simple question

[twq]

Hi,

 

I have three simple questions as follows:

 

1. How can I model Zero Valent Iron in REACT?(Since there only Fe²⁺ Or Fe³⁺ that could be selected in basis species)

 

2. How do I add reaction trace? I cannot get the results like that from the tutorial(in "Using GWB"), I did not know which input file was that you used in REACT.

 

3 What does "Fe(OH)3(ppd)" mean?

 

Thanks a lot

 

Tianwei

[Brian Farrell]

Hi Tianwei,

 

1.In most thermo datasets, Fe++ is a basis species and Fe+++ is a redox species. Depending on the purpose of your model, you can choose to decouple Fe+++ from Fe++ (to treat them independently, perhaps to implement redox transformation kinetcs, etc.) or leave coupled. You'll want to do the same thing for zero-valent iron. This is somewhat complicated, however, because iron in the 0 oxidation state is only found as a solid under geochemical conditions.

 

To do so, you'll need to add a fictitious redox species Fe(0)(aq) which is coupled to the basis species Fe++. Then, you would add a mineral Fe(0) which is formed from the Fe(0)(aq) species. You should be able to find in the literature a log K for the reaction between the zero valent iron mineral and Fe++, but you'll need to define a log K for the two reactions you're specifying. The log K for the reaction between Fe(0) and the fictitious Fe(0)(aq) should be arbitrarily chosen such that Fe(0) is very stable. The log K for the reaction between Fe++ and Fe(0)(aq), when combined with the reaction between Fe(0) and Fe(0)(aq), should equal what you find in the literature.

 

You should take a look at section 17.5 in the Geochemical and Biogeochemical Reaction Modeling text for a very similar example (adding a coupling reaction for zero-valent sulfur). The dataset described in the text, thermo+S0.dat, is included in the GWB installation directory for your reference.

 

2. A reaction trace is a projection of a reaction path model (calculated in React) onto an activity diagram calculated in Act2 or Tact. When you run a model in React, it should create a text output file (React_output.txt) and a plot output file (React_plot.gtp) in your current working directory. The plot file (React_plot.gtp, but not the plot itself!) can be overlain onto your activity diagram either by dragging it onto your Act2 or Tact diagram, or by going to File -> Open -> Reaction Trace... in Act2 or Tact (you may need to browse to find the correct gtp file). You'll need to make sure that the plot file (and of course your reaction path model) contains within it information that matches the axes of your activity diagram. For example, you can't overlay a reaction trace onto an Ed-pH diagram unless your reaction path model includes pH and a measure of oxidation state. For more information, see sections 5.5 or 6.5 in the GWB Essentials Guide.

 

3. I believe (ppd) means "precipitated", so Fe(OH)3(ppd) refers to a freshly precipitated ferric iron hydroxide mineral.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

[twq]

Hi Brian,

 

Thanks for your quickly reply.

 

For your replying to question 1, I need to think it over, it was really complicated.

For your replying to question 2, I know this and i run the model successfully, but the result (the graph) I got was different from that in tutorial(in "using GWB"), i wonder if my input script file is the same as yours, could you send me the input script file you used in tutorial (part "How do I add a reaction trace "in "Using GWB")? or tell me which file was that you used in install directory.

 

Regards

 

Tianwei

[twq]

Hi Brian,

 

I attached the script file for tracing the reaction path. As I said before, I could not get the same results like Fig.15.8 in book 'Geochemical and Biogeochemical Reaction Modeling' written by Craig M. Bethke. I am confused, could you tell me why?

 

 

Best regards

 

 

Tianwei

Ch15_pyrite-modified.rea

[Brian Farrell]

Hi Tianwei,

 

The script you've attached includes the "dump" command, which was not used in the model plotted in figure 15.8. The dump command removes the minerals in equilibrium with the fluid (pyrite and hematite) during the beginning of a reaction path model. In this case, oxygen fugacity rises quickly and pH changes little. If you go back to the text, you'll see that the dump command is added as a variation of the original calculation. In the original calculation, plotted in 15.8, the presence of pyrite served to buffer the fluid's oxidation state.

 

Regards,

Brian

[twq]

Hi Brian,

 

I got it, Thanks a lot.

 

 

Tianwei