Bioremediation means to use a biological remedy to abate or clean up contamination. This makes it different from remedies where contaminated soil or water is removed for chemical treatment or decontamination, incineration, or burial in a landfill. Microbes are often used to remedy environmental problems found in soil, water, and sediments. Plants have also been used to assist bioremediation processes. This is called phytoremediation. Biological processes have been used for some inorganic materials, like metals, to lower radioactivity and to remediate organic contaminants. With metal contamination the usual challenge is to accumulate the metal into harvestable plant parts, which must then be disposed of in a hazardous waste landfill before or after incineration to reduce the plant to ash. Two exceptions are mercury and selenium, which can be released as volatile elements directly from plants to atmosphere. The concept and practice of using plants and microorganisms to remediate contaminated soil have developed over the past thirty years.
The idea of bioremediation has become popular with the onset of the twenty-first century. In principle, genetically engineered plants and microorganisms
|Microbial population||Suitable kinds of organisms that can biodegrade all of the contaminants|
|Oxygen||Enough to support aerobic biodegradation (about 2% oxygen in the gas phase or 0.4 mg/liter in the soil water)|
|Water||Soil moisture should be from 50–70% of the water holding capacity of the soil|
|Nutrients||Nitrogen, phosphorus, sulfur, and other nutrients to support good microbial growth|
|Temperature||Appropriate temperatures for microbial growth (0–40˚C)|
|pH||Best range is from 6.5 to 7.5|
can greatly enhance the potential range of bioremediation. For example, bacterial enzymes engineered into plants can speed up the breakdown of TNT and other explosives. With transgenic poplar trees carrying a bacterial gene, methyl mercury may be converted to elemental mercury, which is released to the atmosphere at extreme dilution. However, concern about release of such organisms into the environment has limited actual field applications.
Natural bioremediation has been occurring for millions of years. Biodegradation of dead vegetation and dead animals is a kind of bioremediation. It is a natural part of the carbon, nitrogen, and sulfur cycles. Chemical energy present in waste materials is used by microorganisms to grow while they convert organic carbon and hydrogen to carbon dioxide and water.
When bioremediation is applied by people, microbial biodegradation processes are said to be managed. However, bioremediation takes place naturally and often it occurs prior to efforts to manage the process. One of the first examples of managed bioremediation was land farming (refers to the managed biodegradation of organic compounds that are distributed onto the soil surface, fertilized, and then tilled). Many petroleum companies have used it. High-molecular-weight organic compounds (i.e., oil sludges and wastes) are spread onto soil and then tilled into the ground with fertilizer, as part of the managed bioremediation process. Good conditions for microbial biodegradation are maintained by controlling soil moisture and soil nutrients. In 1974 R.L. Raymond was awarded a patent for the bioremediation of gasoline. This was one of the first patents granted for a bioremediation process.
Since about 1980, prepared bed systems have been used for bioremediation. In this approach, contaminated soil is excavated and deposited with appropriate fertilizers into a shallow layer over an impermeable base. Conditions are managed to obtain biodegradation of the contaminants of concern.
Composting has been used as a bioremediation process for many different organic compounds. It is widely employed to recycle nutrients in garden and yard waste. A finished compost can be used as a soil conditioner. Extending composting technology to new bioremediation applications requires experiments. The biodegradation process must be effective within the context of existing environmental conditions, and odors and gases that are generated by the process have to be strictly controlled.
In Situ Bioremediation
In situ processes (degrading the contaminants in place) are often recommended because less material has to be moved. These processes can be designed with or without plants. Plants have been used because they take up large quantities of water. This helps to control contaminated water, such as a groundwater contaminant plume , in the soil. Aerobic (oxygen-using) processes may occur in the unsaturated layer of soil, the vadose zone, which is found above the water table. The vadose zone is defined as the layer of soil having continuously connected passages filled with air, while the saturated zone is the deeper part where the pores are filled with water. Oxygen moves in the unsaturated zone by diffusion through pores in the soil. Some plants also provide pathways to move oxygen into the soil. This can be very important to increase the aerobic degradation of organic compounds.
Fate of Various Organic Contaminants
Petroleum-contaminated soil has been remediated in situ with plants added to enhance the degradation processes. The biodegradation of phenol, oil, gasoline, jet fuel, and other petroleum hydrocarbons occurs in soil. When plants are present, soil erosion is reduced and more microbes are present in the plant root zone. Methyl tertiary butyl ether (MTBE), used in gasoline to enhance the octane rating of the fuel, is difficult to remediate because it is very soluble in water and is hard to break down using microbes normally present in soil. In vegetation-based bioremediation, MTBE is moved from the soil to the atmosphere along with the water that plants take up from soil and release to the air. The MTBE breaks down rapidly in the atmosphere. Benzotriazoles, used as corrosion inhibitors in antifreeze and aircraft deicer fluids, are treated by plant-based bioremediation. The benzotriazole adsorbs or sticks to the plant roots and ends up as part of the plant biomass. Trichloroethylene (TCE) is a common chlorinated solvent that is biotransformed in the soil. It can be taken up by plants along with water. Then the TCE diffuses into the atmosphere where it is destroyed by atmospheric processes.
Bioremediation requires good nutrient and environmental conditions for biodegradation. When oxygen is needed for oxidation of the organic contaminants, bioventing (pumping air into the soil) is often used. Sometimes, fertilizers are added to the soil. In certain places irrigation is necessary so that plants or microbes can grow.
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Bioremediation Discussion Group. Available from http://www.bioremediationgroup.org .
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Larry Eugene Erickson and Lawrence C. Davis