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Question about Eh-pH diagram


dong5600
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Dear GWB:

 

I have a question about making Eh-pH diagram to predict iron secondary minerals formed during iron reduction under anaerobic condition. My questions include:

 

1) When we choose the pair for Eh, is oxygen-e- the only choice? I could find other selections for Eh, but oxygen does not seem correct for anaerobic reaction, which typically exhibit negative Eh values.

 

2) For some of the solid phases, the ones showed up in the Eh-pH diagram were not observed in our study, while some of our observed ones did not appear. Is there any way I can check, modify or add some mineral information? Is solubility the only information needed for such analyses?

 

Any suggestion is highly appreciated!

 

Yiran

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Hi Yiran,


Even though dissolved oxygen might be so low as to be immeasurable in your fluid, O2(aq) activity is still a perfectly valid variable for thermodynamic calculations. If you don't want the program to write reactions in terms of O2(aq), though, you can simply swap it out of the basis. You might swap it out for a variety of species, such as O2(g), the e-, H2(aq), H2(g), or even activity ratios, such as Fe+++/Fe++. Then you'd constrain the activity, fugacity, Eh, pe, or activity ratio. Does this answer your question?


As for your second question, you can suppress stable minerals or species that show up on the diagram in order to consider less stable species. To do so, go to the Config - Suppress... dialog in Act2. Within an Act2 run, you can also temporarily modify the log Ks of any reaction(s) by going to the Config - Alter log Ks dialog.


For a more permanent solution, you can go into the thermo dataset that you're using and permanently modify the log Ks of any reaction(s), so that your new data will be used in any calculation that you make. And if you need to consider species that aren't in the thermo dataset you're using, you can add reactions and log Ks for your species of interest to the thermo dataset. The TEdit program is very useful for performing such modifications. To prevent confusion, it's always a good idea to save modified datasets with a unique filename, rather than overwriting the datasets we distribute.


For more information, please see the suppress and alter commands in the GWB Reference Manual, as well as the TEdit section of the GWB Essentials Guide.


Hope this helps,


Brian Farrell

Aqueous Solutions LLC
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Brian:

 

Thank you so much. Your suggestion is very helpful. Just to follow up, you mentioned some alternative species "such as O2(g), the e-, H2(aq), H2(g), or even activity ratios, such as Fe+++/Fe++". So far, I only found choice of Eh or pe under O2 pair (maybe I miss something else. I am wondering if there is another redox pair better fitting anaerobic condition. Is e- available for pairing with H2? Also, for Fe+++/Fe++, can we do this for pairing with ferric solid mineral with Fe++ as mineral is way less soluble than Fe+++ itself?

 

Thanks a lot!

 

Yiran

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Hi Yiran,

 

Since you mentioned you were interested in Eh-pH diagrams, I thought you were referring to selecting axes variables that indicate the oxidation state. The introductory section of the Using Act2 chapter in the GWB Essentials Guide illustrates how to set up various types of redox-pH diagrams, including those with log a O2(aq), log f O2(g), Eh, and log a Fe+++/a Fe++ axes. As you can see, they're all quite similar.

 

From your last message, I believe you're referring to something else. Can you please clarify?

 

Thanks,

Brian

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Hi, Brian:

 

Thanks a lot. For the last message, what I meat was the state and the forms of iron in my reaction system. For example, lepidocrocite (a kind of FeOOH) was used as the electron acceptor, hydrogen was used as the electron donor. When the reaction occurs, I don't feel the Fe(III) and Fe(II) ratio can be simply expressed as Fe+++/Fe++ because the majority of Fe(III) stay in the solid phase. However, when using diagram species for lepidocrocite <--> Fe++, I could only define activity as 1 but could not input Fe(II) concentrations (e.g., Fe++=10 mM). Which forms for these iron species I should set to reflect the conditions in my reaction system?

 

Thanks!

 

Yiran

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Hi Yiran,

 

If you want an Eh-pH diagram, you should set H+ with pH units on the x axis. For the y axis, add O2(aq), swap it out for the e-, and set Eh units.

 

For what you're doing, it's most common to just leave Fe++ as the diagram species and set the activity roughly equal to the molal concentration of Fe++. You'll likely have to suppress stable phases, such as Hematite, to allow a metastable mineral like lepidocrocite to form.

 

Hope this helps,

Brian

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