Fe(III) and redox calculation under siderite + fixed p(CO2) equilibrium

[mp]

Dear Phreeqc users,

I would appreciate it very much if somebody could help me understand how Phreeqc calculates Fe(III) (and the redox state) in a system where only an Fe(II) mineral is defined to be at equilibrium, but no Fe(III) or oxygen buffer is specified. In the example below pure water (in initial equilibrium with atmospheric O2, but containing no Fe) is equilibrated siderite at atmospheric p(CO2):

#----------------------
SOLUTION 1
pe 4
redox pe
temp 25.0
-units mol/kgw

pH 7.0 #charge

EQUILIBRIUM_PHASES 1
Siderite 0.0 #fix Fe+2
CO2(g)  -3.5 #fix CO3-2

END
#----------------------

Using the phreeqc.dat the following redox and speciation of the Fe system is calculated upon equilibration with the mineral and gas:

                                       pH  =   7.468      Charge balance
                                       pe  =  -3.489      Adjusted to redox equilibrium

Fe(2)         7.295e-05
   Fe+2            7.106e-05   6.646e-05    -4.148    -4.177    -0.029    -22.20
   FeHCO3+         9.507e-07   9.348e-07    -6.022    -6.029    -0.007     (0) 
   FeOH+           6.281e-07   6.176e-07    -6.202    -6.209    -0.007     (0) 
   FeCO3           3.090e-07   3.090e-07    -6.510    -6.510     0.000     (0) 
   Fe(OH)2         1.545e-10   1.545e-10    -9.811    -9.811     0.000     (0) 
   Fe(OH)3-        1.715e-13   1.687e-13   -12.766   -12.773    -0.007     (0) 
Fe(3)         1.866e-11
   Fe(OH)3         1.440e-11   1.440e-11   -10.842   -10.842     0.000     (0) 
   Fe(OH)2+        3.867e-12   3.803e-12   -11.413   -11.420    -0.007     (0) 
   Fe(OH)4-        3.924e-13   3.858e-13   -12.406   -12.414    -0.007     (0) 
   FeOH+2          4.180e-16   3.908e-16   -15.379   -15.408    -0.029     (0) 
   Fe+3            2.389e-21   2.060e-21   -20.622   -20.686    -0.065     (0) 
   Fe2(OH)2+4      5.386e-30   4.110e-30   -29.269   -29.386    -0.117     (0) 
   Fe3(OH)4+5      4.982e-39   3.266e-39   -38.303   -38.486    -0.183     (0)


I have the impression that for a meaningul Fe(3) and redox calculation one would need to specify an additional constraint, for example a Fe(3) equilibrium phase (say goethite) or impose p(O2)... My question is therefore whether the Fe(3) total concentration, its speciation, and the redox calcuated are meaningful.

Thank you very much.


-mrek

[dlparkhurst]

Here is the concept. You have the follow reactions that involve e-.

Fe+2 = Fe+3 + e-
H2O + e- = 0.5 H2 + OH-
CO2 + 8e- + 6H2O = CH4 + 8OH-
H2O =  0.5O2 + 2H+ + 2e-

If you write the mass action equations, you can write an expression for pe for each one. For example:

pe = log(a(Fe+3)) - log(a(Fe+2) - log(K)

To achieve equilibrium, the reactions progress to the point that all of the equations produce the same pe.

[mp]

Thank you for the explanation. I still wonder though, in terms of phase rule (and whether the system is fully constrained), I imagine we have the following situation (assuming H2O at constant mass):

Fe++ fixed by siderite
HCO3- fixed by p(CO2)
H+ calculated from charge balance

Now, O2 (and pe) can be calculated from the Fe+++/Fe++ redox couple, but this requires a constraint on Fe+++ (which is a master species). Alternatively, O2 or H2 could be specified (thus fixing Fe+++). I wonder if I am missing something...


Thank you,



-mrek

[dlparkhurst]

I don't really want to get into the phase rule. There is at least one more degree of freedom in your system. I have chosen to add H2(g) and Hematite to the list of EQUILIBRIUM_PHASES. The result is an invariant solution composition. Adding CH4 to the result does not change the solution composition (although the volume of solution changes).

SOLUTION 1
    temp      25
    pH        7
EQUILIBRIUM_PHASES 1
    CO2(g)    -3.5 10
    Siderite  0 10
    H2(g)     -10 10
    Hematite  0 10
SAVE solution 1
END
REACTION 1
    CH4(g) 1
    0.1 moles
RUN_CELLS
    -cells 1
END

[mp]

Excellent. Many thanks.