Regarding gas fugacity at high temperature (80C) in Act2

[Polly Tsai]

Dear Brian,

I am not sure what the value of gas fugacity (like CO2) at high temperature (80C) in pure water (no presence of complexing ions) should be input for uranium (UO2) pH/Eh diagram? I tried to input fugacity of CO2 <-> HCO3- using arbitary data, like 1, 0.8...0.3 and the plot was not the same as that of technical report. Please refer to the attached Act2 raw data and the technical report.

Thanks again for your kind help.

 

Polly

 

U_80C in pure water_pH vs pe.ac2

[Brian Farrell]

Hi Polly,

The CO2 fugacity you specify should be unique for your particular calculation. Looking at the figure you’ve provided from the report, no species with carbon predominate, so I’m reasonably certain that’s not the missing link (in this type of calculation, at least). 

Act2’s Results pane (as well as the “Act2_output.txt” file that you can access by clicking “View Results” from the Plot pane) includes a list of all the species and minerals included in the calculation. If you look there, you’ll notice that UO2(OH)3- and UO2(OH)4--, which predominate in the upper right of the technical report’s diagram, but not yours, are missing. If you look in the thermo dataset, you’ll see they only have log K values specified at 25 C, so they won’t be considered in your 80 C calculation. Act2’s “extrapolate” function can be used to estimate log K values outside their known values. This can be quite dangerous, especially when only a single log K value is known, but I believe that’s what has been done in the report.

For more information, please see the Thermo Datasets chapter in the GWB Reference Manual, as well as the extrapolate command in the GWB Command Reference.

Regards,

Brian Farrell
Aqueous Solutions 
 

[Polly Tsai]

Dear Brian,

Thanks for your response. Yes, my plot without consideration of gas fugacity is not the same as that of the above technical report.

Please refer to the attached plot and raw data. As to the termo dataset, ver.8.0 of GWB was used in the technical report, while mine is ver.12.0. Is it possible for different version of thermo dataset?  In addition, you can find the it's also not the same plot for 25C and 80C in the technical report. Please refer to the two figures of Technical report_25C and 80. So, I think they should consider  the logk value in their 80C.

So far, I still don't know how to do in the next step.

 

Polly

 

Plot without fugacity.docx

U_80C in pure water_pH vs pe_wo fugacity.ac2

[Brian Farrell]

Hi Polly, 

Like I said, you need to use the “extrapolate” option to match your 80 C plot to the technical report. When set to 80 C, Act2 by default doesn’t load the UO2(OH)3- and UO2(OH)4-- species that you see in the report because they have log Ks only at 25 C. If you extrapolate these log K values to higher temperatures, Act2 will load them, and you’ll get the same diagram.

Regards,
Brian

[Polly Tsai]

Hi, Brian,

Thanks so much for your instruction. The pH/Eh diagram at 80C has been successfully plotted after selecting the "alter logk" of config. I wondered GWB would automatically calculate the species.

 

All the best,

Polly