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Re: PHREEQC question

> We're using PHREEQCI for speciation modelling.  When we give the program
an input solution and then perform a batch reaction the program
automatically adjusts the pe in the resultant solution.  Presumably this is
to maintain charge balance.  Is there a way to over-ride this in order for
the resultant solution to have a pe which we specify even if there will be
a charge imbalance?

PHREEQC does not easily allow fixed activities of individual species. In
part, this is by design because maintaining a fixed pe in your case, while
allowing pH to vary can move the solution out of the stability field of
water. You can also implicitly be requiring many moles of mass transfer to
maintain a fixed activity. Usually, the change in pe from reaction is
telling you something.

However, although you can not simply specify a constant pe, you can define
a phase that reacts until you get the pe that you want. Thus, you have to
explicitly add something to solution to adjust the pe. The following is a
bit artificial, but organic matter reacts with water, the reaction would
normally produce methane, CO2, and a very low pe. The phase "Fix_pe" is
defined in PHASES and required to equilibrate with the solution with
EQUILIBRIUM_PHASES. However, the definition in EQUILIBRIUM_PHASES specifies
that oxygen (O2) be added or removed from the solution until the saturation
index of -7 is reached for the mineral. This combination of definitions
causes oxygen to be added until the pe is 7.0. The oxygen allows all the
carbon to be CO2 rather than half as methane.


      pe 7
      CH2O 1
      1 mmol
      Fix_pe   -7   O2
    e- = e-
    log_k  0.0
David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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