It is wonderful what you can really do with Geochemist’s workbench. In this exercise, I am going to use the known concentrations of sea water and use Geochemist’s workbech to perform a little exercise.
Geochemist workbench has different modules. In this particular exercise, I am going to use the SpecE8 module. This module is similar to PHREEQC and similar task can be performed with that powerful tool also.
Now to model sea water composition, we are going to make some assumptions:
- CO2 fugacity controls the pH of the sea water which can be written in terms of the reaction: H+ + HCO3- = CO2 + H2O. Log fugacity of CO2 in the atmosphere = -3.5
- We also assume that the sea water dissolved oxygen (O2(aq)) is in equilibrium with atmospheric oxygen (O2(g)). or, f(O2)=.2
So, in our model we do not specify any pH and let the model predict the pH of the sea water. It would be nice to see how close we get to the actual pH of the sea water.
Analysis of sea water composition is widely available in numerous geochemistry text books and over the web. I used the sea water composition as presented in the website: http://www.seafriends.org.nz/oceano/seawater.htm
Chemical analysis of Sea Water:
chemical ion |
valence
|
concentration
ppm, mg/kg |
part of
salinity % |
molecular
weight |
mmol/
kg |
Chloride Cl |
-1
|
19345
|
55.03
|
35.453
|
546
|
Sodium Na |
+1
|
10752
|
30.59
|
22.990
|
468
|
Sulfate SO4 |
-2
|
2701
|
7.68
|
96.062
|
28.1
|
Magnesium Mg |
+2
|
1295
|
3.68
|
24.305
|
53.3
|
Calcium Ca |
+2
|
416
|
1.18
|
40.078
|
10.4
|
Potassium K |
+1
|
390
|
1.11
|
39.098
|
9.97
|
Bicarbonate HCO3 |
-1
|
145
|
0.41
|
61.016
|
2.34
|
Bromide Br |
-1
|
66
|
0.19
|
79.904
|
0.83
|
Borate BO3 |
-3
|
27
|
0.08
|
58.808
|
0.46
|
Strontium Sr |
+2
|
13
|
0.04
|
87.620
|
0.091
|
Fluoride F |
-1
|
1
|
0.003
|
18.998
|
0.068
|
Before doing anything, I used the GSS module of the Geochemist’s module to check for the Charge Imbalance Error. It came out to be 0.023%. This gave me confidence that we have all of the major ions that we need to properly construct the geochemical model. Table 1 shows the result that I got.
Now it is time to construct the geochemical model using the SpecE8 model. Many people like to hand code all commands into the Geochemist’s workbench. I find it easier just to use their user friendly interface. However, sometime you need to know some simple tricks to set up the basis panel. “SWAP” could be little confusing in the beginning. To construct the model we would have to swap fugacity of CO2 with H+ ion concentration. This will keep the f(CO2) constant (so at atmospheric equilibrium). We also need to SWAP between atmospheric O2 and dissolved O2. Figure 1 shows how to construct the basis for sea water speciation model.
Once the basis is ready, we can run the model. Below I am going to focus on some of the results and at the end I am going to attach the complete output file as TXT. you can download an play with the txt if you like:
Part 1 Result: This part gives you the some of the important parameters. First of all, the model has predicted a pH of 8.34 for the sea water. In reality the pH varies between 7.8 to 8.5 depending on the place of sea water collection. Secondly, the CBE is 0.04%. This again tells you that the chemical analysis of the sea water was great. It gives you the carbonate alkalinity 122.22 mg/kg sol’n as CaCO3 and also tells you that you have a Na-Cl water.
