W0101
Development of Tunnel and Cavity Type Inorganic Ion Exchangers for Nuclear
Waste Remediation. Abraham Clearfield, Damodara Poojary, Franz Gingl,
Elizabeth Behrens, Anatoly Bortun, Lyudmila Bortun., Chemistry, Texas A&M
University, College Station, TX 77842-3012
There exists large quantities of liquid hazardous nuclear wastes that require
treatment and disposal. This waste contains varying amounts of HNO3
for acid waste solutions or high levels of NaNO3 and NaOH for
alkaline wastes. The highly radioactive species constitute a small fraction of
the total and need to be removed and immobilized to minimize the amount of
highly radioactive material to be stored in underground vaults. The remaining
low level waste can then be disposed of as grout and stored above ground. In
order to remove the highly radioactive species, particularly
137Cs+ and 90Sr, highly selective radiation
resistant ion exchangers are required. We have found that certain silicates,
titanates and phosphates, whose structures contain small cavities and tunnels
have the ability to remove the required species in the presence of high levels
of background electrolytes.
We have synthesized a potassium titanium silicate of composition
K3H(TiO)4(SiO4)3*xH2O1
that has a structure similar to the mineral pharmacosiderite with a cubic unit
cell, a=7.7644(3) Å,
.
The potassium phase was converted to the Cs+ and
H3O+ forms without change of symmetry. Each structure
contains a three dimensional tunnel system filled by the exchangeable ions and
water molecules. Substitutions of germanium for silicon and/or titanium
results in formation of tetragonal distortions with a=11.215(2), c=7.9705(2)
Å for
K3H(TiO4)(GeO4)3*4H2O
and similar changes for
K3H(TiO)3.5(GeO)0.5(GeO4)2.5*4H2O.
However, these substituted forms show a remarkable increase in thin
selectivity for Cs+ and Sr2+. The mechanism of the
exchange reactions and reasons for the changes in selectivity will be
presented.
Similar substitutions have been carried out with another tunnel structure
titanium silicate and a small cavity compound
Na2Zr(SiO3)3. The use of ab initio structure
solutions from powder data and of Rietveld methods in this type of research
will be presented.