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Chromium
Pollution and Bioremediation
By: Sumit
Yadav, O. P. Shukla & U. N. Rai
Introduction:
Tannery
effluent is a major source of aquatic pollution in India with high chemical
oxygen demand (COD), biological oxygen demand (BOD), and hexavalent
chromium. There are a large number of tanneries scattered all over the
country but the main areas of their concentration are Tamilnadu, Uttar
Pradesh and West Bengal.
Chromium, a steel-grey, lustrous, hard and
brittle metal, occurs in nature in bound forms that constitute 0.1–0.3 mg
kg-1 of the Earth’s crust. It has several oxidation states ranging from Cr
(-II) to Cr (+VI), the trivalent and hexavalent states are the most stable
ones. A maximum acceptable concentration of
0.05 mg/L (50 µg / L) for chromium in drinking water has been
established on the basis of health considerations.
Hexavalent chromium [Cr (VI)] compounds are being used in a wide variety of
commercial processes and unregulated disposal of the chromium containing
effluent has led to the contamination of soil, sediment, surface and ground
waters. In trace amounts, chromium is considered an essential nutrient for
numerous organisms, but at higher level, it is toxic and mutagenic. Nearly
80% of the tanneries in India are engaged in the chrome tanning processes.
Most of them discharge untreated wastewater into the environment. In such
aqueous waste, Cr(VI) is present as either dichromate (Cr2O72-)
in acidic environments or as chromate (CrO4- ) in
alkaline environments.
Impact On
Environment:
In
humans, exposure to hexavalent chromium salts for periods of 2 to 26 years
has been implicated as a cause of cancer of the digestive tract. High levels
of chromium and zinc in soil have been correlated with regional incidences
of stomach cancer. Based on exposure to chromium via inhalation, the
International Agency for Research on Cancer has categorized “chromium and
certain chromium compounds” in Group 1: sufficient evidence for
carcinogenicity in humans and animals.
In
plants, high levels of Cr supply can inhibit seed germination and subsequent
seedling growth. The deleterious effect of Cr is less pronounced on seed
germination than on seedling growth. Barley seeds germinated and grew well
at Cr(VI) levels of up to 100 mg kg-1
in soil but were always slower in development due to Cr inhibition of
diatase, which is responsible for mobilizing the reserve starch necessary
for initial growth. At 500 mg kg-1
Cr no seed germination occurred. In another study, Cr(VI) concentrations up
to 2 mM supplied as K2Cr2O7
(588 mg kg-1
Cr) did not affect germination of pea seeds significantly. However, high
levels of organic matter (5%) and/or low levels of soil pH significantly
reduced Cr toxicity on germination due to the extremely low bio-availability
of Cr under these conditions.
One
potentially important source of increasing Cr levels in plant shoots is
foliar application of Cr. Interestingly, research has shown that when Cr(III)
or Cr(VI) is applied through the leaf surface, on lettuce and bean plants,
they were not translocated from the leaves to other plant parts..
Surprisingly, Cr(III) was absorbed more rapidly than Cr(VI). The lack of Cr
transport in plant tissues might be due to the localization of Cr in leaf
cells as well as the tendency of Cr ions to bind or precipitate in an
insoluble form.
Bioremediation of chromium:
Some research workers suggest that by using Cr
(VI) contaminated groundwater to irrigate organic matter rich soil they
could remove Cr from water by reduction and precipitation in the soil as Cr
(III). Their initial investigations in a soil column study indicated that
application of relatively large volumes of water spiked with 1 mg l‑1
Cr (VI) yielded outflow Cr levels. Chromate adsorption accounted for
<1% of the total immobilized Cr and the amount taken up by alfalfa
shoots was <0.5% of the total added. In a study Losi et al. (1994)
examined the processes responsible for Cr
(VI) reduction in soil. They found that organic matter content, bioactivity,
and oxygen status were among the important factors. Under aerobic,
field-moist conditions, organic matter rich soil reduced 96% of added Cr
(VI). Sterile soils receiving similar amendments reduced only 75% of the
original Cr (VI), demonstrating the importance of the presence of soil
microorganisms in conjunction with a readily available carbon source. These
studies emphasize the role of soil organic matter in the reduction of Cr
(VI) to Cr (III). Organic matter enhances the reduction of chromate in soil
by increasing microbial activities. Realizing the potential importance of
soil microorganisms in reducing Cr (VI) in contaminated soils, several
groups attempted to identify and isolate microorganisms. Recent researchers
suggest that Cr-resistant microorganisms are present in all soils even in
those with no history of Cr-contamination. Bacterial populations resistant
to as much as 500 mg L‑1
Cr (VI) and fungal populations
resistant to 1000 mg L‑1
Cr (VI) were directly isolated from 2 uncontaminated soils. In an attempt to
evaluate the use of Cr-resistant bacteria for the bioremediation of Cr
(VI)-contaminated soils researchers have isolated a population of P.