- Temperature = 25.0 C Pressure = 1.013 bars
- pH = 8.344 log fO2 = -0.699
- Eh = 0.7253 volts pe = 12.2612
- Ionic strength = 0.635602
- Charge imbalance = 0.040118 eq/kg (7.153% error)
- Activity of water = 0.982060
- Solvent mass = 1.000000 kg
- Solution mass = 1.037308 kg
- Solution density = 1.030 g/cm3
- Chlorinity = 0.566009 molal
- Dissolved solids = 35966 mg/kg sol’n
- Hardness = 6371.71 mg/kg sol’n as CaCO3
- carbonate = 122.22 mg/kg sol’n as CaCO3
- non-carbonate = 6249.49 mg/kg sol’n as CaCO3
- Rock mass = 0.000000 kg
- Carbonate alkalinity= 122.22 mg/kg sol’n as CaCO3
- Water type = Na-Cl
Result part 2: this part details the mollalities and log activities of the different species present in sea water. We can easily tell that the sea water is dominated by free Cl- and Na+ ion followed by Mg++, K+, SO4– and other complex species.
Aqueous species molality mg/kg sol’n act. coef. log act.
—————————————————————————
Cl- 0.5491 1.877e+004 0.6290 -0.4617
Na+ 0.4780 1.059e+004 0.6726 -0.4928
Mg++ 0.04301 1008. 0.3171 -1.8652
SiO2(aq) 0.02210 1280. 1.1689 -1.5877
K+ 0.01029 387.8 0.6290 -2.1890
MgCl+ 0.009901 570.4 0.6726 -2.1766
Ca++ 0.006497 251.0 0.2479 -2.7930
CaCl+ 0.004146 301.9 0.6726 -2.5547
H6(H2SiO4)4– 0.004042 1490. 0.1709 -3.1606
NaH3SiO4 0.003871 440.7 1.0000 -2.4121
NaCl 0.002793 157.4 1.0000 -2.5539
HCO3- 0.001506 88.57 0.6913 -2.9826
H3SiO4- 0.001270 116.4 0.6726 -3.0686
Mg(H3SiO4)2 0.001194 247.0 1.0000 -2.9228
Br- 0.0008568 66.00 0.6290 -3.2685
MgH3SiO4+ 0.0005698 65.60 0.6726 -3.4165
NaHCO3 0.0004503 36.47 1.0000 -3.3465
MgHCO3+ 0.0002160 17.77 0.6726 -3.8377
O2(aq) 0.0002159 6.660 1.1689 -3.5980
MgH2SiO4 0.0001743 19.89 1.0000 -3.7588
Sr++ 0.0001529 12.92 0.2102 -4.4928
MgCO3 0.0001173 9.533 1.0000 -3.9308
KCl 5.778e-005 4.152 1.0000 -4.2382
CO3– 5.454e-005 3.155 0.1907 -4.9829
CaHCO3+ 3.911e-005 3.812 0.7147 -4.5536
CaH3SiO4+ 3.823e-005 4.982 0.6726 -4.5899
F- 2.903e-005 0.5317 0.6519 -4.7229
CaCO3 2.754e-005 2.658 1.0000 -4.5600
Mg2CO3++ 2.653e-005 2.778 0.2102 -5.2536
MgF+ 2.419e-005 1.010 0.6726 -4.7886
Ca(H3SiO4)2 1.668e-005 3.703 1.0000 -4.7778
NaCO3- 1.588e-005 1.271 0.6726 -4.9714
CO2(aq) 1.116e-005 0.4734 1.0000 -4.9524
MgOH+ 7.113e-006 0.2833 0.6726 -5.3202
OH- 3.426e-006 0.05617 0.6519 -5.6510
H4(H2SiO4)4—- 2.379e-006 0.8725 0.0007 -8.7707
CaH2SiO4 1.736e-006 0.2246 1.0000 -5.7603
NaF 8.028e-007 0.03250 1.0000 -6.0954
SrHCO3+ 7.507e-007 0.1076 0.6726 -6.2968
CaF+ 5.709e-007 0.03251 0.6726 -6.4157
NaOH 4.507e-007 0.01738 1.0000 -6.3462
