After reading this article you will learn about:- 1. Atmospheric Remediation 2. Phytoremediation of the Soil 3. Microbial Degradation of the Wastes 4. Antibody Based Bioremediation 5. Conclusion and Future Prospects of Bioremediation.
Atmospheric Remediation:
The green plants have always attracted the attention of human beings, specially for their utility and aesthetic values. In fact they have been considered to be essential for the very existence of life on this planet as they provided food directly or indirectly for all life forms. Their air purifying quality has also been realized ever since it was known that in daytime, they absorb CO2 and emit O2.
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The sink potential of green plants for CO2 has been often exploited for reducing the concentration of this waste gas in the indoor atmosphere. The volatile emissions from flowers, fruits and leaves of many plant species creating a fragrant environment around them has also fancied human beings since ages. In late 1960s and early 70s several investigators performed rigorous experiments to demonstrate that green plants could absorb significant amounts of air pollutants such as SO2, O3, PAN, NO2, H2S etc. through their leaves.
Based on experimental data from 10 plant species it was estimated that the rate of absorption of NO2 from an atmosphere of 0.1 ppm NO2 was 13 µg NO2 dm2 h-1, which was sufficient for satisfying about 23% of the total nitrogen requirement of plants on global level.
The plant species vary in their toxic responses to the air pollutants and quite a few species are visibly tolerant to normal levels of atmospheric pollutants. However, episodic release of very high concentrations of pollutants in some localities cause severe damages to the vegetation.
It is also important to note that sulfurous and nitrogenous pollutants enrich plants with the essential nutrients S and N, and in conditions of soil nutrient deficiency, the pollutants specially the nitrogenous gases may act as an alternate source of nutrient. Realizing the sink potential of green vegetation, the city architects and planners always advocate for green spaces and green belts in and around cities.
The plants can also be utilized for monitoring the air pollutants in urban and sub-urban localities. The sensitive species of plants develop identifiable symptoms of injury, specially on their leaves even during short exposure to pollutants. These species can be used as indicator species for determining the presence, intensity and distribution of air pollutants. Some common indicator species are shown in Table 1.
In a few studies, pigeons have been used for bio monitoring Pb concentration in the ambient atmosphere, specially in the urban atmosphere. Pigeons have a small habitat, a small body size, a high rate of metabolism and a high inhalation rate and therefore can be used for monitoring air pollution.
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In a study by Schildermann (1997), it was found that the pigeons inhabiting the high traffic density area of Amsterdam had higher level of Pb, in blood and in lung and liver tissues than those from low traffic density. Similar studies in other cities also demonstrated a correlation between body Pb content and the vehicular traffic density.
Phytoremediation of the Soil:
Plant growth has profound effect on physical, chemical and biological properties of the soil in its immediate vicinity. Sometimes these changes may bring about substantial changes in the life supporting activities of the soil.
Some specific types of plants may be raised in otherwise barren soil and over the years the quality of soil may change and other plant species may be grown. All kinds of contaminants, organic, inorganic and heavy metals may be removed from the soil by growing suitable plant species.
1. Removal of Organic Contaminants:
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The organic contaminants in the soil include industrial and urban effluents and the agro-chemicals such as insecticides, pesticides, etc. If these contaminants are in biologically accessible forms, they can be removed from the soil by the use of specific plant species. These plant species must have a high capacity for absorption and retention and should also have a high biomass production rate.
In a patented process of soil remediation, carrots are used for removing insecticide residues dichlorodiphenyl- trichloroethane from the contaminated soils. The carrot roots absorb this chemical from the soil in large amounts. These roots are harvested, solar dried and then incinerated to destroy the chemical.
2. Removal of Heavy Metals:
Soil contamination with heavy metals has become a world-wide problem, leading to losses in agricultural yield and hazardous health effects as they reach the food chain. Heavy metals, unlike organic pollutants, cannot be chemically degraded or biodegraded by the micro-organisms in the soil. Both essential as well as non-essential heavy metals can accumulate in soils partly in the form of agro-chemical derived waste and mostly in the form of urban and industrial effluents.
While essential heavy metals such as Ni, Cu, Zn, Mn and B, are utilized by the plants for their metabolic activities, non-essential heavy metals Cd, Hg and Pb, have no role in plants or for that matter in other life forms and thus they are harmful in even smaller quantities to all life forms. Cadmium is a toxic metal that can accumulate in the human body with a half-life exceeding 10 years.
