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.
GEMs and Environmental Safety
The risks and health
hazards associated with the use of GEMs are highly controversial and debatable issues.
The new organism (GEM), once it enters the environment, may disturb the
ecological balance and cause harm to the habitat. Some of the GEMs may turn
virulent and become genetic bombs, causing great harm to humankind. Because of the risks involved in the use of GEMs, so far,
no GEM has been allowed to enter the environmental fields. Thus, the use of
GEMs has been confined to the laboratories, and fully controlled processes of
biodegradation (usually employing bioreactors).
About the Author: Ihsan
Ur Rahman is a student of BS Environmental Science at Abdul Wali Khan
University, Mardan. He is one among the founder of 3 Stars Youth Council. He is
also an environmental and social activist and a debater.
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