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Polly Tsai

Chemical compositions of pore water in bentonite using React module

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Dear Brian,

 

I performed some calculations to obtain the initial chemical composition of pore water in bentonite (MX-80) using GWB code. These calculations have been done by equilibrating the Forsmark groundwater (Table 5.4 in the ppt file) with bentonite with a porosity of 43%, including equilibrium with bentonite accessory minerals as gypsum and quartz (equilibrium with carbonate minerals have also been calculated when considering the MX-80 bentonite), and exchange and surface reactions (Table 5.3 in the ppt file). Table 5-3 and Table 5-4 shows the thermodynamic data used. And according to the above description, enclosed please find the attachments of script, thermo data set and sorbing surface. However, I don’t know why the error message was shown as “bad surface reaction” after running. The other question is how to select SiO2 (aq) or Si(OH)4(aq) in the base pane based on the known concentration of Si in Forsmark groundwater for this case.

Thanks for your kind assistance in dealing with my questions.

 

 

Best regards,

 

Polly

 

 

Query_about_porewater_chemistry_to_Brian_20200218.pptx SKB pore water calculation.rea thermo_phreeqc_Polly.tdat SOH_Polly.sdat IonEx_Polly.sdat

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Hi Polly,

For the two-layer surface complexation dataset, you need to specify a site density or densities for each sorbing mineral. If you don’t, the program can’t account for the existence of the surface. If Ferrihydrite contains both of the sites you defined, >(w1)SOH and >(w2)SOH, your entry might look like this:  

Ferrihydrite
     surface area= 600.0000 m2/g
     2 sorption sites
     >(w1)SOH      site density=      .0050 mol/mol mineral
     >(w2)SOH      site density=      .2000 mol/mol mineral

You should, of course, supply values appropriate for your sorbing mineral. In GWB14, by the way, you can alternatively specify site density in sites/nm2, as you sometimes see in the literature. 

You should also remove the entry for >(w) in the surface species section. I’m not sure if it was intentionally put in the dataset like this, or it was a stub of an entry that you didn’t finish, but there’s no  reaction, stability, or mole weight and that’s causing a problem.

Finally, the surface dataset has a field where the user specifies the thermo dataset to use with it. The aqueous species that are included in the surface reactions, as well as the sorbing minerals, are drawn from the thermo dataset you specify. If you plan to use this surface dataset with your custom thermo dataset, it might be best to specify that custom thermo dataset within the surface dataset. Note that the sorbing mineral you’ve chosen, Ferrihydite, is not in your custom thermo dataset, so you may need to make further modifications to either your thermo or surface dataset.

You’ll probably have to make these modifications in a text editor, like Notepad, since TEdit can’t open the unproperly formatted dataset. 

As for your thermo dataset, I don’t think you should include SiO2, H4SiO4, and Si(OH)4(aq) as separate basis entries. They all represent essentially the same thing. Pick one and write all reactions in terms of that species. 

Hope this helps,

Brian Farrell
Aqueous Solutions
 

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Dear Brian,

Thanks so much for your response. I have already revised surface complexation dataset and changed the file name as AlOH_Polly because the sorbing mineral was montmorillionate of MX-80 bentonite and the complexing site was AlOH. However, the results showed "Residuals too large, 645-th interation". How do I solve the problem?Enclosed please find the script, thermo dataset and sorbing surface. By the way, if I cancelled  Cl as charge balance (system default), there was a result, but I could not make sure it was correct or not.

Good luck,

 

Polly

 

SKB pore water calculation.rea AlOH_Polly.sdat IonEx_Polly.sdat thermo_phreeqc_Polly.tdat

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