SrCO3 2.146e-007 0.03054 1.0000 -6.6684
CaOH+ 1.063e-007 0.005848 0.6726 -7.1459
H2SiO4– 8.788e-008 0.007972 0.1709 -7.8234
Mg2OH+++ 2.483e-008 0.001571 0.0700 -8.7598
H+ 5.640e-009 5.480e-006 0.8037 -8.3436
SrF+ 5.085e-009 0.0005227 0.6726 -8.4659
KOH 4.588e-009 0.0002482 1.0000 -8.3384
SrOH+ 5.478e-010 5.526e-005 0.6726 -9.4336
HF 1.267e-010 2.444e-006 1.0000 -9.8972
Mg4(OH)4++++ 7.598e-013 1.210e-007 0.0225 -13.7677
HF2- 1.127e-014 4.239e-010 0.6726 -14.1202
HCl 1.244e-015 4.371e-011 1.0000 -14.9053
H2F2 4.303e-020 1.660e-015 1.0000 -19.3663
ClO4- 6.569e-024 6.298e-019 0.6519 -23.3683
SiF6– 4.689e-033 6.422e-028 0.1709 -33.0962
H2(aq) 4.078e-045 7.924e-042 1.1689 -44.3218
CH4(aq) 4.853e-149 7.506e-145 1.1689 -148.2462
CH3COO- 1.989e-154 1.132e-149 0.6913 -153.8616
MgCH3COO+ 5.197e-155 4.176e-150 0.6726 -154.4565
NaCH3COO 2.904e-155 2.297e-150 1.0000 -154.5370
SrCH3COO+ 9.039e-158 1.278e-152 0.6726 -157.2161
HCH3COO 3.556e-158 2.059e-153 1.0000 -157.4490
CaCH3COO+ 2.168e-158 2.072e-153 0.6726 -157.8362
Ca(O-phth) 0.0000 0.0000 1.0000 -300.0000
Na(O-phth)- 0.0000 0.0000 0.6726 -300.0000
H2(O-phth) 0.0000 0.0000 1.0000 -300.0000
H(O-phth)- 0.0000 0.0000 0.6726 -300.0000
(O-phth)– 0.0000 0.0000 0.1709 -300.0000
Part 3:-Saturation index : mineral saturation index An index showing whether a water will tend to dissolve or precipitate a particular mineral. Its value is negative when the mineral may be dissolved, positive when it may be precipitated, and zero when the water and mineral are at chemical equilibrium. The result shows that although the system has acquired internal equilibrium within the fluid, there are still 23 metastable mineral phases present. However, it is important to remember that the minerals could be more soluble that the derived values based on the LLNL database.
log Q/K log Q/K
—————————————————————-
Antigorite 86.4430s/sat Bischofite -7.3648
Tremolite 27.4628s/sat KNaCO3^6H2O -7.6905
Anthophyllite 23.2014s/sat Antarcticite -7.8763
Sepiolite 18.6718s/sat SrCl2^2H2O -7.9548
Talc 16.5211s/sat Na2SiO3 -8.1699
Chrysotile 9.7027s/sat CaCl2^4H2O -8.6398
Diopside 4.5557s/sat Portlandite -8.6988
Dolomite-ord 3.5432s/sat Ca(OH)2(c) -8.6988
Dolomite 3.5432s/sat SrCl2^H2O -9.4232
Quartz 2.4116s/sat Ca5Si6O17^3H2O -9.7641
Huntite 2.3004s/sat Carnallite -9.9444
Tridymite 2.2458s/sat MgOHCl -10.0385
Chalcedony 2.1404s/sat MgCl2^4H2O -10.2774
Dolomite-dis 1.9988s/sat Ca2SiO4^7/6H2O -10.7288
Strontianite 1.9515s/sat Ca2SiO4(gamma) -11.3250
