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352 4. Adsorption and Ion Exchange

Bitumen immobilization/asphalt blending Bitumen (asphalt) is a generic term used to
cover a wide range of high molecular weight hydrocarbons, generated in crude-oil process-
An e ing. Asphalt blending has many advantages for certain types of wxample is the astes.
organic wastes for which asphalt is a much better solidifying material than pozzolanic
binders (Freeman, 1998). Furthermore, asphaltic binders can be used in construction works.
Bitumen processes can be held either as batch or as continuous operations. In the f irst
case, the steps of drying and mixing the dried material in molten bitumen are in ed, olv v
whereas in continuous operation, the spent material is introduced as slurry to equipment
that continuously mixes the bitumen at the same time. Then, the bitumen mixture flo ws
into a suitable storage container and is solidified upon cooling. Neilson and Colombo
v (1982) hae presented the main features of the process for ion-exchange resin w astes.

Polymer immobilization The polymers used in the immobilization of spent materials
can be classified into two main categories: thermoplastic and thermosetting polymers. The
first type is fed in the form of a solid, and then melts upon heating and combines with the
waste. On the other hand, thermosetting polymers are supplied in a liquid form and are
then polymerized to a solid form, combining with the waste upon heating or in the pres-
ence of catalysts.
Spent resins are generally compatible with the polymer matrix material. Generally the ,
polymer and the resin do not interact chemically. The immobilization of spent ion-exchange
v
resins in polymers is a common application all oer the world. Epoxy resins, polyesters,
polyethylene, polystyrene and copolymers, polyurethane, phenol-formaldehyde, and poly-
styrene are among the polymers used (IAEA, 1988). Inorganic materials are generally not
immobilized using polymers because they are more acceptable to other immobilization
matrices such as cement.
Properties of immobilized waste forms

v
The properties of the immobilized waste form are decisie for the quality of disposal or
storage for a long period of the spent material. Depending on the procedure followed, there
is a variety of mechanical, chemical, or thermal features obtained. As mentioned earlier ,
cement waste forms hae a very stable structure with excellent mechanical strength.
v
However, improper formulations may lead to defects in the waste-form structure. The
characteristics of bitumen waste forms are closely related to the nature of the solidifying
material that has been used. The exposure of the waste form to heat or pressure may lead
er
v
to its softening. Moreo although bitumen waste forms are w rehydration of aterproof,
,
organic ion-exchange media may occur upon prolonged exposure to water and subsequent
failure of the waste form due to swelling (IAEA, 2002).
In many cases, the mechanical properties of a waste form are an important part of the
waste acceptance criteria for storage and/or disposal of the wThe waste form may aste.
undergo a series of tests, such as a determination of its compressie strength, to demon- v
strate that it will maintain its integrity oer the required period of time. v
The resistance of the waste form to biodegradation in the environment during long-term
storage or disposal is another feature of major importance. The action of microor ganisms
may lead to the release of the components of the waste form into the en or to vironment,
the formation of gases such as hydrogen or methane that may be of concern for the design