Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 185

8.2.4.1 Illite Clay

Illite clay, (OH)4Kx(Al4Fe4Mg4Mg6)(Si8-xAl)O20, has proven to be an effective
additive in cementitious waste forms for retarding the release of the soluble radio-
isotope 137 Cs. 29-32 Illite was shown to be an effective selective sorbent for 137 Cs in
the early 1960s. 33-35 Illite has a relatively low equivalent exchange capacity among
clays, but the gap between illite layers is apparently ideal to allow cesium ions to
diffuse between the silicate sheets and essentially irreversibly trap these ions. The
cesium ions must remain mobile and not sorbed on the external ion exchange sites
to become irreversibly trapped in this manner. For example, a concentration of a
few molar of sodium ions is enough to replace the cesium ions sorbed on the external
ion exchange sites and allow this intercalation diffusion to occur. Illite increases the
ANSI/ANS-16.1-2003 cesium leachability index for a slag-cement-flyash grout from
about 8 to about 10. This represents a decrease of two orders of magnitude in the
effective diffusion coefficient for this waste form.

8.2.4.2 Crystalline Silicotitanate

Crystalline silicotitanate (CST) is a synthetic sorbent that has been developed and
marketed recently as a highly specific sorptive agent for 137 Cs. CST increases the
ANSI/ANS-16.1-2003 cesium leachability index for a slag-cement-flyash grout from
about 8 to about 11, a decrease of three orders of magnitude in the effective diffusion
coefficient. In addition, CST increased the 90 Sr leachability index from 10 to 12.
The high cost of CST makes this synthetic sorbent less attractive for S/S than the
natural alternatives, despite its high loading capacity and specificity for 137 Cs.

8.2.4.3 Clinoptilolite and Other Zeolites

The cation exchange capabilities of zeolites may make them an effective barrier to
radionuclide migration. 36 For the same reason, they have been proposed as a treat-
ment agent to sorb/extract cations from waste, 37-41 to immobilize cations by mixing
directly with waste, 42-46 and to immobilize cations by mixing in a waste grout. 28
Natural zeolites form from weathering volcanic glass, such as clinoptilolite, ideally
(Na, K, Ca 0.5 ) 6 Si 30 Al 6 O 72 •24H 2 O. 47 The reported cation selectivity for four natural
zeolites follow: 40

Chabazite: Cs > NH 4 > Pb > Na > Cd > Sr > Cu > Zn
Clinoptilolite: Cs > Pb > NH 4 > Na > Sr > Cd Cu Zn
Mordenite:

Pb > Cs > NH 4 > Na > Cd

Phillipsite: Cs > Pb > NH 4 > Na > Sr > Cd > Zn

These results imply that Cd, Sr, Cu, and Zn may not be effectively immobilized
for the Na- form of these zeolites or for high Na-bearing wastes.
Synthetic mordenite and two other synthetic zeolites effectively sorbed uranium
from solution, but not natural clinoptilolite and another synthetic zeolite. 41 This relative