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I have a question about how pE is calculated in Spec8.  I am having some issues with getting pE results based off of some low negative Eh values that are being put in as inputs (the lowest being -240 mV).  Spec8 will not calculate pE or anything else including species activities and mineral saturations unless I lower the Eh.  What role does pH play in this?

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Hello Kevin,

The GWB calculates pe with the Nernst equation. For more details please refer to the Geochemical and Biogeochemical textbook, section 3.3.8 the pe and Eh. The Eh constraint sets the oxidation state of your system for all redox reactions unless the reaction is decoupled. If you want to see an example of this, please see section 7.3 Redox Disequilibrium in the GWB Essentials guide. A pH is required for constraining the reaction e-  + H+  + .25 O2(aq)  = .5 H2O in your system.

It's hard to diagnose the issue with your input file without seeing it. Is the Results pane reporting an error when you trigger the calculation? We can take a closer look if you attach a copy of the input file that's not producing results according to your expectations.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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I have a question about the geochemistry of groundwater around contaminated sites, such as landfills.  I am using the Geochemists Workbench software to model certain sites, using the model to produce Pourbaix Diagrams to assess whether the groundwater is supersaturated with certain minerals.  Most of our sites have issues with dissolved cobalt and nickel.  The diagrams for one of my sites indicates that the groundwater is supersaturated with respect to cobalt ferrite, limiting the dissolved cobalt concentrations in the groundwater. However, I cannot find much, if any, literature on this precipitate relative to its presence in natural environments like groundwater.   I don’t believe the cobalt ferrite precipitate is actually forming at the site.  The same goes for nickel ferrite (trevorite) which is a rare mineral.  I can suppress these compounds within the model and, when this is done, the dominant precipitate forming is hematite, which I do believe is most likely occurring.

 

Do you have any information or insight into these two ferrite compounds and whether or not these can actually form in groundwater and remain stable? 

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I have no specific insight about trevorite (perhaps someone else on the forum will), but in calculating equilibrium models it's important to remember that the most thermodynamically stable minerals might not be likely to form, especially within relatively short timeframes. Using your experience to suppress minerals that fall in this category is common in geochemical modeling.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC

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