Ion-Exchange handling data for several minerals

[Frank Bok]

Hello,

I would like to model a scenarion involving several minerals that are capable of cation exchange (Montmorillonite, Illite, Smectite, ...). I also collected some mineral-specific exchange parameters and want to set up a Ion-Exchange surface file (*.sdat). Is there a way to connect the different exchange reactions and their coefficients to the corresponding minerals? For surface complexation modelling data, this can be done quit nicely in one single file by defining different minerals with their surface groups. But for Ion-Exchange reactions I haven't found a solution. Any help is highly appiciated!

Many thanks in advance and all the best!

Frank

[Jia Wang]

Hello Frank,

Unfortunately, unlike the two layer and three layer surface models, ion-exchange datasets do not have exchange sites that can be linked to specific minerals. I am not sure the exact problem you're investigating, but if you want to model exchange on just one mineral at a time, you can load in the ion-exchange dataset prepared with the corresponding mineral swapped into your system. Then repeat with another mineral and the corresponding ion-exchange dataset. You can specify the cation or anion exchange capacity in terms of total electrical equivalents or electrical equivalents per mass of rock, in which case the program multiplies this value by the total mass of equilibrium and kinetic minerals, as well as inert mineral mass.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Frank Bok]

Hello Jia,

thanks for your answer. Actually I want to model the retardation of ions in a natural soil sample where I know the mineral composition: Quartz (76.0%), Albite (5,7%), Orthoclase (3,4%), Microcline (6,6%), Illite (2,6%), Kaolinite (2,6%), Montmorillonite (3,1%). With your help I managed to include them all into a calculation at the same time using kinetics. I also created an surface complexation modelling dataset using an bottom-up approach. But wand to include ion-exchange for several minerals - at least Montmorillonite, Illite and Kaolinite - because Ion-Exchange is a major contribution process on retadration. So my problem is to include mineral specific Ion-exchange data and I am looking for a way to simultaniously include Ion-exchange data for Montmorillonite, Illite and Kaolinite. Is this somehow possible?

I just wonder if this could be realized by having different exchange-sites (>X:Na, >Y:Na, >Z:Na)? But then the exchange capacity is not mineral-specific.

Many thanks and best regards,

Frank

[Jia Wang]

Hello Frank,

Thank you for providing clarifications. You can definitely load in multiple ion-exchange datasets with a different name for each exchange site like you described above. If your mineral volume is not changing, you can set an absolute value for the exchange capacity instead of that eq per grock. That way, the capacity is not affected by the mass of minerals you have present in your system. 

Just one more note. To load multiple surface datasets of the same model type, make sure that a unique identifier is set for the "Surface type". This is the third line from the top of the file and by default set to IonEx when you create a new ion-exchange dataset.  

For example, the default dataset header lines look like: 

For example, the default dataset header lines look like: 
Dataset of surface reactions for gwb programs
Dataset format: may20
Surface type: IonEx
Model type: ion-exchange  gaines-thomas

Changing the surface type to a unique identifier by adding _Y:

Dataset of surface reactions for gwb programs
Dataset format: may20
Surface type: IonEx_Y
Model type: ion-exchange  gaines-thomas

Hope this helps,
Jia

 

[Frank Bok]

Dear Jia,

many thanks for this helpful tip!

Best regards,
Frank