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Dear Bryan,

Please i'm in need of help with Eh vs Ph plot with act2. I'm trying to reproduce a thermodynamic model that was done  with SUPCRT92  with Geochemist Work Bench's thermodynamic Model.

I realise that my result is not the same, even after using same Delta G values that was used in SUPCRT92. Please any ideas why the results are not the same please? is there any way you can help me. I want to reproduce the same thing that was done on SUPCRT92.

 

 

 

snaptool.PNG

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Hi Jude,

It's really hard to tell without any knowledge of how your model or the originals were set up. One guess is that the originals considered only inorganic carbon, whereas you're forming various organic species under reducing conditions. You can decouple the various carbon redox species from the HCO3- basis species to limit your calculation to inorganic carbon. For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide.

If you have more questions, you'll have to provide your Act2 input file, your thermo dataset, and more information about how the original diagrams were made.

Regards,

Brian Farrell
Aqueous Solutions

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Hello Brian,

Thank you for your prompt response.

I've tried to decouple it as you advised, it's not giving me the same thing as the model.

The data base i used  is the thermo.com.V8.R6+.tdat which is said to be gotten from SUPCRT92 . I've also attached my input file as well.

To be honest, i saw model i'm working on in a  paper which i attached below  there are no other information on how the Model was built.

I look forward to your favourable assitance with this Brian.

Regards

Jude

7 hours ago, Brian Farrell said:

Hi Jude,

It's really hard to tell without any knowledge of how your model or the originals were set up. One guess is that the originals considered only inorganic carbon, whereas you're forming various organic species under reducing conditions. You can decouple the various carbon redox species from the HCO3- basis species to limit your calculation to inorganic carbon. For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide.

If you have more questions, you'll have to provide your Act2 input file, your thermo dataset, and more information about how the original diagrams were made.

Regards,

Brian Farrell
Aqueous Solutions

 

Hello Brian,

Thank you for your prompt response.

I've tried to decouple it as you advised, it's not giving me the same thing as the model.

The data base i used  is the thermo.com.V8.R6+.tdat which is said to be gotten from SUPCRT92 . I've also attached my input file as well.

To be honest, i saw model i'm working on in a  paper which i attached below  there are no other information on how the Model was built.

I look forward to your favourable assitance with this Brian.

Regards

Jude

 

 

act 2 inputfile.png

MAG-4_Burton.Machel.Qi_SEMP_1993.pdf

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On 7/11/2019 at 11:33 PM, Brian Farrell said:

Hi Jude,

It's really hard to tell without any knowledge of how your model or the originals were set up. One guess is that the originals considered only inorganic carbon, whereas you're forming various organic species under reducing conditions. You can decouple the various carbon redox species from the HCO3- basis species to limit your calculation to inorganic carbon. For more information, please see 2.4 Redox couples and 7.3 Redox disequilibrium in the GWB Essentials Guide.

If you have more questions, you'll have to provide your Act2 input file, your thermo dataset, and more information about how the original diagrams were made.

Regards,

Brian Farrell
Aqueous Solutions

Hello Brian,

Please help me have a look at my reply.

Thanks

Jude

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Hi Jude,

Ok, you showed an Eh vs. log a HCO3- diagram before (Figure 1D/2D). Now you want to make an Eh-pH diagram (Figure 1A/2A) instead? If so, based on the caption to Figure 1A, you need to set the HS- activity to 10^-12 and add HCO3- with an activity of 10^-2. For each different diagram you want to make, you need to pay close attention to the conditions indicated in the figure caption.

After getting the geochemical constraints right, the most important step is to make sure the thermodynamic data you’re using in Act2 matches that used in the original reference. There are potentially multiple different sources of data in thermo.comV8.R6+.tdat. It’s also possible the information in SUPCRT has changed over the years. I think you’ll need to calculate the log K for every relevant reaction from the delta G o formation values listed in Table 2 and see if they match the log K values in the GWB thermo dataset. If not, you can permanently modify the log Ks in the thermo dataset, or you can temporarily change them within an Act2 run using the “Alter log K” feature. 

Finally, it looks like the publication used a relatively short list of species in its calculation. Act2, on the other hand, will load every possible species that can be formed based on the specific basis – in your case Fe++, H2O, e-, H+, HCO3-, and HS-. You may need to suppress certain species, like FeHCO3+ or FeCO3(aq), if they appear in Act2 but not in the original calculation.

For more information, please see the “alter” and “suppress” commands in the Act2 chapter of the GWB Command Reference. Please see as well the Using Act2 and Using TEdit chapters in the GWB Essentials Guide.

Regards,
Brian

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