Impact of gypsum equilibrium on pyrite oxidation

[emkau]

Dear PhreeqcUsers, dear Mr. Parkhurst,

I am stuck with a finding I cannot explain and would like to ask for some ideas / hints .
Basicly I am looking at Pyrite oxidation, calcite dissolution and gypsum precipitation. After gypsum equilibrium is reached, Ca2+ and SO4-2 leave the solution and precipitate as gypsum (Ca+2 + SO4-2 = CaSO4). However, pH as well as Fe(tot) concentrations of the solution also stagnate, even though more oxygen is introduced into the system. Why is that?
Since this is a very basic process, I expect there are already some similar questions here, but I was not able to find them. If you do, please send me the link.

Thank you already in advance
emkau

Code: [Select]

SOLUTION 1 Pure water           
   

EQUILIBRIUM_PHASES 1
        Calcite     0       10
        Pyrite      0       10
        Goethite    0       0
        Gypsum      0       0             
        CO2(g)      -3.4    0             

REACTION 1
        O2      1.0
        1.0 moles in 50 steps
END

[dlparkhurst]

I think you have reached an invariant point, or, alternatively, you have zero degrees of freedom in the Gibbs' Phase Rule.

Too many hours have been spent arguing about the phase rule, but in practical terms, if you allow 10 moles of each of the minerals, you can take the solution at the end of your reaction, add any combination of H, O, Ca, S, Fe, and C, and you will not change the solution composition (I think). You must have six components, 6 phases (minerals +  solution), and no degrees of freedom (T&P are fixed), but don't quote me.

[MichaelZ20]

All the Fe produced in pyrite oxidation precipitates with goethite.
If one phase is removed (CO2(g), for example), Fe increases.

SOLUTION 1 Pure water           

EQUILIBRIUM_PHASES 1
        Calcite       0       10
        Pyrite        0       10
        Goethite    0        0
        Gypsum     0        0             
        CO2(g)     -3.4     0             

REACTION 1
        O2      1.0
        10.0 moles in 50 steps
   
USER_GRAPH 1
-headings RXN Fe Goethite Pyrite
-axis_titles "Added O2, mol/kgw", "Total Fe, mol/kgw", "Mass change of Fe minerals, mol/kgw"
10 GRAPH_X RXN
20 GRAPH_Y TOT("Fe")
30 GRAPH_SY EQUI_DELTA("Goethite"), EQUI_DELTA("Pyrite")
END