There are other ways to set this up, but it is simplest if you consider the consumption of organic carbon as the kinetic reaction. Then all you must do is add the correct amount of CH2O and let equilibrium do the rest. Signs may be confusing, but positive "moles" times positive coefficient of CH2O is positive, which means CH2O is added to the solution. Fe(OH)3(a) dissolves to equilibrium in response to the addition of CH2O. Carbon from CH2O must end up as C(IV)(carbonate species) and/or C(-IV) (methane), because those are the only aqueous species for carbon that are defined to the program. With Fe(OH)3(a) present, the carbon ends up in carbonate (oxidized) and Fe(OH)3 is reduced because of thermodynamics. Another reason to set it up this way is that the number of surface complexation sites can be related (proportional) to the number of moles of an EQUILIBRIUM_PHASE, so as you develop this model, you can allow for surface sites that diminish as Fe(OH)3 dissolves. SOLUTION 0 pH 7 Na 10 C 10 charge # note total C ends up at 1.2 mmol for charge balance at pH 7 EQUILIBRIUM_PHASES Fe(OH)3(a) 0.0 .01 KINETICS Fe(OH)3_reduction -formula CH2O 1 -steps 86400 172800 345600 691200 1382400 # 1, 2, 4, 8, 16 days RATES Fe(OH)3_reduction -start 10 m_feoh3 = EQUI("Fe(OH)3(a)") 20 k = .05/(24*3600) # constant /sec 30 rate = k*m_feoh3 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 "Roden, Eric E" <Eric.Roden@xxxx To: "'dlpark@xxxxxxxx'" <dlpark@xxxxxxxx> gov> cc: In-Reply-To: <1B1080D3ADDF8D4AB59832DE0A02C52A6BAE59@xxxxxxxxxxxxxxxx> Subject: Phreeqc modeling of kinetic Fe(OH)3 reductive dissolution 02/13/02 11:20 AM Dear Dr. Parkhurst: I wonder if I might trouble you with a question regarding the use of Phreeqc (version 1.5.08 for Windows) for a paper I'm working on to be submitted to Applied Geochemistry. This is actually the same problem, more-or-less, which I had written to you about sometime ago, as I was just starting to try my hand at mixed equilibrium-kinetic geochemical modeling. I wish to simulate changes in pH and mineral precipitation accompanying kinetic reductive dissolution of hydrous ferric oxide [HFO, assumed to be represented by Fe(OH)3] by dissmilatory Fe(III)-reducing bacteria in HCO3-buffered medium. An approximate set of starting conditions for such a simulation is as follows: pH 7 10 mM NaHCO3 10 mmol/L Fe(OH)3 We have good evidence that bacterial HFO reduction follows first-order kinetics, with a rate constant of ca. 0.05/d in our normal culture systems. Hence, the initial idea would be to predict changes in pH and dissolved inorganic carbon speciation assuming that HFO reductive dissolution took place according the following reaction: Fe(OH)3(s) + 0.25 CH2O + 1.75H+ => Fe2+(aq) + 0.25HCO3- + 2.5H2O in which the labile organic carbon (CH2O) is present in excess and therefore does not need to be tracked during the simulation, and in which Fe(OH)3 consumption is depicted as a first-order reaction RFe(OH)3(s) = -k[Fe(OH)3(s)] Alternatively, the reaction could be modeled as a surface-area controlled process according to a more standard formulation such as RFe(OH)3 = -k(m/m0)^n Ultimately I would like to include other heterogeneous reactions and their impact on pH changes during HFO reduction, namely: (i) H+ complexation by Fe(OH)3 surfaces, whose abundance would be declining as reductive dissolution takes place, e.g. as a first approximation in direct proportion to molar/mass concentration and an assumed surface area of 600 m2/g; (ii) precipitation of FeCO3(s) (siderite) and/or Fe(OH)2. I have made some initial attempts to set-up a Phreeqc simulation of the simple case where only reductive dissolution of Fe(OH)3 takes place, but I am uncertain about how to depict the parallel consumption of the mineral phase and H+, together with production of Fe2+(aq) and HCO3-. I would be very grateful if you could provide assistance in setting-up this problem. If I can get this to work, I can think of a great many uses for Phreeqc in understanding and interpreting experimental studies of bacterial Fe(III) oxide reduction and associated aqueous/solid-phase geochemical reactions. 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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