Page 197 - Soil Degradation, Conservation and Remediation
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186 6 Soil Pollution
soil. These organisms belong to at least 11 different prokaryotic divisions (Glazer
and Nikaido 2007 ). Bioremediation has been used at a number of sites worldwide,
including Europe, with varying degrees of success. Techniques are improving
as greater knowledge and experience are gained, and there is no doubt that biore-
mediation has great potential for dealing with certain types of site contamination.
A list of some contaminants potentially suitable for bioremediation is given below
(Vidali 2001 ):
Chlorinated solvents Trichloroethylene, perchloroethylene
Polychlorinated biphenyls (PCBs) 4-chlorobiphenyl, 4,4-dichlorobiphenyl
Chlorinated phenol Pentachlorophenol
BTEX Benzene, toluene, ethylbenzene, xylene
Polyaromatic hydrocarbons Naphthalene, anthracene, fluorene, pyrene, benzo(a)pyrene
(PAHs)
Pesticides Atrazine, carbaryl, carbofuran, coumaphos, diazinon,
glycophosphate, parathion, prpham,2,4D
Contaminant compounds are transformed by living organisms, individually or in
mixtures, through reactions related to their metabolic processes. For bioremediation
to be effective, microorganisms must enzymatically attack the pollutants and convert
them to harmless products. It can be effective only where environmental conditions
permit microbial growth and activity. The microorganisms employed for biodegrada-
tion of organic pollutants of soil may be classified into the following groups:
1. Aerobic : Efficient aerobic degraders are Pseudomonas , Alcaligenes , Sphingomonas ,
Rhodococcus , and Mycobacterium . They have been used to degrade pesticides and
hydrocarbons, both alkanes and polyaromatic compounds. Many of these bacteria
use the contaminant as the sole source of carbon and energy.
2. Anaerobic : Anaerobic bacteria are less used than aerobic bacteria. They can effectively
be used for bioremediation of polychlorinated biphenyls (PCBs) in river sediments,
dechlorination of the solvent trichloroethylene (TCE), and chloroform.
3. Ligninolytic fungi : Fungi such as the white rot fungus Phanerochaete chrysosporium
have the ability to degrade an extremely diverse range of persistent or toxic organic
pollutants. Common substrates used include straw, sawdust, or corn cobs.
4. Methylotrophs : Aerobic bacteria include those that utilize methane for carbon
and energy. The initial enzyme in the pathway for aerobic degradation, methane
mono-oxygenase, has a broad substrate range and is active against a wide
range of compounds, including the chlorinated aliphatics trichloroethylene and
1,2-dichloroethane.
Degrading microbes and contaminants must be in contact for degradation to be
effective. Since the microbes and the contaminants are not uniformly spread in soil,
achieving an effective contact between the two is difficult. However, some bacteria
are mobile and exhibit a chemotactic response, sensing the contaminant and moving
toward it. Filaments of fungi grow toward the contaminant. It is possible to enhance
the mobilization of the contaminant utilizing some surfactants such as sodium
dodecyl sulfate (Vidali 2001 ).