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Cd accumulation in soil now poses a major environmental and human health problem and an effective solution of the problem is to be found. Use of plants and also of bacteria, algae and fungi to remove heavy metals and other contaminants from polluted soils and water, known as phytoremediation is a biological approach in cleaning the environment.
The heavy metals accumulated in the plant biomass can then be extracted out and removed from the environment. Compared with the extraction and reburial of heavy metal contaminated soil, the removal of heavy metal by phytoremediation is about 1000 fold less expensive.
It is particularly suitable when the concentration of the heavy metal in soil is low. For this heavy metal tolerant plants are to be used, which can accumulate large quantities of heavy metals without any significant adverse effect of the heavy metal on their growth and physiology.
These are known as hyper accumulators of the metal, accumulating more than 1000 p.g heavy metal per g of dry weight in their aerial parts. About 400 taxa of terrestrial plants have been identified as hyper accumulators of various heavy metals, about 300 of which are Ni accumulators and about 16 species are Zn accumulators.
These species are usually endemic to metalliferous soils. One of the common plant species is the Indian mustard (Brassica juncea) which is a high bio-mass crop plant and is able to accumulate Cd up to 1100 µg/g dry mass in the shoots and 6700 µg/g in the roots. Examples of some hyper absorbers of heavy metals are given in table 2.
Another approach in the phytoremediation of soil for heavy metals has been the increased uptake and volatilization of the heavy metal by the plant. Volatilization of Se by several plant species has been observed by Banuelos (1993) and by Zayed and Terry (1994).
These species acquire Se from the soil even if the concentration of the element is low and then release it in the atmosphere in the gaseous form. This property has been exploited for the removal of Hg from the soil. Transgenic Arabidopsis thaliana plants containing mercury reductase gene have been created, which are capable of absorbing and volatilizing Hg from the soil.
The heavy metal tolerant plants have developed different types of strategies to tolerate high concentrations of heavy metals. The most important of course is the induced synthesis of phytochelatins which can bind to heavy metals and sequester them in the vacuoles.
Phytochelatins are small polypeptides, made up of amino acids, glutamate and glycine. They have a structure (y-glutamyl-cysteinyl) n-glycine, where n is 2 to 11. The induction of phytochelatin synthesis in response to heavy metal such as by Cd, Cu, Zn, Pb, Hg, Ni, Bi, Ag and Au and also by the multiatomic anions SeO4 SeO32- and AsO43-has been demonstrated in quite a few investigations.
In mustard, which is a hyper accumulator of Cd, most of the heavy metal is accumulated in the trichomes on the leaf surface. It is to be noted that trichomes are the site of accumulation of Mn and Pb also in bean plants, and the metal binding protein gene is localized in the trichomes. Thus, the presence of phytochelatin has a direct link with the accumulation of heavy metals.
Removal of Heavy Metals from Effluents:
Both domestic and industrial effluents often contain heavy metals which contaminant water reservoirs to an extent that they become unfit for consumption as well as for irrigation purposes. The most efficient and inexpensive method of the removal of heavy metals and other contaminants from the effluents of industries is the construction of wetlands around the discharge site.
The plants to be grown in these wetlands may be carefully selected, which will have a significant potential for the absorption and accumulation of contaminants. For example, a wetland vegetated mainly with Scirpus robustus, Polypogon monspeliens and Typha latifolia is very efficient in treating oil refinery effluent, removing about 90% of the selenium existing in the wetland.
The physicochemical and the biological properties of the wetland may be suitably modified to increase the metal removal capability of the plants inhabiting the wetland. In a recent study, it has been demonstrated by de Souza (1999) that rhizoshere bacteria can increase the efficiency of selenium and mercury phytoremediation by promoting the accumulation of these metals in the tissues of Scirpus robustus and Polypogon monspeliensis.
Removal of Heavy Metals through Biosorption:
Biosorption method of removal of heavy metals from effluents is a new but a cost effective method of bioremediation. In this method, dead biomass from the naturally abundant and/or waste biomass of algae, moss, fungi or bacteria is used to sequester toxic heavy metals for the removal of contaminants from the industrial effluents.
Although many biological materials bind heavy metals, only those having selectivity for the heavy metals and having a high absorption potential are used in bio sorption process. Several proprietary bio sorption processes such as AlgaSORB and AMT-Bioclaim using plant biomass were developed in the early 1990’s. For preparing the bio sorbents, the dead biomass is usually pretreated by washing with acid and/or bases before drying and granulation.
