RE: Using Phreeqc for modeling redox reactions

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

I think your reaction is essentially correct. You need to remove the
EQUILIBRIUM_PHASES with O2(g). Initially I think the reaction should be as
follows:

At the anode    Fe = Fe+3 + 3e-

At the cathode  CrO4-2 + 3e- = Cr+3, with the stoichiometry determined by
equality of electrons.

However, you have NO3 in the system and that confuses things. If you
simplify the system, you can see the reaction you want. Comment out the
N(5) and reduce the amount of iron in the reaction by an order of
magnitude. You will see that you make Cr(3) and Fe(3). If you add an excess
of Fe, you eventually reduce all of the CrO4, make Fe+2 out of all the
Fe+3, and start to reduce SO4, CO3, or water.

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



                                                                                                            
                      "Mukhopadhyay,                                                                        
                      Biswajit"                To:       'David L Parkhurst' <dlpark@xxxxxxxx>              
                      <BiswajitM@xxxxxx        cc:                                                          
In-Reply-To: <73EE64E70EC5D211B50D0090273F22508A6C2A@xxxxxxxxxxxxxxxxxxx>
                      m>                       Subject:  RE: Using Phreeqc for modeling redox reactions     
                                                                                                            
                      01/14/03 04:36 PM                                                                     
                                                                                                            
                                                                                                            




David:
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-
(1)
At the cathode (reduction reaction):      2H2O + 2e-  *  H2 * + 2OH- (aq)
(2)
 <<...OLE_Obj...>>
How can I model this in Phreeqc?
Regards,

Biswajit

> -----Original Message-----
> From:            David L Parkhurst [SMTP:dlpark@xxxxxxxx]
> Sent:            Tuesday, January 14, 2003 5:47 PM
> To:        Mukhopadhyay, Biswajit
In-Reply-To: <73EE64E70EC5D211B50D0090273F22508A6C2A@xxxxxxxxxxxxxxxxxxx>
> 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
>
>