Trichloroethylene (TCE) belongs to a family of synthetic chlorinated hydrocarbons manufactured as solvents with a greatly reduced potential for fire

Transcription

1 Trichloroethylene (TCE) belongs to a family of synthetic chlorinated hydrocarbons manufactured as solvents with a greatly reduced potential for fire or explosion. These halogenated organics may have served their purposes all too well; the stability and solvent properties of these compounds led to their application in electronic components, and dry cleaning, as fumigants for insect and rodent control, for the cleaning of septic tanks and military hardware, and in the manufacture of plastics (1). Since World War II, TCE has been used as the standard solvent for degreasing metal parts and textiles and demand for TCE may increase with U.S. Environmental protection Agency approval of TCE as an alternative for chlorofluorocarbon 113 and methyl chloroform. TCE is now present in 27.9% of hazardous waste sites in USA (it is one of the 10 most common pollutants detected at such sites), and this volatile organic compound is second only to trihalomethanes (i.e., bromodichloromethane, bromoform, and chloroform) as the most frequently detected compound in municipal ground-water supplies (2). The widespread use of these compounds and concomitant careless handling, storage, and disposal coupled with the chemical stability make TCE and related molecules among the most frequently detected groundwater contaminants in the United States. TCE is a suspected carcinogen and is one of the most commonly detected volatile organic contaminants in soil (3). Ironically, the relatively stable nature of TCE has doubtlessly saved many lives that would have been lost to fire or explosion in industrial accidents, but these same properties have led to widespread groundwater contamination that carries its own consequences for human health. Fortunately, both aerobic, and anaerobic biological processes can degrade TCE in the environment. A variety of aerobic microorganisms have been shown to degrade TCE when grown under conditions in which specific substrates induce relevant degradation activities. These inducers include various aromatic and aliphatic hydrocarbons such as toluene, phenol, isopropylbenzene, propane and methane (4). These are co-metabolized with TCE by the microorganism. Concerns over the presence of TCE in groundwater are heightened by reports that some anaerobic bacteria biotransform TCE to vinyl chloride. The latter compound is well documented to be tumorigenic in mammals (5). In recent years, the estimated annual vinyl chloride production in the world was 27 million tons (6) Due to the established toxicity of this product, the Environmental Protection Agency has a nonenforceable maximum 1

2 contaminant level goal of 0 µg liter -1 for TCE in drinking water supplies. Presently, the enforced maximum contaminant level for TCE is 5 µg liter -1 (7). Chloroform (CF), a colourless sweet-smelling, dense liquid is a trihalomethane and is considered somewhat hazardous. Several million tons are produced annually as a precursor to Teflon and refrigerants, but its use for refrigerant is being phased out. Trichloromethane (chloroform) is a halogenated organic which is common groundwater contaminant. The compound is used as industrial solvent, is volatile, and is transported into the environment by accidental spillage, leaking storage tanks, improper disposal, and landfill leachates (8). CF is a suspected human carcinogen and migrates relatively rapidly (9). Much effort have been made to physicochemically or biologically remove the compound from polluted sites. Although CF has been recognized as a recalcitrant compound (10), it can be aerobically degraded by methanotrophic organisms, ammonia-oxidizing organisms, and a recombinant pseudomonad expressing soluble methane monooxygenase. Anaerobic systems may provide an alternative to aerobic systems for CF degradation. CF can be degraded to CO 2 and dichloromethane by methanogenic enrichment cultures and pure methanogenic cultures and also by nonmethanogenic anaerobic cultures (9). Bacteria which can degrade TCE may also be able to degrade CF (11). TCE and CF were chosen as the target compounds in this study because they are related, differing only in the presence of an additional carbon atom and a carbon-carbon double bond in TCE. H H (a) (b) Fig 1. The chemical structure of (a) trichloroethylene and (b) chloroform. The necessity to clean up recalcitrant organic pollutants introduced by human activities into the environment led to the development of several biological treatments, which are perceived as being more cost effective and environment-friendly than other technologies 2

