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Bioremediation:
Ecotechnology for the Present Century
Anil K. Gupta1,
Mohammad Yunus2
and Pramod K. Pandey3
Bioremediation is one of
nature's prudent ways to purify the polluted environment and that degraded by
the anthropogenic activities. Although the term 'bioremediation' may be recent,
but the process in not new as its origin relates to the origin of life when the
first organism was stressed by certain compounds and it evolved the process to
convert such compounds in less harmful forms by adopting certain detoxifying
mechanisms in order to overcome the stress. The present day bioremediation
technologies are based on the processes and potentials of almost all types of
life forms, viz., plants (phytoremediation), microorganisms (microbial
remediation) and animals (zooremediation). The approach under demand and
discussion now can be termed as 'ecoremediation' to express the wider range of
objectives and scope. However, for optimizing the maximal benefit
sustainability of such technologies, requires an effective policy directives
supported by social perception and must necessarily incorporate the application
of environmental impact assessments, intellectual property rights and
cost-benefit analysis for commercial viability.
PHYTOREMEDIATION
Phytoremediation is a
cost-effective, simple and sustainable beneficiary technique to remove
pollutants from the environmental components - air, water or soil, using
plants. Phytoremdiation targets include contaminant metals, metalloids,
petroleum hydrocarbons, pesticides, explosive or toxic gases, chlorinated
solvents and a range of industrial by-products. Commercially viable
phytoremediation systems for clean-up of shallow aquifers and water-borne
contaminant are now well in practice. Phytoremediation may be defined as 'the
process of restoration of the quality of environment by the application of
plants'. It is the use of 'green plant based systems' to remediate contaminated
soils, sediments, water or air. Sometimes it is also referred as 'greenclean'.
Certain techniques that end with the volatilization are used for the
remediation of volatile forms of contaminants. These techniques are
collectively called ` as 'phytovolatilization'. Recently, certain transgenic
plants have been shown to reduce mercury from the more hazardous ionic and
methylated forms to elemental mercury that is later volatilized. Similar
technique is also applicable for selenium. 'Under the scope of 'phytostabilization',
agronomic techniques are used to stabilize contaminated sites in-situ. It
mainly serves to check the spread of contaminant in the environment to reduce
further damage.
'Phytoextraction'
involves the use of plants to extract contaminants from the environment. During
this process, metals are transported from media to the plant parts by
absorption through roots. It can be used for rernediation targeting at lead,
radionuclides with chromium, arsenic and mercury. A technique for extraction of
precious metals with high economic importance such as nickel, copper, etc., is
evolved and called 'phytomining'. Plants selected for bioremediaiton purpose
are expected to possess one or more of the following properties:
-
Taking up metals from
soil particles and/or soil solution into their roots.
-
Binding the metals into
their root tissue (physically or chemically).
-
Transporting the metals
from roots into the growing shoots.
-
Preventing or
inhibiting the metals from leaching out of the soil.
Such plants must not only
accumulate metals but should also be fast growing in different conditions and
lend themselves to easy harvesting. The binding of metals in plant tissue
involves certain proteins known as metallothionins. Metal hyperaccumulating
species adopt different strategies to avoid metal toxicity, the most important
being the vacuolar accumulation of the heavy metal. Amongst various
endogenously produced molecules that help in this process are the
phytochelatins known for sequestering heavy metals in the vacuoles. Members of
the family Brassicaceae have shown tremendous potential for cleaning up of
polluting metals. The pine tree has been noted for absorbing Beryllium whereas
Acedium
plants were demonstrated for vanadium accumulation.
Plants that can trap or
filter out the dust content or can absorb the gaseous pollutants of the
atmosphere to detoxify them or precipitate the metals are used for the purpose
of atmospheric phytoremediation or air pollution attenuation. For application
to dust attenuation, plants are expected to possess the following
characteristic features:
-
Dense canopy- compact
branching, closely arranged leaves.
-
Broad leaves.
-
Leaves with rough and
hairy surface.
-
Leaves with wax and
other sticky material deposition.
The biochemical features
of the resistant plants are mainly based on following terms:
1.�����
Intercellular pH and buffering capacity (provided by inorganic salts, organic
phosphates, proteins, amino acids like histidine, cysteine, polyamines, etc.).
