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Ion diffusion in zeolites and other molecular sieves plays an
important role in ion exchange processes, as well as adsorption
separations and catalysis. Molecular simulations have been very
useful in understanding diffusion of neutral molecules in zeolites,
but simulations of ion diffusion are scarce.
Our interest in this topic came through a collaboration with
Argonne National Laboratory. The cold war has left a great deal of
nuclear waste in many forms, and
the storage and disposal of this radioactive waste poses a significant
environmental problem. One of the processes developed at Argonne
involves preprocessing the radioactive waste and
then storing the radionuclide ion-pairs in zeolite A or sodalite.
It is important to understand the microscopic processes inside the
zeolite to get parameters that can be used to predict the long-term
behavior of this waste form and hence the viability of this storage
method. In our work, we used molecular
simulation to study diffusion of the ions inside the
zeolite, especially transition-state theory.
The goal was to aid in the judicious choice of the
best zeolite as an immobilization matrix
for the ions. The diffusivity parameters obtained from
simulations could be used in continuum models to predict the behavior
of the zeolite waste form over its expected lifetime of a few thousand
years.
Amit Gupta
worked on ion diffusion in sodalite as one portion
of his PhD thesis. This work was funded by Argonne National
Laboratory.
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