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Add new mineral for calculating saturation index of secondary mineral


dong5600
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Hi, Brian:

 

I want to calculate saturation index for the secondary minerals formed from reduction of lepidocrocite (gamma-FeOOH). I found the logK information for different minerals in the thermo file, but lepidocrocite is not there and the logK values were given to different temperatures (e.g., 0-300 degree C) in the system. Now, I found solubility for lepidocrocite at room temperature in another paper, is there any way I can integrate such information into the system and fill the logK values for other temperatures?

 

Please advice. Thanks a lot!

 

Yiran

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Hi Yiran,

 

You can edit GWB's thermodynamic datasets to include additional minerals or species. You can also modify the equilibrium constants (log Ks) within the datasets. The easiest way to modify a dataset is to use TEdit. For more information, see section 9, Using TEdit, in the GWB Essentials Guide, which you can access from the help menu of any GWB program. It may also be helpful to take a quick look at the Thermo Datasets Appendix to the GWB Reference Manual.

 

How much thermodynamic data do you have for lepidocrocite? If you only have the log K at one temperature, perhaps 25 C, you can enter that value in the 25 C field and leave the other temperatures at their default value of 500. That indicates to the program that no data is available for the other temperatures. If you have other information, like the standard enthalpy change, then you could use the Van't Hoff equation to calculate the equilibrium constant at some of the other principal temperatures, like 0 C and 60 C.

 

A couple tips. Be sure to save your new dataset with a unique file name, like thermo+lepidocrocite.tdat. After modifying the dataset, be sure to load it into SpecE8 (or whatever GWB program you're using) by going to File -> Open -> Thermo Data... and browsing for your new dataset.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions

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Hi Yiran, glad that helped.

 

Best,

Brian

Hi, Brian:

 

I have a follow-up question about calculating the equilibrium constant for the custom minerals. I did some calculations of lepidocrocite based on the Van't Hoff equation, the thermodynamic values I found for this mineral and other components for the dissolution reaction FeOOH+3H+--->Fe+++ +2H2O. The calculated Keq seems very small (logKeq=-0.00046 at 25 degree C), much lower than the value reported for dissolution constant (logKsp=log([Fe+++]/[H+]^3)) reported in literature (logKsp=1.39 reported in Schwertmann, Solubility and dissolution of iron oxides, Plant and Soil, 130, 1-25). For this reaction, my understanding about the Keq and Ksp is that they are the same. I am curious which values I should use. Because for another mineral I was planning to add, there is no solubility information but free energy, so I have to use the energy method to calculate equilibrium constant and am concerned about the accuracy.

 

Please advise and thanks a lot!

 

Yiran

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Hi Yiran,

 

Unfortunately I don't know which value is closer to reality. It's common for some variability to exist in thermodynamic data like this. When you get around to the free energy calculations, try to compare a few different sources to see how much variability there is.

 

FYI, if you're interested in the effect of a particular mineral's stability on your calculation results, you can use the "alter log K" function to temporarily adjust the value that the GWB programs use. Just go to Config -> Alter log K to access the dialog.

 

Best,

Brian

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Hi Yiran,

 

Unfortunately I don't know which value is closer to reality. It's common for some variability to exist in thermodynamic data like this. When you get around to the free energy calculations, try to compare a few different sources to see how much variability there is.

 

FYI, if you're interested in the effect of a particular mineral's stability on your calculation results, you can use the "alter log K" function to temporarily adjust the value that the GWB programs use. Just go to Config -> Alter log K to access the dialog.

 

Best,

Brian

Brian:

 

Thanks a lot for your suggestion. I will search for more thermodynamic information. For the logK you suggested, I have another question for it. I was trying to set the logK for one of the new minerals (carbonate green rust), since I could not find H and S for it, I was not able to use Van't Hoff equation to calculate logK at other temperatures. So, I input the value at 25 degree and left others 500. When I was calculating for the reactions at 37 degrees, the system reported errors and did not give any result. Is that mean the setting temperature have to be the same as the reaction temperature?

 

Thank you again!

 

Yiran

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Hi Yiran,

 

Can you clarify what program you're using and what type of errors you encountered?

 

If you set your temperature equal to one of the principal temperature for which you have data, 25 C for example, then the programs will use those log Ks directly. If you set temperature to some other value, though, the program will interpolate (but not extrapolate) between principal temperatures. If the program can't interpolate to find a value for a particular reaction, then it won't load that reaction. So if you have data at 25 C and 60 C and set your system to 37 C, the program will interpolate between the two known values. Because you don't have a value at 60 C, though, the program has to ignore your mineral. One solution is to use the "extrapolate" option (from the Config - Alter log K dialog) to consider all reactions at all temperatures. This can be a little dangerous, though. A potentially better strategy would be to assume that the log K for your green rust doesn't change much over a small temperature range and simply set the log K at 60 C equal to the value you set for 25 C. This way, the program will load that mineral at any temperature between 25 C and 60 C, using the same value for the log K, but will not consider it outside that range. For more information, please see 6.33, extrapolate, in the GWB Reference Manual.

 

Hope this helps,

Brian

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Hi Yiran,

 

Can you clarify what program you're using and what type of errors you encountered?

 

If you set your temperature equal to one of the principal temperature for which you have data, 25 C for example, then the programs will use those log Ks directly. If you set temperature to some other value, though, the program will interpolate (but not extrapolate) between principal temperatures. If the program can't interpolate to find a value for a particular reaction, then it won't load that reaction. So if you have data at 25 C and 60 C and set your system to 37 C, the program will interpolate between the two known values. Because you don't have a value at 60 C, though, the program has to ignore your mineral. One solution is to use the "extrapolate" option (from the Config - Alter log K dialog) to consider all reactions at all temperatures. This can be a little dangerous, though. A potentially better strategy would be to assume that the log K for your green rust doesn't change much over a small temperature range and simply set the log K at 60 C equal to the value you set for 25 C. This way, the program will load that mineral at any temperature between 25 C and 60 C, using the same value for the log K, but will not consider it outside that range. For more information, please see 6.33, extrapolate, in the GWB Reference Manual.

 

Hope this helps,

Brian

Thanks a lot, Brian. I will check it. The second solution sounds reasonable to me. I used the 25 degree temperature logK for green rust, but it did not show on the secondary minerals after I calculate the saturation index. I think what you mentioned may explain it.

 

Will try again and thanks a lot!

 

Yiran

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