Page 195 - Soil Degradation, Conservation and Remediation

184 6 Soil Pollution

oxidation and decomposition, vitrification, etc. (Sharma and Reddy 2004 ). There
are in situ (on-site) and ex situ (off-site) remediation techniques. In situ methods
are used directly at the contamination site so that soil does not need to be excavated,
and therefore the chance of causing further environmental harm is minimized. Most
of these methods are, however, relatively expensive and slow (Ward et al. 2003 )
or limited by the production of secondary waste streams that require subsequent
disposal or treatment.
Attempts of soil washing and solvent extraction have been made as ex situ methods.
In these techniques, water and other solvent mixtures, including dichloromethane,
ethanol, methanol, and toluene, have been utilized (Rababah and Matsuzawa
2002 ). Additionally, surfactants have also been tried. Surfactants such as TWEEN
40, TWEEN 80, Brij 30, DOWFAX 8390, and STEOL 330 have shown to be effective
for PAH removal from soil (Ahn et al. 2008 ). Recent studies have advocated the
use of vegetable oil as a nontoxic, biodegradable, and cost-effective alternative to
these conventional solvents and surfactants (Gong et al. 2006 ). Successful removal

of PAHs from soil with efficiencies above 80 % has been reported. Pizzul et al.
( 2007 ) reported the use of rapeseed oil on the degradation of polycyclic aromatic
hydrocarbons in soils by Rhodococcus wratislaviensis .
The hydrogen peroxide oxidation technique has been employed for the remediation
of organic pollutants in soils with more success. It is relatively fast, taking only days
or weeks; the contaminants are treated in situ and converted to harmless substances
(e.g., H 2 O, CO 2 , O 2 , halide ions). Hydrogen peroxide can be electrochemically
generated on-site, which may further increase the economic feasibility and effective-
ness of this process for treatment of contaminated sites. Natural iron oxide minerals
(hematite Fe 2 O 3 , goethite FeOOH, magnetite Fe 3 O 4 , and ferrihydrite) present in soil
can catalyze hydrogen peroxide oxidation of organic compounds. Disadvantages

include difficulties controlling in situ heat and gas production. Low soil permeability,
incomplete site delineation, and soil alkalinity may limit the applicability of the
hydrogen peroxide oxidation technique (Goi et al. 2009 ).
Permeable reactive barrier (PRB) technology, using iron metal or zerovalent iron
0
(Fe ) as reactive media, has been very effective in dehalogenation (detoxifi cation)
of organic contaminants in groundwater. A PRB consists of installing a trench
0
perpendicular to the path of groundwater flow and filling it with Fe (e.g., iron fi lings).

As the contaminant-laden groundwater passes through the PRB, the organic
0
contaminants react with Fe and are dehalogenated into nontoxic forms (Gillham
0
and O’Hannesin 1994 ; Sharma and Reddy 2004 ). Several studies investigated Fe as
an effective reductant in treatment of chlorinated ethylenes, halomethanes,
nitroaromatic compounds, pentachlorophenol, chlorinated pesticides such as DDT,
polychlorinated biphenyls, atrazine, and other organic compounds containing reducible
0
functional groups or bonds. With this experience with Fe , nanotechnology has
emerged as an efficient tool of remediation of soils polluted with organic pollutants.

The technology involves the synthesis of nanoscale iron particles (NIP) and their
application to contaminated soils. The reaction pathways of NIP with target
halogenated organic contaminants are similar to that of zerovalent iron commonly

used in a PRB technology. However, due to their infinitesimally small size, NIP can