I'm not sure what you are trying to do with your modeling, but here are a few comments. It seems like you may not need solid solutions at all. If you know the ion exchange parameters of the mixture of clays, you could treat the system as having one set of exchange sites. You would also need the appropriate half-reaction exchange constants for the various cations in your system. Initial conditions can be set with the EXCHANGE data block; it is usually easiest to set the initial exchanger composition by specifying it to be in equilibrium with a given solution. Alternatively, you can model the system as a set of several separate exchangers, say X, Y, and Z. In this case you would need to know the number of exchange sites of each type and the half-reaction equilibrium constants of each cation with each type of exchange site. Finally, it might be possible to treat the montmorillonite as a solid solution, if never really thought this through. I've always worked with simple stoichiometries like strontium for calcium. You might have to be careful with the equations that define each solid solution component to get the stoichiometry right. You would also need log K's for all of the endmember montmorillonites. Assuming it is more than a binary solid solution, you must assume ideality, which may not be a good choice. If I'm on the wrong track, send me some more email and maybe we can work it out. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 "Karen Parker" <KEP656@xxxxxxxx To: dlpark@xxxxxxxx ham.ac.uk> cc: In-Reply-To: <1F2325958E6@xxxxxxxxxxxxxxxxx> Subject: Solid solutions and PHREEQC 05/02/01 05:12 AM Hi I wonder if you could please help. I am trying to model a column expt using PHREEQC The column is a mixture of sand and clay ( mixtures of illite, kaolinite, chlorite or montmorillonite) with known ionic exchange parameters. I am adding to this a binary chloride salt mixture and want to model the breakthrough curves using exchange and transport keywords. I have tried using the solid_solutions keyword to set the initial conditions in the column but am unable to do so. My only other thought I guess is to set solution 1-x using the formula for the clays. Could you tell me if I am on the right track, sorry but I am new to this type of modelling and I am trying to learn as I work through so any help would be greatly received Thankyou for your time Karen KAREN. E. PARKER Hydrogeology Research Group School of Earth Sciences Aston Webb Building The University of Birmingham Edgbaston Birmingham B15 2TT United Kingdom Tel: +44 (0)121 4144407 Fax: +44 (0)121 4144942 Email: K.E.Parker@xxxxxxxxxx
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