Redox couple calculation

[IanB]

Hello,

I have some questions about how PHREEQC comes up with it's redox couples and the associated pe/Eh values. I am inputting concentrations for approximately 20 different elements and compounds (including S(-2) as H2S, S(6) as SO4, O(0) as DO, C(-4) as dissolved methane, C(4) as alkalinity-HCO3) for hundreds of groundwater samples. We also have measured values for pH, temperature, and ORP which has been converted to pe and input as a parameter as well. The output file for the intial_solution simulation contains redox couples, usually a mix of C(-4)/C(4), O(-2)/O(0), and S(-2)/S(6) and includes pe/Eh values for each redox couple.

My questions are:-how does PHREEQC come up with these redox couples and the pe/Eh values if i already entered the concentrations and pe?
                          -are the redox couples listed in any specific order? (they appear to be alphabetical from visual inspection)
                          -is there a way to determine the predominant redox couple in the solution?

Here is an example of what I input and received:
DATABASE C:\Program Files\USGS\Phreeqc Interactive 3.3.3-10424\database\wateq4f.dat
   TITLE Full run inlcuding all samples. For this run Fe+Mn will be entered as their element and not a specific valence state.
   SOLUTION_SPREAD
       units    mg/l
 Description      XDOAXXX 2015XXXX   
 Number         1
 Temperature   8.4   
 pH               7.24
 O(0)             0.21
 S(-2)         0.0212     
 pe             6.998541729
 C(-4)         0.0005
 Br                0.06   
 Cl               1.462301717
 F              0.280983066
 N(5)             0.061746186
 P              0.08
 S(6)           4.80765984
 Alkalinity   as HCO3- 90.917118         
 Na               4.611405754
 Mg               1.802219908
 Al               0.024177859
 K               0.587014199
 Ca               25.79710201
 Mn               0.024326378
 Fe               0.010559215
 Ni               0.00119
 Cu               0.005502245
 Zn               0.009534816
 As               0.000795
 Se               0.00228
 Sr               0.122681883
 Cd               0.000085
 Ba               0.127571329
 Pb               0.000075
 U                0.000185


Beginning of initial solution calculations.
-------------------------------------------

Initial solution 1.    XDOAXXX 2015XXX

-----------------------------Solution composition----------------------------

   Elements           Molality       Moles

   Al                8.962e-07   8.962e-07
   Alkalinity        1.490e-03   1.490e-03
   As                1.061e-08   1.061e-08
   Ba                9.290e-07   9.290e-07
   Br                7.510e-07   7.510e-07
   C(-4)             3.117e-08   3.117e-08
   Ca                6.437e-04   6.437e-04
   Cd                7.563e-10   7.563e-10
   Cl                4.125e-05   4.125e-05
   Cu                8.660e-08   8.660e-08
   F                 1.479e-05   1.479e-05
   Fe                1.891e-07   1.891e-07
   K                 1.501e-05   1.501e-05
   Mg                7.414e-05   7.414e-05
   Mn                4.429e-07   4.429e-07
   N(5)              4.409e-06   4.409e-06
   Na                2.006e-04   2.006e-04
   Ni                2.027e-08   2.027e-08
   O(0)              1.313e-05   1.313e-05
   P                 2.583e-06   2.583e-06
   Pb                3.620e-10   3.620e-10
   S(-2)             6.613e-07   6.613e-07
   S(6)              5.005e-05   5.005e-05
   Se                2.888e-08   2.888e-08
   Sr                1.400e-06   1.400e-06
   U                 7.773e-10   7.773e-10
   Zn                1.459e-07   1.459e-07

----------------------------Description of solution--------------------------

                                       pH  =   7.240   
                                       pe  =   6.999   
                        Activity of water  =   1.000
                           Ionic strength  =   2.394e-03
                       Mass of water (kg)  =   1.000e+00
                    Total carbon (mol/kg)  =   1.726e-03
                       Total CO2 (mol/kg)  =   1.726e-03
                         Temperature (°C)  =   8.40
                  Electrical balance (eq)  =   6.150e-06
 Percent error, 100*(Cat-|An|)/(Cat+|An|)  =   0.19
                               Iterations  =  11
                                  Total H  = 1.110139e+02
                                  Total O  = 5.551139e+01

---------------------------------Redox couples-------------------------------

   Redox couple             pe  Eh (volts)

   C(-4)/C(4)          -3.4134     -0.1907
   O(-2)/O(0)          14.4406      0.8067
   S(-2)/S(6)          -3.2791     -0.1832

We are using the program mostly for the speciation/mineral SI calculations to understand our groundwater chemistries but would like to know whether we can use these redox couples. We have not specified any equilibrium phases or any redox couples for specific elements.

Any help would be greatly appreciated.

Cheers,
Ian Bragdon

[dlparkhurst]

-how does PHREEQC come up with these redox couples and the pe/Eh values if i already entered the
concentrations and pe?

You can calculate a pe from concentrations for a pair of redox states for an element. A redox state is defined in SOLUTION (or SOLUTION_SPREAD) by an element name with a redox valence in parenthesis: Fe(2) or S(6), for example. For any element that has two or more redox states defined in a SOLUTION, PHREEQC will calculate a pe and Eh.

SOLUTION_MASTER_SPECIES defines all of the redox states that can be entered. SOLUTION_SPECIES has reactions that create all secondary master species (those elements with parentheses in SOLUTION_MASTER_SPECIES) from the primary masters species (those elements without parentheses in their names). These reactions may use O2(aq) or other species, but they can be reduced to reactions that contain e-. Reactions with e- can be used to calculate pe or Eh.

If a redox state is defined, then the total is distributed among the species of that redox state. The activities of the secondary master species can then be used to calculate pe and Eh for each redox couple.

The specified pe is not used in the distribution of species for a redox state. The specified pe is used to distribute a total concentration of an element, say Fe (no parentheses), between Fe(2) and Fe(3).

 -are the redox couples listed in any specific order? (they appear to be alphabetical from visual
 inspection).

They are sorted alphabetically by element and, within an element, by lowest redox state of the couple. For example:
   C(-4)/C(4)          -3.4134     -0.1907
   N(-3)/N(0)          -3.0456     -0.1701
   N(-3)/N(3)           6.4635      0.3611
   N(-3)/N(5)           6.9441      0.3879
   N(0)/N(3)           15.9726      0.8923
   N(0)/N(5)           12.9379      0.7227
   N(3)/N(5)            8.3857      0.4684
   O(-2)/O(0)          14.4406      0.8067
   S(-2)/S(6)          -3.2791     -0.1832


-is there a way to determine the predominant redox couple in the solution?

Redox states of natural environments tend to fall within Berner's classification scheme: Oxic, post-oxic, sulfidic, and methanic, and roughly in the sequence of electron acceptors O2, NO3, MnO2, FeOOH, SO4, CH2O. There may be some overlap, but also, you can get mixtures of redox environments when sampling. This reference can get you started in the literature: McMahon, P. B., and F. H. Chapelle. "Redox processes and water quality of selected principal aquifer systems." Groundwater 46.2 (2008): 259-271.