Industrialization and increased use of chemicals in manufacturing processes have resulted in the generation of a large amount of chemical wastes. These waste products are most often being dumped on land and in water bodies. This continued dumping has definitely speeded up the process of land contamination to such an extent that remediation is not a choice anymore but in fact, a necessity.
The most prevalent of these industrial pollutants are hydrocarbons, poly chlorinated biphenyls, ammonia, metals like lead, mercury, chromium, chlorinated solvents, halogenated organic solvents, non-chlorinated pesticides and herbicides. These substances almost always have been identified to have toxic properties (xenobiotics); their effects may range from slightly harmful to potentially dangerous to the health of humans and animals.
Recent successes in the field of bioremediation, with extensive use of microorganisms are considerable cause for relief. Certain pollutants like hydrocarbons have been degraded successfully and the soil thus effectively detoxified.
Bioremediation is the process of harnessing the natural occurrence that depends on the microbes to go about their regular life processes. These processes include metabolic pathways that utilize these toxins as a source of energy and convert them into non-toxic substances like ethylene and chloride.
This kind of clean up can be done ex-situ or in-situ. In-situ processes involve the degradation of the xenobiotics at the site of pollution itself. As long as the essential nutrients are made available, they will be able to degrade the xenobiotics. An example is the cleanup of oil spills in oceans. Ex situ bioremediation involves the transportation of polluted material, in this case, soil, from the site to the laboratory for detoxification processes. However, this can prove costly as it may involve transportation of large amounts of the waste. It may also disturb the site.
So, for soil contamination, commonly used remediation methods are in situ remediation, land farming and slurry phase bioreactors. In situ techniques include bioventing and phytoremediation.
Bioventing: This process involves the circulation of air through the soil thus facilitating aerobic degradation. But, this may take a very long time to happen.
Phytoremediation: This is remediation of polluted soils using plants. Some plants have the capacity to reduce and degrade pollutants into harmless substances, near the rhizosphere. This may also take several years.
Landfarming or solid phase soil reactor involves mixing the soil with microbes and nutrients. It is most commonly used for remediation of some aromatic compounds. Slurry phase bioreactors subject the polluted soil to optimal conditions of nutrition, temperature etc. such that the reaction is accelerated.
Bioremediation of polluted soils can be done by three techniques- biostimulation, bioaugmentation and natural attenuation.
Biostimulation: This is the process of stimulating the microorganisms that are already present in the polluted soil. The required stimulation can be achieved by the addition of nutrients like nitrogen or phosphorus which act as electron donors. Supplementing the soil with co-substrates like methane can help degrade some toxic compounds. These methods promote the growth of the natural microbial population present in the soil.
The disadvantage of these methods is that the process of addition of the nutrients can vary depending on the surface topology. The irregularities on the surface may also cause uneven distribution of the added nutrients and may thus lead to uneven growth of microbes on the surface.
Another method is the addition of surfactants to aid the dispersal of the hydrophobic compounds thus exposing the hydrophobicity of toxins present in the soil.
Bioaugmentation:This is the process of addition of specific strains of microorganisms to the affected soil. This method is used in the cases where the pollutant is a complex molecule that may not be broken down by the native population of microbes. Some pesticides, TNT and some complex hydrocarbons may have to be remediated by this method.
The ideal combination of microbial strains that can achieve the best remediation results are referred to a consortia or a cocktail of microbes. These may or may not include genetically modified organisms.
This process is very commonly utilized in sewage treatment plants that have a problematic bioreactor population.
Natural Attenuation: This is the process of bioremediation that occurs naturally without any meddling by humans. Only observation of the natural processes is allowed.
Effect of Microorganisms:
Most remediation processes involve the conversion of the xenobiotic compounds into non-toxic ones, by the process of degradation, thus protecting the environment. But, some microbes work by activating the xenobiotic. This may prove deadly if a non-toxic xenobiotic is converted into a more toxic one. Also, genetically modified organisms have to be used with caution as they are being introduced into the ecosystem (in case of in situ methods).
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