ACT2 Module - Stability Diagram

[tarang]

Hi,

 

I am trying to create an iron sulfide and iron oxide stability diagram and having difficulty inputting both a sulfate concentration and H2S concentration. The species I am interested in analyzing contains 0.004 mol/L H2S and 2.1 mol/L Na2SO4 and both contribute to the formation of troilite and pyrite, but I cannot seem to model them together. The module only lets me specify one or the other in the Basis Page under "in the presence of." Please see the attached file for more information.

 

Thank you!

Stability Diagram_Iron Sulfide and Iron Oxide.ac2

[katezat]

Hello Tarang,

 

Thank you for attaching your Act2 diagram, it was very helpful. There are two options for you to include sulfate and sulfide depending if you are assuming they are in equilibrium or not. If you are assuming sulfate and sulfide are in equilibrium (and that the form of the complexing sulfur species depends on the Eh and pH conditions), then you should set SO42- to speciate over x-y. This is what you have done. It’s called a mosaic diagram. You can include labels for the sub-fields of the plot where the different sulfur species (HSO4-, SO42-, H2S, HS-, S2-,etc.) are predominant. To do so, right click on your diagram, select view and check the mosaic labels box.

 

Alternatively, you can choose to decouple H2S from SO42- to allow both species to coexist since they will no longer be in equilibrium. To decouple the redox couples go to Config > Redox Couples > select HS-/SO42-. Then you can independently add HS- to the “in the presence of field” and swap in H2S(aq).

 

Kind regards,

 

Katelyn

[tarang]

Katelyn,

 

Thanks for the help! That makes sense, and now I know what speciate is doing. I updated the information, and now I'm getting pyrite as the dominant type of iron sulfide scale present. However, I know other scales can form, so I tried using the suppress feature. Is that a function of stability? So pyrite is the most stable, and the second most stable is pyrrhotite (see attached)?

 

Also, I looked through the mineral species, and the only iron sulfide compounds I saw were pyrrhotite, troilite, and pyrite. I am also aware of greigite, mackinawite, and marcasite. Is there a way to incorporate them as well?

 

Thank you!

Stability Diagram_Iron Sulfide and Iron Oxide_Suppress.ac2

[katezat]

Hi Tarang,

 

You are correct, the suppress feature is a function of stability. If you suppress a mineral phase, the next most stable species will have a chance to form in its place, as in your attached example.

 

The reason the other iron sulfide compounds (greigite, mackinawite, and marcasite) are not included in the mineral species is because those minerals are not included in the thermodynamic dataset loaded by the program (thermo.tdat by default). Thermo datasets are fully editable, though. To add a new mineral, for example, you just need an equilibrium constant corresponding to a reaction to form the mineral from species in the dataset. You can use the TEdit program (under the support tab on the GWB dashboard) to modify a dataset in this way. You can find a tutorial on how to edit thermo datasets here. Also, in the Geochemical and Biogeochemical Reaction Modeling text there is an example calculation that specifically shows how to add mackinawite to a thermo dataset (Chapter 18 Example Calculation).

[spring2015]

Hi, This topic was really helpful. I have a question I would appreciate if you could guide me. How could you calculate activity of Fe3+ and activity of SO42-?

[katezat]

Hi Spring2015,

 

You can use SpecE8 to determine the distribution of mass among aqueous species at equilibrium. SpecE8 will figure the concentration, activity, and activity coefficient of aqueous species based on the fluid composition you define. For more info, see Section 7.1 in the GWB Essentials Guide.

 

Kind regards,

 

Katelyn

[spring2015]

Thanks a lot for your reply. I have some more questions so I am going to build another title. I hope to receive your guidance.