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Eh-pH Diagrams with different main ions


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Hello,

 

I am trying to figure out the stability field of a mineral (Jarosite). When I try to plot the Eh-pH diagram with the "diagram species" as Fe(+2), I am not getting Jarosite, but with the same composition with a different diagram species such as K+ or SO4(-2) I am getting Jarosite stability fields.

 

Why is it ? What are the implications of using different diagram species in Act2?

 

Thank you

 

P.S. Attached are the the Eh-pH diagrams

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post-12208-0-37253600-1434920567_thumb.png

post-12208-0-58675600-1434920569_thumb.png

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Hello Kaushik,

 

An activity diagram is a simple representation of a chemical system. It shows the predominant form of the main (diagram) species as a function of two variables (Eh and pH in your case), optionally accounting for complexation with additional ligand species. When K+ is the main species, everything that plots on the diagram must include potassium. When SO4(2-) is the main species, everything that plots must include sulfur. When Fe++ is the main species, everything that plots must include iron.

 

It is possible for a mineral like Jarosite, which contains potassium, sulfur, and iron, to plot on a diagram with any of the above-mentioned species as the main species, but it won’t unless it is the predominant form under the conditions of the diagram. Jarosite is a stable mineral phase when you diagram K+ or SO4(2-) but not one of the most stable mineral phase for iron. In any activity diagram, you can suppress a species to allow the next most stable species to form in its place. If you want Jarosite to plot on your diagram with Fe++, you will need to suppress (config> suppress) some stable species.

 

Kind regards,

 

Katelyn

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  • 2 weeks later...

Hi Katelyn,

Thank you so much for your reply. I was contemplating on your reply and my problem statement. I would highly appreciate if you can help me with understanding the working of GWB.

 

Below is the crux of the problem statement and my doubts:

 

Problem statement: I am taking the general composition of sea-water and trying to find the Eh-pH diagram for the above composition with Fe and Ca as the diagram species.

 

Problems/Doubts:

 

1. The input asked by GWB Act2 are ACTIVITIES (effective concentration), rather than concentrations which values we have. Do I directly provide the concentrations and the software itself takes into account the activity calculations or I need to calculate the activities separately and then provide the same? If I have to calculate the activities, then how can I calculate the activities of the individual components?

 

2. When I am plotting the Eh-pH diagram, what does the BLUE shade and the YELLOW shade imply? If BLUE implies Aqueous fields and Yellow implies Mineral fields, what is the implication of species like FeSO4 or FeSO4(+) in aqueous regions? I am not able to understand how FeSO4 can exist as a molecule in aqueous field. Probably it should dissociate into Fe and SO4(-2). If not then what is the significance of FeSO4 or FeSO4(+) in aqueous field?

 

Thank you so much

post-12208-0-52614900-1435749497_thumb.jpg

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Hello Kaushik,

 

1) When you select species to diagram in Act2, you need to enter the activity not concentration values. In dilute solutions, it is sometimes safe to assume the activity is equivalent to molality. In fact, this is likely what you assumed when you learned to make activity diagrams by hand. If that assumption is not valid for your system, you can use GSS or SpecE8 to calculate the activity of individual species in solution. You just need to pick an appropriate pH and oxidation state as constraints. You can read more about calculating the activity of free species in the GWB Essentials Guide GSS and SpecE8 chapters.

 

2) You are correct, aqueous species are colored blue and mineral species are yellow. In real systems it’s very possible to have complex aqueous species like FeSO4+ or FeSO4; not all species are completely dissociated. Consider a strong acid like HCl. It will almost completely dissociate to H+ and Cl-, but there is still a small amount of the HCl complex at most pH values (and if the pH is low enough, you’ll have more HCl than H+ or Cl-). A weak acid like carbonic acid (H2CO3) will dissociate to an even lesser extent, so you can find different complexes such as H2CO3, HCO3-, CO3--or CO2 all in solution. In natural systems, it’s very common to find free species and complexes in solution. Typically the higher the ionic strength, the more complex species you’ll find. For more info, take a look at section 6.3 Red Sea Brine in the Geochemical and Biogeochemical Reaction Modeling text. Pay close attention to the comparison between complexing in three different waters: Amazon River, Seawater, and Red Sea brine.

