Re: Modeling Waite et al. 1994 in Phreeqc

["David L Parkhurst" <dlpark@xxxxxxxx>]

> Am I correct that as in the mass action equation only the H+ activity has
an
exponent of 2, Phreeqc should provide similar results to Waite et al.?

I think you are right. The mole-fraction activity for surface species would
not affect the mass-action equation in this case.

> I set it up as follows but I run into problems that the total H balance
of the
system is distorted. I would be glad if you have any suggestions.



I don't have a copy of Waite and others close at hand and I have pretty
much forgotten their paper. How is the total H distorted? Thinking about
what phreeqc does, there is a mole balance equation for H and =Fe sites. I
think the sum of the species in the calculated distribution of species must
conserve both H and sites, so I don't know where the error would be.

What are you comparing to? If you are comparing to calculations by Waite, I
wonder if there is an error in their total H condition when they use the
funny mass-action equation. I am not sure if they used that mass-action for
a good reason or just by necessity because they didn't use a more
reasonable activity convention for bidentate sorbed species.

Let me  know a little more about the total H balance being "distorted" and
I'll try to give you a better answer.

David




David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled