pe and O2 equilibration

[Gaulty]

SOLUTION_SPREAD
    -units    mg/l
  pH               Alkalinity      Cl    S(6)       Al       Ba      Ca             Mg        K      Na      Fe
       mg/l as Ca0.5(CO3)0.5    mg/l    mg/l     mg/l     mg/l    mg/l    mg/l charge     mg/l    mg/l        
 7.5                       85       7    1190    0.028    0.012     233            143     5.17    15.5    0.12
 2.6                               19    4000      119    0.003     370            375     0.76     8.8     178
 8.4                      179     1.7     616     0.02    0.005     207           62.4     3.14    2.69    0.01
 6.4                     70.5     0.1     3.1     0.15     0.01    24.2            3.9      0.6    1.09    0.31
 5.6                                      0.6                      0.17          0.025    0.017    0.02        
END
MIX 1
    1    79
    2    2.5
    3    6.5
    4    10
    5    2
END
USE mix 1
EQUILIBRIUM_PHASES 1 atmosphere
    Co2(g)    -2.5 10
    O2(g)     -0.68 10
    Fe(OH)3(a) 0 0
    Calcite   0 0
    Barite    0 0
    Gypsum    0 0
SAVE solution 6
END

Hi there,

Trying to run a mixing simulation and want to equilibrate my mixed solution with the atmosphere. No information on redox parameters for the system so have left default pe = 4. It is my understanding that the equilibration step balances charge and redox with pH and pe.

Resulting pe for my input solution above is  14.6. This always seems to happen when I equilibrate with O2.   I assume it is incorporating O(-2)/O(0) which results in obtaining a very high (and seemingly unrealistic) pe.

I found the link below, which seems to suggest my pe calculation is correct, however, it seems very high.
http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00466.html


Any ideas?

[dlparkhurst]

If you have any O2 in solution at equilibrium, even nanomoles, you will have a high pe. Because you have specified O2(g), 14 is a typical value for equilibrium with the atmosphere, but it does vary with pH.

[Gaulty]

OK. Thanks David!

[shona.rubens]

I know this post has been dormant for quite some time, but I was hoping you could shed some light on why you used -2.5 for your value for CO2. I realize this is the log of the partial pressure making the partial pressure of CO2 0.003 although should it not be 0.0003?

I question this as in my model using a value of -2.5 actually is beneficial, but I want to understand the reasoning behind this value.

Any background you could give me would be fantastic.

Thanks

[dlparkhurst]

For water in equilibrium with the atmosphere, -3.4 is the current value for log PCO2.

If the water is in contact with soil CO2, then the log partial pressure can be as high as -1 for a very productive root zone.

[Gaulty]

For the particular pit lake I was modeling pCO2 was -2.5.