SURFACE parameters

[Charlie]

Q. 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?

A. 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.

example:

SURFACE 1
-equilibrate with solution 1
Surfa_w  1.0   1000    0.33    #surface binding site, sites (or site density) , specific area per g, g of mineral

The manual describes these parameters:

surface binding site--Name of a surface binding site.
sites--Total number of sites for this binding site, in moles; applies when -sites_units is absolute.
(site density--Site density for this binding site, in sites per square nanometer; applies when -sites_units is density)
specific area per g --Specific area of surface, in m2/g. (Default is 600 m 2 /g)
grams --Mass of solid for calculation of surface area in g.

surface area is grams times specific_area_per_gram (Default is 0 g).

In the example above, surface area is 1000 m2/g * 0.33 g = 330 m2

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Edited from PHREEQC thread (http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00084.html)