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akram

Ploting Eh-pH for Fe_Hg_H2O system

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I tried to plot Fe_Hg_H2O system in Act2 but I don't get Fe. SO, I used Fe++ to plot it and I obtain the diagram wich doesn't have iron solid on it and it is sound strange. In addition, when I talked to my adviser,he told me should I get iron solid on it.Finally, I would like to ask about how I can get Fe in database in order to plot my diagram.

 

I am looking in advance to hearing from you.

 

Best regards,

 

Akram

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Hello Akram,

 

Could you specify what type of diagram you are hoping to create? Some sort of redox-pH diagram or a solubility diagram, for example? Are you referring to metallic iron, Fe(0), or some iron oxide minerals?

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC

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Hello Farrell,

 

I would like to create a solubility diagram. Yes, I am referring to metallic iron, Fe(0)

 

 

Regards,

 

Akram

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Hi,

 

When you choose Fe++ as the main "diagram species" in Act2, you're considering not only the free Fe++ species, but any Fe complexes or minerals in the thermo dataset that can be formed from the current Basis. The Basis will include Fe++, your diagram axes (O2(aq) and H+, for redox state and pH, respectively), and any additional complexing species ("in the presence of" field). Since you have Fe++ (ferrous iron) and oxygen in your system, you can combine these to consider the various Fe+++(ferric iron) species and minerals.

 

You should also be able to consider Fe(0), metallic iron, in the same way as long as it is listed in the thermo data. The default GWB dataset, thermo.dat, does not have an entry for Fe(0) but thermo.com.v8.r6+.dat does. Try loading this dataset and making a redox pH diagram for Fe++. You'll need to turn the "water limits" off in order to see where Fe(0) will plot.

 

If you wanted to edit your own thermo dataset to include Fe(0), you would need to add a reaction for a new mineral "Fe(0)" written in terms of the default Basis species - likely Fe++ and O2(aq).

 

You should take a look at a few sections of the GWB User's Guides for more information. The Act2 section of the GWB Essentials Guide will be very useful. In the Reference Manual, try looking up the "water_limits" keyword and the Thermo Datasets Appendix for details on editing a thermo dataset.

 

Hope this helps,

Brian

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Hi

 

I wanted to edit my own thermo dataset to include Fe(0)but I don't know how to write in terms of the default Basis species - likely Fe++ and O2(aq). As you maintained in the last email.

 

Regards,

 

 

Akram

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Hi Akram,

 

Depending on the dataset you're using, Fe++ and O2(aq) are likely to be Basis species. You'll need as well H2O and H+ to balance the reaction. The conventions you'll need to follow are described in the Thermo Datasets Appendix to the GWB Reference Manual.

 

You might take a look at the reaction for Fe as it's written in thermo.com.V8.R6+.dat. Negative coefficients indicate that the balancing Basis species should appear on the left side of the reaction, along with whatever species or mineral you're interested in - Fe here. You'll likely want to verify that the equilibrium constants are appropriate for your application. You might search for log K's in the literature (be sure they're for reactions balanced the same way you enter the reaction in the dataset), or for standard state free energies of formation so that you can calculate a log K.

 

Editing the database this way will allow you to make redox-pH diagrams, but for more in-depth analyses of redox chemistry, you might want to take a look at this topic, and the Redox Equilibrium chapter in the Geochemical and Biogeochemical Reaction Modeling text.

 

Hope this helps,

Brian

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Hi

 

I tried what you told me but I don't get it. I am confuse about it and it is hard to get Fe in it. Also, I would like to get swap e- for CO2 OR H2O instead of O2(aq). So, could you help me with this ,please?

 

Regards

 

Akram

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Hi,

 

If you're having a hard time adding Fe to the thermo dataset, could you please attach what you have so far so I can take a look? In the meantime, I would use thermo.com.v8.r6+.dat, which does have an entry for elemental Fe. You'll need to do a few things to make the elemental Fe field visible on your plot. First, you'll need to uncheck the "water limits" option so that Act2 will consider the areas where conditions are either so reducing or so oxidizing that water is not stable. You do this by right-clicking your plot and selecting "View...", then uncheck "water limits". Next make sure the range of your axes is appropriate to consider where Fe would be stable.

