Re: Solubility Study using PhrqeeQC

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

I'm not sure what you need to do, but I'll make a few comments. PHREEQC can
probably mimic your experiment fairly well, put in mixture of solution 1
and solution 2; add minerals A and B; etc.

To get the solutions, you can use MIX to generate the mixture, say .03 of
SOLUTION 1 and .001 of SOLUTION 2. If it is not a mixture, you can define a
mass of water (-water in SOLUTION) that is .03 to give you approximately 30
mL.  You should use your initial pH values for the solution(s).

The key is what you want to do with the clay minerals. (1) If you want them
to react to equilibrium, then you should include them both in one
EQUILIBRIUM_PHASES data block. (2) If you want to dissolve one and
precipitate the other, you might add the dissolving phase incrementally
with REACTION and assume the precipitating phase equilibrates
(EQUILIBRIUM_PHASES). There are other possibilities assuming secondary
phases like gibbsite or kaolinite could form. Alternatively, you could
incrementally precipitate one phase (REACTION) and assume the other
dissolves to equilibrium (though you could generate negative concentrations
if you are not careful). (3) Another alternative is to one or both minerals
in as kinetic reactions (KINETICS and RATES) and adjust the rate constants
to try to match your results.

I would strongly suggest that before you get too deeply into the forward
modeling, you try some inverse modeling (INVERSE_MODELING). That is,
calculate how much mole transfer of different phases would be needed to go
from your initial solution to your final solution. You can include of
number of possible reactant and product phases and determine multiple
models that account for your final solution composition.

David


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225