Environmental Engineering Reference
In-Depth Information
air sparging, however in the early 1990s, risks of spreading of contaminants by this
technique were regarded as a disadvantage. (Johnson et al.
1993
).
In conclusion the source oriented physical techniques for volatile contaminants
were shown to be less effective and were replaced by more effective treatments in
the path of these contaminants.
Chemical treatment by
In Situ Chemical Oxidation
(ISCO) has been described
at field scale since about 2002 (Hartog et al.
2004
; Plaisier et al.
2003
). In Situ
Chemical Oxidation (ISCO) involves the introduction of chemical oxidants into the
soil to destroy organic contaminants (e.g., chlorinated organic solvents or fuels).
A wide variety of chemical oxidant types exist. Most frequently used for ISCO
are hydrogen peroxide (H
2
O
2
, often in Fenton's Reagent mixture with ferrous
iron), permanganate (as KMnO
4
or NaMnO
4
) and ozone (O
3
). Recently, persul-
fate (Na
2
S
2
O
8
) has emerged as an alternative oxidant for ISCO. These chemical
oxidants are stronger than those that occur in biological oxidation reactions. Due
to their higher oxidative strength they can oxidize a wider variety of contami-
nants at a faster rate. In addition, complete oxidation of the target contaminant
generally occurs without the formation of potentially harmful intermediate contam-
inants. Due to the non-biological nature of the oxidation process, high contaminant
concentration, including NAPL phases, can be oxidized unhampered by toxicity
effects. Despite the wide range of reaction characteristics for the multitudes of
chemical oxidant types available, general key issues relating to the sustainability
and cost-effectiveness of ISCO can be identified. First the loss of oxidants through
reactions with the natural soil oxidant demand (e.g., components such as organic
matter and iron sulphides) should be minimized. A successful ISCO application
therefore requires optimal oxidant loading (dose concentration and delivery) for a
particular contaminated soil system to maximize cost-effectiveness and minimize
soil disturbance (Haselow et al. 2003; Mumford et al. 2004; Nelson et al. 2001).
The application of strong oxidants may lead to the unwanted mobilization of met-
als, as due to the oxidation of organic matter the binding capacity is expected
to decrease and also the formation of contaminants as byproducts might occur
(Crimi and Siegrist
2003
,
2004
). Other researchers point to potential reduction of
soil permeability as the faster reaction rates promote the accumulation of reaction
precipitates and gas formation (Lee et al.
2003
). Also some research groups are
concerned about the recovery of biological soil functions (Ecosystem Services),
including Natural Attenuation capacity, after ISCO application (Christ et al.
2005
;
Sahl et al. 2007). However, others promote the combination of ISCO and biodegra-
dation. The rationale for this is to reduce the amount of chemical oxidants and to aim
at partial oxidation followed by biodegradation of the intermediate contaminants.
Further monitoring procedures need to be developed to improve the monitoring of
the remediation process (Cave et al.
2007
).
Most In Situ remediation technologies focus on organic contaminants, as in gen-
eral for heavy metals there are no other options than mobilization or immobilization
of metals. For In Situ mobilization of metals, only a limited number of studies have
been performed. In a sandy soil, Jansen et al. (2004) were able to mobilize zinc in
a sandy soil at pH
=
4. The acidic effluent downstream of the site was treated by
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