 

I'm not sure what you mean by swapping e- for CO2 OR H2O instead of O2(aq). Try taking a look at some of the examples on our Diagrams webpage. If you click the Act2 icon, you'll open up Act2 with all of the necessary commands already specified. You can then adjust the plot to suit your needs.

 

Hope this helps,

Brian

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Hi

 

I have added Fe but i have hard time to get Fe-H2O-Hg system and I don't know how to get Hg in the diagram. So, I attached the code and the diagram in order to get help from you.

 

I am looking forward to hearing from you

 

Akram

 

Fe_CO2_MINERALS_SUPPRESSED.ac2

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Hi Akram,

 

You've selected Fe as the main system to diagram, on Eh pH axes, in the presence of CO2 and H2S. Every species, mineral, or gas which will plot on this diagram must include Fe, and can optionally include H, O, C, or S. If you unsuppressed Siderite (FeCO3) or Troilite (FeS), for example, these could form. Hg does not form any sort of aqueous complex, gaseous species, or mineral with Fe, at least according to the currently loaded thermo data, so adding Hg++ to the basis (in the "in the presence of" field) will not affect your diagram. To see this, you can look read through the thermo dataset itself (go to File - View - thermo....) or, a little easier, go to Config - Show..., choose Fe++ (or Hg++) as the filter, and a list of all species which include Fe++ (or Hg++) will appear. There are none which include both Fe and Hg, so nothing with Hg will show up on your diagram. If you are aware of some species that exists, you would need to add this to the thermo data. (Keep in mind that surface species are not considered in these types of diagrams, so Act2 will not consider a surface complex of Hg++ onto an iron mineral like HFO).

 

Hope this helps,

Brian

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Hi,

 

One more item to note. You've swapped Fe(s) for Fe++, and specified an activity of 0.0001. This is not much different from saying the activity of Fe(OH)3(s) or Hematite is 0.0001 - it's a tricky thing to do. Specifying the activity of an aqueous species, like Fe++, is much easier however. Since this is a redox pH diagram, Act2 will use combinations of the Basis species Fe++, O2(aq) (or e-), H+, and H2O to form all of the species, minerals, and gases which can appear on the diagram. In your case, Fe+++, Fe++, Fe(OH)3(s), FeO(s), and Fe(s) are predominant. Thus, there is no need to swap in Fe(s) to ensure it appears in your diagram.

 

Cheers,

Brian

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Hi

 

I would need to add Hg to the thermo data. So, How can I add this to the thermo?

 

Regards,

 

Akram

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Hi Akram,

 

What sort of species are you hoping to consider in your diagram? Do you know the oxidation state of Fe and Hg in this species?

 

Once you have the appropriate information, you can start adding entries to your thermo data. The Appendix I've mentioned is a good place to see how this should be done. An example in which Phenol(aq) is added to a thermo dataset is given in our Tutorials webpage (go to Using GWB - How do I edit thermo datasets?). For your information, Phenol is a carbon molecule in a different oxidation state from HCO3-, the default Basis species used to represent carbon. It is added as a new Redox species, which is basically a Basis species in an alternative oxidation state.

 

Since you're interested in Fe, take a look at some of the Fe species in the thermo datasets. Fe++ is a Basis species and Fe+++ is a redox species. All aqueous species and minerals which include ferrous iron (like FeCl2 and Siderite) are written in terms of Fe++, while those which contain ferric iron (FeCl3 and Hematite) are written in terms of Fe+++. Magnetite is an interesting case, in that it includes both ferrous and ferric iron. If you take a look at the entry for Magnetite in the thermo data, you'll see that it is written in terms of both Fe++ and Fe+++.

 

Try looking through the thermo data in this way. You'll need to go through the above-mentioned Appendix as well. Finally, I would encourage you to read Chapter 7 (Redox disequilibrium) of Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text.

 

Hope this helps,

Brian

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Hi Brian,

 

Hope you are fine!

 

I am hoping to consider Iron and mercuric ion in my diagram but I don't want oxygen on it. Yes, the oxidation of Fe is Fe++ and Hg is Hg++.

In addition, I have tried what you mentioned in previous email but I have problem about how to add entries to thermo data even if I read the Appendix that you have mentioned. Please, could you add it or help me to add it because I have hard time to add it.

