Re: -equilibrate command for PHREEQC

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

> I am trying to use the equilibrate command under SURFACE for PHREEQC.  I
have a water composition that I am assuming is in equilibrium with
ferrihydrite.  I'm trying to model the composition of the ferrihydrite and
compare to the actual composition from samples I digested.  When I use
the -equilibrate command, the initial surface composition is virtually
empty
of sorbed species, possibly because there is no ferrihydrite.

You are right, there is no ferrihydrite initially in equilibrium_phases 2.
It generates a little 1e-30, just to avoid zero divides and other numerical
problems.

> But the solution is supersaturated wrt ferrihydrite.  So I don't
understand why the
initial surface does not have ferrihydrite.

You need to consider your conceptual model. The initial surface calculation
is done before any reactions--that is, before precipitation of
ferrihydrite--when both an initial surface and a reaction are defined in
the same simulation. END statements define simulations.

You could take your initial solution and equilibrium_phases 2, let them
react, and save equilibrium_phases 2. Then you could define your surface to
be in equilibrium with the initial solution and there would be some sites
to react with. You could also save the solution as well as the
equilibrium_phases, and then do the initial surface calculation with the
new solution that is in equilibrium with the phases and the new
equilibrium_phases. After this initial surface calculation, you would have
equilibrium among the solution, the surface, and the equilibrium_phases.

I'm a little suspicious of your fixing the pH and pe. I think you should
let the reactions determine that. Equilibrium with calcite and a fixed
oxygen partial pressure seems more reasonable than a fixed pH and pe.

David

> If you could provide any
suggestions I would appreciate it very much.  The input and database files
are attached.



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