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# Rate constant for the kinetic oxidation of Fe(II) by oxygen

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Hi,

As part of my lecture on reaction kinetics at University of Bayreuth, I have discussed the rate law of the abiotic oxidation of Fe(II) by molecular oxygen (Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters Wiley-Interscience, New York ) with my students. The reaction is Fe++  + .25*O2(aq)  + 2.5*H2O  -> Fe(OH)3(ppd)  + 2*H+   and  the rate law is given by:   rate_con * activity("OH-")^2 * pO2 * molality("Fe++")   with a rate constant of 8 * 10^13 M-2 atm-1 min-1 (at 20°C). I am now trying to simulate the kinetic oxidation of Fe(II) according to this rate law using GWB with my students.

For initial rates, the rate law above should equal the example presented at the GWB academy which uses the buit-in rate equation rate_con * activity("OH-")^2 * molality("O2(aq)") * molality("Fe++")  * (1-Q/K). The rate constant provided by GWB for this reaction is 7*10^2 kg mol-1 s-1. I understand that the latter rate constant refers to reaction with dissolved O2 on a basis of seconds at (probably?) 25°C, while the constant given above refers to a rate law expression with at O2 in air, on a basis of minutes at 20°C.

I was wondering how these tow values compare, i.e. how I can calculate the rate constant from Stumm and Morgan in the units given by GWB. I tried to consider Henry’s constant (again, at 25 °C?) and the transformation from min to seconds, but this gives a completely different number. Also, I was wondering if the rate constant given by GWB refers to a temperature of 25°C (which is then adjusted by Arrhenius’ law, I assume?). If both rate constants cannot be readily compared, do you have a reference for the value used in GWB?

The script I want to use with my students is given below, and it pretty much aligns with the file provided on the GWB homepage.

Thanks a lot

Kerstin

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Hello Kerstin,

The GWB example for Fe2+ oxidation adds  OH-, O2(aq), and Fe2+ as promoting species based on the rate law. If you want to use the same rate law as described in Stumm and Morgan, you can substitute O2(g) for O2(aq) as a promoting species. For the rate constant, you would need to convert from /min to /s. For more details regarding kinetic rate laws, please refer to chapter 4 Kinetic Reaction Paths in the GWB Reaction Modeling User Guide. In particular, I think you will be interested in section 4.6 Kinetics of Redox Reactions.

If a constant value is provided in the rate constant field, that value is applied across the temperature range of your simulation and does not vary. Note the example input file is set at 10 C, not 25 C. The rate constant used in the example is for demonstrating the principles for kinetic redox reactions, not presenting kinetic parameters as facts.

Hope this helps,
Jia Wang

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