Environmental Engineering Reference
In-Depth Information
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Free-standing water
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Low-strength final waste form products
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Poor durability
Some constituents common in radioactive, hazardous, and mixed wastes such
as chlorides and nitrates are known “poisons” to the hydration reaction.
2
Higher
concentrations of waste containing these constituents are more likely to result in
process or performance failures. This phenomenon is exacerbated by the need to
optimize waste loading efficiency, driven by concerns over treatment cost and limited
disposal capacity for treated wastes.
Long-term performance in disposal is dependent on many factors and may
degrade over time due to changes in the chemical and physical environment. For
example, stability of contaminants is dependent on high solution pH, and while
waste forms contain reserve alkalinity, exposure to neutral or acidic groundwater
leachates will eventually lower the pH, resulting in higher solubility and enhanced
leaching. Solidified cement grout waste forms reduce the surface area of contaminant
exposed to percolating groundwater, reducing the net leach rate. However, compared
with alternative S/S treatment techniques, cement-based waste forms are more
porous, allowing enhanced leaching. Cement grout waste forms containing evapo-
rator salt concentrates or ion exchange resins have been shown to swell and crack
upon exposure to saturated conditions typical in shallow land disposal at many
sites.
3,4
Cracking or disintegration of the monolithic waste form structure rapidly
increases the available surface area exposed to leachant, with similar consequences
for the mobilization of contaminants. Swelling and cracking can occur in organic
polymer waste forms as well, especially in soft materials with low strength (e.g.,
bitumen). Polymer waste forms with higher tensile and compressive strength (e.g.,
polyester styrene, epoxy, polyethylene) tend to resist swelling and maintain mechan-
ical integrity even with relatively high (e.g., 40 to 60 wt%) salt loadings.
Investigation and development of organic polymers for SS of wastes have been
conducted over the past 25 years to provide improvements in waste loading efficiency
and performance, as well as process economics, compared with conventional cement
grout technologies. This chapter discusses some common organic polymers and their
use in S/S. In general, organic polymer encapsulation technologies can be divided
into two main categories: thermosetting and thermoplastic polymers.
Since organic thermosetting and thermoplastic polymers are chemically inert,
they do not react chemically with inorganic or radioactive waste constituents to
chemically stabilize the waste. However, with the inclusion of additives that react
with waste constituents and reduce contaminant solubility, organic polymers can be
considered true S/S technologies.
Encapsulation of small solid waste particles (< 60 mm) distributed homoge-
nously throughout the organic polymer matrix is known as microencapsulation. In
this case, individual waste particles are fully surrounded and encapsulated by the
polymer matrix. For wastes containing larger particles (> 60 mm) such as debris or
contaminated lead, clean polymer can be placed around the waste to reduce leach-
ability in a process known as macroencapsulation. Typically, thermoplastic polymers
such as high-density or low-density polyethylene are used for macroencapsulation
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