Cristobalite 1.8611s/sat CaCl2^2H2O -11.8283
Enstatite 1.7577s/sat CaCl2^H2O -11.9552
Amrph^silica 1.1259s/sat K2CO3^3/2H2O -12.5017
Magnesite 1.0593s/sat SrCl2(c) -12.5775
Calcite 0.8550s/sat Larnite -12.7860
CaSi2O5^2H2O 0.7341s/sat Rankinite -13.4744
Aragonite 0.6901s/sat MgBr2^6H2O -13.6887
Forsterite -0.1057 SrBr2^6H2O -13.9048
Monohydrocalcite -0.1467 Ca4Si3O10^3/2H2O -14.0754
Ca2Si3O8^5/2H2O -0.3435 Sr(OH)2(c) -14.3787
Fluorite -1.2800 Merwinite -14.9477
Wollastonite -1.3251 Hydrophilite -15.5350
Brucite -1.6324 MgCl2^2H2O -15.6990
Artinite -1.6564 Ca2Cl2(OH)2^H2O -16.1839
Nesquehonite -1.6630 Sr2SiO4(c) -18.4683
Pseudowollastoni -1.7160 Ca6Si6O18^H2O -18.7464
Halite -2.5473 Lime -18.8076
Monticellite -2.6387 MgCl2^H2O -19.0437
Hydromagnesite -2.8020 KMgCl3^2H2O -19.5978
MgF2(c) -3.1325 SrBr2^H2O -19.8539
SrSiO3(c) -3.4642 Ca3Si2O7^3H2O -21.7603
Sylvite -3.6101 SrBr2(c) -23.3666
Ca5Si6O17^21/2H2 -4.2514 Chloromagnesite -24.7928
Gaylussite -4.3245 Tachyhydrite -26.8322
Pirssonite -4.4636 KMgCl3 -26.8883
Mg2Cl(OH)3^4H2O -5.3251 SrO(c) -28.9331
SrF2(c) -5.4002 Ca4Cl2(OH)6^13H2 -30.5893
Kalicinite -5.5036 Ca3SiO5 -33.9276
Na2Si2O5 -5.6438 Na4SiO4 -36.7405
Akermanite -5.7759 MgBr2 -37.0691
Ca5Si6O17^11/2H2 -6.0696 K8H4(CO3)6^3H2O -46.8361
SrCl2^6H2O -6.1348 Na6Si2O7 -57.8214
KBr -6.5701 Graphite -75.5362
NaBr -6.6932 O-phth acid(c) -596.6112
Part 4: Gas Fugacities: This part calculates the different gases in sea water.
Gases fugacity log fug.
———————————————–
O2(g) 0.2000 -0.699
Steam 0.03075 -1.512
CO2(g) 0.0003162 -3.500
H2(g) 6.169e-042 -41.210
CH4(g) 3.750e-146 -145.426
Part 5: Initial basis and elemental composition
Original basis total moles moles mg/kg moles mg/kg L/kg
——————————————————————————-
Br- 0.000857 0.000857 66.0
Ca++ 0.0108 0.0108 416.
Cl- 0.566 0.566 1.93e+004
F- 5.46e-005 5.46e-005 1.00
H+ -0.0169 -0.0169 -16.4
H2O 55.6 55.6 9.65e+005
HCO3- 0.00247 0.00247 145.
K+ 0.0103 0.0103 390.
Mg++ 0.0553 0.0553 1.29e+003
Na+ 0.485 0.485 1.08e+004
O2(aq) 0.000216 0.000216 6.66
SiO2(aq) 0.0466 0.0466 2.70e+003
Sr++ 0.000154 0.000154 13.0
Elemental composition In fluid Sorbed
total moles moles mg/kg moles mg/kg
——————————————————————————-
Bromine 0.0008568 0.0008568 66.00
Calcium 0.01077 0.01077 416.0
Carbon 0.002465 0.002465 28.54
Chlorine 0.5660 0.5660 1.934e+004
Fluorine 5.460e-005 5.460e-005 1.000
Hydrogen 111.1 111.1 1.080e+005
Magnesium 0.05527 0.05527 1295.
Oxygen 55.66 55.66 8.585e+005
Potassium 0.01035 0.01035 390.0
Silicon 0.04663 0.04663 1263.
