Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 117

TABLE 6.2.3
Levels of Contaminants in Soil and
Wastewater and Results of Leaching Tests

Contaminant Cd Cr Pb Hg

In soil (ppm) 1044 1310 2457 1002

TCLP result (mg/L) 0.18 0.13 < 0.2 0.0015

Leaching index ND * ND 16.4 16.6

* ND: Not detected, below detection limit.

For Ceramicrete, the waste is pretreated with sodium or potassium sulfide to stabilize
Hg. 5

6.2.4.2 Stabilization of Fission Products

The stabilization of some fission products such as 99 Tc and 137 Cs has been a challenge,
since these contaminants are found mostly in salt waste streams and are readily
soluble in groundwater. Several laboratory studies demonstrated that Ceramicrete
technology can be used to effectively stabilize such fission products.
Technetium is highly mobile in its soluble Tc +7 oxidation state. Also, because it
is volatile at moderately elevated temperatures, waste streams containing technetium
must be stabilized at ambient temperatures. Ceramicrete stabilization of 99 Tc parti-
tioned from high-level tank wastes was demonstrated by Singh et al. 10 The waste
stream was a product of a complexation-elution process that separates 99 Tc from
High-level waste (HLW) such as supernate from salt waste tanks at Hanford and
Savannah River. Adding SnCl 2 to the binder mixture and then slurrying with water
reduced technetium from its +7 to +5 valence state. The concentration of 99 Tc in
the waste form varied from 40 to 900 ppm. The slurry was mixed for 20 min, set
into a hard ceramic waste form, and cured three weeks before being leach tested.
The ANSI/ANS-16.1-2003 leachability index on technetium has been consistently
between 13.3 and 14.6, higher than the index for cesium and strontium from glass. 11
The waste forms had a compressive strength of about 30 MPa and proved durable
in an aqueous environment. These results demonstrated the effectiveness of a reduc-
ing environment in the Ceramicrete matrix for contaminants that tend to oxidize and
leach out.
In another study, debris waste, produced from scraping the internal surface of
pipes from the K-25 Gaseous Diffusion plant at Oak Ridge destined for demolition,
was stabilized. The main contaminants were technetium and uranium with techne-
tium concentrations of 33,886 and 1,750 pCi/g (2020 and 104 ppm) in the two
samples tested. Again SnCl 2 was used as the reductant to suitably stabilize techne-
tium. The technetium leachability indices from the ANSI/ANS-16.1-2003 test were
found to be 12 and 17.7, consistent with values obtained for the other waste streams
described above. These examples illustrate the effectiveness of Ceramicrete stabili-
zation of technetium by adding the reductant, SnCl 2 .