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I am trying to show the reaction release of CaO2 to O2 for remediation of benzene-impacted groundwater in a system. However, I cannot find an equilibrium constant value for the reaction of CaO2. How can I go about having this reaction in GWB? What is the best way to show benzene degradation through use of added O2?

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Hello C. Penna,

I would suggest you add the species CaO2 as a redox species into the thermo database to start. If CaO2 species is generally unstable then you can put in sufficiently large Log K values to ensure that CaO2 is thermodynamically unstable. For more information, please refer to section 9 in the GWB Essentials Guide for adding new entries into the database. I recommend you save the database with a new file name such as ‘thermo+CaO2.tdat’ to preserve the original file. 

Then in the GWB application you are using for your simulation (React, X1t, X2t), use the decouple dialog to decouple CaO2. This allows you to add in CaO2 as a basis species in your basis pane. You can then specify a kinetic redox reaction and an intrinsic rate constant in the reactants pane for your system to regulate the rate at which CaO2 dissociates in your system. 

Additionally, you might also want to consider a custom rate law as well for your reaction. GWB defaults to a built in general rate law form but you can customize your own rate law according to observations or experimental data. For more information, please refer to Section 5 in the GWB Reaction Modeling Guide. 

If you have any further questions, please include your input files. 

Best Regards,

Jia Wang
 

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Hi Jia,

Thanks for the response. I was able to successfully add in CaO2 as a redox within the thermo dataset. And then was able to create a coherent expression in Rxn for CaO2 and H2O, however I am having issues with the React application. I have tried altering the input values and seem to keep coming up with errors, mostly stating that my "porosity of node 0 is too small". I am not sure how to proceed from here. Could you provide feedback on any issues with my parameters?

Thank you,

C

CaO2penna.rea

thermopenna.tdat

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Hello C. Penna,

I took a quick look at your script and it seems like you have set a combined volume for the system to be greater than 100 percent. I suggest you leave medium properties at default settings unless you are specifying a porous medium from your field site or experiment. The program will calculate the porosity and volume minerals base on your entries in the basis pane. 

You might also want to start at 0 for the rate constant to make sure that entries in the basis pane and the rest of the system is set up correctly before starting to add in your redox reaction. Once your initial system is set up in the way you want and is able to run without the reaction, then increase the rate constant. Additionally, I noticed that you have O2(g) fugacity fixed right now in the Reactants pane. In this case, the fixed fugacity is calculated from the initial O2(aq) concentration in your basis pane. Usually a fixed gas reactant is set for modeling systems buffered in nature by contact with a gas reservoir such as the atmosphere or in a controlled laboratory setting. If you fix the fugacity of O2(g), your O2(aq) concentration will  to equilibrate with the fugacity which would not change throughout the simulation. You might want to reconsider this constraint for your system.

Hope this helps.

Best regards,

Jia 
 

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