Page 222 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 211

the contaminants are simply moved to a different place, where they must be monitored,
the risk of spreading contaminated soil and dust particles during removal and
transport of contaminated soil, and the relatively high cost. For different heavy
metals, extracting solutions for soil washing may be different. Several classes of
chemicals used for soil washing include surfactants, cosolvents, cyclodextrins,
chelating agents, and organic acids.

Soil Flushing

In soil flushing, an aqueous solution is injected into the contaminated zone of soil

followed by extraction of groundwater and elutriate (flushing solution mixed with
the contaminants) and aboveground treatment and discharge. The goal of this in situ

flushing is to enhance the solubility or mobility of contaminants and accelerating
the remediation process. Flushing solutions may include water or surfactants,
cosolvents, acids, bases, oxidants, chelants, and solvents which percolate through the
soil, and soluble compounds present in the soil are dissolved. The elutriate is pumped
out of the contaminated zone into a water treatment system to remove pollutants.

Encapsulation
The basic principle is the underground construction of an impermeable vertical
barrier to allow the containment of gases and liquids. A variety of construction
methods such as cutoff slurry walls using mainly cement–bentonite–water slurries,
thin walls, sheet pile walls, bored-pile cutoff walls, jet grouting curtains, injection
walls, and frozen barriers have been developed. Encapsulation is recommended as
the easiest way to safely dispose of metal polluted soils and hazardous wastes

including hospital wastes. Contaminated soils and wastes are filled in leakproof
containers. When the container is three-quarters full, a material such as cement
(mortar), plastic foam, or clay is poured into the container until completely fi lled.
After the material has hardened, the container is sealed and may be landfi lled,
stored, or buried. It is also possible to encapsulate chemical or pharmaceutical waste
together with sharps (WHO 1999 ).

Solidification/Stabilization

Solidification is done through the addition of binding agents to a contaminated
material to impart physical stability to contain contaminants in a solid product.

Stabilization (fixation) involves the addition of reagents to the contaminated soil to
produce more chemically stable constituents. The general approach for solidifi cation/
stabilization treatment processes involves mixing or injecting treatment agents to
the contaminated soils. Inorganic binders such as clay (bentonite and kaolinite),

cement, fly ash, blast furnace slag, calcium carbonate, Fe/Mn oxides, charcoal, zeolite,
and organic stabilizers such as bitumen, composts, and manures, or a combination
of organic–inorganic amendments may be used. The dominant mechanism by which
metals are immobilized is by precipitation of hydroxides within the solid matrix