Environmental Engineering Reference
In-Depth Information
should be avoided for sites near population centers and at places with unique
meteorological conditions that promote the rapid deposition of volatile com-
pounds. Hence, the consequences of releasing the metals to the atmosphere need to
be considered carefully before adopting this method as a remediation tool (Suko
et al. 2006 ; Padmavathiamma and Loretta 2007 ).
17.4.6.4 Rhizofilteration/Phytofilteration
Plant assisted technique, which involves the use of both terrestrial and aquatic
plants, to absorb, concentrate, and precipitate contaminants in the aqueous system
has low contaminant concentration in their roots or seedlings (blastofilteration)
(Dushenkov et al. 1995 ; Prasad and Frietas 2003 ). Mechanisms involved in rhi-
zofilteration include chemisorption, complexation, ion exchange, micro precipi-
tation, hydroxide condensation onto the biosurface, and surface adsorption
(Gardea-Torresdey et al. 2004a , b ). Root exudates and changes in rhizosphere pH
also may cause metals to precipitate onto root surfaces. As they become saturated
with metal contaminants, roots or whole plants are harvested for disposal (Flath-
man and Lanza 1998 ). Rhizofilteration can partially treat industrial discharge,
agricultural runoff, or acid mine drainage. It can be used for Pb, Cr, Cd, Ca, Cu,
Ni, and Zn, excess nutrients, and radionuclides (U, Cs, Sr), which are primarily
retained with the roots (Ensley 2000 ) (Table 17.5 ).
17.4.6.5 Phytodegradation
It involves the breakdown of organics to simpler molecules that are incorporated
into the plant tissues. Plant contains enzymes or enzyme cofactors that can
breakdown and convert ammunition wastes, chlorinated solvents such as trichlo-
roethylene and other herbicides (Newman and Reynolds 2004 ). Various plant
species that can degrade aromatic rings in the absence of microorganisms have
been described by Dec and Bollag ( 1994 ) and Singh and Jain ( 2003 ). Polychlo-
rinated biphenyls (PCBs) have been metabolized by sterile plant tissues. Phenols
have been degraded by plants, such as horseradish, potato (Solanum tuberosum),
and white radish (Raphanus sativus) that contain peroxidases (Roper et al. 1996 ).
Poplar trees (populus species) are capable of transforming trichloroethylene in soil
and ground water (Newman et al. 1997 ). The enzymes are usually dehalogenases
(transformation of chlorinated compounds), peroxidases (transformation of phe-
nolic compounds), nitroreductases (transformation of explosives and other nitrated
compounds), nitrilase (transformation of cyanated aromatic compounds), and
phosphatases (transformation of organophosphates pesticides (Boyajian and Car-
riera 1997 ) (Table 17.6 ).
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