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About the Kaolinite sorption setting


zixuan
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Hey, I'm a new one to learn use GWB to reproduce results of Michael Essington(2017), Adsorption of Antimonate by Kaolinite, especially the part of antimony and phosphate. 

There are several questions.

1) As for SiO2(aq), what should I deal with this entry? I have swapped Al3+ to kaolinite in 10g/L. But I don't know how to set a concentration for SiO2. I tried to set a ratio of 1 for SiO2:Al3+, is it right?

2) And when I run the program, such problems would occur. 

Solving for initial system.
 Swapping >AlOH2+ in for >AlOH
 Swapping >SiO- in for >SiOH
Newton-Raphson didn't converge after 999 iterations, max residual =           1, Xi = 0.0000
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
---------------------------------------------------------
 >SiO-                   215.3            1    3.539e-09

--------------------------------------------------------

3)So, I tried to delete the >AlO- or >SiO- from my sorbing surface dataset and ran it again, which could get a Gtplot, but it seemed to take K+ and NO3- as main sorbate. And the figure was totally different from one of ME.

 

I have attached my dataset and reaction data. I will appreciate if anyone could take a look. Thanks a lot.

P+0.01KNO3.rea Kaolinite-P.sdat

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Hello Zixuan,

I took a quick look at your input script and surface dataset and here are a couple of suggestions to help you get started. 

Check to make sure the log Ks in the dataset are in the correct format for reactions written in GWB. You might need to negate the log Ks from the paper if the reaction is written in the reversed direction. For more information on triple-layer model datasets, see section 2.5.2 in the GWB Essentials guide. For surface dataset formatting, see section 4 in the GWB reference guide. 

I think you would need to provide a SiO2(aq) concentration for your reaction. Does the author provide any information regarding the setup of the model? If you want to reproduce the same results, you should try to follow as closely as possible the author's set up. I would also suggest that you check that the log Ks for aqueous reactions in the thermo dataset match the ones provided in the paper as well. 

Hope this helps,
Jia Wang

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  • 2 weeks later...
On 4/16/2021 at 4:40 AM, Jia Wang said:

Hello Zixuan,

I took a quick look at your input script and surface dataset and here are a couple of suggestions to help you get started. 

Check to make sure the log Ks in the dataset are in the correct format for reactions written in GWB. You might need to negate the log Ks from the paper if the reaction is written in the reversed direction. For more information on triple-layer model datasets, see section 2.5.2 in the GWB Essentials guide. For surface dataset formatting, see section 4 in the GWB reference guide. 

I think you would need to provide a SiO2(aq) concentration for your reaction. Does the author provide any information regarding the setup of the model? If you want to reproduce the same results, you should try to follow as closely as possible the author's set up. I would also suggest that you check that the log Ks for aqueous reactions in the thermo dataset match the ones provided in the paper as well. 

Hope this helps,
Jia Wang

Dear Jia Wang,

Thanks for your reply. And sorry for my late reply.

I have successfully simulated the adsorption of antimony on Kaolinite after revising logKs.

Thanks for your suggestion.

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  • 1 month later...

Excuse me, I cannot get an ideal simulation result as for PO43- sorption.

In the paper, it took a TLM model to simulate this process with inner-sphere species(>AlOPO32-) and outer-sphere species (>AlOH2+:HPO42-).

But the result was still not optimum. I have attached my sdat, this was established on the base of thermo.dat. 

Meanwhile, the result, solution settings and paper also have been attached.

doi:10.2136/sssaj2016.12.0402

The reference result of paper is Fig.7.(b)

I would be appreciate if someone could help me. Thanks!

2.png

Kaolinite-p-original.sdat p.rea

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Hello Zixuan,

I think you're pretty close with your script. The software calculates the speciation of your fluid based on the component concentrations prescribed in the basis pane. I noticed that you had set the activity ratio of SiO2(aq)/Al3+ to 1. This is not a particularly robust way of performing these simulations as each component may speciate to different extents. Holding these ratios constant might not be particularly realistic and probably contributed to your convergence issue initially. Since the paper did not provide initial SiO2(aq) and Al3+ concentrations in the fluid, you can try setting a negligible concentration for both and set up Kaolinite as a kinetic mineral with a zero rate constant. This way, you are effectively setting up kaolinite as an inert surface, as it will not dissolve or precipitate with a reaction rate of 0 but available for surface complexation.

