Environmental Engineering Reference
In-Depth Information
(Mushtaq and Khan
2010
). Cr exists in several
oxidation states and the two most stable forms
present in soils are Cr(VI) and Cr(III). Among
these two, Cr(III) is considered as less toxic in
comparison to bioavailable Cr(VI) compounds in
the form of chromate (CrO
4
− 2
) and dichromate
(Cr
2
O
7
− 2
) (Messer et al.
2006
). Cr (VI) can be
toxic to plants up to concentrations of 0.5 mg L
− 1
in solution and 5 mg kg
− 1
in soil (Turner and Rust
1971
). Cr(VI) is a very toxic, powerful epithe-
lial irritant and an established human carcinogen
by International Agency for Research on Can-
cer (IARC
1980
), the Environmental Protection
Agency (EPA
1984
), and the World Health Or-
ganization (WHO
1988
). Toxicity of Cr has been
studied in many plants and its excess amount
causes inhibition of chlorophyll biosynthesis in
terrestrial plants (Vajpayee et al.
2000
), affected
germination process, plant growth, yield and total
dry matter production; causes deleterious effects
on plant physiological processes such as photo-
synthesis, water relations, and mineral nutrition
(Shanker et al.
2005
); generates reactive oxygen
species (ROS) and alters metabolic enzymes
(Yadav
2010
); and leads to nutrient imbalance,
wilting of tops, and root injury (Scoccianti et al.
2006
; Yadav
2010
). Cr phytotoxicity also affects
fodder nutritive value (Sangwan et al.
2014a
); ac-
tivities of nitrogen metabolism enzymes (Sang-
wan et al.
2014b
) and carbohydrate, protein, and
guar gum content of cluster bean (Sangwan et al.
2013
). The activities of antioxidant enzymes, viz.,
superoxide dismutase (SOD), catalase (CAT),
ascorbate peroxidase (APX), and glutathione re-
ductase are also significantly affected by Cr(VI)
treatment in wheat (Subrahmanyam
2008
), and
no seed formation was observed even at 1.0 mM
Cr(VI) (Sharma et al.
1995
). It has been reported
that the toxic property of Cr(VI) originates from
the formation of ROS, i.e., superoxide radical,
hydrogen peroxide and hydroxyl radical, and in
higher concentrations, these ROS produce cyto-
toxic effects due to their ability to oxidize lipids,
proteins, and nucleic acids (Shanker et al.
2004
;
Panda
2007
; Pandey et al.
2009
). In order to
mitigate deleterious effects of ROS, plants pos-
sess complex defense mechanisms that involve
both enzymatic and nonenzymatic antioxidants
(Panda
2007
). The simultaneous action of vari-
ous antioxidant enzymes is essential for regula-
tion of ROS levels within the cell (Shanker et al.
2004
; Panda
2007
). Nonenzymatic antioxidants
such as ascorbate and glutathione (GSH) also
play an important role in preventing oxidative
stress (Noctor and Foyer
1998
). Considering
the negative effects of Cr(VI), the development
of efficient, cost-effective, and environmentally
sound methods for removing Cr(VI) from con-
taminated sites or alleviation of its phytotoxic
effects is important to safeguard the quality of
drinking water, agricultural products, and the en-
vironment (Diwan et al.
2008
). In India, Cr(VI)
contamination is a big problem around various
industries using Cr compounds, which causes
considerable negative impact on crop produc-
tion. This problem further gets exacerbated due
to the use of Cr-contaminated water by farmers
in irrigation. Thus, the cleanliness of the environ-
ment for safer food production is a major con-
cern. Therefore, methods are needed to alleviate
Cr toxicity, and also to decrease the Cr content in
crops, which may be helpful to minimize health
risks (Tripathi et al.
2012
).
6.2
Promising Approaches
for Amelioration of Chromium
Phytotoxicity and Its Removal
from Contaminated Sites
In this section of the chapter, we have discussed
several studies in support of alleviation of toxic
effects of Cr(VI) to understand the best possible
way to avoid the phytotoxic effects of Cr(VI).
Most studies have been focused on application
of plant growth regulators (PGRs), metal chela-
tors, modification of soil nutrients, and Silicon
(Si) application. Further, a variety of other meth-
ods have also been developed for remediation of
contaminated soil to protect plants from phyto-
toxic effects of Cr. Some commonly used soil
remediation methods are chemical immobiliza-
tion (Kumpiene et al.
2008
), phytoremediation
(Memon and Schroder
2009
), and soil washing
(Davezza et al.
2011
). Among them, chemical
immobilization is a cost-effective and promising
Search WWH ::
Custom Search