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Polly Tsai

Regarding activities of diagram species and the presence of anions

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Dear Brian,

I would like to make a pH/Eh diagram of uranium (like UO2, uraninite) in the deep groundwater

i.e., under surface of 400-500m, and have two questions listed as below.

1.  In my case study, the UO22+ was selected in the diagram species, but I don't know the value of activity that I should input and there is no reason to input the value in the GWB essentials guide. Is it arbitrary? I also referred to the other technical report and the activity of "1.0e-06" was used. But I still have no idea for this value, because the different ionic concentration will have an influence on the result of making Eh/pH diagram.

2. The second question, also in this module, if UO22+ is in the presence of the following table 1, i.e., the sources of anions. However, I don't make sure the correctness of input parameters in Act2 of GWB, please refer to the attachment. The result that I plotted (Polly) is different to the technicial report for the same input parameters. Please also refer to the attachment.

Table 1

Cl_tot (mol/L)

8.55e-04

C_tot (mol/L)

1.15e-03

S_tot (mol/L)

9.59e-05

N_tot (mol/L)

4.03e-05

P_tot (mol/L)

2.05e-06

Si_tot (mol/L)

6.90e-04

F_tot (mol/L)

2.11e-04

 

input parameters in GWB.png

Results.png

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Hi Polly,

First, verify that the thermo dataset you’re using matches that used in the technical report.

I think a huge difference is that the diagram depicted in the technical report is an example of a “mosaic diagram”. The complexing HCO3- ion, for example, might be allowed to speciate as a function of pH into CO2(aq) or CO3--. To accomplish this, the diagram is divided into several parts in which one of those forms of carbon predominates. Within each subdiagram, reactions are written in terms of the predominant form of the complexing ligand. You can recognize a mosaic diagram by its shape – the slope of the bounding line between a pair of species is normally constant, but in a mosaic diagram it might change abruptly. You can choose to speciate over the conditions of the x axis only (pH here), y axis only (pe), or both. 

You should check as well the redox coupling and choice of oxidation states in the diagrams. Your analysis specifies only N, not the valence of nitrogen species. Do you know whether all the nitrogen exists in one oxidation state, such as nitrate, or as several? If it’s only nitrate, for example, you would decouple the redox species NO3- from the basis species NH3(aq), then add NO3- (not NH3(aq). In this case, speciate over y would have no effect, because no coupling reactions are enabled for the ligand. You should consider your choice of redox coupling reactions together with the degree of speciation you specify for complexing ligands.

Finally, you may need to suppress one or more species that are thermodynamically favored, but may have been excluded from the report.

Perhaps the paper, or an appendix, reveals more information about the details of the calculation? 

For more information, please see 2.4 Redox couples and 5.3 Mosaic diagrams in the GWB Essentials Guide. For information on suppressing species, please see 3.54 Suppress in the GWB Command Reference, as well as the example in 6.2 Solubility diagrams in the GWB Essentials Guide.

Please post on the front page of the GWB forum, not the archive of old posts.

Regards,

Brian Farrell
Aqueous Solutions
 

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Dear Brian,

Thanks so much for your prompt reply. The dataset, thermo.com.V8.R6+.dat was used for the technical report and my case study.

I should try the function of mosaic diagram in the GWB.

 

Polly

 

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