Speciation with different partial pressure of oxygen

[Moji]

Hello Everyone,

I was wondering if anyone has any suggestion/explanation of why I am getting similar Se speciation in the presence of two different partial pressure of oxygen (PO2 0.01 and 0.1%- with PCO2 4%). I have searched and used some suggestion from the forum on how to approach it, however in both cases, I am getting the same speciation.

#
SOLUTION 1
    temp      11
    pH        7
    pe        4
    redox     O(-2)/O(0)
    units     ppb
    density   1
    K         0 charge
    Alkalinity 300 ppm     #mg/L as CaCO3
    C         1 CO2(g)     0.602
    Ca        180 ppm
    Cl        60 ppm
    Fe        100
    Mg        150 ppm
    N(-3)     0.2 ppm
    N(5)      10 ppm
    O(0)      1 O2(g)      -2
    S(6)      500 ppm
    Se        780
    -water    1 # kg
PHASES
pH_Fix
    H+ = H+
    log_k     0
EQUILIBRIUM_PHASES 1
    CO2(g)    0.6 10
    Calcite   0 10
    Halite    -20 10
    pH_Fix    -7 HCl       10
        -force_equality
END
SOLUTION 1
    temp      11
    pH        7
    pe        4
    redox     O(-2)/O(0)
    units     ppb
    density   1
    K         0 charge
    Alkalinity 300 ppm     #mg/L as CaCO3
    C         1 CO2(g)     0.602
    Ca        180 ppm
    Cl        60 ppm
    Fe        100
    Mg        150 ppm
    N(-3)     0.2 ppm
    N(5)      10 ppm
    O(0)      1 O2(g)      -1
    S(6)      500 ppm
    Se        780
    -water    1 # kg
PHASES
pH_Fix
    H+ = H+
    log_k     0
EQUILIBRIUM_PHASES 1
    CO2(g)    0.6 10
    Calcite   0 10
    Halite    -20 10
    pH_Fix    -7 HCl       10
        -force_equality
END
#

[dlparkhurst]

You are still more oxidizing than the transition to a lower Se redox state. Define the same concentrations of Se(6), Se(4), and Se(-2) to see the pe's where the Se transitions occur.

[Moji]

Thanks! I noticed what I have to do to find out the transition pe value. To follow up if you don't mind asking. I would like to add Pyrite (as Phase) with its associated reaction plus adding it to equilibrium phase (minteq.v4):

Pyrite
   FeS2 + 2H+ + 2e- = Fe+2 + 2HS-
   log_k   -18.5082
   delta_h   49.844   kJ

Here is the code that I am looking at (Simplify it-only one PO2=0.1%):

#
SOLUTION 6
    temp      11
    pH        7
    pe        4
    redox     O(-2)/O(0)
    units     ppb
    density   1
    Alkalinity 300 ppm
    C         1 CO2(g)     0.4
    Ca        180 ppm
    Cl        60 ppm
    Fe        100
    K         0 charge
    Mg        150 ppm
    N(-3)     0.2 ppm
    N(5)      10 ppm
    O(0)      1 O2(g)      -1
    S(6)      500 ppm
    Se(-2)    600         # Similar Concentrations for different species
    Se(4)     600
    Se(6)     600
    -water    1 # kg
PHASES
pH_Fix
    H+ = H+
    log_k     0
Pyrite
    FeS2 + 2H+ + 2e- = Fe+2 + 2HS-
    log_k     -18.5082
    delta_h   49.844 kJ
EQUILIBRIUM_PHASES 6
    CO2(g)    0.6 10
    Calcite   0 10
    Halite    -20 10
    Pyrite    0 10
    pH_Fix    -7 HCl       10
        -force_equality
END
#

Q1- Produced Fe2+ from Pyrite would oxidize to Fe3+ and I would like PHREEQC to calculate Iron species. Should I add Iron oxidation reaction in Phase? and Change Fe2+ and Fe3+ concentrations to 1ppm in equilibrium with Pyrite with SI=0 ? My measured total Iron is 100 ug/L.

Q2- The current model suggested Se (2-) would be the dominant Se, while the PO2 is at 0.1% which is high. Any though on why it would be the reason?

Thanks for your time!

[dlparkhurst]

No, you should not rewrite the pyrite equation. If you do it correctly, the resulting equilibrium will be the same.

Pyrite and oxygen will not coexist at equilibrium. Depending on the relative amounts, either all of the oxygen or all of the pyrite will be consumed. If you want to oxidize a specific amount of pyrite, use REACTION.

You have done something wrong with Se(-2). A much lower pe is necessary, corresponding to P(O2) similar to sulfide. I' m guessing 1e-20 or lower.