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)
Please note that some U.S. Geological Survey (USGS) information accessed through this page may be preliminary in nature and presented prior to final review and approval by the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the sole responsibility of the user.
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
The URL of this page is:
Last modified: $Date: 2005-09-13 21:04:21 -0600 (Tue, 13 Sep 2005) $
Visitor number 1993 since Jan 22, 1998.