Environmental Engineering Reference
In-Depth Information
nonrecyclable toxic by-products are at the levels that present an unacceptable risk
to the sensitive biosphere (Sheoran et al.
2008
; Sikdar et al.
1998
)
Historically, soil pollution has been of very little concern, and its contamination
is a relatively recent issue when, actually, it began a long time ago, particularly
after the industrial revolution, even though only recently has mankind become
aware of its dimension, persistence, and harmful effects (Castelo-Grande et al.
2010
). Pollutants can be built up in the soil directly or indirectly from several
sources, such as industrial emissions, mining and smelting of metalliferous ore,
electroplating energy and fuel production, and sludge dumping. A wide range of
inorganic and organic compounds cause contamination, and these include heavy
metals, combustible and putrescible substances, hazardous wastes, explosives, and
petroleum products. There has been increasing concern over the last few decades
regarding organic pollutants, which includes PCBs such as dioxin, polycyclic
aromatic hydrocarbons (PAHs) such as benzoapyrene, nitroaromatics such as tri-
nitrotoluene (TNT), and linear halogenated hydrocarbons such as trichloroethylene
(TCE). Major components of inorganic contaminants are heavy metals (Ghosh and
Singh
2005
). Heavy metals cannot be destroyed biologically and are present in
soils as free metal ions, soluble metal complexes (sequestered to ligands),
exchangeable metal ions, organically bound metals, precipitated or insoluble
compounds such as oxides, carbonates, and hydroxides, or they may form part of
the structure of silicate materials (indigenous soil content) (Davies et al.
2001
).
Contaminated soils are the target of several thousands of pollutants that vary in
their compositions and concentration. Elevated concentration of these pollutants
impair the functioning of human population and pose significant risk to the
development of flora and fauna (Scullion
2006
). Contaminated soil fails to support
crop growth and negatively affects its development because of interference of
phytotoxic contaminants with metabolic processes and sometimes also leading to
plant death (Hoffmann
1983
). Besides this, soil toxicity disrupts biological cycling
of nutrients and also affects the hydrosphere compromising with the quality of
drinking water resources, and threatening the aquatic ecosystem (Bilek
2004
).
Human beings are also at risk from polluted soils, thus the magnitude of pollution
in our soils calls for immediate action (Friberg et al.
1986
; Knasmuller et al.
1998
;
Nathanail and Earl
2001
). When contaminant concentrations in soil are too high
for natural biodegradation to occur, cleanup action is warranted (Sikdar et al.
1998
). Thus, in response to a growing need to address this environmental con-
tamination, many remediation technologies have been developed to treat soil
contaminated by various pollutants, including in situ (treating relatively undis-
turbed soils) and ex situ (treating excavated soils) methods either onsite or in
designated soil treatment facilities. In situ has the advantage of minimal disruption
to activities on site or on adjacent land. Ex situ approaches generally offer greater
scope for managing conditions to optimize treatment efficiency and for controlling
potential spread of pollutants. In situ methods are favored over the ex situ tech-
niques due to their low cost and reduced impact on the ecosystem (Reed et al.
1992
).
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