Re: Solid solutions and PHREEQC

["David L Parkhurst" <dlpark@xxxxxxxx>]

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