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Kinetics of redox reactions

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[admin note: this post represents recovered forum content after a critical hard drive failure - it is not in it's original form]



Feb 5 2007, 06:07 PM

I am trying to reproduce Figure 4.2 in the Reaction Modeling Guide of GWB without success. I am running the Professional version 6.03 and I think that I have all the most recent patches installed. I am using the GUI to reproduce the command line given in the text under Kinetic Reaction Paths subsection Kinetics of Redox Reactions. The script is attached. The problem is that my script does not reproduce the curves shown in Figure 4.2. The example command given in the text corresponds to a problem in which Fe++ in a laboratory vessel oxidizes to Fe(OH)3© in the presence of atomosheric O2. A rate constant and powers of catalyzing species are provided in the example command line. In Figure 4.2 the Fe++ concentration decreases from 30 mmolal to 5 mmolal in 20 days. The Fe+++ concentration rises correspondingly. In my script the Fe++ concentration decreases to zero in about 5 days. However, Fe+++ does not rise much above zero, rather Fe(OH)3© precipitates as one would expect.


Can anyone figure out why my script yields different results than the exmple in the guide?


Thank you,


Dusty Earley

Attached File(s)

Fe__Kinetics_Fig4_2.rea ( 616bytes ) Number of downloads: 4

Fe__Kinetics_Fig4_2.rea ( 616bytes ) Number of downloads: 4



Feb 6 2007, 11:47 AM

Hi Dusty:


I can find three minor differences between your script and the one given in the User's Guide:


1. The simulation in the Reaction Modeling Guide is run at 10C, whereas yours is at 25C. This difference is responsible for most of the discrepancy between the two results.


2. You've defined the concentrations for Na+, Cl- and Fe++ as "free" concentrations, whereas the manual defines them as bulk. In general, the "free" designation should be reserved for setting mineral masses.


3. You have an extra line in the "Reactants" tab, in which you titrate 1 ug Fe(OH)3 (ppd) into the basis.


#2 and #3 skew your results in very minor ways, if at all.


BTW. - if you want to reproduce scripts given in the User's Guide, an easy way is to open the User's Guide as a PDF file via the "Help" menu, and then cutting and pasting the script lines directly into the Command tab. You'll find that the GUI auto-populates with the text you've pasted in.


Hope that helps,


Tom Meuzelaar

RockWare, Inc.



Feb 6 2007, 05:20 PM



Thanks this does help solve the problem with Fe++ concentration decay. However, I am still puzzled why Figure 4.2 shows Fe+++ concentrations increasing. Shouldn't these concentrations stay low due to Fe(OH)3 precipitation at pH=7. I still seem to be missing somthing.









Good question. The Fe3+ in the system is indeed all bound in the ferric-hydrous mineral phase. The graph in the Reaction Modeling Guide (on p. 35) is actually plotting Fe+++ component in the system, not Fe+++ in the fluid. To reproduce this in GtPlot, simply click "Plot - Components - In System (Fluid+Rock)". If you just plot Fe+++ in solution ("Plot - Species- Concentrations", "Y-Axis - Consider - Fe+++"), you'll notice that ferric iron in solution is at extremely low concentrations.


I hope that helps,




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