BIOREMEDIATION

Bioremediation

Bioremediation refers to the process of using microorganisms to remove the environmental pollutants i.e. the toxic wastes found in soil, water, air etc. Microbes serve as scavengers in bioremediation for environmental clean-up of wastes.Bioremediation basically involves the conversion of complex organic molecules to simpler (and mostly non-toxic) ones. Other names used for bioremediation are biodegradation, bio-treatment, bio-reclamation and bio-restoration.

Xenobiotic

Most of the reactions of bioremediation involve Xenobiotic.Xenos means foreign. Xenobiotic broadly refer to the unnatural, foreign and synthetic chemicals such as pesticides, herbicides, refrigerants, solvents and other organic compounds.  Microbial degradation of xenobiotic assumes significance, since it provides an effective and economic means of disposing of toxic chemicals, particularly the environmental pollutants.

Pseudomonas: The Predominant Microorganism for Bioremediation

The microorganism Pseudomonas occupies a special place in bioremediation. Members of the genus Pseudomonas (a soil microorganism) are the most predominant microorganisms that degrade xenobiotic.  About 40-50 microbial strains of micro­organisms, capable of degrading xenobiotic have been isolated.  Different strains of Pseudomonas are capable of detoxifying more than 100 organic compounds, have been identified.  The examples of organic compounds are several hydrocarbons, phenols, organophosphates, polychlorinated biphenyls (PCBs) and polycyclic aromatics and naphthalene.

Examples of microorganisms used in bioremediation

Pseudomonas species: aliphatic and aromatic hydrocarbons, naphthalene, xylene etc. Mycobacterium species: benzene, branched hydrocarbons. Candida species: polychlorinated biphenyls. Streptomyces: halogenated hydrocarbons

Factors Affecting Bioremediation

Several factors influence the process of bioremediation/biodegradation, Chemical nature of the xenobiotic, Capability of the individual microorganism, Nutrient supply, Oxygen supply, Temperature, PH and Redox potential

Consortia of microorganisms for Biodegradation

A particular strain of microorganism may degrade one or more compounds. Sometimes, for the degradation of a single compound, the synergetic action of a few microorganisms (i.e. a consortium or cocktail of microbes) may be more efficient. For instance, parathion (cholinesterase inhibitor used as an insecticide) is more efficiently degraded by the combined action of Pseudomonas aeruginosa and Pseudomonas stulzeri.

Recalcitrant Xenobiotic

There are certain compounds that do not easily undergo biodegradation and therefore persist in the environment for a long period (sometimes in years). They are labelled as recalcitrant. There may be several reasons for the resistance of xenobiotic to microbial degradation. They may chemically inert (highly stable). Lack of enzyme system in the microorganisms for biodegradation. They cannot enter the microorganisms being large molecules. The compounds may be highly toxic or result in the formation highly toxic products that kill microorganisms.

There are a large number of recalcitrant xenobiotic compounds e.g. chloroform, insecticides (DDT, lindane), herbicides (dalapon) and synthetic polymers (polystyrene, polyethylene, polyvinyl chlorine).

It takes about 4-5 years for the degradation of DDT (75-100%) in the soil. A group of microorganisms (Aspergillus flavus, Mucor aternans, Fusarium oxysporum and Trichoderma viride) are associated with the slow biodegradation of DDT.

Types of Bioremediation

The environmental clean-up process through bioremediation can be achieved in two ways:

• In Situ Bioremediation
• Ex Situ Bioremediation

In situ bioremediation

In situ bioremediation involves a direct approach for the microbial degradation of xenobiotics at the sites of pollution (soil, ground water). Addition of adequate quantities of nutrients at the sites promotes microbial growth. When these microorganisms are exposed to xenobiotics (pollutants), they develop metabolic ability to degrade them. The growth of the microorganisms and their ability to bring out biodegradation are dependent on the supply of essential nutrients (nitrogen, phosphorus etc.). In situ bioremediation has been successfully applied for clean-up of oil spillages, beaches etc.

Ex Situ Bioremediation

The waste or toxic materials can be collected from the polluted sites and the bioremediation with the requisite microorganisms (frequently a consortium of organisms) can be carried out at designed places. This process is certainly an improvement over in situ bioremediation, and has been successfully used at some places.

Advantages of in situ bioremediation:

Cost-effective, with minimal exposure to public or site personnel. Sites of bioremediation remain minimally disrupted.

Disadvantages of in situ bioremediation:

Very time consuming process. Sites are directly exposed to environmental factors (temperature, Oxygen supply etc.). Microbial degrading ability varies seasonally

Advantages of ex situ bioremediation:

Better controlled and more efficient process.Process can be improved by enrichment with desired microorganisms. Time required is short.

Disadvantages of ex situ bioremediation:

Very costly process. Sites of pollution are highly disturbed.  There may be disposal problem after the process is complete.

Types of Reactions in Bioremediation

Microbial degradation of organic compounds primarily involves:

• Aerobic
• Anaerobic
• Sequential

Aerobic bioremediation

Aerobic bioremediation involves the utilization of Oxygen for the oxidation of organic compounds. The compounds may serve as substrates for the supply of carbon and energy to the microorganisms. Two types of enzymes namely mono-oxygenases and di-oxygenases are involved in aerobic biodegradation.  An oxygenates is any enzyme that oxidizes a substrate by transferring the oxygen from molecular oxygen O2 (as in air) to it.

Anaerobic bioremediation

Anaerobic bioremediation does not require Oxygen supply. The growth of anaerobic microorganisms is slow and so is the degradation processes. Anaerobic bioremediation is cost-effective, since the need for continuous Oxygen supply is not there.

Sequential Bioremediation

In the degradation of several xenobiotic, both aerobic and anaerobic processes are involved. This is often an effective way of reducing the toxicity of a pollutant. For instance, tetra chloromethane and tetrachloroethane undergo sequential degradation. Chloromethane can be biodegraded by reductive dechlorination under anaerobic conditions as well as by oxidation under aerobic conditions. The tendency of chloroethenes to undergo reductive DE chlorination decreases with a decreasing number of chlorine substituents, whereas with less chlorine substituents chloroethenes more easily undergo oxidative degradation.