Rate of Mn-Oxidation in 'catalysis'

[KMitra]

Dear GWB Team,

I attended GWB workshop in 2020 where we were given the 'catalysis' code to simulate Mn(II) oxidation on Fe(OH)3. I had a few doubts regarding the same:

1. How did we get the rate constant = 20000 ? Was it from a paper or just a number chosen for showing the process?

2. Why did we not include activation energy? Is it combined in the above number?

3. Why was Fe(OH)3(ppd) used? Can other minerals be used? 

4. How did we get the surface area of Fe(OH)3?

5. Why MnO2 (aq) has the log k = 9?

Thank you!

[Jia Wang]

Hello,

The rate constant chosen is to demonstrate the process in the exercise. You should use a rate constant that describes the conditions of your system. 

The default option in the GWB is to provide a rate constant. Alternative, you can provide the activation energy and pre-exponential factor for the program to calculate the rate constant using the Arrhenius equation. The advantage to the latter method is that it can account for temperature variation while the first does not. Please take a look at chapter 4.1 GWB Reaction Modeling User Guide for more information. 

Yes, you can use other minerals. Fe(OH)3ppd is used as a catalyst surface and it's relatively inert in this example, precipitation/dissolution doesn't interfere with the reaction of interest. 

The surface area is determined by the user, typically through some estimation or lab measurement of sample. This specific surface area was used as an example for demonstration. 

The redox species MnO2(aq) is a fictive species added so that the simulation can account for redox disequilibrium between Mn(IV) and other Mn species. A log K (e.g. -9)  is set so that a small amount of MnO2(aq) would be present when in equilibrium with pyrolusite. Note that this simulation is only interested in Mn aqueous species and Mn-minerals don't really matter. If you're not interested in including MnO2(aq) in your work, you can also consider the example for Autocatalytic oxidation of manganese in the Geochemical and Biogeochemical Modeling textbook. This example shows the promotion of Mn2+ oxidation as manganese precipitates. The thermo dataset is adjusted differently since Mn(IV) is not of interest. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC