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Davies and Debye Huckel with same water activity model

Peter S

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Is there a database that can calculate activity coefficients with the Debye-Huckel model and the Davies equation while maintaining the same model for the activity of water?

I am trying to determine how Fe concentrations, buffered by the reaction:

Pyrite + 4Fe2+ + 4H2O = 8H+ + Magnetite + Pyrrhotite

change as a function of activity coefficient model.

So far I have used thermo.dat for the Debye-Huckel model, and PHREEQC for the Davies equation ( I set ion size = 0 for all charged species). I fix pH at a constant value. The differences in calculated activity of Fe2+ are small, but it would be great if there were no difference at all.

Thank you


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I don't believe you can do that. If your fluid is dilute, the water activity might be 1 in either case, though. And if you're setting pH, that leaves a unique Fe++ activity at a given temperature (just make sure the stability of the minerals is the same in each database). You could use Rxn to figure that activity, and if you specify ionic strength you can solve for the Fe++ concentration.

Hope this helps,

Brian Farrell

Aqueous Solutions LLC

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  • 3 years later...

GWB 2022 includes more flexibility in how water activity is calculated within the various Debye-Huckel methods (b-dot, phreeqc/wateq4f/minteq, davies). With the B-dot extended Debye-Huckel model, specifically, you can now use the method of Wolery from newer versions of EQ3/6, the method of Garrels and Christ used in PhreeqC, or simply set water activity to 1. The methods are set in the thermodynamic dataset following the value for H2O's "ion size" parameter. More information is available in section 3.4.2, Basis species, in the Thermo datasets chapter of the GWB Reference Manual. So, to make a closer comparison between the B-dot model used in thermo.tdat and the Davies equation in your modified thermo_phreeqc.tdat, you could set a version of thermo.tdat to use the Garrels and Christ method for water activity, since that is the method which is already used in thermo_phreeqc.tdat. 

These features are only available in GWB 2022 and in datasets with the "mar21" format. As a final note, the "debye-huckel" label can be replaced with "b-dot" in GWB 2022 beginning with the "mar21" format. The old label is from a time when the B-dot Debye-Huckel was the only non-virial model included in the GWB.


Brian Farrell
Aqueous Solutions

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