Human being – the most selfish living being on Earth - in the name of development, is known to be spoiling the ecosystem. The spoilage includes excessive use of natural resources, destruction of nature, development of hazardous substances detrimental to the ecosystem, etc. Persistent Organic Pollutants (POPs) are a group of hazardous chemicals that remain intact in the environment for long periods, accumulate in the fatty tissue of living organisms and are toxic to humans and wildlife. POPs circulate globally and can cause damage wherever they travel. The twelve hazardous chemicals: DDT, Aldrin, Dieldrin, Endrin, Chlordane, Heptachlor, Hexachlorobenzene, Mirex, Toxaphene, Polychlorinated Biphenyls (PCBs) , Dioxins, and Furans are grouped under POPs. In order to address the issue of POPs, a Stockholm Convention was organized by UNEP 2001. Its mandate is to protect the human health and environment from POPs. In implementing the Convention, Governments are supposed to take measures to eliminate or reduce the release of POPs into the environment. In this context, the problem of environmental contamination due to PCBs has become a great concern because of accumulation of these highly toxic compounds in the tissues of living beings leading to serious damage to human health.
PCBs are also commonly known as chlorobiphenyls and produced by direct chlorination of biphenyl. There are no known natural sources of PCBs. They were first synthesized in United States in 1929. They are either oily liquids or solids that are colorless to light yellow. They were used as coolants and lubricants in transformers, capacitors, and in other electrical equipment, surface coatings, printing inks, carbonless duplication paper, waxes, etc. During the time of their manufacturing, no effective mechanisms of their disposal were identified and therefore, even now they are found widely distributed in the environment, as they do not easily break down. While industrial use of PCBs has been sharply restricted, significant quantities of PCBs are still being released into the environment from waste dumps and failure of old electrical equipment. PCB contamination has been observed in drinking water sediments, wastewater, foods and aquatic organisms and especially in fish. Some PCBs can exist as a vapor in air. Generally their concentrations in the environment are low but are noticed in increasing quantity higher up in the food chain because their residues continue to accumulate in the tissues of living organisms.
Most importantly, although the use of PCBs have been banned all over the world it is being formed as a byproduct during the manufacture of various types of polymeric products such as adhesive, plastic, polythene, pesticides, rubber, and related polymeric compounds. Besides, they are also being formed during incomplete incineration as well as due to burning of waste materials in the dust bins. Thus it has been observed that several congeners of PCBs are detected in the industrial discharges of various types of industries especially those who are involved in manufacture of polymeric compounds, like, plastic, adhesive, pesticides, rubber etc. as well as in the incomplete waste materials.
Many websites for example :
describe the effect of PCBs on human health and how to protect oneself from this hazardous chemical.
Since large number of hydrogen atoms are present on the biphenyl nucleus, many different chlorinated compounds (termed "congeners" - isomers of different homologous series) are possible. As many as 209 congeners of the PCBs could be theoretically produced. However, due to steric restrictions, only about half of this number actually occur. Based on their structure, PCBs are grouped into :
Coplanars – those compounds having chlorinated substitutents in both para positions and any/all meta positions. There are 20 coplanar PCBs of which three are very toxic and non-orthochlorinated. Depending on the number of Cl atoms on the biphenyl rings, other group of PCBs is divided into mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona- and deca-chlorobiphenyls.
Clophen A-50 is a technical grade chemical compound containing about 40 different congeners of PCBs. It is well known in commercial circles and to people skilled in the art.
Efforts to control the PCBs are on. Natural microbial populations do not easily remove PCBs. However, some PCB congeners are found to be transformed by both anaerobic and aerobic bacteria. The aerobic degradation of PCBs is generally limited to less--chlorinated congeners (five or fewer chlorines per biphenyl molecule) by an enzymatic mechanism involving deoxygenation of the aromatic ring. The more chlorinated congeners are generally recalcitrant to aerobic degradation. Studies on the biodegradability of several isomers of PCBs have found decrease in degradability with the increase in chlorine substitution. An isomer with four Cl is not easily degraded. The position of the chlorine is also important. Ortho positioning of two chlorines on a single ring greatly inhibits degradation.
The scientists at NIO have identified a novel marine bacterium from Arabian Sea designated as Psedumonas CH07 having degrading properties of congeners. The strain is capable of sustaining growth in a medium containing 100-ppm final concentration of Clophen A-50. The scientists have also set a microbial process for the degradation of PCBs present in Clophen A-50 and other non-ortho or mono-ortho chlorinated biphenyls (coplanar), ortho-substituted chlorinated biphenyls and sterically hindered chlorinated biphenyls PCBs present in Clophen A-50 using the novel marine bacterium Psedumonas CH07.
The microorganism was isolated and subjected to intense anthropogenic activity following routine microbiological methods. Many isolates were randomly selected and purified for further studies. The purified isolate was tested for growth on seawater nutrient agar containing Clophen A-50 to a final concentration of 10, 50 and 100 ppm. The bacterium was grown in sizable quantity for identification and experimentation.
To characterize the isolate, several biochemical tests were carried out. Presence of various enzymes viz. lipase, gelatinase, amylase, oxidase, catalase, and urease, decarboxylases (Arginine and Ornithine) was examined. Utilization of gluconate, pyruvate, citrate, and cellobiose, utilization of glucose, sucrose, mannitol, arabinose, rhamnose, nitrate reduction, MR (methyl red), VP (Voges Proskaeur), H 2 S production and oxidation-fermentation was also examined. The isolate was finally identified as Pseudomonas.
In order to test the effect of this microorganism pure solvent of PCBs was essential. Following standard procedures the scientists prepared such PCB solvent samples. These were further subjected to gas chromatographic analyses to identify concentrations of different congeners of PCBs in the samples.
The pure PCB sample and purified isolates of the bacterial strain at hand; the next obvious step was detoxification of the PCBs. The microbial detoxification of PCBs begins with dechlorination. This involves the stepwise removal of Cl atoms and their replacement with hydrogen atoms. However, as described above, most naturally occurring microbially mediated dechlorination process exhibit limited specificity. The dechlorination is generally restricted to removal of para- or meta-chlorines located adjacent to other chlorines, hence residual meta- and/or para-chlorines remain. In natural and genetically non-altered microorganisms the extensive and desirable removal of all meta- and para-chlorines does not occur, and the end products contain 3-; 2,5-; and 2,3,5-chlorophenyl groups or 4-, 2,4-, and 2,4,6,-chlorophenyl groups.
The unique characteristics of Pseudomonas CH07 that the NIO Scientists isolated are noteworthy. This strain exhibited its ability to degrade two of the three coplanar PCB congeners which are the most toxic of all the PCB congeners. The potential of Pseudomonas CH07 to attack chlorobiphenyls having 5 Chlorine atoms is very important in the degradation and detoxification process of both sterically hindered and coplanar classes of PCBs.
The fight with the hazardous substances would continue. This contribution is one step ahead in the research process and making our environment and ecosystems cleaner and better for the generations to come. |
