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Vol. 9 No. 2 - April 2003

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


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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