> 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
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