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How to set Ion exchange on minus coefficient


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Hello, everyone


I am trying to reproduce heavy metal adsorption on kaolinite from some of the thesis

But I could not understand how to set ion exchange sites.


For example, I tried the SCMs and ion exchange of kaolinite (Gu and Evans,2008)



In this thesis, using ion exchange sites and SOH sites

When I try to make an ion exchange site using Tedit, I could not register Na+ and H+ exchange

There is 2equation remain in the thesis 

(XH + Na+ ⇄ XNa +H+                     K = -2.02)

(2XH+ + Cd2+ ⇄ X2Cd + 2Na+             K=  1.22)

But This soft show the error when I set the coefficient is lower than 0

Please tell me other ways or solutions to reproduce these ion exchange models


Thank you for your cooperation.



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I do not have access to the paper linked so I am not able to see more information on selectivity coefficients or equilibrium constants for these exchange reactions.

Unlike a reaction in the thermodynamic or a two-layer surface complexation reaction database, the ion-exchange type dataset ask users to provide a selectivity coefficient that is a linear value. The selectivity coefficient should not be a negative value. The selectivity coefficient serves as an equilibrium constant for the mass action equation corresponding to the ion exchange reaction. Perhaps the authors published values for Log K instead of linear values? Perhaps double check the paper for more details.

Additionally, I should also note that the GWB carries mass action equations for ion exchange reactions in terms of activities rather than molalities of the aqueous species. When taking coefficients from literature, you should double check units and convert from total molality to free activity as needed. Please see section 2.6.4 in the GWB Essentials User Guide for more information on how The GWB models ion-exchange.

If you still are encountering issues with this, please provide more context to the studies and details with regards to what you are trying to model.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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Dear Jia

Thank you for your reply.

One of My study's purposes is to make a list of kaolinite surface complexation like ferrihydrite did by Dzombak and Morel 1992.

Several kaolinite adsorption studies, using exchange sites sorb some heavy metal to reproduce low pH sorbing metals. 
so now I try to reproduce them in GWB modeling to verify whether the equilibrium constants this thesis using can use or not.

The thesis uses log K value. I will try to fix them to linear value and use them or not.
Thank you for your cooperation. 


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  • 3 weeks later...

Dear all

As advised, we verified whether we could reproduce the prediction by modifying the equilibrium constant to a linear value, but still could not reproduce the experimental results described in the paper.

Various papers on the adsorption of positive ions on kaolinite have used both specific adsorption and adsorption by permanent negative charge, but we have not been able to reproduce the experimental results in these papers.

We are currently working on a list of equilibrium constants for kaolinite.
As a first step, we are checking to see if GWB can represent the experimental results described in the paper using the equilibrium constants and other statuses given in the paper.

After the confirmation, we are planning to verify how much the experimental results are in error with the results described in other papers and select the appropriate ones.

In the relevant paper, the ionic strength is predicted at 0.1, 0.01, and 0.001 with NaNO3, the cadmium concentration is 4.88e-5mol/L, and kaolinite is 7.8g/L input. On top of this, the pH is adjusted between 3 and 10.

REACT was validated with a mixture of one type of permanent negative charge and one specific adsorption site, and one type of permanent negative charge and one specific adsorption site.

Send PDF of the paper, GWB script, adsorption data, and thermodynamic data. Please can someone help me with this?



1-s2.0-S0016703707005698-main.pdf GuonGu.rea Gu_etal.sdat Gu_inout.sdat Gu_outer.sdat GWB_ThermoddemV1.10_15Dec2020.tdat

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Hello Arata,

Thank you for posting the paper and the files to troubleshoot with. Here are some suggestions for you.

Since the amount of metals added to various experiments, in this case cadmium, is the total present, you should enable the "sorbate include" option in your input file (Config -> Options... check "sorbate"). This will distribute the total mass of cadmium between your mineral surface and solution. If not, the software will calculate an additional amount of cadmium sorbed to the mineral surface separate from the fluid. For more information, please see the “sorbate” command in the GWB Command Reference.

The React file you have attached didn't include any kaolinite. Perhaps you had previously set 7.8 g/l in the Reactants pane?

A minor consideration is that FITEQL uses the Davies activity model to calculate species activity, while the thermo dataset you have attached uses the B-dot method. This may cause some minor differences in your case.

The biggest issue here is the ion-exchange convention used in the paper does not match the standard method used in most geochemical modeling software, including the GWB. The exchange species in the mass action expressions for equations 1 and 2 use concentration units, just like aqueous species (as denoted by the brackets). In a typical ion-exchange model, such as Gaines-Thomas, however, the site activities are given in terms of the fraction of the total electrical equivalents of exchange capacity occupied by the ion. Unless you can convert the constants to one of the conventions used in the GWB, you won't be able to replicate those reactions in the GWB (or other conventional modeling programs) using the ion-exchange model.

However, the equations shown in the paper for ion-exchange are similar to one of the conventions for a non-electrostatic surface complexation model. Your combined dataset Gu_inout.sdat should work for these purposes, as long as you change the convention for polydentate reactions to “stoichiometric”, to match the method in the paper. For more information, please see section 2.6.8 Polydentate sorption in the GWB Essentials Guide.

You should note, however, that the stoichiometric method is considered outdated and is not inaccurate when the amount of sorbing mineral differs from the original experimental conditions. For a more robust surface complexation model, you should convert the equilibrium constants for polydentate reactions (in this case, the ion exchange reaction for Cd) to either the hiemstra-vanriemsdijk or appelo-postma protocol. In the hiemstra-vanriemsdijk model, the mass action equations recast the molal terms in terms of mole fractions of sorbing sites. The appelo-postma model uses site coverage rather than that of mole fractions. For a quick primer on polydentate sorption, please see section 10.2.5 Multidentate Complexes in the Geochemical and Biogeochemical Reaction Modeling text, third edition. Also, the paper "Mass Action Expressions for Bidentate Adsorption in Surface Complexation Modeling: Theory and Practice" paper published by Wang and Giammar, 2013, which is cited in the text, provides much more information on the subject and describes a method for converting log Ks from the stoichiometric to more useful conventions.

Hope this helps,

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Dear Jia

Finally, My calculated data approached as the model insisted in the thesis after following all of your advice.

Also, I tried to convert log Ks to describe the multidentate complexes as hiemstra-vanriemsdijk model and got the same results

Thank you for your very helpful suggestion.




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