Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 126

Capitalizing on the ability of sulfur to react with toxic metals and form stable
sulfides led to the development of a related technology known as Sulfur Polymer
Stabilization/Solidification. This technology first reacts the waste to chemically
reduce the metals to metal sulfides and then physically encapsulates the stabilized
waste in a microencapsulated final waste form. SPSS has been effectively applied
for the treatment of elemental mercury and mercury-contaminated soil, sludge, and
debris and has been licensed for the treatment of mercury generated as a result of
mining operations. 23,33 While the U.S. Environmental Protection Agency (EPA) con-
sidered an Advanced Notice of Proposed Rulemaking that would allow SPSS treat-
ment for disposal of elemental mercury, it has yet to approve any technology for
this purpose. 34
Sulfur polymer-microencapsulated final waste forms exceeded minimum recom-
mended standards 37 for the Toxicity Characterization Leaching Procedure (TCLP)
of the U.S. EPA and the short-term tests 35,36 recommended by the U.S. Nuclear
Regulatory Commission (NRC) for low-level radioactive waste forms to demonstrate
compliance with 10 CFR 61. Testing included compressive strength, water immer-
sion, leaching, freeze/thaw durability, radiation stability, and biodegradation. For
radioactive contaminants, leaching was conducted using the American Nuclear Soci-
ety protocol, ANSI/ANS-16.1-2003, and the American Society of Testing Materials
(ASTM) accelerated leaching test, ASTM D-1308. Sulfur polymer has a relatively
low permeability and, consequently, as seen in Table 6.3.3, leachability indices of
10 to 14, compared to the NRC minimum leachability index criterion of 6. 38 SPC-
microencapsulated mixed waste incinerator ash, containing as much as 7 wt% lead
at waste loadings up to 43 wt% ash, passed TCLP. 19 For SPSS-encapsulated mercury
and mercury-contaminated soil, sludge, and debris, mercury concentrations below
the Universal Treatment Standard (UTS) level of 0.025 mg/L have been routinely
achieved. 39,40 As seen in Figure 6.3.3, treating a sludge with a mercury concentration
of 5000 mg/L resulted in leachates below this UTS limit for leachant pH ranging
from 2 to 10.

TABLE 6.3.3
Leach Indices for Sulfur Polymer-
Microencapsulated Waste Forms 38

Leachability Index
60 Co

Waste Type Waste Loading

137 Cs

Sodium sulfate
Evaporator concentrate

25 12.5 10.6

40 10.7 9.7

Incinerator ash 20 14.0 11.2

40 14.6 11.1