Organic Polymers for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 81

applications. For mixed waste lead and debris, the Environmental Protection Agency
(EPA) has identified macroencapsulation as the best demonstrated available tech-
nology (BDAT). Macroencapsulation may be deployed by extruding a plastic layer
around consolidated waste or by hermetically sealing the waste within plastic sleeves
or containers.
Organic polymers can be applied for in situ remediation or ex situ treatment of
waste. However, those polymers with lower viscosity at ambient temperature, e.g.,
thermosetting resins, are generally more suitable for in situ applications than ther-
moplastic resins. For example, polyester styrene can be readily pumped into sub-
surface soil via jet grouting techniques, penetrating and filling voids. In these cases,
the monomer is pumped in two separate batches; one premixed with catalyst and
the other with promoter, so the polymerization reaction is triggered in a controlled
fashion within the soil and no solidified product is left to solidify within process
piping. The use of thermoplastic polymers for in situ application would require
heating large masses of soil to maintain the liquid state of the polymers during
injection and mixing, and thus is not practical or cost-effective. The following
subsections discuss in more detail three thermosetting polymer systems used for S/S
(urea-formaldehyde [UF], polybutadiene, and polyester resins) and two thermoplas-
tics (bitumen and polyethylene).

5.2 THERMOSETS

Thermosetting polymers are formed when a liquid monomeric resin is reacted in
the presence of a catalyst and promoter to form polymerized organic chains that
intermesh and form a stable, monolithic solid. Once polymerized to a solid, ther-
mosetting polymers cannot be reformed. They can be adapted for encapsulation of
waste by mixing waste constituents with the monomer prior to polymerization.
Thermosetting polymers can be used to treat solid waste or liquids (if mixed as an
emulsion with the monomer). Thermosets do require a chemical reaction to initiate
polymerization, and as such, like hydraulic cement grouts, are susceptible to inter-
actions between the waste and the binder. However, thermosets are more prone to
difficulty in the presence of organic contaminants and are less likely to be affected
by inorganic constituents commonly found in radioactive wastes. Polybutadiene, and
polyester are all examples of thermosetting polymers.

5.2.1 U REA -F ORMALDEHYDE

UF is a solidification process based on the condensation polymerization of UF
thermosetting resin that occurs upon the addition of an acidic catalyst. 5 It was a
popular commercial technique marketed by several solidification vendors in the
1970s to treat both solid and aqueous low-level radioactive wastes from nuclear
power plants. Liquid waste was treated by forming an emulsion in the low-viscosity
UF liquid and then adding the catalyst to induce solidification.
The UF resin binder consists of an aqueous emulsion of partially polymerized
monomethylol and dimethylol urea with a small amount (less than 3 wt%)
of formaldehyde, which is reacted under neutral or alkaline conditions. Partial