How to calculate the pe to calculate the saturation index?

[spring2015]

Hi all,

I am trying to calculate the saturation index of galena in solution. In my project I mix 2mM Lead nitrate with 7.8 Na2S and adjust the pH to 5.5, I entered these numbers in geochemist's workbench and attached it to this message. But I do not know the pe number. Would you please check the file I attached to see if the what I have entered correct or not? So why galena is under saturation?

For example is it OK I swap s with SO4-2?

What about the pe?

 

Pb-S.sp8

[katezat]

There are a variety of methods to constrain the oxidation state, including setting the pe, Eh, oxygen fugacity, dissolved oxygen concentration, equilibrium with a mineral, or the ratio of oxidized to reduced species. Rather than setting a specific pe value, though, I think you first need to conceptualize your problem. You may want to see how different conditions affect your system by setting up a reducing or oxidizing environment. If the fluid is open to the atmosphere then you could set the oxygen fugacity and you’d end up with most of your sulfur present as sulfate. Or if you’re working in an anaerobic chamber and you know that most of the sulfur is present as sulfide, you could decouple HS- from SO4—and set the sulfide concentration directly.

 

Try taking a look at sections 7.2 (Equilibrium models) and 7.3 (Redox disequilibrium) in the GWB Essentials Modeling Guide. For even more information, Chapters 6 and 7 in the Geochemical and Biogeochemical Reaction Modeling text should be useful.

 

A potentially interesting calculation would be to set up a reaction path model (you’d need React for this) to simulate the addition of Na2S to an initially oxygenated fluid. You could even control whether the fluid remains in contact with the atmosphere or if it’s isolated at the beginning of the path. In this way, you could see how the addition of Na2S would affect the oxidation state of the fluid, and as a result, the saturation state of galena.