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Re: kinetic problem

First, do you realize that you can not have equilibrium with silica,
gibbsite, kaolinite, kspar, and kmica simultaneously? The phase rule only
allows 3 of the phases to be in equilibrium simultaneously. With a
substantial amount of silica and gibbsite, I would not be surprised if you
reach a steady state where the chemistry does not change until you have
dissolved the silica or gibbsite. You are generating a lot of reactions
even in the absence of your infilling solution.

Second, the PHREEQC is not good at integrating the rate expressions you
have chosen. It does not handle stiff equations very well under some
conditions. Because the integration is explicit, the rate for k-mica does
not account for the change in composition from the rate for kaolinite (for
example). This can necessitate small internal time steps and it is possible
that the method will not converge. I would suggest using SI in place of SR
in your rates. I think this will make the rates smoother. SR can change
many orders of magnitude with a small change in concentration.

Finally, I would use a short column until you are happy with the chemistry.


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

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