RE: Using Phreeqc for modeling redox reactions

["David L Parkhurst" <dlpark@xxxxxxxx>]

If you run this file you will see that the amount of Fe+3 produced is equal
to the amount of Cr+3. I have removed nitrate and reduced the amount of
iron to simplify the problem.

If you include nitrate, then it will consume some electrons and you may not
get the reduction of Cr that you expect.

As you add more iron, other electron acceptors get into the act and also
the pH increases. The model says that Fe(OH)4- exists even in the presence
of sulfide. I'm not sure I believe that, but that is another issue. The
number of electrons is conserved. It may not be calculating the reactions
that you expect, but I am convinced that it is calculating the correct
thermodynamic reactions for the aqueous model.


TITLE MW-15 Precision Groundwater reaction with 1X Fe+2
    SOLUTION 1
    temp      23.41
    pH        7.0
    pe        O2(g) -0.68
    units     mg/L
    density   1
    Al        0
    Alkalinity 292.9 as HCO3-
    Ca        310
    Cl        730
    K         3.3
    Mg        160
    Si        36 as SiO2
#    N(5)      12 as NO3-
    Na        600
    C(4)      0 as CO3-2
    S(6)      1500 as SO4-2
    Cr(6)       86.00   # Andco Cr determination
SAVE solution 1
END
USE Solution 1  # Note no reaction is specified separately
REACTION 1
Fe  1
#4.96e-3  # This is 1X amount
4.96e-4

END

This may be a better model because it precipitates solids and keeps the pH
a bit lower.

TITLE MW-15 Precision Groundwater reaction with 1X Fe+2
    SOLUTION 1
    temp      23.41
    pH        7.0
    pe        O2(g) -0.68
    units     mg/L
    density   1
    Al        0
    Alkalinity 292.9 as HCO3-
    Ca        310
    Cl        730
    K         3.3
    Mg        160
    Si        36 as SiO2
#    N(5)      12 as NO3-
    Na        600
    C(4)      0 as CO3-2
    S(6)      1500 as SO4-2
    Cr(6)       86.00   # Andco Cr determination
SAVE solution 1
END
USE Solution 1  # Note no reaction is specified separately
REACTION 1
Fe  1
4.96e-3  # This is 1X amount

EQUILIBRIUM_PHASES
Fe(OH)3(a) 0 0
Cr_hydroxide 0 0
END


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled