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Gypsum saturation


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I calculated the gypsum log K values at 25 °C within the Rxn function and obtained Log K = (-4.4433), corresponding to 6 mM / 1.033 g/l of gypsum to reach saturation. However, most reviewed literature suggests a 2 - 2.5 g/l range. Then when I used React, I only reached saturation after titrating 3.55 g/l of gypsum in H2O at 25°C. I'm unsure if there's an error in my calculation or assumptions. Why am I observing gypsum precipitation only after adding 3.55 g/l?

Additionally, I've noticed "smectite reykjanes" precipitation in some of my models, but I couldn't find direct information about this type of smectite in the literature. The TEdit bibliography has no information on this. Do you have a reference for this mineral?


I appreciate your help. 


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Hello Beth,

I think there are a couple of factors here that can explain your observation. The first being that the speciation calculation in React (and other GWB apps) accounts for complexes. Assuming that you have a very simple system, containing only water, calcium, and sulfate, the program will calculate the distribution of mass in solution. Complexes that form will reduce the amount of free ions in solution. When I tried with the three component system above and titrated in gypsum, Ca++ and SO4-- complexes strongly and formed a relatively large quantity of CaSO4 in solution. This falls in line with your observation that more gypsum need to be added to the system before it becomes in equilibrium, since there are not as much free ions in solution as you think. If you suppress CaSO4 in the run, you will see that gypsum begins to accumulate in your system earlier, after about 2.25 g have been titrated into 1 liter of solution. You can find the Suppress... dialog under the "Config" menu in React.

Perhaps the more important consideration here is that the mass action law accounts for the activity of species, not concentration. Assuming that you are using the default thermo dataset, thermo.tdat, React will use the bdot method, a variation of the Debye-huckel equation, to calculate the activity coefficient of various species. You can plot variable type "Species activity" to see values for Ca++ and SO4--. If you multiple them at the point at which gypsum starts to accumulate (i.e. gypsum becomes in equilibrium with your fluid), you will find that it is equal to the equilibrium constant. If you had assumed that the concentration is equal to activity and the solution deviates away from ideality, you will see very different results than expected. If you would like more details regarding activity models compatible with the GWB, please see section 7.4 in the GWB Essentials Guide. You might also find the Equilibrium models lesson on the GWB Academy helpful.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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Hello again,

With regards to the thermodynamic dataset, I do not have any additional information regarding the smectite-reykjanes clay but perhaps someone else can help. If you are working with thermo.tdat, you can find the data sources, as cited by LLNL, at the bottom of the file. If you are opening the dataset using TEdit, GWB's thermodynamic dataset editor, you can view the sources listed in the "Header" pane under the Bibliography section.

Hope this helps,

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