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How to set a specific surface area in ion exchange simulation using Spec8


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I use Spec8 to do an ion exchange simulation (please see this: https://www.e-education.psu.edu/png550/node/836), and have some questions.

1) In the simulation, they give a value of "specific surface area". Can I add this condition in Spec8? (And how to do it?)

2) There are some different results between Spec8 and the website. Is it because we do not use the same thermo data or maybe others reason?

Thank you for your help.





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Hi Betty,

As far as I am aware specific surface area isn’t used for ion exchange calculations. That's true in the GWB, at least. You specify the cation or anion exchange capacity for an ion exchange surface in SpecE8’s Sorbing Surfaces dialog. You can specify the CEC or AEC in terms of total electrical equivalents. Or, you can specify them in terms of 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.

It’s hard to say definitively. Your website mentions first solving problems assuming no aqueous complexation of the cations (i.e. complete dissociation), and then making a second calculation including aqueous complexes (e.g. CaCl+). You should make sure you’re comparing simulations with the same assumptions, like those above. To ignore complexes, you’ll want to use the convenient Suppress feature in SpecE8. For more information please see the Suppress command in the SpecE8 chapter of the GWB Command Reference.

You should also verify that the underlying thermodynamic and surface data matches the original source. At a quick glance, I think the selectivity coefficients for the ion exchange reactions you’ve specified are incorrect. The website has a table of log K values. The GWB, however, requires that selectivity coefficients for the ion exchange model be expressed in the surface dataset as linear, not logarithmic values. Please see the Preamble and Bibliography of the IonEx.sdat example installed in the GWB’s Gtdata folder for more information. Please see as well section Ion Exchange in the Using TEdit chapter of the GWB Essentials Guide.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC

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