Speciation of solution chemistry

[Anvj0705]

Hii,

I am trying to calculate the saturation indices of minerals at certain chemistry of solution.

In brief about the experiment....
            I did a glass dissolution experiment at 90 degree. I collected sample in a certain duration of time to measure the chemistry of the solution.
Now I want to know the saturation indices of possible mineral in my solution. I have only pH, Temp. and cation concentration in input file. In output data, the percentage error is huge. Can I use this this data.

I wanted to know that... Can I use this simulated saturation indices?

Code: [Select]

#EXP-6_3_ all SI vs Time
 
SOLUTION 2 #HRS
 temp 90
 ph 8.7
 units ppm
K 29.04
Na 15.14
Al 0.38
Ca 0.5
Mg 0.09
Si 1.31
 
SOLUTION 4
 temp 90
 ph 8.7
 units ppm
K 11.66
Na 6.43
Al 0.36
Ca 0.95
Mg 0.35
Si 2.35

SOLUTION 6
 temp 90
 ph 8.7
 units ppm
K 11.59
Na 6.79
Al 0.37
Ca 1.29
Mg 0.41
Si 4.41

SOLUTION 8
 temp 90
 ph 8.7
 units ppm
K 12.15
Na 7.11
Al 0.33
Ca 1.51
Mg 0.71
Si 6.11

SOLUTION 10
 temp 90
 ph 8.7
 units ppm
K 11.96
Na 6.93
Al 0.49
Ca 1.69
Mg 0.83
Si 7.24

SOLUTION 12
 temp 90
 ph 8.7
 units ppm
K 12.13
Na 7.4
Al 0.64
Ca 1.79
Mg 0.96
Si 8.49

SOLUTION 14
 temp 90
 ph 8.7
 units ppm
K 11.78
Na 7.02
Al 0.68
Ca 2.37
Mg 1.11
Si 9.29

SOLUTION 16
 temp 90
 ph 8.7
 units ppm
K 11.96
Na 7.03
Al 0.5
Ca 2.13
Mg 1.06
Si 9.3

SOLUTION 18
 temp 90
 ph 9.1
 units ppm
K 1.54
Na 6.12
Al 1.98
Ca 3.15
Mg 1.44
Si 13.32

SOLUTION 20
 temp 90
 ph 9.1
 units ppm
K 27.83
Na 13.76
Al 1.34
Ca 5.33
Mg 1.67
Si 14.25

SOLUTION 22
 temp 90
 ph 9.1
 units ppm
K 27.67
Na 13.69
Al 1.56
Ca 5.1
Mg 1.71
Si 17.09

SOLUTION 24
 temp 90
 ph 9.1
 units ppm
K 28.39
Na 14.11
Al 1.9
Ca 5.59
Mg 1.69
Si 21.98

SOLUTION 28
 temp 90
 ph 9.1
 units ppm
K 26.4
Na 12.97
Al 1.68
Ca 6.26
Mg 1.7
Si 21.71

SOLUTION 32
 temp 90
 ph 9.1
 units ppm
K 3.45
Na 12.12
Al 2.1
Ca 7.28
Mg 1.68
Si 23.4


SELECTED_OUTPUT
     -file Output.txt
     -reset false
-pH true
-activities H+ Mg+2 MgOH+ Al+3 Al(OH)4- Al(OH)3 Al(OH)2+ AlOH+2 Na+ Ca+2 K+ H4SiO4
-saturation_indices Brucite Gibbsite Kaolinite Ca-Montmorillonite

[dlparkhurst]

No saturation indices will be calculated for carbonate, sulfate, or chloride minerals.

So, the only minerals that you could consider are pure aluminosilicate and oxyhydroxide minerals. I suppose you can use the saturation indices for those minerals. The ionic strength will be too low, which affects the activity coefficients. It would be a little better if you include an anion to allow a better estimate of ionic strength, whichever anion you think is predominant.

You will have a hard time interpreting your data if calcite, dolomite, or sulfate minerals are important reactants. Calcite can be an important reactant even in aluminosilicate dominated rocks. 

[Anvj0705]

How would the percentage error be explained if I am using the SI values of these minerals as mentioned in the script?

[dlparkhurst]

You will have to decide.

[Anvj0705]

In the continuation of my Pyrite dissolution experiment. I tried to calculate the solubility calculation of pyrite at wide pH range.
I run the following script.

Code: [Select]

# Title Pyrite Dissolution

PHASES
Fix_H+
H+ = H+
log_k 0

SOLUTION 1
temp 25
units mol/kgw
O(0) 1 O2(g) -0.7
-water 1 #kg
END

USER_GRAPH 1
    -headings               pH Fe
    -axis_titles            "pH" "% Pyrite dissolved"
    -chart_title            "Pyrite solubility curve"
    -axis_scale y_axis      auto auto auto auto auto
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X -LA("H+")
20 GRAPH_Y TOT("Fe")*100/0.00416666666666667
  -end
    -active                 true
END

USE SOLUTION 1
SELECTED_OUTPUT 1
    -file                 Only Pyrite.dat
    -totals               Fe
    #-molalities         H+ OH- Na+
    -saturation_indices   Hematite Goethite Magnetite Ferrihydrite(am) Ferryhydrite Fe(OH)2(cr) FeO(s) Maghemite(ord) Maghemite(disord)

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -2.0 HCl 10
Pyrite 0 4.167E-3
#Goethite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -2.5 HCl 10
Pyrite 0 4.167E-3
#Goethite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -3.0 HCl 10
Pyrite 0 4.167E-3
#Goethite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -3.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -4.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -4.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -5.1 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -5.8 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -6.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -6.6 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -7.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -7.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -8.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
#Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -8.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -9.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -9.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -10.0 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -10.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -11.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 0 0
Magnetite 0 0
END
I kept the over saturated phases in equilibrium with solution and allow them to get precipitated. but my experimental data is not matching well at some pH range.

Now I am thinking to allow these phases at the target SI where solubility is matching with my experimental data Like following script....

If it is fine to do, How would I get the amount of moles of precipitated phase?
Is it possible to consider Specific surface area of mineral in solubility calculation?

Code: [Select]

USE solution 1
EQUILIBRIUM_PHASES 1
Fix_H+ -8.5 NaOH 10
Pyrite 0 4.167E-3
#Goethite 0 0
Hematite 2 0
Magnetite 7.2 0
END

[dlparkhurst]

EQULIBRIUM_PHASES calculates equilibrium (or the target SI); it does not depend on surface area.

If you set a target SI, the amount of the mineral will adjust to the specified SI or dissolve completely. The amount dissolved or precipitated is given by EQUI_DELTA("mineral")with an equal change in the moles of EQUI("mineral"). The amount is also shown in the print file.

[Anvj0705]

Thank you for your response.

I attempted to set the target saturation index (SI) of a specific mineral to achieve the desired iron concentration in solution. However, in the output file, the EQUI_DELTA("mineral") command shows the target SI or equilibrium phase values as 0.00.
Here I could not able to attach the file, otherwise I would have attach print file/experimental plot.

[dlparkhurst]

Code: [Select]

SOLUTION
O(0) 1 O2(g) -0.7
END
USE solution 1
EQUILIBRIUM_PHASES
Pyrite -5 10
USER_PRINT
10 PRINT "Pyrite SI: :", SI("Pyrite")
20 PRINT "Delta pyrite, moles: ", EQUI_DELTA("Pyrite")
END