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RE: Using Phreeqc for modeling redox reactions

Thanks a lot for your reply.  I think the reason I see the difference in two
approaches I was taking lies in your explanation that in the Reaction
calculation Phreeqc does not differentiate between Fe(II) and Fe(0).  But in
the electrochemical process I am actually introducing ferrous iron into the
solution directly instead of as a salt such as FeSO4.  The reactions are:

At the anode (oxidation reaction):         Fe *  Fe+2 (aq) + 2e-
At the cathode (reduction reaction):      2H2O + 2e-  *  H2 * + 2OH- (aq)
How can I model this in Phreeqc?


> -----Original Message-----
> From:	David L Parkhurst [SMTP:dlpark@xxxxxxxx]
> Sent:	Tuesday, January 14, 2003 5:47 PM
> To:	Mukhopadhyay, Biswajit
> Subject:	Re: Using Phreeqc for modeling redox reactions
> > 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.
> If you include the O2(g), you are assuming that sufficient oxygen enters
> the beaker to maintain atmospheric partial pressure of oxygen at all
> times.
> My guess is that the gas-water transfer of oxygen is relatively slow and
> you might be better off not using the O2(g). If the transfer were really
> fast, you would expect to see red iron hydroxide forming as well.
> Note that in the reaction calculation, PHREEQC does not consider any
> charge
> that you might enter ("Fe+2"). It ignores the charge and is actually
> adding
> elemental Fe to the solution, which is ok, that is the reaction you want.
> The elemental iron must oxidize to Fe+2 (or Fe+3) and in the process
> something must be reduced. If you include the oxygen, then oxygen will be
> reduced, whereas leaving oxygen out will force the reduction the most
> thermodynamically preferred electron acceptor available, Cr(IV) I should
> think.
> 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

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