Environmental Engineering Reference
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toxic form by mixing soil or waste with chemical binders, such as cement, sulfide,
and phosphate binders, polyester resins, or polysiloxane compounds to create a
slurry, paste or other semi-liquid state, and is allowed time to cure into solid form
(Wang et al. 2012 ). Additives through which solidification is achieved are either
cement-based, pozzolon-based, the thermoplastic methods, the organic polymeri-
zation methods, the encapsulation method, and organophillic clay-based (Wang
et al. 2012 ). Among these methods, cement-based solidification/stabilization is of
increasing importance as option for remediating contaminated sites because of its
low material and equipment cost.
This technique is suitable for contaminant in shallow depths and of large vol-
ume, and is not suitable for metals, which are not highly soluble, and do not form
hydroxides, such as arsenic, chromium, and mercury. In situ solidification/stabil-
ization techniques are preferred since labor and energy costs are lower, but site
conditions, such as bedrock, large boulders, clay, and oily patches may cause
mixing problems (Mulligan et al. 2001 ).
17.2.4 Vapor Extraction and Air Sparging
Vapor extraction and air sparging techniques are based on the manipulation of
pollutant distribution between liquid and vapor phases. These treatments promote
the volatilization of pollutants (e.g., benzene, toluene, ethyl benzene, xylene, and
chlorobenzenes) in unsaturated and saturated zones. Extracted gases or vapors may
be adsorbed onto activated carbon or treated (e.g., by oxidation). The effectiveness
of vapor extraction systems may be extended to semi-volatile pollutants by
injection of heated air or heating by microwave/radiowave, and rates of extraction
may be improved by increasing air flow rates to a point when mass transfer limits
volatilization (George et al. 1992 ; Park et al. 2005 ). Air sparging also known as
in situ air stripping or in situ volatilization induces partitioning of dissolved and
free-phase contaminants into the vapor phase, and increases in dissolved oxygen
can stimulate aerobic degradation. Benzene removal by air sparging has been
shown by Adams and Reddy ( 2003 ). Both these approaches are less well suited to
fine textured soils because of restricted rates of movement in the mobile phases
and increased distances over which volatile organic contaminants have to diffuse
through an aqueous phase. Also treatment rates are slower in soils with higher
organic contents (Gomez-Lahoz et al. 1995 ).
17.2.5 Vitrification
Vitrification of molten glass is another method of solidification/stabilization pro-
cess requiring thermal
energy. It uses heat of up to [ 1,000 [ 1,600 C-
[ 2,000 C to destroy organic pollutants by pyrolysis and immobilize most of the
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