Introduction
Leather industry is one of the
important industries in India as it earns a high
proportion of foreign exchange. It is also the
major reason for the environmental influx of chromium(Cr).
The effluent and sludge disposed from these industries
into rivers and onto land has led to extensive
degradation of productive land (Ramasamy, 1997).
Cleaning up of the Cr contaminated sites is a
challenging task. Phytoremediation involves the
use of plants to remove toxic substances from
the environment. The use of plants has been investigated
for a wide variety of chemical substances, including
metal and organic contaminants, in various media,
most commonly soil and water. It is an emerging
technology that can be considered for remediation
of contaminated sites because of its cost effectiveness,
aesthetic advantages, and longterm applicability.
Though many small herbs have been successfully
proved to accumulating heavy metal ions like chromium,
the burden of harvesting and disposing the one
season plants poses greater difficulty in applying
the bioremediation. Hence large plants with long
period of life and soil covering and transpiration
potentials could be the best choice. On this basis
an attempt has been made to use Azadirachta
indica with economic timber value, applying
hydroponic studies, pot culture and by field experiments
for the remediation of chromium contamination.
Methodology
Chromium concentrations were
selected based on previous hydroponic studies
of annual plants (Shanker et al., 2004).
Chromium was used at levels such as 10, 25, 50,
100, 150mM in the experiments. Soil amended with
various levels of tannery sludge (10%, 20%, 40%,
60%, 80% and 100% were prepared and filled in
to the pots and pot with only garden soil served
as control. All the experiments were repeated
twice. Based on these results, field trials were
conducted to evaluate the metal accumulation.
Measurement of chromium content (mg Kg-1)
was made on plant parts according to Jones et
al. (1991).
Results and discussion
Plants have the genetic potential
to clean up soil contaminated with toxic metals.
Some plants can take up, translocate, and tolerate
increased levels of certain heavy metals that
would be toxic to any other known organisms.
Identification of metal hyperaccumulator species
has been an impetus for Phytoremediation research.
In the present study, a significantly high accumulation
of heavy metals was found in various parts of
the Azadirachta indica,grown on hydroponic
and tannery sludge amended soil. It was observed
that under normal control conditions, the concentration
of Cr in various parts of the plant was <
1 mg/Kg dry weight (DW) of plant tissues. The
accumulation of hexavalent Cr in hydroponics
experiment of Azadirachta indica was
found maximum in shoots followed by roots and
leaves which increased with increase in chromium
amendments (Fig.1). Of the total amount of chromium
accumulated by Azadirachta indica 95.16%
was in the shoots and 4.63% in roots. However,
chromium accumulation in the leaf was quiet
low or very negligible when compared to stem
and roots(Fig.2). Presence of Cr in the external
environment does not lead to any changes in
the growth and development pattern of the plants
at the initial low concentrations (Table1).
While, Hasselgren (1999) found stem biomass
production of three willow clones was enhanced
by sludge application rate; it also led to more
uniform growth and a greater shoot number than
in control plants. But Sinapsis alba
showed reduction in plant height at 200 –
400 mg chromium per kilogram soil (Hanus and
Tomas, 1993).
In pot studies also the test
plant accumulated higher amount of chromium
in the stem followed by roots and leaves; but
the chromium accumulation in the shoot was over
2 and 28 fold when compared to the root and
leaf respectively (Table2). The accumulation
of Cr increased with increase in sludge amendments
and exposure periods as observed by Hasselgren
(1999). Whereas, Huffman and Allaway (1973)
have reported as much as 98% chromium accumulation
in the roots of bean plants.
The reason for the high accumulation
in roots of the plants could be because chromium
is immobilized in the vacuoles of the root cells
(Shanker et al., 2004). Khan (2000)
further reported the potential of mycorhiza
in protecting tree species Populus euroamericana,
Acacia arabica and Dalbergia sisso
against the harmful effects of chromium contaminated
tannery effluent polluted soil.
In the field experiment, the test plant also showed
higher accumulation of Cr in stem tissues. Thus
Azadirachta indica was not only able
to tolerate very high concentrations of chromium
but also showed appreciable growth over control
plants (Fig.3). Similar results have been recorded
in 16 Salix clones grown in a field trial (Watson,
2002; Pulford et al., 2002). Therefore,
Azardirachta indica could be potentially
exploited in phyto remediation practices like
soil reclamation, phyto extraction of metals like
Cr from tannery effluent amended soil.
