Environmental Engineering Reference
In-Depth Information
the inherent growth dynamics of the plant, as
well as in response to variations in the heavy
metal levels and availability in the surrounding
water and soil (Hardej and Ozimek 2002 ).
Pleurotus australis showed a very high adsorp-
tion affi nity for Cu, Cd, Ni, Pb, and Zn from
aqueous solutions (Southichak et al . 2006 ).
Interactions between the microbial community,
species composition, soil/sediment characteris-
tics, and hydrology play an important role in the
types and rates of remediation reactions that
occurred in the contaminated environment.
Williams et al. ( 1999 ) mentioned that successful
long-term phytoremediation must carefully con-
sider the management implications of secondary
succession to avoid ecological shifts away from
the optimal plant community structure.
metal concentrations as a function of metal
uptake and biomass production plays an impor-
tant role in achieving regulatory acceptance.
Hypothetically, metal removal can be accounted
by determining metal concentration in plant,
multiplied by the biomass produced, and compar-
ing it with the reduction in soil metal concentra-
tions. There are many factors that make it
challenging in the fi eld. One of the constraints for
commercial implementation of phytoremediation
has been the disposal of contaminated plant
material. After cropping, the plant is removed
from the contaminated site that leads to accumu-
lation of huge quantity of hazardous biomass.
This hazardous biomass should be stored or dis-
posed properly so that it does not pose any risk to
the environment. Biomass is nothing but stored
solar energy in plant mass, which is also termed
as materials having combustible organic matter.
Biomass contains carbon, hydrogen, and oxygen
and is known as oxygenated hydrocarbons. The
main constituents of any biomass material are
lignin, hemicellulose, cellulose, mineral matter,
and ash. It possesses high moisture and volatile
matter constituents, low bulk density, and calo-
rifi c value. The percentage of these components
varies from species to species. The dry weight of
B. juncea for induced phytoextraction of Pb
amounts to 6 tonnes per hectare with 10,000-
15,000 mg/kg of metal in dry weight (Blaylock
et al. 1997 ). Handling of huge quantity of this
type of waste is a problem and hence need vol-
ume reduction (Blaylock and Huang 2000 ).
Plant-associated microbes play a very impor-
tant role in heavy metal removal from industrial
waste-contaminated soils. However, uptake of
metals by microbes and host plants is only a tem-
porary removal process, and the harvesting and
removal of biomass are an essential step for
effective removal of heavy metals from contami-
nated environment. Therefore, this aspect needs
separate attention. Weis and Weis ( 2004 ) men-
tioned that senescent plant tissues may be sources
of metals released through leaching or can be
sinks for metals through litter adsorption or
microbial immobilization. They also stated that
the extent of uptake and the distribution of metals
within plants have important effects on the
6
Harvesting and Recycling
of Plant Biomass
The role of plants in the phytoremediation of
heavy metals were considered to be a successful,
low-cost, cleanup option to ameliorate the quality
of contaminated soil (Gopal 2003 ). They not only
assimilate pollutants directly into their tissues but
also they act as catalysts for purifi cation reactions
by increasing the environment diversity in the
root zone and promoting a variety of chemical
and biochemical reactions that enhance purifi ca-
tion (Jenssen et al. 1993 ). Vajpayee et al. ( 2001 )
demonstrated this concept following metal
uptake tests with Vallisneria spiralis , a freshwa-
ter submerged, rooted wetland species that were
tested for Cr accumulation in microcosms and
were found to effectively remove Cr by adsorp-
tion and absorption into plant tissues, hence, con-
cluded that V. spiralis may be suitable as Cr
accumulators in constructed wetlands, where
biomass could be safely harvested and disposed
off. Harvested plant biomass may also be used in
biogas production, resulting in a safe and advan-
tageous disposal.
Phytoremediation involves repeated cropping
of plants in contaminated soil, until the metal
concentration drops to acceptable level. The abil-
ity of the plants to account for the decrease in soil
 
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