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Negative H+ and O2 Results


shensel
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Hello,

I have reacted some water with a mineral assemblage and get negative amounts of H+ and O2 in the "Original Basis" section post reaction. What does this mean? If I then pickup the entire system, it seems that portlandite and magnetite have replaced my H+ and O2 in the Basis tab. Why does this happen? I've attached the .rea and reaction output .txt files here.

Thanks,

Scott Hensel

01_Minteq_React_output.txt 01_Minteq.rea

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Hello Scott,

The values reported in the Original Basis reflect the bulk concentration of each component in the system, which can consist of negative masses. The concentration of any species themselves may be very small but not negative. As an example, imagine a system consisting of only the components of H2O and H+. With this system you can form species H2O, H+, and OH-, where OH- = H2O - H+. In a highly alkaline system, which your attached script is, the amount of OH- is going to be greater than H+, and the overall solution is described by a positive amount of water component and negative amount of H+ component. For more discussion on this, please refer to Chapter 3 of the Geochemical and Biogeochemical Reaction Modeling text.

When you use a pickup command for the entire system, this will set the fluid composition and any minerals in equilibrium with the fluid as the new Basis. In this case, Portlandite and Magnetite are swapped into the basis for H+ and O2(aq) components respectively. React performs the swapping automatically at each time step to find the equilibrium state, where no minerals are supersaturated.

As a simple example, you can run the following script in a new React:

SiO2(aq) = 10 mg/kg
go

In the Results pane, you will see that React had swapped in Quartz for SiO2(aq) since the fluid was supersaturated with respect to Quartz and a small amount of mineral is precipitated. If you now pickup the results as your basis, you will get Quartz swapped into the system for SiO2(aq) and the amount of mineral precipitated. If you would like a more detailed example of this, please refer to Chapter 6 of the Geochemical and Biogeochemical Reaction Modeling text.

Btw, I have moved your post to the front page, where we get alerts when a new post is added.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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Hi Jia, 

Thanks for the reply, that explanation helped quite a bit. So if I'm thinking about this right, H+ is swapped specifically for portlandite because of how H+ relates to OH-, and because OH is in the formula for portlandite? 

Also thanks for moving my question to the front page, I'll post any future questions there. 

 

Thanks,

Scott

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Hello Scott,

You're welcome. Portlandite is swapped into the system because the mineral is in equilibrium with the fluid at the end of the simulation. If you look at the reaction for portlandite in the thermodynamic dataset, you can see H+ in its reaction. In any of the reaction modeling applications, you can swap in a mineral for a component that it's composed of to constrain the concentration by being in equilibrium with that mineral.

Best,
Jia

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