Biotechnological Aspects of Soil Decontamination - Biotechnologies and Biomimetics for Civil Engineering - page 370

17.2.2 Isolation and Containment
Contaminants can be isolated and contained, to prevent further movement, to
reduce the permeability of the waste to less than 1 9 10 -7 m/s (as required by the
USEPA), and to increase the strength or bearing capacity of the waste (USEPA
1994 ). This technique consists of the use of barriers that inhibit the migration of
contaminants to the neighboring uncontaminated site. Physical barriers made of
steel, cement, bentonite, and grout walls can be used for capping, vertical, and
horizontal containment. Capping is a site specific proven technology, which uses
synthetic membranes to reduce water infiltration. Horizontal barriers restrict the
downward movement of metal contaminant within the soil, whereas vertical
barrier reduces the migration from one site to another. These barriers are made of
slurry walls, grout, or geomembrane curtains, and sheet pile walls. It is the least
expensive approach, but leaves the contaminant in place without treatment. The
selection of each technology is site-specific. They are beneficial where the area of
contaminant is shallow but large. More research is required to match reactive
media with contaminants, model life time performance, optimize retention times,
and develop methods for regeneration of reactive media.
In terms of risk management, these above mentioned approaches aimed to
control the pathway linking hazard and receptor without treating the source of the
hazard. Remediation practices emphasized containment rather than treatment.
17.2.3 Solidification/Stabilization
Solidification process is a nondestructive physical method to immobilize the
contaminants by encapsulating them in a solid of high structural integrity, while
stabilization includes chemical reactions to reduce contaminant mobility. It is also
known as waste fixation through both physical and chemical means. Some variants
like liquid monomers that polymerize, pozzolans, bitumen, fly-ash, asphalt, and
cement are injected to encapsulate the soils. Capping or jacketing or complete
coating of the contaminated sediment with sandy material, such as clean sediment,
sand, or gravel, which decreases the direct contact area between the water and the
contaminated sediment, is what is done under encapsulation (Peng et al. 2009 ).
Two ways of encapsulation are (i) microencapsulation (ii) macroencapsulation.
Port land cement, pozzolans, or lime/hydrated lime, and organic polymers may be
used for microencapsulation, whereas concrete, organic materials (polythene,
polyesters, etc.), sulfur cement, etc. can be used for macroencapsulation.
Some researchers have reported that a good cap thickness was approximately
50 cm; and through capping the sediment by sands materials, the heavy metal
concentration in water could reduce to 80 %. The cost of implementing this tech-
nology is dependent on the lithology of the site and the depth of the contaminant. As
the depth of contamination increases, so does the cost (Khan et al. 2004 ).
The stabilization converts the contaminants into less soluble immobilized and less