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Phreeqc simulations



Dear David:

I wanted to let you know that I figured out why the apparent level of Fe(II)
production was altered when I included Hfo surface complexation in the
simulations of Fe(OH)3 reductive dissolution - which is that the default
database contains constants for Fe2+ sorption onto Hfo surfaces. I created
an altered database in which these reactions were omitted and the results
(in terms of Fe(II) accumulation) were identical to those obtained in the
absence of surface complexation (but of course there was a significant
alteration in the evolution of pH). So I'm all squared away on that front. I
wonder if I could ask a basic question about how Phreeqc works, so as to
better understand how to use the code to do other kinds of biogeochemical
simulations (which I'm doing a lot of these days as part of various
DOE-sponsored subsurface metal-radionuclide contaminant transformation
studies). Considering the second version of the Hfo reductive dissolution
simulation, in which the reaction is handled with the following KINETICS and
RATES blocks:

KINETICS
Fe(OH)3_reduction
	-m 0.035
	-m0 0.035
	-formula  CH2O 0.25 Fe(OH)3 1.0
	-steps 1.728e6 in 200 steps
SURFACE 1
	-equilibrate with solution 0
	Hfo_wOH Fe(OH)3_reduction kinetic 0.2 5.33e4
RATES
Fe(OH)3_reduction
   -start
10 m_feoh3 = m
20 k = 0.03/(24*3600)
30 rate = k*m_feoh3
40 moles = rate * TIME
50 SAVE moles
   -end

The question is, exactly what is happening during the reaction simulation?
Are small quantities of Fe(OH)3 and CH2O being allowed to react to
equillibrium at each time step, with the quantity of reactants dictated by
the rate expressions in the RATES block? I assume that this must be the
case, since the KINETICS block does not contain information on the amount of
HCO3- liberation or H+ consumption associated with the overall reaction:

Fe(OH)3 + 0.25 CH2O + 1.75H+  =>  Fe2+ + HCO3- + 2.5H2O

Although this approach is of course quite acceptable, I'm wondering whether
it is possible to use Phreeqc in a more generic way, so that the consumption
of reactants and production of end-products associated with the reductive
dissolution reaction are accounted for explicitly, without having to assume
global redox equilibrium. The reason I ask is that I do not commonly think
about biogeochemical reaction systems in terms of global equilibrium;
rather, instinct leads me to consider everything as a kinetically-controlled
reaction - with the exception of aqueous phase speciation and surface
complexation reactions. Hence, I would like to learn to use Phreeqc in this
more general way, which I assume is possible if the input file is
constructed properly. Thanks very much in advance for your input.

Best regards, Eric

Eric E. Roden
Current address (01/07/02 to 05/10/02):
Analytical Microbiology
Pacific Northwest National Laboratory
900 Battelle Blvd, Mail Stop P7-50
Richland, WA   99352
(509) 373-1043 (office)
(509) 376-5154 (lab)
(509) 376-1321 (fax)
(509) 627-0118 (home)

Permanent address:
The University of Alabama
Department of Biological Sciences
A122 Bevill Bldg 7th Ave
Tuscaloosa, AL 35487-0206
(205) 348-0556 (office)
(205) 348-1813 (lab)
(205) 348-1403 (fax)
(205) 349-1134 (home)



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