Page 197 - Soil Degradation, Conservation and Remediation
P. 197

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 ).
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