMPERATURE 1
    17  22  27  32  37  42  47 
    52  55
USER_PUNCH 1
    -headings EQUI_DELTA SI
    -start
10 PUNCH EQUI_DELTA("Calcite")*GFW("CaCO3")*1000, SI("Calcite")
    -end
SELECTED_OUTPUT 1
    -file                 selected_output_2.sel
    -high_precision       true
    -simulation           true
    -step                 true
    -active               true
    -user_punch           true
END
And the output is

Code: [Select]

        EQUI_DELTA                   SI
3.247977226930e+00   0.000000000000e+00
7.925892612165e+00   0.000000000000e+00
1.361880076427e+01   0.000000000000e+00
2.028311248191e+01   0.000000000000e+00
2.786366134803e+01   0.000000000000e+00
3.629080775855e+01   0.000000000000e+00
4.547920935385e+01   0.000000000000e+00
5.532828320181e+01   0.000000000000e+00
6.150796213941e+01   0.000000000000e+00





Do I need to consider all the minerals with saturation index > 0 in the EQUILIBRIUM_PHASES section?

Code: [Select]

EQUILIBRIUM_PHASES 1
    Calcite   0 10
    Anhydrite 0 10
    Al(OH)3(a) 0 10
    Goethite  0 10
    Gibbsite  0 10
    Hematite  0 10
    ......
    ......
    ......
   


[dlparkhurst]

What did you learn from the script I posted?

[mukherjeeindrani09]

I guess I should consider minerals with saturation index = 0 in the EQUILIBRIUM_PHASES section? But I did not find any such minerals when calculated the saturation indices using PHREEQC.

[dlparkhurst]

Not the answer I was hoping for.

I'm not here to teach geochemistry, so I will only give you only one lesson--saturation indices.

The following ignores uncertainties in the SI calculation.

If a mineral is undersaturated (negative SI), then the mineral could dissolve if present. The mineral cannot precipitate.

If a mineral is supersaturated (positive SI), then the mineral could precipitate. The mineral cannot dissolve.

If a mineral has SI ~ 0, it could mean that the mineral is reacting fast enough to stay near equilibrium (or it could be coincidental due to other reactions).

All statements are conditioned by kinetics. A mineral reaction may be thermodynamically favored (as indicated by the SI), but reaction simply does not proceed at a significant rate. For example, although dolomite readily dissolves, it does not form under most near-surface aquifer conditions. 

So, your job is to determine which minerals are present and likely to dissolve, which minerals may or may not be present, but are likely to precipitate, and whether CO2(g) or O2(g) (a gas reservoir) is involved.

Knowing nothing about a system, I would start by considering CO2(g) and calcite reactions.

The answer I was looking for is that at the original temperature of your water, calcite must dissolve to reach equilibrium. If you take that equilibrated water and heat it up, calcite precipitates.