Processes > Inverse modelling

Calculation of amount of precipitated minerals

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dlparkhurst:
Consider the results of this script.


--- Code: ---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
--- End code ---

mukherjeeindrani09:
Thank you @dlparkhurst

I have tried with below code


--- Code: ---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
--- End code ---

And the output is

--- Code: ---        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

--- End code ---





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


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

--- End code ---

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.

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