3 (12). Compared to physicochemical treatment, biological treatments are considered more effective and economical for remediation on a large site polluted with relatively low chloroform concentration. In such a context, the use of microorganisms for degradation of recalcitrant xenobiotics has been suggested as a potential tool to improve or accelerate the remediation of polluted sites. This is called Bioremediation. Micro-organisms are known to use many substrates as their source of energy and play a major role in the natural degradation processes that take place around us. This helps to break down the waste materials and convert it into less toxic forms which will mostly be in sync with the environment. This process is generally termed as biodegradation. In bioremediation process the microbes are deliberately used to clean up the environment and degrade waste or chemicals that are hazardous to the environment and can affect the different life forms in the immediate ecosystem. Usually the exposure of toxic materials in the environment automatically attracts toxic materials degrading microbes to the area and the biodegradation process does occur, but takes a long time to clear off the mess, and by that time it would do extensive damage to the surrounding life forms, including humans who consume the fish from the water bodies polluted by toxic materials. Bioremediation was proposed even earlier to help clean up oil spills, but people didn't have much knowledge about the process at that time, and there was no experimental evidence to prove its quick action and better efficiency. Now we know that there are large number of microbes that can break down oil through a special metabolic pathway to form carbon dioxide and fatty acids as by products. In bioremediation, the natural biodegradation action is speeded up by either introducing exogenous microbial population, or stimulating the indigenous microbial population by providing favourable growth conditions like optimum temperature, nutrients, aeration, etc. The Pseudomonas species is commonly used for oil degradation. The invention of the 'super bug' Pseudomonas putida, a gram negative rod shaped soil bacteria by Ananda Mohan Chakrabarty has the capability of cleaning up about threefourth of the oil pollution in spillage sites. The super bug is a genetically engineered organism whose plasmid contains the genes responsible for oil degradation from four different species of bacteria and is the first living organism to be patented (13). The wide field study showed positive results with the higher rate of toxic materials 3

4 degradation at polluted sites by natural population of microbes. Still, bioremediation is considered as the most reliable and quick way of treating toxic materials. The speedy clean-up can not only save the environment and living organisms, but also have other advantages like reducing economic impacts, handling legal liabilities and settlement of claims. Bacteria can degrade and thereby detoxify a wide range of hazardous synthetic compounds. Our goal is to harness these abilities to remove toxic chemicals from polluted water. A promising method to accomplish this goal lies in the use of immobilized bacterial cells. Immobilized cells have been defined as cells that are entrapped within or associated with an insoluble matrix. Mattiasson (14) discussed six general methods of immobilization: covalent coupling, adsorption, biospecific affinity, entrapment in a threedimensional polymer network, confinement in a liquid-liquid emulsion, and entrapment within a semipermeable membrane. Under many conditions, immobilized cells have advantages over either free cells or immobilized enzymes. By preventing washout, immobilization allows a high cell density to be maintained in a reaction vessel. Catalytic stability can be greater for immobilized cells, and some immobilized microorganisms tolerate higher concentration of toxic compounds than do their nonimmobilized counterparts. One potential disadvantage of immobilization is the increased resistance of substrates and products to diffusion through immobilization matrices. Owing to the low solubility of oxygen in water and the high local cell density, oxygen transfer often becomes the rate limiting factor in the performance of aerobic, immobilized cell system (15). However, proper selection of immobilization technique and supporting materials is needed to minimize the disadvantage of immobilization. One of the most suitable methods for cell immobilization is entrapment in calcium alginate, because this technique is simple, nontoxic and cheap. Beads of calcium alginate are prepared under mild conditions and used extensively for microencapsulating and entrapping cells. Alginate, a natural polymer, has attracted researchers owing to its ease of availability, compatibility with hydrophobic as well as hydrophilic molecules, lack of toxicity, and attractive adhesive and mechanical properties. Alginate gels are biodegradable in physiological conditions, and chemically erasable in basic (ph > 7) 4

5 aqueous environments. Due to the biocompatibility of alginate polymers, they were used for the formation of membranes and thin-films potentially useful in bioseparation and other biorelated applications. Because of alginate ability to be ionically cross-linked with multivalent metal cations entrapping biomolecules into the biopolymer matrix, numerous reports have been published on the encapsulation of proteins/enzymes, DNA, cells and other biomolecular species in alginate gels with the retention of their full biological activity. The alginate gel is most notably used as films or microcapsules that can release components passively or in response to changed environmental conditions, through the controlled degradation of the assembly (16). In work summarized here we isolate, identify and characterize one bacterial strain responsible for the high TCE and CF-degrading activity and its utilization of trichloroethylene and chloroform as the sole carbon source.the degradation of trichloroethylene and chloroform by free cells as compared to immobilized cells in calcium alginate and agar has been investigated and a comparative study of their reusability is also performed in batch and semi continuous degradation. 5

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