RE: Phreeqc modeling of kinetic Fe(OH)3 reductive dissolution

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

> However, I would in fact like to
try to depict the Fe(OH)3 reductive dissolution reaction kinetically, i.e.
in a manner analogous to how Postma and Appelo (2000) modeled MnO2
reduction
by Fe2+, according to a standard surface-area controlled reaction
mechanism.

The input file does model kinetically controlled Fe(OH)3. The rate is
dependent on the amount of Fe(OH)3 that is present, but is controlled by
titrating the rate determined amount of reductant. You can add the factor
to account for m/m0 in the rate statement 30 to account for surface area
effects. I like this approach because it adds only one reactant and does
not assume any stoichiometric relation between CH2O and iron.

> Could you show me how to set this one up? And then field an important
question: within this kinetically-controlled framework is it still possible
to model surface complexation by the diminishing Fe(OH)3?

It sounds like you want something more like the following. However, it
assumes that Fe(OH)3 is the only reductant (CH2O and Fe(OH)3(a) are
stoichiometrically related), if you start with some O2 and nitrate in the
system, you will probably generate organic matter, which is not good.

You will have to pick a rate expression that you like. I think Postma and
Appelo used a factor of 1-SR in their expressions. However, I think the
reaction would proceed even if 1-SR were 0, because it is still may be
thermodynamically feasible to oxidize organic matter and reduce iron.

I've included the surface connection to the kinetic reaction. You would do
something similar for the previous example I sent except to connect the
surface to "Fe(OH)3(a) equilibrium_phase".

This is probably workable for an input data set:

SOLUTION 0
      pH    7
      Na    10
      C     10 charge
KINETICS
Fe(OH)3_reduction
      -m .01
      -m0 .01
      -formula    CH2O .25 Fe(OH)3 1
      -steps 86400 172800 345600 691200 1382400 # 1, 2, 4, 8, 16 days
SURFACE 1
    -equilibrate with solution 0
    Hfo_wOH Fe(OH)3_reduction kinetic           0.1    6
RATES
Fe(OH)3_reduction
      -start
10 m_feoh3 = M
20 k = .05/(24*3600)    # constant /sec
30 rate = k*(M/M0)^.6*M*(1-SR("Fe(OH)3(a)"))
40 moles = rate*TIME    # TIME in sec
50 SAVE moles
      -end
END


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225