Different O(0) molality between LLNL and Sit

Conceptual Models > Database selection and modification

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Jeonghwan Hwang:
Hello, this is Jeonghwan Hwang.

I want to check the database effects for porewater simulation.
I used LLNL and Sit database and made a simple solution.

================================================
DATABASE C:\PHREEQC\database\Sit.dat
EXCHANGE_MASTER_SPECIES
    Z   Z-
EXCHANGE_SPECIES
    Z- = Z-
       log_k   0.0
    Z- + Na+ = NaZ
       log_K 0.0
    Z- + K+ = KZ
       log_k 0.6
    2Z- + Ca+2 = CaZ2
       log_k 0.41
    2Z- + Mg+2 = MgZ2
       log_k 0.34
SURFACE_MASTER_SPECIES
    Mont_wa   Mont_waOH
    Mont_wb   Mont_wbOH
SURFACE_SPECIES
    Mont_waOH = Mont_waOH
    log_k 0
    Mont_waOH + H+ = Mont_waOH2+
    log_k 4.5
    Mont_waOH = Mont_waO- + H+
    log_k -7.9
    Mont_wbOH = Mont_wbOH
    log_k 0
    Mont_wbOH + H+ = Mont_wbOH2+
    log_k 6.0
    Mont_wbOH = Mont_wbO- + H+
    log_k -10.5
KNOBS
-tolerance 1e-16
end
SOLUTION 1
    units mol/L
    pH   7.2
    pe   -2.42
    Temp   15
    C   2.20E-03 as HCO3
    Ca   2.33E-02
    Cl   1.53E-01
    Fe   3.31E-05
    K   8.75E-04
    Mg   9.30E-03
    Na   8.88E-02
    S   6.80E-03
    Si   1.85E-04
    -water 0.1
GAS_PHASE 1
-fixed_volume
-volume 0.33
CO2(g) 0.000308
      O2(g) 0.2095
EXCHANGE 1
       NaZ   0.84645
       CaZ2 0.10581
       KZ 0.023513
       MgZ2 0.047025
SURFACE 1
    -sites_units absolute
    Mont_waOH 0.0627
    Mont_wbOH 0.0627
    no_edl
EQUILIBRIUM_PHASES 1
    Quartz 0   1.3063
    #K-feldspar 0 0.39469
    Pyrite    0   0.0091552
    Gypsum    0   0.08068
Calcite 0 0
Siderite 0 0
Dolomite 0 0
END
================================================

In the results, I showed a significant difference between LLNL and Sit.
Especially, the O(0) concentration in Sit database showed a negative value.

When I showed the precise results, the O(0) in sit database has various species, as below
================================================
O(0) -4.326e-19
CO 4.326e-19 4.326e-19 -18.364 -18.364 0.000 (0)
S2-2 8.834e-28 2.604e-28 -27.054 -27.584 -0.530 (0)
S3-2 1.571e-34 4.631e-35 -33.804 -34.334 -0.530 (0)
S4-2 0.000e+00 0.000e+00 -41.692 -42.222 -0.530 (0)
S4O6-2 0.000e+00 0.000e+00 -42.047 -42.578 -0.530 (0)
S3O6-2 0.000e+00 0.000e+00 -48.228 -48.758 -0.530 (0)
S5-2 0.000e+00 0.000e+00 -49.807 -50.337 -0.530 (0)
Cl2 0.000e+00 0.000e+00 -55.818 -55.818 0.000 (0)
HSO5- 0.000e+00 0.000e+00 -63.146 -63.279 -0.133 (0)
S5O6-2 0.000e+00 0.000e+00 -65.908 -66.438 -0.530 (0)
O2 0.000e+00 0.000e+00 -70.266 -70.266 0.000 (0)
S2O8-2 0.000e+00 0.000e+00 -78.323 -78.853 -0.530 (0)
ClO4- 0.000e+00 0.000e+00 -157.862 -157.985 -0.123 (0)
================================================

However, the LLNL and the other (I used PHREEQC, Wateq4f, and Minteq DB to compare) have simplified O2 concentration, only.

================================================
O(0) 0.000e+00
O2 0.000e+00 0.000e+00 -70.451 -70.418 0.032 (0)
================================================

Can anyone give me some advice to explain Sit DB?
I want to make the results of Sit same to LLNL.

Thank you
Sincerely,

Jeonghwan Hwang

dlparkhurst:
sit.dat has some errors for the polysulfides, polythionates, and other redox species. When O2 or e- is used in the reaction that defines an aqueous species, that species should either be defined as a secondary master species , or -mass_balance should be used to define which redox states the species belongs to.

Consider S2-2. You could either consider this aqueous species as its own redox state as follows:

--- Code: ---SOLUTION_MASTER_SPECIES
S(-1) S2-2 2.0 S(-2) S
SOLUTION_SPECIES
2.000HS- - 1.000H2O + 0.500O2 = S2-2
log_k 32.450
delta_h -212.123 #kJ/mol
# Enthalpy of formation: 35.04 #kJ/mol #04CHI
-analytic -4.7123E+0 0E+0 1.10799E+4 0E+0 0E+0
END

--- End code ---

or, you could consideer it part of the S(-2) redox state as is done in llnl.dat:

--- Code: ---SOLUTION_SPECIES
2.000HS- - 1.000H2O + 0.500O2 = S2-2
log_k 32.450
delta_h -212.123 #kJ/mol
# Enthalpy of formation: 35.04 #kJ/mol #04CHI
-analytic -4.7123E+0 0E+0 1.10799E+4 0E+0 0E+0
-mass_balance S(-2)2

--- End code ---

You would need to change the definitions in sit.dat of all of the species that show up in the distribution of species for O(0) except the O2(aq) species. O2(aq) should be the only species that appears in the O(0) output.

As an aside, I am not a big fan of including the polysulfide definitions in the database. My feeling is that they are transitory species that are unstable relative to S(-2) and S(6) (or perhaps native sulfur).

Jeonghwan Hwang:
I changed the code in light of your comments, and the results showed a better one.
I also think about your comments about polysulfide.

Thank you for your advice!
Sincerely,

Jeonghwan Hwang

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