> 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
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