Jump to content
Geochemist's Workbench Support Forum

labradorite dissolution kinetic model


Andrew L
 Share

Recommended Posts

I would like to use the labradorite dissolution rate provided in Carroll and Knauss (2005, Chemical Geology) in a kinetic model in GWB. The rate is given by

 

r = k * 10(-Ea/(2.303RT)) * [(aH+3n/aAl3+n)KT/(1+KT(aH+3n/aAl3+n))]

 

where the rate constant k is 10-5.69 mol cm-2 s-1, the activation energy Ea is 10.06 kcal mol-1, n is 0.31, and KT is a complex formation constant that varies with temperature. How do I incorporate this rate equation in GWB?

 

Thanks!

Link to comment
Share on other sites

Hi,

 

You'll start out as you would if you were to use the GWB's built-in rate law. On the Reactants pane, click "add" - Kinetic - Mineral... and select labradorite from the pulldown. Then, change the pulldown next to "rate law" from "built in" to one of the options for custom rate laws, like "script file". In that case, you write your rate law script in the BASIC language. You'll need to define your custom variables (like R, n, and KT) within your script, but some of the parameters that are used in the built in rate law, like rate constant, can be specified within the GUI if you'd like. You can access any of the "internal parameters" in Table 5.1 of the GWB Reaction Modeling Guide in your rate law. Examples include the temperature (TK for Kelvin, TC for Celcius), rate constant that you defined (rate_con), current pH (pH), etc. You can also access a variety of "helper functions" that are found in Table 5.2 of that same guide. For example, activity("H+") or activity("Al+++").

 

For the temperature-dependent formation complex constant, KT, I would set the value directly at first, then once you're comfortable with the rate law you could set up a polynomial equation within your script to evaluate KT as a function of temperature.

 

For more details and examples, please take a look at section 5, Custom Rate Laws, in the GWB Reaction Modeling Guide.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

Link to comment
Share on other sites

I tried to write the rate law as a script directly into the input file. I included the values of R, T, n, and KT directly into the script and assigned the values of k and Ea in the GUI. I also multiplied the rate equation by the surface area (which was assigned in the GUI) and (1 - Q/K). When I try to run the model, I get an error message indicating my rate law cannot be evaluated. The model is attached, and I would appreciate help. Thanks!

anorthite dissolution.rea

Link to comment
Share on other sites

Hi,

 

After checking out your input file I went to the Reactants pane, clicked the "+" next to "script", then clicked "Edit..." to open up the script dialog. After deleting the first ( kinetic Anorthite rate_law = { ) and last ( } ) lines I'm able to start running your model. FYI, you'd use commands like that if working from the command pane, but not when using the GUI. Try opening the two attached input files and comparing the command input (go to the Command pane) with the GUI input (go to the Reactants pane the above-mentioned rate law dialog).

 

The model does fail pretty early in the calculation, though. I played around a bit and one of a couple different changes let the model run to completion. Decreasing the timescale quite a bit worked, and so did setting a pH buffer (on the Reactants pane, click add - fixed - pH). Do either of these changes seem reasonable? Are the pre-exponential factor and other terms that you entered all correct?

 

Regards,

Brian

anorthite dissolution_Alt1.rea

anorthite dissolution_Alt2.rea

Link to comment
Share on other sites

Thank you for your help, Brian. Either of the changes that you suggested seem reasonable. I hadn't thought too much about the specifics of the model; I was just trying to get it to run. I believe all of the other terms are correct. Thanks again.

Link to comment
Share on other sites

Brian,

 

What is the purpose of the phrase "RETURN rate?" I can get the model to run with or without this phrase, but it provides different results. Should I be including the phrase "RETURN rate" when using the script dialog option? Thanks in advance for your help!

Link to comment
Share on other sites

Hi Andrew,

 

I'm glad to hear that you're making progress. Regarding the phrase "RETURN rate", the script needs to return some value for the reaction rate which is done with the return statement. Your React simulation will likely still run without the RETURN statement, but the reaction rate will just be zero over the entirety of the reaction path. You can take a look at the Kinetic parameter "_ dissolution rate" in Gtplot to verify this.

 

Regards,

Brian

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
×
  • Create New...