The granular material either as such or after immobilization with some support material such as silica is packed up in sorption columns. The metal bearing effluent is then passed through the sorption column and the heavy metals are taken up by the sorption material in the column.
The sorption material can be regenerated after washing with the acid and/or base (Fig. 1). The mechanism of the retention of heavy metals by the sorption material, which varies according to the material (Table 3), is apparently through cation exchange process.
Bio sorption method has following advantages over conventional methods of removing toxic metals from industrial effluents:
1. It is a cheaper method.
2. It has a high detoxifying efficiency in dilute effluents.
3. There is no nutrient requirement for obtaining the biomass.
4. Smaller volumes of effluent can be used.
Microbial Degradation of the Wastes:
Microorganisms in the environment are known to decompose the dead and decaying matter and convert it in to simpler organic molecules and gases. This property of the micro-organisms is now being utilized for managing the solid and liquid dissolved wastes. Composting of agricultural, municipal and industrial wastes is an easy method of producing products from the wastes, which are valuable for crop plants. Limited use of bacteria, microalgae, fungi, yeasts and plants have been made for degrading waste products.
The micro-organisms utilize C and N in the waste to synthesize their cellular constituents. It has been shown that the bio-transformation of waste paper to ethanol by recombinant bacteria is cost effective compared with the conventional process using yeast and added enzymes.
Degradation of oil spills and other hydrocarbons in the ocean and other water bodies and also in the soil can be safely achieved through the stimulation of micro-organisms and also through the introduction of more efficient and vigorously growing bacteria and other microorganisms in that environment.
Some highly efficient hydrocarbon degrading bacteria are Xanthobacter autotrophicus, Pseudomonas aeruginosa, P. stutzeri and Acinetobacter sp. The growth of these bacteria in hydrocarbon rich environment is stimulated by the addition of ammonium sulphate, which provides nutrient N and S.
Antibody Based Bioremediation:
Antibodies are glycoproteins of the immunoglobin family, which are capable of binding non-covalently and reversibly with a specific antigen. They are produced in higher animals by the cells of the lymphoid series. Genetically transformed plants can also be induced to synthesize specific antibodies.
Antibody and active fragments of antibodies can also be used for the sensitive detection and efficient removal of organic pollutants from the environment (Harris, 1999). Antibodies can bind specifically with their antigen and the antigen can be separated as antigen antibody complex from the mixture of the pollutants.
In recent years, it has been shown that active fragments of antibodies from a single antibody can also be produced. This provides an opportunity to increase the remediation efficiency of the antibody. Genetically engineered antibodies can be produced in a variety of cultured cells and organisms. But cost wise the most useful organism for producing antibodies will be transgenic plants or viral vectors (Table 4).
In a typical bioremediation procedure, the antibodies can be taken as a column and the water contaminated with organic substances (say pesticide) are poured through the column. The column can retain the specific antigen and let other components of water flow down. For example, atrazine pollution can be remedied by passing polluted water through a column of immobilized anti- atrazine single chain antibodies.
Once the column is saturated with atrazine, it can be recycled by eluting the atrazine with high salt solution, the pollutant then being recovered in a concentrated form for disposal or even for reuse as herbicide. It is possible that a single column could be recycled at least 100 times.
Conclusion and Future Prospects of Bioremediation:
Bioremediation through the use of plants and micro-organisms is undoubtedly an inexpensive and attractive methodology of cleaning the environment, specially because it involves familiar process of plant and microbial cultivation. The potential of such a technology at large scale has to be realized and improved.
The plants to be selected for phytoremediation should have some specific properties such as extensive root growth with in depth soil penetration and an elevated potential for absorption, retention and tolerance of the toxic contaminants. It should also have readily harvestable high shoot biomass.
The plant breeders have been employing traditional breeding techniques to produce plants with desired characteristics. However, in most cases the characteristics selected were either incompletely expressed or it was reverted back after a few generations. Now, it is possible to isolate or to synthesize a gene with specific characteristics and to produce a genetically transformed organism (transgenic) with the introduction and expression of the specific gene.
This technology of genetic engineering is certainly going to play the most important role in bioremediation of the environment, in years to come. Analysis of genetic mutants may also help in understanding the process and in selecting suitable plants and micro-organisms to be used in bioremediation.