Table
: 1 Growth parameters of Azadirachta indica
as influenced by Cr(VI) in nutrient medium after
120 h of treatment.
Parameter
|
Control
|
Cr (VI)
(10 µM)
|
Cr (VI)
(25 µM)
|
Cr (VI)
(50 µM) |
Cr (VI)
(100 µM) |
Cr (VI)
(150 µM)
|
| |
9.48 ±0.6
|
9.12 ±0.1
|
8.42 ±0.6
|
7.63 ±0.3
|
6.91 ±0.5
|
6.75 ±0.6
|
|
|
|
|
|
|
3.06 ±0.4
|
|
16.3 ±2.1 |
16.7 ±0.9
|
15.6 ±1.1 |
14.68 ±1.9
|
14.7 ±2.3
|
13.73 ±2.3
|
|
|
0.028 ±0.003 |
0.028 ±0.006 |
0.029 ±0.006 |
0.024 ±0.004
|
0.015 ±0.006
|
0.012 ±0.006
|
|
|
|
|
|
|
|
0.041 ±0.018
|
Table: 2 Accumulation
of Cr * in A. indica treated with tannery
sludge in pot experiment.
Plant parts
|
|
Tannery
sludge ( % )
20
|
|
|
|
|
|
Root
|
48.43 ± 0.90
|
71.43 ± 0.95
|
94.26 ± 0.86
|
110.40 ± 0.72
|
253.03 ± 1.35
|
341.66 ± 1.60
|
|
Shoot
|
|
451.43 ± 1.25
|
597.93 ± 0.47
|
785.26 ± 1.27
|
896.36 ± 2.85
|
1021 ± 2.25
|
Leaves
|
17.03 ± 1.15
|
21.93 ± 0.41
|
25.50 ± 0.72
|
29.16 ± 1.30
|
35.00 ± 1.50
|
36.66 ± 0.85
|
*mg/Kg tissue (DW)
Table:
3 Accumulation of Cr (VI) in different parts
of A. indica in field experiment.
Plant
– parts
|
Cr mg/Kg
(DW)
|
|
16.3 ± 0.02 |
|
5.2 ± 0.02 |
|
|
Fig.
1. Azadirachta indica grown at various
concentrations of chromium (VI).
Fig.
2. Accumulation of Chromium (VI) in different
parts of A.indica.
Fig.3.
Growth of Azadirachta indica plants
in tannery sludge amended soil
References:
Hasselgren K. 1999. Utilisation
of sewage sludge in short-rotation energy forestry:
a pilot study. Waste Manage Res;17:251–62.
India. Fourth international conference on the
biogeochemistry of trace elements.
Jones,B., Wolf,B., Mills,H.A.,
1991. Plant analysis handbook: a practical sampling,
preparation, Analysis and Interpretation Guide.
Micro-Macro International, Athens, GA. University
of California, Berkeley, USA. June 23-26, pp
771-772.
Khan AG, Kuek C, Chaudhry TM,
Khoo CS and Hayes WJ. 2000. Role of plants,
mycorrhizae and phytochelators in heavy metal
contaminated land remediation. Chemosphere;41:197–207.
Pulford ID, Riddell-Black D
and Stewart C. 2002. Heavy metal uptake by willow
clones from sewage sludge-treated soil: the
potential for phytoremediation. Int. J. Phytoremediation;4:59–72.
Ramasamy, K. 1997. Tannery
effluent related pollution on land and water
ecosytems in Raskin, I. and B.D. Ensley (Eds.),
2000. Phytoremediation of Toxic Metals: Using
Plants to Clean up the Environment, John Wiley,
New York.
Shanker A. K, Djanaguiraman,
M., Sudhagar, R., Chandrashekar, C.N and Pathmanabhan
G. 2004. Differential antioxidative response
of ascorbate glutathione pathway enzymes and
metabolites to chromium speciation stress in
green gram (Vigna radiata (L) R Wilczek, cv
CO 4) roots. Plant Sci , 166:1035–
43.
Watson C, Pulford ID and Riddell-Black
D. 1999. Heavy metal toxicity responses of two
willow (Salix) varieties grown hydroponically:
development of a tolerance screening test. Environ
Geochem Health;21:359–
64.
Watson, C. 2002. The phytoremediation
potential of Salix: studies of the interaction
of heavy metals and willows. PhD thesis, University
of Glasgow.
