General Guidelines for S/S of Wastes - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 10

2.2 SCIENTIFIC BASIS OF S/S

S/S is typically a process that involves the mixing of a waste with a binder to reduce
contaminant leachability by both physical and chemical means and to convert the
hazardous waste into an environmentally acceptable waste form, which goes to a
landfill or is used in construction. Stabilization processes and solidification processes
have different goals. Stabilization attempts to reduce the solubility or chemical
reactivity of a waste by changing its chemical state or by physical entrapment
(microencapsulation). Solidification systems attempt to convert the waste into an
easily handled solid with reduced hazards from volatilization, leaching, or spillage.
The two are often discussed together because they have the common purpose of
improving the containment of potential pollutants in treated wastes. Combined
processes are often termed “waste fixation” or “encapsulation.”
Solidification of waste materials is widely used for the disposal of radioactive
waste. Many developments relating to solidification originated from low-level radio-
active waste disposal. Regulations pertaining to disposal of radioactive waste require
that the wastes be converted into a free-standing solid with a minor amount of free
water. Most processes used for nuclear waste include a step in which granular ion
exchange waste and liquids are incorporated in a solid matrix using a cementing or
binding agent (for example, portland cement, organic polymers, or asphalt). The
resulting block of waste, with relatively low permeability, reduces the surface area
across which the transfer of pollutants can occur.
In hazardous waste disposal and site remediation, treated material must meet
certain standards for safe land disposal, by removing the hazardous characteristic
for characteristically hazardous waste or by formal delisting for listed waste. For
the toxic characteristic, this usually requires passing concentration-based standards
using the USEPA TCLP test.

To accomplish this goal, a variety of strategies may
be used to prevent contaminant leaching, including neutralization, oxidation/reduc-
tion, physical entrapment, chemical stabilization, and binding of the stabilized solid
into a monolith. Appropriate treatment strategies should (1) treat waste, or contam-
inated sites, to be chemically inert and nonleachable, to the extent possible and (2)
be economical.
A binder is often used to stabilize the contaminants in the waste or contaminated
sites and to remove the free liquid. In cases where the waste is extremely soluble
or no suitable chemical binder can be found, the waste may be contained by encap-
sulation in some hydrophobic medium, such as asphalt or polyethylene. This may
be done either by incorporating the waste directly in the partially molten material
or by forming jackets of polymeric material around blocks of waste.
Many types of binder have been developed for S/S of wastes, as discussed, but
not all have been employed in remedial action on uncontrolled waste sites. The most
commonly used binders are discussed in Chapters 4, 5, and 6. Portland cement is
most commonly used because of its availability and low cost. Supplementary cement-
ing materials such as coal fly ash and ground blast furnace slag are often used to
partially replace portland cement, to improve the performance of the treated wastes,
and to reduce the cost of the binder.

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