> I am trying to use the equilibrate command under SURFACE for PHREEQC. I have a water composition that I am assuming is in equilibrium with ferrihydrite. I'm trying to model the composition of the ferrihydrite and compare to the actual composition from samples I digested. When I use the -equilibrate command, the initial surface composition is virtually empty of sorbed species, possibly because there is no ferrihydrite. You are right, there is no ferrihydrite initially in equilibrium_phases 2. It generates a little 1e-30, just to avoid zero divides and other numerical problems. > But the solution is supersaturated wrt ferrihydrite. So I don't understand why the initial surface does not have ferrihydrite. You need to consider your conceptual model. The initial surface calculation is done before any reactions--that is, before precipitation of ferrihydrite--when both an initial surface and a reaction are defined in the same simulation. END statements define simulations. You could take your initial solution and equilibrium_phases 2, let them react, and save equilibrium_phases 2. Then you could define your surface to be in equilibrium with the initial solution and there would be some sites to react with. You could also save the solution as well as the equilibrium_phases, and then do the initial surface calculation with the new solution that is in equilibrium with the phases and the new equilibrium_phases. After this initial surface calculation, you would have equilibrium among the solution, the surface, and the equilibrium_phases. I'm a little suspicious of your fixing the pH and pe. I think you should let the reactions determine that. Equilibrium with calcite and a fixed oxygen partial pressure seems more reasonable than a fixed pH and pe. David > If you could provide any suggestions I would appreciate it very much. The input and database files are attached. 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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