Composition of sea water: speciation model using Geochemist’s Workbench

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:

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.

Figure 2: Bar Chart of SpecE8 Model

Figure 3: Durov Diagram of Sea Water

Figure 4: Piper Diagram of SpecE8 model of Sea Water

Figure 5: Radial Plot of SpecE8 Sea Water speciation

Figure 6: Stiff Diagram of SpecE8 model of sea water