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Jeonghwan Hwang

Why does pH in my model rapidly decrease?

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Hi, this is Jeonghwan Hwang,

Thank you for reading.

I want to make a reactive transport modeling of fluid in montmorillonite-Ca.

At first time, i need to make the converged initial condition and use react with 'thermo.com.V8.R6+'.

1) I used the initial water composition with field data. HCO3- ion was selected to balacing ion because balancing with Cl- could not converge the model.

2) I put the reactants 'montmor-Ca'  and put the rate 0.1 volume%/yr

After run,  I saw that the pH of fluid was rapidly decreased at first term of my model.

At first, i think my model should increase the pH, because the reaction of montmorillonite-Ca need H+ ion.

Can anyone please answer to my question?

Thank you.

 

Sincerely

Jeonghwan Hwang

React-question.rea

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Hello Jeonghwan,

Thank you for attaching your script. I have looked at your script and noticed a couple of things that might help explain why the pH decreases initially. If you look at the primary dissolution of Montmor-Ca with its default species using Rxn, then you do indeed expect the reaction to consume H+ and consequently for the pH to increase. However, in your React model, the output text file shows that the pre-dominant Al species is AlO2-. When you swap the Al+++ species with AlO2- and recalculate the reaction for Montmor-Ca dissolution in Rxn, you will see H+ is on the right hand side of the equation and thus indicating the production of H+ in your system.

Additionally, other minerals precipitating in your React model might also influence the consumption or the production of H+ in your system. For example, you can see that the initial system is saturated with respect to Talc and precipitates in React’s calculation. (For more information about React’s treatment of the initial system , please refer to section 2.3 in the GWB Reaction Modeling Guide.) If you return to the Rxn application, you can balance the reaction with Talc using the predominant species in fluid and see that the precipitation of Talc will increase H+ in solution. Your model subsequently precipitates other minerals such as Diaspore, Saponite-Ca, Stilbite, etc with the addition of Montmor-Ca. I would suggest you check whether the precipitation of these minerals is consuming or producing H+ using Rxn as mentioned above. This will most definitely influence the pH of your system.

Lastly, it is rare to constrain the initial H+ concentration in mg/kg. Did you get this from a calculation? Perhaps check if the pH of your field measurements matches with what you entered for the H+ constraint?

Hope this helps.

Best regards,

Jia Wang

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Hello, thank you for your help.

I find that AlO2- mainly affect to decrease pH in my model.

So, I made a model which i want to make by suppress the AlO2-.

 

Next time, i wanted to know whether my react model can be an equilibrium condition .

So, I extended my model time from 30 yr to 500 yr.

And I find that pH of my model converged to 8.3 after almost 100 yr.

 

I have 2 questions in my model.

1) Can i suppress AlO2- in my model? Is there any physical or chemical problem of my model to represent real environment? I don't know well of suppress... 

2) Can I say that my model has an equilibrium condition of pH? 

Thank you

Sincerely

 

Jeonghwan Hwang

REACT0928.rea

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Hello Jeonghwan,

Thank you for attaching your script. Suppress is a very useful feature to allow for less thermodynamically favorable minerals or species to form in your system by suppressing the more stable minerals from forming. React consider all minerals available from the database when making calculations unless otherwise specified. You can refer to the GWB Command Guide and the Reaction Modeling guide to learn more about this feature. With regards to your first question, you can certainly suppress AlO2- in your model but does this make sense in the context of the system you’re modeling and your field observations? I am not sure what your system is so it is difficult to determine what would be the most appropriate to suppress. Perhaps your field data will help to give you some suggestions with regards to what is appropriate for suppressing? I also noticed that you have unchecked precipitation in REACT0928.rea. This restricts minerals from precipitating even when they are supersaturated. Is this a constraint you meant to set? 

As for your second question, I think you mean to ask if your model has a pH of 8.3 at equilibrium? In simple reaction paths, where one or more reactants are gradually added to the system is called a “titration path”. The system’s equilibrium state is calculated as it steps forward in reaction progress. So React actually calculates the equilibrium state of your reaction at every time step. You can refer to chapter 3.1 “Titration paths” for more information.

Hope this helps.

Best Regards,

Jia Wang

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