2.�����
Tolerance
of enzymes mainly comprised of enzymes of antioxidant system.
3.�����
Metabolic
detoxification mechanism.
4.�����
Recovering
ability.
Thus, plants on the basis
of their atmospheric phytoremdiation capacity can be grouped in three
categories:
-
Less absorption + strong tolerance = strongest resistance.
-
Less absorption + weak tolerance = fair resistance.
-
More absorption + strong tolerance = ideal plants for use as mitigator
species.
Floating macrophytes are
well suited to treat biodegradable wastes with even biological oxygen demand of
the order of 200 mg/I. However it doesn't survive the metal-bearing
non-biodegradable wastes for long.
MICROBIAL BIOREMEDIATION
Microbial bioremediation
is defined as the process by which microorganisms are stimulated to rapidly
degrade hazardous organic contaminants to environmentally safe levels in soils,
subsurface materials, water, sludge, and residues. Microbes deal with poisonous
chemicals by applying enzymes to convert one chemical into another form and
taking energy or utilizable matter from this process. The chemical
transformations generally involve breaking of large molecules into several
small molecules in simpler form. In some cases the by-products of bacterial
bioremediation are not only harmless but may prove useful. For example, methane
is derived by the bacterial degradation of sulphite liquor, a waste product in
paper manufacturing.
Supported with sufficient
nutrients and a terminal electron acceptor for metabolism, almost all the
natural organics are biodegradable within a range of extent, but even simpler
organic compounds fail to favor any microbial activity owing to unfavourable
conditions like, extremes in temperature or pH, presence of toxicants or
antimicrobial agents, lack of water, nutrient scarcity, absence of electron
acceptor, etc.
Thus, the ultimate goal
of bioremediation is conversion of undesirable organic compounds into innocuous
materials, usually carbon dioxide, water, inorganic salts, and biomass.
However, when biodegradation of compounds is incomplete, it may lead to
accumulation of undesirable byproducts. Microbial bioremediation is
particularly useful for treatment of wastes and wastewater from municipal
areas, food processing, agriculture sector, and more recently the hazardous
wastes from various origins.
Bioremediation of
xenobiotics�
The term 'xenobiotics' is
derived from the Greek word 'Xenon' that means 'a strange', and thus, denoted a
material that is not naturally occurring in the biosphere. Major sources of
xenobiotic compounds are agrochemical and petrochemical compounds. Aromatic
compounds and pesticides are the most significant among them. In the
bioremeditation of xenobiotic compounds, the microbial dehalogenation is an
important process as a large number of such compounds contain halogen group in
their structure. Dehalogenation also facilitates further degradation of the
compound due to cleavage of the carbon-halogen bond under the catalytic action
of dehalogenases enzyme. It is important to mention that many oxygenases act as
dehalogenases by actively catalyzing the dehalogenation step.
Lignases, a group of
extracellular enzymes, include lignin peroxidase, arylmethoxy demethylase and
phenol oxidase, well studied in white rot fungi Phanerochaete chrysosporium.
Such enzymes are efficient it scavenging a range of hazardous polyaromatic
hydrocarbons that are structurally diverse and normally considered resistant to
microbial degradation. Detoxification mechanisms for organomercurials have also
been reported well.
Desulphurizafion of
fossil fuels
Inorganic sulphur removal
from coal is achieved through the oxidation of sulphur by microbes and
heterotrophic microorganisms. Direct oxidation of inorganic sulphur by
Thiobacillus
sp. is
membrane bound
reaction and requires direct contact of the substrate with the bacterium.
Hence, attachment of the culture to coal particle is the absolute requirement.
Studies have been undertaken for removal of organic sulphur from coal and oil
using a variety of organisms in mixed and pure cultures of heterotrophic
bacteria. However, sulphur removal has also been reported under anaerobic
microbial action.
Precipitation or
biotransformation of metals and radionuclides
Microbial activity can
biologically mediate the precipitation of metals from aqueous solutions.
