Anaerobic Oxidation of Pyrite

[avadhoot]

I have a groundwater with water type Na-HCO3 that is close to unconfined portion of aquifer and it undergoes pyrite oxidation where the initial sulphate increases and reaches elevated values at distal zones. The resultant water type is in confined portion of the aquifer and has water type of Na-HCO3-SO4. However since there is little to no oxygen present in the confined portions, I am trying to find out how would denitrification could couple with anaerobic pyrite oxidation such that resultant SO4 concentrations are elevated. I tried to include Nitrogen species to simulate this anaerobic pyrite oxidation, however I encountered error:

Using Cl1 standard precision optimization routine.

Post_mortem examination of inverse modeling:

   ERROR: equality not satisfied for C(4), -7.103741e-03.
   ERROR: inequality not satisfied for N(5) eps-, 4.924347e-03

Summary of inverse modeling:

   Number of models found: 0
   Number of minimal models found: 0
   Number of infeasible sets of phases saved: 1
   Number of calls to cl1: 3
------------------

Below is my code

Code: [Select]

TITLE Inverse Modeling Na-HCO3 to Na-HCO3-SO4 Anerobic Pyrite Oxidation

SOLUTION 1 Sodium Bicarbonate Water
    temp      25
    pH        8.1
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 388
    Ca        24
    Cl        4
    Fe        0.173
    K         3
    Mg        16
    Na        136
    S(6)      132
    N(5)      5
    -water    1 # kg
END

SOLUTION 2 Sodium Bicarbonate Sulphate Water
    temp      25
    pH        8.8
    pe        4
    redox     pe
    units     mg/l
    density   1
    Alkalinity 927.9
    Ca        2.59
    Cl        113.08
    Fe        0.21
    K         2
    Mg        1.06
    Na        824.37
    S(6)      769.57
    N(5)      0.5
    -water    1 # kg
END

INVERSE_MODELING 1
    -solutions      1        2
    -uncertainty    0.15     0.15
    -phases
        Calcite
        Dolomite
        NaX
        KX
        Goethite
        Halite
        Pyrite
        NH3(g)
        NH4X
    -minimal
    -tolerance         1e-10
    -mineral_water     true
END
I have the following questions that perplex me on how to understand anaerobic pyrite oxidation:
1. Which reactions do I need to consider for this process?
2. Since there is likely microbial activity, how can I include that as probable coupled process?
3. How do I calculate minimum oxygen concentration that would be required to initiate pyrite oxidation?

Thanks

[dlparkhurst]

You need to add O2(g) as a reactant.

Pyrite oxidation could consume any available oxygen.