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Re: discrepancies between sorbed and solvated Se in PHREEQC

> The output from a sorption routine in PHREEQC shows that I have a certain
number of moles of selenium species sorbed to Hfo_w.  However the solution
composition shows the same number of moles present after sorption as was
initially specified in the input solution.  Why is there no apparent
decrease in disolved Se?

I'd have to see the files, but if the surface is in equilibrium with a
solution and that solution was replaced with an identical solution then
there would be no change. This could easily happen if you specify the
surface composition to be that which is in equilibrium with solution 1
(SURFACE; -eq solution 1) and then react the surface with solution 1.

> Does PHREEQC consider sorbed species to be in solution?

No, they are considered part of the solid phases.

> Also, I'm confused about the specification of sites and surface area in
the surface data block.  After reading the FAQ's on the website I have the
impression that the number of surface sites is either a number of moles of
sites to which 1 kg of solution will be added (FAQ 60) or the number of
moles of sites present on 1 mole of Fe (FAQ 61).  Which is correct?

Depends on if the number of sites is dependent on the moles of an
equilibrium phase (or kinetic reactant). Please read line 2 and line 3 on
pp 163 and 164 of manual.

> If I specify the moles of a site present (and the mass of solid if sites
are related to moles of solid) why is it necessary to specify surface area.

Surface complexation depends on the charge density (charge per area). The
surface area is required for all surface complexation calculations. If you
just define a number of moles of sites, you must define the corresponding
surface area; if you define the number of sites related to an equilibrium
phase, then you need to define the surface area per mole of the phase.


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

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