(120e) Two-Site Equilibrium Model for Ion Exchange between Multivalent Cations and Zeolite-a in a Molten Salt

A two-site equilibrium model which previously only
accommodated monovalent cations has been extended to include divalent and
trivalent cations for ion exchange between zeolite-A and molten chloride salts,
a process being considered for concentrating nuclear fission products into high
level waste forms.  Equilibrium
constants were determined by fitting the model to equilibrium data sets for ion
exchange between zeolite-A and Cs ternary salt (CsCl-LiCl-KCl), Rb ternary salt
(RbCl-LiCl-KCl), Na ternary salt (NaCl-LiCl-KCl), Sr ternary salt (SrCl₂-LiCl-KCl),
and U ternary salt (UCl₃-LiCl-KCl).  The results reveal a good fit between the experimental data
sets and the model.  The two ion
exchange sites, framework sites and occluded sites, demonstrate different
relative selectivities for the cations. 
It was found that Sr²⁺ is the preferred cation in the ion
exchange site, and Cs⁺ is the preferred cation in the occlusion
site.  Meanwhile, Li⁺
has the highest combined selectivity when both ion exchange and occlusion sites
are considered.  Interestingly,
divalent and trivalent species are more preferred in the ion exchange site than
the monovalent species.  Further
detailed results will be discussed and compared with other models previously
reported in the literature.