Hope this helps,
Jia Wang

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On 6/3/2021 at 6:02 AM, Jia Wang said:

I think you're pretty close with your script. The software calculates the speciation of your fluid based on the component concentrations prescribed in the basis pane. I noticed that you had set the activity ratio of SiO2(aq)/Al3+ to 1. This is not a particularly robust way of performing these simulations as each component may speciate to different extents. Holding these ratios constant might not be particularly realistic and probably contributed to your convergence issue initially. Since the paper did not provide initial SiO2(aq) and Al3+ concentrations in the fluid, you can try setting a negligible concentration for both and set up Kaolinite as a kinetic mineral with a zero rate constant. This way, you are effectively setting up kaolinite as an inert surface, as it will not dissolve or precipitate with a reaction rate of 0 but available for surface complexation.

Thanks for your help!

I edited the data and simulated again, though the convergence in my last graph disappeared and the tendency was very close to that on paper, the total sorbed fraction was still incorrect.

In 0.01KNO3 it should begin with 90% and in 0.1KNO3 it should begin with about 70%. I had no idea about these differences.

 

p.rea

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Hello Zixuan,

Thanks for providing your updated script. By setting up kaolinite as a kinetic mineral with a zero dissolution rate, you have prescribed a surface available for surface complexation for your experiment without precipitation or dissolution of kaolinite to maintain equilibrium with the fluid. Also, the initial method of setting a Al+++/SiO2(aq) activity ratio to a constant value was causing a large amount of dissolved Al and P species to form aqueous complexes that were not considered as a part of the complexation reaction. 

Here are a few suggestions for you after looking at the paper. If you are using the default thermo.tdat for your run, you would want to double check that the thermodynamic data is consistent. For example, the default dataset doesn't include the reaction for KPO3-- or KH2PO4-. You should also check that the log K data is consistent with those provided in the paper. You will also notice that species not included in the reactions listed in the paper may form because the GWB considers all the reactions possible in the dataset unless it is suppressed. You may find it helpful to either suppress all the species other than the one considered in the paper or can create a new thermo dataset by extracting a subset of the relevant reactions from thermo.tdat.  

The paper did not provide the activity model used in its modeling with FITEQL. thermo.tdat uses the B-dot activity model, an extended version of the Debye-Huckel equation, to calculate the activity of species. You can find this information in the Headers pane when you open the dataset in TEdit or the activity model line if you open the file in a text editor. If a different activity model was used in the paper, then it may explain some of the differences observed. 

Other details include whether or not charge balancing was utilized in the original calculation and if so, was the simulation charge balanced with an anion or cation. The GWB uses a molal or mole fraction (for polydentate complexes) standard state in surface complexation reactions. Some programs may use a molar standard state instead, which may result in a small difference when comparing simulation results across software. 

I would recommend communicating with the corresponding author(s) to see if you are able to obtain more details regarding their model if you would like to improve your results. 

Hope this helps,
Jia Wang

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10 hours ago, Jia Wang said:

Hello Zixuan,

Thanks for providing your updated script. By setting up kaolinite as a kinetic mineral with a zero dissolution rate, you have prescribed a surface available for surface complexation for your experiment without precipitation or dissolution of kaolinite to maintain equilibrium with the fluid. Also, the initial method of setting a Al+++/SiO2(aq) activity ratio to a constant value was causing a large amount of dissolved Al and P species to form aqueous complexes that were not considered as a part of the complexation reaction. 

Here are a few suggestions for you after looking at the paper. If you are using the default thermo.tdat for your run, you would want to double check that the thermodynamic data is consistent. For example, the default dataset doesn't include the reaction for KPO3-- or KH2PO4-. You should also check that the log K data is consistent with those provided in the paper. You will also notice that species not included in the reactions listed in the paper may form because the GWB considers all the reactions possible in the dataset unless it is suppressed. You may find it helpful to either suppress all the species other than the one considered in the paper or can create a new thermo dataset by extracting a subset of the relevant reactions from thermo.tdat.  

The paper did not provide the activity model used in its modeling with FITEQL. thermo.tdat uses the B-dot activity model, an extended version of the Debye-Huckel equation, to calculate the activity of species. You can find this information in the Headers pane when you open the dataset in TEdit or the activity model line if you open the file in a text editor. If a different activity model was used in the paper, then it may explain some of the differences observed. 

Other details include whether or not charge balancing was utilized in the original calculation and if so, was the simulation charge balanced with an anion or cation. The GWB uses a molal or mole fraction (for polydentate complexes) standard state in surface complexation reactions. Some programs may use a molar standard state instead, which may result in a small difference when comparing simulation results across software. 

I would recommend communicating with the corresponding author(s) to see if you are able to obtain more details regarding their model if you would like to improve your results. 

Hope this helps,
Jia Wang

Dear Jia Wang,

Thanks for your patient explanation and checking.

But I don't think it should be ascribed to the lack of constants of KH2PO4 and KPO42-. Actually, I had added these 2 constants before I posted to you, but there was no change in the result.

Perhaps, like what you said, it's better to consult with the corresponding author concerning the activity model, charge balance and molal units.

It's really helpful, thank you!

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