 

Kind regards,

 

Katelyn

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Hi Katelyn,

 

Thank you for the solution. I knew about "Degree of Dissociation" but clearly forgot it while applying.

 

I then understand that the "Mineral Species" that show up on the Eh-pH diagram is only telling us about the "Thermodynamic Stability" and nothing about the "Saturation" of the same and so with the Aqueous species. Whether the particular species will be precipitated in the solution or it will remain in aqueous phase will be governed with the SATURATION INDEX of the species in the solution. If I understand correctly, we cannot decide about a mineral's solubility in an Eh-pH diagram. Is it correct what I understand?

 

I also have a huge number of sample concentration whose Activity Diagrams I need to plot. Is there a simpler way to input all the solutions at once and plot the Activity diagrams?

 

Thank you

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Hello Kaushik,

 

Minerals plotted on an Eh-pH diagram are saturated under those specific oxidation and pH conditions. However, I wouldn’t recommend using an Eh-pH diagram to get information on mineral saturation. If you are interested in calculating the saturation index for minerals in your samples, you can easily calculate it using GSS (click +analyte > Calculate… > Variable type: Mineral saturation).

 

I’m not exactly sure what you mean, but there is no automated way to produce a large number of Eh-pH diagrams. Do your samples differ very much? I don’t think you would find it very meaningful to make multiple slightly different Eh-pH diagrams. Instead, you might want to create one representative Eh-pH diagram for your data and plot your other samples as scatter data on the diagram. You can take a look at Section 3.6.5 Scatter data in the GWB Essentials Guide to learn how to plot GSS data on diagrams.

 

Kind regards,

Katelyn

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Hi Katelyn,

 

Thank you for the constant help and the demo version for my professor. I am currently working on React and would soon buy a professional license package. GWB is wonderful.

 

I have a trifle problem regarding using React. I am trying to see the different mineral assemblages being precipitated when seawater reacts with Basalt. For the same, I have specified the composition of seawater using different ions and set water 1kg in the Basis pane. On the reactants pane I have specified the different minerals by weight %.

 

Now I would like to change the water:rock ratio, which now i guess is 1:1, to different ratios like 10:1, 100:1 and so on. How can I do it?

 

Thanks again for your immense support.

Kaushik

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Hello Kaushik,

 

I’m glad to hear you’re enjoying the free student version and demoing our Professional package.

 

There are a few ways you can modify the amounts of water or rock in your system. If you just want to change the amount of water in your system you can simply modify the mass on the Basis pane (1kg -->10kg).

 

If you need to change the amount of any mineral reacted into the fluid, you can do so for each individual mineral on the Reactants pane. Or, if the proportions of each mineral will remain the same, you can simply adjust the “reactant times” value. It’s a multiplier (default 1) for the mass of any reactant on the Reactants pane, whether it’s solvent water, species, minerals, or gases. Let’s look at a fluid mixing example. If you have 1 kg solvent plus some dissolved solutes in your initial system (the Basis pane) and 1 kg solvent plus some solutes in the Reactants pane and set the "reactants times" to 1 you will end up with 2 kg solvent plus some dissolved solutes in the mixture. If you change the “reactant times” to 2 you will end up with 3kg solvent plus some dissolved solutes in the mixture.

 

It sounds like you’ve configured the minerals in basalt as simple reactants to be added to the fluid. This is what we call a titration model. Basically, you start with a fluid (or fluid-rock equilibrium system), to which the model reacts a small amount of the reactant minerals at a time, taking a number of steps to add the entire reactant mass. So, if you plot some value vs. the variable “mass reacted”, you’ll see how your system evolves after each incremental addition of the minerals making up basalt to the fluid. A reaction path model, then, is not just about the final result. There is information available at each step of the process. Maybe this is all you need?

 

Take a look at section 2.2.2 Titration Models in the Geochemical and Biogeochemical Reaction Modeling text to see if this setup is appropriate. React can model alternate configurations, as described in Chapter 13, that may be more appropriate.

 

Kind regards,

Katelyn

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