mendocina from a sewage sludge and used it for the reduction of Cr (VI)
to Cr (III) in a soil microcosm study. Their results indicate that P.
mendocina was able to immobilize 100 mg kg‑1
Cr (VI) in 8 h by reducing it to Cr (III). The Cr (VI)- contaminated soils,
after the microbiological treatment, supported growth of wheat seedlings
without exerting any toxic effects, illustrating the usefulness of the
microbiological treatment in the bioremediation of chromate-contaminated
sites.
Bioaccumulation and bio-sorption:
Many microbes
by cellular activities and/or their products significantly contribute in
these biogeochemical cycles. Biotechnological approaches to the abatement of
toxic metal pollution consist of selectively using and enhancing these
natural processes to treat particular wastes. The processes by which the
microorganisms interact with the toxic metals enabling their removal/and
recovery are bio-sorption, bioaccumulation and enzymatic reduction.
Phytoremediation:
Phytoremediation of Cr pollution can be
achieved by extraction of the metal from polluted soils into harvestable
plant tissues (phytoextraction), by the accumulation of the element in the
root tissues of aquatic plants growing in contaminated water (rhizofiltration),
or by the in situ detoxification of the metal through plant-based
chelation, reduction, and oxidation mechanisms (phytodetoxification).
Phytoextraction:
Research on
the phytoextraction of Cr from contaminated soils and sediments has been
scarce. Very few plant species such as Sutera fodina, Dicoma
niccolifera and Leptospermum scoparium have been reported to
accumulate Cr to high concentrations in their tissues. Attempts are being
made to use promising aquatic plant species for the phytoextraction of Cr
from contaminated tannery sludge, the ability of three plant species (Scirpus
lacustris, Phragmites karka and Bacopa monnieri) to
absorb, translocate and concentrate Cr in their tissues. For the
phytoextraction process to be effective substantial amounts of Cr removed
from the root medium must be translocated to the harvestable plant parts so
that it can be completely removed from the contaminated site. Clearly, more
research needs to be done in this area to utilize available Cr
hyper-accumulator plant species.
Molecular
approaches in remediation of chromium:
Another
approach is to use molecular techniques to genetically engineer plants that
can hyperaccumulate Cr and other heavy metals. In a recent study it was
found that there is a high correlation in Cr content in shoots of many plant
species with the content of other heavy metals such as cadmium, copper,
nickel and zinc. Traits impacting the accumulation of these heavy metals in
plant shoots were found to be associated. Using recent molecular approaches,
it will be possible to evolve plants suitable for phytoremediation of soils
and waters contaminated with Cr and/or other heavy metals.
Conclusion:
Thus chromium
bioremediation through micro-organism or plants may be the best-suited
technology in present context to clean up Cr contaminated sites and these
technologies are eco-friendly and cost effective. Cr (VI) is readily
immobilized in soils by adsorption, reduction, and precipitation processes,
with only a fraction of the total Cr concentrations available for plant
uptake. When taken up by plants, >99% of the absorbed Cr is retained
in the roots where it is reduced to Cr (III) species in a short time.
Phytotoxic levels of Cr in most plants seem to limit its accumulation in the
food chain. Because most plants have low Cr concentrations, even when grown
on Cr rich soils, the food chain is well protected against Cr toxicity. In
regions, where Cr (VI) contamination of the environment represent a major
area of concern, the use of Cr-hyper-accumulator plant species or
Cr-reducing microorganisms may represent a cost efficient and highly
effective technology for the removal and detoxification of the toxic forms
of Cr.
The
authors Sumit Yadav, O.P. Shukla and U.N. Rai, are associated with the
Ecotoxicology and Bioremediation Group of the National Botanical Research
Institute, Lucknow -226 001, India |
