RE: Using Phreeqc for modeling redox reactions

["Mukhopadhyay, Biswajit" <BiswajitM@xxxxxxx>]

Sorry that I could not express what I was looking from you.  I used the
amount of Fe that was actually used in the experiments.  If I specify this
amount (this amount was determined from reaction stoichiometry/reaction 3 I
sent you yesterday and applying Faraday's Law) and do not use
EQUILIBRIUM_PHASES O2(g) then Phreeqc calculates more Fe(II) in solution
than Fe(III).  But in reality, there should not be no Fe(II) in the
solution.  Why the calculated solution composition is not consistent with
what is perceived from the oxidation-reduction reaction?  I even tried
specifying the reaction.  Please see the enclosed input files.  But I see no
difference between this run and the run using the input file without
specifying the reaction.  So my question is why Phreeqc is not using the
reaction stoichiometry?  Is it due to the fact that it assumes Fe(0) was
added?  In that event, can I circumvent the problem by pretending that FeSO4
was added and giving the log_k of the dissociation reaction FeSO4 = Fe+2 +
SO4-2?
 <<PR_MW_15_Fe_1X_a.pqi>>  <<PR_MW_15_Fe_1X_b.pqi>>  
<<PR_MW_30_Fe_1X_a.pqi>>  <<PR_MW_30_Fe_1X_b.pqi>> 

I also do not understand why for lower Cr content, higher dose is necessary.
I am looking into the kinetics of this reaction.

Thanks for the help.

Biswajit
> -----Original Message-----
> From:	David L Parkhurst [SMTP:dlpark@xxxxxxxx]
> Sent:	Wednesday, January 15, 2003 5:15 PM
> To:	Mukhopadhyay, Biswajit
> Subject:	RE: Using Phreeqc for modeling redox reactions
> 
> 
> I'm not sure what you want. I think your model of the thermodynamics of
> iron and chromium reactions is adequate. You could add equilibrium with
> Cr_hydroxide and Fe(OH)3(a) to simulate the formation of the precipitates.
> 
> As far as the lower concentrations of Cr taking more Fe, I can't think of
> any reason that should happen thermodynamically. Perhaps it is a kinetic
> phenomenon at the electrode.
> 
> David
> 
> 
> 
> 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
> 
>

Attachment: PR_MW_15_Fe_1X_a.pqi
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Attachment: PR_MW_15_Fe_1X_b.pqi
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Attachment: PR_MW_30_Fe_1X_a.pqi
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Attachment: PR_MW_30_Fe_1X_b.pqi
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