Certain bacterial extracellular products may interact with free or sorbed metal
cations forming insoluble metal precipitates. The major mechanism for such
precipitation is through the formation of hydrogen sulphide and the
immobilization of metal cations as metal sulphides. Certain fungi
that produce oxalic acid facilitate immobilization of metals
as metal oxalate crystals. Microbes can also catalyze a range of metal
transformations and are useful for waste treatment. The transformations include
oxidation, reduction and alkylation reactions. Ferrous and manganese ions can
be deposited by bacteria, fungi, algae or protozoa, in the oxidation reactions.
Bioremediation of the
indstrial effluents
Microorganisms like bacteria, protozoan, blue-green algae, are the main drivers
of the biological treatment processes for industrial effluents and sewage
streams. Two principal types of ,biological treatment are:
(1) Percolating filter, also referred as trickling filter or biological filter,
and (2) activated sludge processes. In the trickling filter process, the
settled sewage or wastewater flows through interstices of the medium having a
large surface area prone to establishment of microbial film. This gelatinous
film contains bacteria, fungi, protozoa, and on the upper surface algae causing
oxidation of biological oxygen demand in the sewage. The success of the
activated sludge process is dependent on the ability of the microorganisms to
form aggregates that can settle. Among the heterogeneous population, bacteria
alone are responsible for removing the dissolved organic matter, whilst others
play 'grazing' role for removing frees-wimming bacteria and help in
sedimentation. Advanced biological reactors have been developed in recent
decades for efficient removal of nitrogen and volatile components including
phenols from waste streams.
Composting
The concept
of composting, originally applicable for organic waste conversion into mulch
and soil conditioner, is now being applied to the hazardous waste treatment.
Composting requires that the material be biodegradable, with an adequate water
content, sufficient quantity of porous substrate to retain heat and to allow
gas exchange. For substrates that contain little nitrogen, nitrogen additions
may be required. Composting leads to three prominent results, i.e., (a) waste
is converted to less complex and relatively more stable material, (b) water
content decreases significantly, and (c) reduction of mass of the residue.
Materials that are amenable to the bioremediation composting.
process include sewage sludge, soils contaminated with diesel fuel or other
petro-products, wastes from brewing, antibiotic fermentations, food processing,
mineral oil, agriculture, etc. Bulking agents, for e.g., fibrous plant
material, wood chips, bark, are. added to increase the porosity so as to help
aeration.
Bioaugmentation
Bioremediation operations may be made either on-site or off-site, in-situ
or ex-situ. Bioaugmentation is the enhancement of decontamination in
the media or waste-biodegradation by seeding of competent microflora and
supplemented with desired level of nutrients. Faster decontamination has been
achieved successfully by stimulation of existing microbial populations or
augmentation with adapted strains. Thus, augmentation refers to establishing
suitable conditions for bioremediation by means of adding nutrients for growth
promotion, addition of terminal electron acceptor (oxygen or nitrate), moisture
level adjustment or raising the temperature.
ZOOREMIDIATION
When the
decontamination of environment or waste treatment is performed through the
activities of animals, the process is known as Zooremediation. Animals used for
this purpose include the range of different arthropods, fishes, other filter
feeders in aquatic systems, anu earthworms in solid organic waste management
systems. Many aquatic animals have been successfully demonstrated for water
pollution treatment, but it has not been encouraged owing to significant
ecological safety reasons. However, earthworm based technology has proved
commercial potentials as the role of earthworms in the conversion of organic
materials and improvement of soil has been observed and appreciated, called as
vermicomposting.
EPILOGUE
The recent
interdisciplinary approach of environmental problem solving through combination
of biotechnology, microbiology, genetic engineering on the sphere of ecological
practices has given rise to promising research and application of
bioremediation tools. As these technologies are quite safe on ecological and
health aspects due to least application of chemical compounds, these are
projected as the ecotechnologies for the present century. However, in case of
newer technologies in general and in particular the cases of genetically
modified organisms, adequate ecological risk analysis and environmental impact
assessment must be a prerequisite before launching the commercial application.
Promotion and appropriate implementation of bioremediation technologies in
industrial, urban and commercial sectors needs a well-defined policy directive
at national level so as to guide the stakeholders and to ensure the responsible
care for environment.
1Head,
Institute of Environmental Sciences;
3Head,
Deptt. of Biotechnology, Bundelkhand
University, Jhansi, India.
2Dean,
School of Environmental Sciences, B.B. Ambedkar University, Lucknow, India |
