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Equilibrium water composition with Ca-montmorilloniteand initial groundwater

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Hi, thank you for reading.

I wanted to make a phase diagram of Ca-montmorillonite, but I know that I have to define the geochemical condition properly.

So, I am going to make a initial water composition of bentonite.

For this reason, I use 'react' and put the initial groundwater composition of Gyeongju deep groundwater at 'Basis' and Montmor-Ca at 'Reactants'.

I used thermo.com.V8R6+ to define Ca-montmorillonite.

The temperature was defined at '130 cel' which was usually used in high-level radiowaste disposal.

Unchecked 'precipitation' at iteration, because I want to make only groundwater condition.

Finally, I defined 300 yr to make a equilibrium condition.


The question is below;

1) why did H+ moles in final results show negative value (-164)? And, can i solve it?

2) the results showed the same pattern with different time scale, 10 yr, 300 yr, 1 yr. why did this results present?

3) are there any method to approach the results more precisely? 


Thank you.



Jeonghwan Hwang

CaB-Raw 0808.rea


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Hi Jeonghwan Hwang

Aqueous species, such as the free H+ or OH- ions, must have positive concentrations. Thermodynamic components (the original basis that you see in the text output file), however, can have negative concentrations. For a discussion, please see section 3.2.2 Components with negative masses in Craig Bethke’s Geochemical and Biogeochemical Reaction Modeling text. FYI, the pH is listed at the top of the block of output for each step in the calculation.

Keep in mind that the theory of chemical equilibrium does not include any information about time. You’ve set up an equilibrium model, so the time span you’ve set is meaningless. If you set time-dependent processes, such as a rate of simple reactant addition, an internal heat source, a dual porosity model with diffusion into a stagnant zone, or a kinetic rate law, however, the time span will actually have meaning and be used in the calculation. I’m not sure how important it is to your calculation, but if you want to consider how a mineral like montmorillonite dissolves with time, you need to set a kinetic rate law. You might be able to find kinetic rate laws and parameters in the literature, but you should be careful to ensure that they are appropriate for the system you’re modeling. For more information, please see Chapter 16 Kinetics of dissolution and precipitation in the GBRM text, as well as Chapter 4 Kinetic Reaction Paths in the GWB Reaction Modeling Guide.

Hope this helps,

Brian Farrell
Aqueous Solutions

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