Re: surfaces in PHREEQC

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

> I have a final question about surfaces in PHREEQC.  I defined an organic
surface for VOCs to sorb to.  The three numbers that define the surface are

(a) the number of moles of sites for sorption, (b) the specific surface
area,
and (c) the mass of the surface.  However, when I explicitly define the
number of moles of sites for sorption that 1L of water would partition to
in
an aquifer with a certain fraction of organic carbon, the other two numbers

don't make a difference (I've varied them extensively and I've seen no
sensitivity to those parameters).  Is another way to do this?

The surface area affects the charge density (charge per area). If you are
using uncharged organic compounds and an uncharged surface, I think you
would not see any effect in change the surface area.

> For example, can I not explicitly define the number of moles of sites for
sorption and do this based on the mass and the specific surface area?

For hydrous ferric oxides the number of surface sites and the surface area
could vary independently. Goethite and ferric hydroxides could have similar
surface areas but different numbers of sites per surface area.

> Additionally, how does PHREEQC treat the K given for the sorption
reaction?

The K is for the mass action expression derived from the sorption reaction.
The mass action equation uses activities, which is the activity coefficient
times the molality for aqueous species, and the equivalent fraction of
sites occupied for sorbed species.

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