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Sorption on Montmorillonite


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I would like to know how I can make a surface complexation model for montmorillonite surface according to by Bradbury and Bayens page 9. http://les.web.psi.ch/publications/abstracts/PSI_Bericht02-10.pdf I think that the instructions in manuals are poorly done and don't include how I have to adjust the database for the complexes if I am going to change the FEOH surface database.

I am using thermoddem database which can be exported to GWB from here http://thermoddem.brgm.fr/recherche.asp and is too big to be attached.

The Surface database is attached here. I know I have to adjust the thermoddem database, I just don't know what are all the reactions I have to add and where that this would work.


Best Regards


Aku Itälä



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Hi Aku,


I can't view either the reference you mention or the thermodem dataset. I have a few suggestions to get you started here, though. On the third line of your surface dataset MX80Surf.dat, the surface type is really just a "code" for your dataset to distinguish between different types of surfaces that you may have loaded. I would use something like "Mont" to describe your dataset so that you can use commands like surface_data remove HFO or surface_potential on Mont = 0.


As long as your thermo dataset has all the metals, etc. for which you would like to model sorption, you shouldn't really have to edit that. What you want to do is add correctly reactions to the surface dataset between aqueous and surface species. So if Hg++ appears in your thermo dataset, you would add a reaction like >(s)XOHg+ + H+ = >(s)XOH + Hg++. The form of the surface complex and reaction logK will come from the literature.


As for the form of your surface basis species, >(s)XOH and >(w)XOH are really just labels. Where you identify the elemental composition of the surface basis species, you could just write



charge= 0.0 mole wt.= 372.5017

2 elements in species

1.000 O 1.000 H


but you do not want X as an element in there, because it's not one. What surface group exactly is causing the binding is not too important (you are calling it "X", after all). What's important is specifying the site density and specific surface area of the sorbing mineral. You also want to make sure that your surface species are similar to the FeOH model (i.e. weak and strong site, and protonated/ deprotonated sites) if that's the template you're going to follow.


Hope this helps,


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Thanks for the answer, I will try what you told. The forum added some ). to the end of the lines, I fixed the questions so that the links work fine now, if you would like to check those.

I need only the protonation/deprotonation reactions for montmorillonite.


Thanks for the help, I got it to work and understand the program a little more again :)





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