SUPCRT and GWB compared for arsenical species

[Juraj Majzlan]

Hello GWB team,

I have some new thermodynamic data for hydrothermal arsenides measured in our laboratory and I would like to insert them into the GWB database. However, I cannot reproduce the values in this database when using SUPCRT. For example, for the reaction

H2AsO3- + 0.5O2,aq = H2AsO4-

I get log K(t = 300 C) = 13.65 with SUPCRTTBL (Zimmer et al. 2016) but GWB has log K(t = 300 C) = 11.92. That is a substantial difference. In addition, the SUPCRTTBL database does not contain AsH3(aq). I cannot reproduce the values for loellingite (FeAs2) listed in the GWB database.

It would be extremely important, as already mentioned in some other posts, to publish the SUPCRT database that is used to generate the GWB values. Otherwise, adding new data with different parameters for the aqueous species and gases will only cause confusion.

Thank you

Juro Majzlan

[Jia Wang]

Hello Juro,

The default GWB dataset (thermo.tdat) is based on the Lawrence Livermore National Laboratory database. Thermo.tdat is converted from the file data0.3245r46 from EQ3/6. You can find information regarding the origin of the dataset at the top of the text file and sources cited at the end of the text file. Or, if you open the dataset in TEdit, you can find both sections in the Header pane. You can contact the original authors regarding compilation method and discrepancies. 

Also, I wasn't able to verify the Log k value for the reaction stated above in thermo.tdat. Perhaps, you are looking at a different dataset? The expanded variant of LLNL dataset, thermo.V8.R6+.tdat, does contain thermo data for H2AsO3- with Log Ks spanning from 0 to 300C. The Log K at 300C is 13.8282 and not 11.92. Perhaps your reaction data was altered at some point? You can download the version installed with the software from the thermo webpage. 

SUPCRTBL and SUPCRT uses mostly the same data sources for their compilation but they are not entirely the same. You can view a summary of their differences here. 
If you would like to use a dataset that's consistent with SUPCRTBL, you can convert one of the ready-to-go PhreeqC datasets to GWB format using the TEdit application. There are several default datasets published that you can choose from. Geothermal.dat is compiled from 0 to 300C using the debye huckel b-dot activity model. To convert geothermal.dat to GWB format, you would need to edit the llnl_aqueous_model_parameters block so that there are only 8 principal temperatures spanning the desired range. For more information on converting datasets, please see section 9.3 Importing PhreeqC datasets in the GWB Essential User Guide. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Brian Farrell]

For anyone interested in importing SUPCRTBL's PhreeqC-format datasets to GWB-format, several updates in GWB 2022 make this easier. Thermodynamic datasets can now contain an arbitrary number of principal temperatures, so there is no longer any need to alter the LLNL_AQUEOUS_MODEL_PARAMETERS block to limit it to eight temperatures. Additionally, gases in the PHASES block are now recognized as gases (as opposed to minerals) if written with ",g" as well as "(g)" in the name. Finally, the GWB apps can now calculate water activity and "CO2" activity coefficients according to the methods used in PhreeqC's "LLNL" ("B-dot") activity model, in addition to the equations originally prescribed by Helgeson (1969). The PhreeqC methods are now the default for PhreeqC "LLNL" datasets converted to GWB-format using TEdit. For more information, please see section 3.4.2, Basis species, in the Thermo datasets chapter of the GWB Reference Manual, as well as 9.3, Importing PhreeqC datasets, in the Using TEdit chapter of the GWB Essentials Guide.

Regards,

Brian Farrell
Aqueous Solutions