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Low temperature database and water activity issue


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Hello,

I have encountered an issue when running the react code I can't explain. For my research I need to run evaporation simulations well below the freezing point of pure liquid water. Therefore, I designed an extension of the Thermo_phrqpitz database down to -80oC (60, 40, 20, 0, -20, -40, -60 and -80oC). I added numerous minerals and aqueous species along with their pitzer parameters, from recent literature publications. However, when I run the code, I encounter a sudden jump in water activity between 239 and 239.1 K, which I can't explain in any way. This jump affects the water activity, the ionic activity and therefore the saturation index, generating precipitations of minerals at much higher water activity than they should (basically from 273 down to 239, the results "make sense" but not below, the water activity is way too high). I attach a figure showing such jump in water activity for a 50/50 mol% Mg(ClO3)2 / Mg(ClO4)2. 

Does anyone has any idea why such jump occurs? 

Thank you

Sincerely

Vincent

 

Water_activity_Mg_ClO3-ClO4.tif

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Hello Vincent, 

It is hard to say for sure without being able to test your input file along with your thermo database. It sounds like the precipitation of mineral(s) is affecting the activity of water you see and not the other way around. You can test this by turning off precipitation in your model under the Config in the menu bar and choose Options... Uncheck precipitation and run your model again. Do you still see the same jump without any minerals precipitation? If this issue persists, please attach (or email us) your input file and your thermo file so we can look into it further. 

Best regards,
Jia Wang

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Hello Vincent,

I am not aware of any thermodynamic database which is able to calculate equilibria down to -80°C. Perhaps it helps to consult

https://apps.dtic.mil/dtic/tr/fulltext/u2/a288456.pdf

In the context of martian geochemistry some publications are available, e. g. https://www.sciencedirect.com/science/article/abs/pii/S0016703707005844

Another hint: look into Craig Bethkes book about geochemical modeling, chapter on activity coefficients. Water activity is calculated from osmotic coefficient, which in turn is subject of Pitzer coefficients for dissolved salts and of Aphi. In pure water the calculation of water activity boils down to the Debye-Hückel equation where the impact of Aphi prevails. I think GWB calculates Aphi temperature dependent (Debye-Hückel coefficients), but I am sure the underlying temperature function has never been tested down to such a low temperature.

In salt solutions situation is even worse. I encountered a similar situation, though for elevated temperatures, where a temperature function for a single Pitzer coefficient beyond 170°C went mad all of a sudden. In your specific situation you had to inspect the temperature function for every single Pitzer coefficient in your database - a formidable task!

Best regards,
Helge

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Helge,

To answer your comments:

- I am aware of the martian geochemistry papers (that one and a whole collection of similar papers). This is the reason why I am trying to develop this database. Marion (the paper you cite) uses a code called FREZCHEM which works under 0oC and can model freezing and evaporation. Interestingly I tried to include ice precipitation in the database but this generates an error ("phase rule violation") when running the code but it still shows the saturation index for water ice, which is all you need to see when ice precipitates. 

- No database is designed to run at such low temperatures. My aim was not to run simulations that low, more around -40oC but I wanted a smooth set of Pitzer parameters over this range of temperature. Some of the parameters I use have been tested this low (some down to -50 -60oC). 

- I know some parameters can behave weirdly beyond their temperature range. This is due to the fact they are fitted with polynomial equations which can be difficult to extrapolate. But in the absence of measurements at such low temperatures, I either used constant values or values measured at these temperatures. 

My issue is not a "weird" Pitzer parameter (at least from what I can see) because the transition is way too sharp (figure I initially attached), unless I am missing something somewhere. 

Sincerely

Vincent

 

 

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It isn't aphi. I checked and the function is smooth around 239K. It begins to drop steeply at ~ -63C and below -83C it doesn't work anymore.

Density function for water looks "normal" at 239K, no rapid changes.

May I have look at the database? At the moment I have no idea 😞

Helge

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Helge

I think I checked aphi in the past and I had not seen any significant change or that abrupt at 239 K (I am still trying to figure out why this temperature since none of my values is defined at this temperature, I go by 20 K increments from 193 to 333 K).

How can I send you the database privately? 

Thank you again for your help

Vincent

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