Sodium 0.4851 0.4851 1.075e+004
Strontium 0.0001539 0.0001539 13.00
Graphical presentation:
The output of the geochemical modeling can be presented in various different ways. Below are some examples.
log Q/K log Q/K
—————————————————————-
Antigorite 86.4430s/sat Bischofite -7.3648
Tremolite 27.4628s/sat KNaCO3^6H2O -7.6905
Anthophyllite 23.2014s/sat Antarcticite -7.8763
Sepiolite 18.6718s/sat SrCl2^2H2O -7.9548
Talc 16.5211s/sat Na2SiO3 -8.1699
Chrysotile 9.7027s/sat CaCl2^4H2O -8.6398
Diopside 4.5557s/sat Portlandite -8.6988
Dolomite-ord 3.5432s/sat Ca(OH)2(c) -8.6988
Dolomite 3.5432s/sat SrCl2^H2O -9.4232
Quartz 2.4116s/sat Ca5Si6O17^3H2O -9.7641
Huntite 2.3004s/sat Carnallite -9.9444
Tridymite 2.2458s/sat MgOHCl -10.0385
Chalcedony 2.1404s/sat MgCl2^4H2O -10.2774
Dolomite-dis 1.9988s/sat Ca2SiO4^7/6H2O -10.7288
Strontianite 1.9515s/sat Ca2SiO4(gamma) -11.3250
Cristobalite 1.8611s/sat CaCl2^2H2O -11.8283
Enstatite 1.7577s/sat CaCl2^H2O -11.9552
Amrph^silica 1.1259s/sat K2CO3^3/2H2O -12.5017
Magnesite 1.0593s/sat SrCl2(c) -12.5775
Calcite 0.8550s/sat Larnite -12.7860
CaSi2O5^2H2O 0.7341s/sat Rankinite -13.4744
Aragonite 0.6901s/sat MgBr2^6H2O -13.6887
Forsterite -0.1057 SrBr2^6H2O -13.9048
Monohydrocalcite -0.1467 Ca4Si3O10^3/2H2O -14.0754
Ca2Si3O8^5/2H2O -0.3435 Sr(OH)2(c) -14.3787
Fluorite -1.2800 Merwinite -14.9477
Wollastonite -1.3251 Hydrophilite -15.5350
Brucite -1.6324 MgCl2^2H2O -15.6990
Artinite -1.6564 Ca2Cl2(OH)2^H2O -16.1839
Nesquehonite -1.6630 Sr2SiO4(c) -18.4683
Pseudowollastoni -1.7160 Ca6Si6O18^H2O -18.7464
Halite -2.5473 Lime -18.8076
Monticellite -2.6387 MgCl2^H2O -19.0437
Hydromagnesite -2.8020 KMgCl3^2H2O -19.5978
MgF2(c) -3.1325 SrBr2^H2O -19.8539
SrSiO3(c) -3.4642 Ca3Si2O7^3H2O -21.7603
Sylvite -3.6101 SrBr2(c) -23.3666
Ca5Si6O17^21/2H2 -4.2514 Chloromagnesite -24.7928
Gaylussite -4.3245 Tachyhydrite -26.8322
Pirssonite -4.4636 KMgCl3 -26.8883
Mg2Cl(OH)3^4H2O -5.3251 SrO(c) -28.9331
SrF2(c) -5.4002 Ca4Cl2(OH)6^13H2 -30.5893
Kalicinite -5.5036 Ca3SiO5 -33.9276
Na2Si2O5 -5.6438 Na4SiO4 -36.7405
Akermanite -5.7759 MgBr2 -37.0691
Ca5Si6O17^11/2H2 -6.0696 K8H4(CO3)6^3H2O -46.8361
SrCl2^6H2O -6.1348 Na6Si2O7 -57.8214
KBr -6.5701 Graphite -75.5362
NaBr -6.6932 O-phth acid(c) -596.6112
What happened to sulfate in your model?