Redox Reactions in Database Under "Minerals"

[MBOTZ]

I noticed there are some Redox reactions in the LLNL database that are actually in the Minerals portion of the database, rather than in the Redox portion of the database.  For example, there is the Au+++/Au+ Redox reaction, but then there is also the Au/Au+ (Gold/Au+) reaction (a redox reaction) in the Minerals portion of the database.  I see this same thing with some species in the Aqueous Species portion of the database (e.g., Hg (aq)/Hg2++).  Does GWB treat the redox reactions differently if they are in the Redox portion of the database vs. in the Minerals or Aqueous Species portions of the database?  Thank you!

[Jia Wang]

Hello,

The reactions in the “Redox couples” section of GWB datasets allows users the flexibility to decouple redox equilibrium between different valence states of the same element during simulation. For example, in thermo.tdat, the basis species for carbon is HCO3-.  Acetate (CH3COO-) and methane(CH4(aq)) are species of alternative oxidation states of carbon corresponding to HCO3- in the basis section. By default, the GWB assumes that these species of different redox states are in equilibrium, which means that a bulk concentration entered for the HCO3- component in the Basis is going to be distributed between all redox states carbon. You can decouple these redox reactions which then will allow you to specify a bulk concentration for each oxidation state separately. You can try this by decoupling the HCO3-/CH4(aq) in a SpecE8 instance (Config -> Redox couples). Once you have done so, you can now add both HCO3- and CH4(aq) components in your Basis. If the reaction for CH4(aq) was specified as a secondary aqueous species in the “Aqueous Species” section, the redox reaction is permanently coupled. You will not have the option to disable redox equilibrium. You can try this by going to CH4(aq) in thermos.tdat, right-click and select Couple, save the dataset with a new name and load it into SpecE8.

Any reactions in the Minerals, Aqueous Species, and Gases section can be a redox reaction, but to take advantage of the flexibility of redox disequilibrium, the redox couple should be specified in the “Redox couples” section. Please note that in the GWB, only aqueous species can be specified in the “Redox couples” section. You cannot add a mineral into the Basis without swapping it for an aqueous component.

If you would like more information regarding redox coupling, please see section 2.4 and 7.3 of the GWB Essentials User Guide. You can adjust redox coupling reactions using TEdit. For more information on this, please see section 9.2.9 in the same guide. You can find more information regarding the structure of GWB thermo datasets in the GWB Reference Manual.

Hope this helps,
Jia