> 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 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
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