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Using Phreeqc for modeling redox reactions

Dear Dr. Parkhurst:

I did some experiments to determine at a bench scale, reduction of Cr(VI) in
a contaminated groundwater by adding Fe(II) through electrolysis.  In such
an experiment, an iron electrode is immersed into a beaker containing the
contaminated groundwater, electricity is passed through the electrodes,
aqueous Fe(II) is directly added to the solution and Cr(VI) is reduced to
Cr(III).  No oxygen is bubbled through the beaker but there was no special
arrangement to do the experiments in a particularly reducing environment to
eliminate the competition between the oxidation reaction of Fe(II) to
Fe(III) and reduction of Cr(VI) to Cr(III).  When I am trying to calculate
the equilibrium species distributions using Phreeqc, I see dramatic
difference in the results if I use EQUILIBRIUM_PHASES O2(g)  -0.68 to denote
that the water is open to atmospheric partial pressure of oxygen and if I
don't use this key word.
My question is, for the experiment described above, is using the key word
EQUILIBRIUM_PHASES with O2(g) Partial Pressure (10^-0.68) the correct
approach.  I enclose the input file and the database [modified phreeqc.dat
as phreeqcc.dat to include all Cr(VI) and Cr(III) species] for your viewing.
Please advise.

 <<PR_MW_15_Fe_1X_c.pqi>>  <<phreeqcc.dat>> 

With kindest regards,


Biswajit Mukhopadhyay, M. S., M. E., Ph. D., D. Sc.
Senior Hydrologist & Geochemist
ecology and environment, Inc.
International Specialists in the Environment
Phone: 214-245-1051

Office Address:  1200 Main Street, Suite 500
                         Dallas, TX 75202

Attachment: PR_MW_15_Fe_1X_c.pqi
Description: Binary data

Attachment: phreeqcc.dat
Description: Binary data

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