 

 

I am looking forward to hearing from you,

 

Regards,

 

Akram

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Hi Akram,

 

Are you still interested in an Eh-pH diagram? The Eh is just another way to think about the oxidation state of your system.

 

Could you please post one of the species or minerals that you would like to add, along with its thermodynamic data?

 

Thanks,

Brian

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Hi Brian,

 

Hope you are fine and enjoying the weather,

 

Yes,I am still interested in an Eh-pH diagram. Also, I would like to add FeCO3 and HgCO3. Also, is there any another program that I should use to get absorption reaction on the surface?

 

Best regards,

 

Akram

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Hi Akram,

 

I believe thermo.com.v8.R6+.dat has FeCO3(aq), and thermo_minteq.dat has an entry for HgCO3(aq) at 25 C. Since you're already using v8.R6+ (as far as I can tell), you might try to add the entry for HgCO3 from the minteq database to v8.R6+. That reaction (HgCO3 (aq) + 2 H2O = 2 H+ + Hg(OH)2 + CO3--), which is valid at 25 C, will have to be rewritten in terms of the Basis species in v8.R6+, so that it looks like HgCO3 (aq) + H+ = Hg++ + HCO3-. You can do this in Rxn by performing Basis swaps (swap the Hg(OH)2 out for Hg++, and the CO3-- out for HCO3-). You'll need the equilibrium constant for that reaction calculated at 25 C.

 

Assuming you add this species to v8.R6+, you'll be able to create an Eh-pH diagram for Fe which considers C complexes, or one for Hg which considers C complexes. You won't be able to consider Fe and Hg together, however, unless you create a diagram with C as the main system, in the presence of both Fe and Hg complexes. That being said, your FeCO3(aq) or MgCO3(aq) complexes will not necessarily show up on your diagram unless they are the most stable species for any given conditions.

 

You can consider surface complexes with Rxn, SpecE8, React, X1t, and X2t. Rxn will allow you to balance reactions involving surface complexes and is a great place to start. You'll want to try surface datasets like FeOH.dat (to be used with thermo.dat, or thermo.com.v8.R6+.dat- please note you may need to edit the names of a few species when using it with v8.R6+) or FeOH_minteq.dat (to be used with thermo_minteq.dat).

 

Try taking a look at Sections 2.5 and 7.5 in the GWB Essentials Guide or Sectoin 3.6 in the Reaction Modeling Guide. Browsing through chapters 9, 10, and 11 in the Geochemical and Biogeochemical Reaction Modeling textbook will also be helpful.

 

Regards,

Brian

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Dear Brian,

Hope you are fine,

 

You mentioned in previous email that reaction (HgCO3(aq)+2 H2O = 2H+ + Hg(OH)2 +CO3-- )

 

which is valid at 25C, but I have found this data when I used Rxn.

 

HgCO3 (aq) + 2 H2O = 2 H+ + Hg(OH)2 + CO3--

 

Log K's:

0 °C: -18.2900 55 °C: -18.2900

12 °C: -18.2900 70 °C: -18.2900

25 °C: -18.2900 85 °C: -18.2900

40 °C: -18.2900 100 °C: -18.2900

 

Polynomial fit:

log K = -18.29 + 5.64e-14 × T - 2.788e-15 × T^2 + 4.38e-17 × T^3 - 2.149e-19 × T^4

 

Equilibrium equation:

log K = - log a[HgCO3 (aq)] + 2 × log a[H+] + log a[Hg(OH)2]

- 2 × log a[H2O] + log a[CO3--]

 

So, could you help me to add them to my pourbaix diagram which I have sent to you

 

Best regards,

 

Akram

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Hi Akram,

 

I figured this was a little tricky, and since we're currently setting up an instructional YouTube channel I made an experimental video walkthrough for you. The video describes how you should move the entry for HgCO3(aq) from thermo_minteq.dat to thermo.com.V8.R6+.dat. I hope this helps.

 

GWB YouTube channel: https://www.youtube.com/user/GeochemistsWorkbench/videos?flow=grid&view=1

 

Please let us know what you think about the channel as a whole. If you subscribe, you'll be able to follow our new content as soon as its posted.

 

Cheers,

Brian

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