Bioremediation

DEFINITION: Waste management technology that employs naturally occurring plants, microorganisms, and enzymes or genetically engineered organisms to clean contaminated environments

The environmentally beneficial and inexpensive waste management strategy of bioremediation enables the degradation of toxic organic and inorganic compounds into environmentally harmless products.

Bioremediation uses biological agents to degrade or decompose toxic environmental compounds into less toxic forms. It is a beneficial and inexpensive strategy for waste management that is environmentally friendly in comparison with other technologies. The products of waste are usually simple inorganic nutrients or gases.

Bioremediation works because, as a general rule, all naturally occurring compounds in the are ultimately degraded by biological activity. Toxic and industrial wastes, and even some chemically synthesized compounds that do not naturally occur, can also be decomposed because parts of their structures resemble naturally occurring compounds that are sources of carbon and energy for biological systems. Wastes are either metabolized, in which case they are used as a source of carbon and energy, or cometabolized, in which case they are simply modified so that they lose their or are bound to material in the environment and rendered unavailable.

Bioremediation can occur in situ (at the contaminated site) or ex situ, in which case contaminated soil or water is removed to a treatment facility where takes place under controlled environmental conditions. Bioremediation can use organisms that naturally occur at a site, or it can be stimulated through the addition of organisms to the contaminated site in a process known as "seeding." Some organisms have been genetically engineered specifically for this purpose; the first ever patented was a bacterium that had been modified to degrade the components of oil. A number of naturally occurring microbes have also been found to be useful in cleaning up oil spills, as they are able to break down the hydrocarbons in oil into byproducts that are then further broken down by other microorganisms.

Techniques

Numerous approaches to bioremediation have been developed. One of the simplest is to fertilize a contaminated site to optimal levels and allow naturally occurring biodegrading populations to increase and become active. Organic contaminants have been mixed with decomposed and partially decomposed organic material and composted as a bioremediation process. In a method analogous to the activated sludge process in treatment, contaminants are mixed in slurries and aerated to promote their decomposition. It is possible to obtain biosolids that are specially adapted for systems because they have previously been exposed to similar organic wastes.

In situ restoration of contaminated is often accomplished through the injection of nutrients and oxygen into the aquifers to promote the and activity of indigenous microorganisms. Trichloroethylene (TCE), for example, is cometabolized by methane-oxidizing and can be bioremediated through the injection of oxygen and methane into contaminated aquifers to stimulate the activity of these bacteria. Nitrate-contaminated aquifers have been successfully treated through the pumping of readily available carbon-containing or into the aquifers to stimulate denitrifying bacteria, which subsequently convert the to harmless nitrogen gas.

Bioreactors have been used in which the is mixed with a solid carrier, or the organisms are immobilized to a solid surface and continuously exposed to the contaminant. This has been used with both bacteria and fungi. For example, Phanerochaete chysosporium, which produces an extracellular peroxidase and hydrogen peroxide (H₂O₂), has been used to cleave various organic contaminants such as in bioreactors.

Highly chlorinated organic contaminants such as TCE and polychlorinated biphenyls (PCBs) resist degradation aerobically, but the contaminants can be dechlorinated by bacteria, which decreases their toxicity and makes them easier to decompose. High concentrations of PCBs in the Hudson River in New York have been dechlorinated to less toxic forms by anaerobic bacteria. Methanogens—anaerobic bacteria that produce methane—have been observed to dechlorinate TCE in anaerobic bioreactors.

One of the problems with some wastes is that they are mixed with radioactive materials that are highly toxic to living organisms. One solution to this problem has been the genetic engineering of radiation-resistant bacteria so that they also have the ability to bioremediate. For example, Deinococcus radiodurans, a bacterium that can survive in nuclear reactors, has been genetically engineered to contain genes for the metabolism of toluene, which will enable it to be used in the bioremediation of radiation- and organic waste-contaminated sites.

Phytoremediation

Phytoremediation is a special type of bioremediation in which plants—grasses, shrubs, trees, and algae—are used to biodegrade or immobilize environmental contaminants, usually metals. Types of phytoremediation include phytoextraction, in which the contaminant is extracted from soil by plant roots; phytostabilization, in which the contaminant is immobilized in the vicinity of plant roots; phytostimulation, in which the plant root exudates stimulate rhizosphere microorganisms that bioremediate the contaminant; phytovolatilization, in which the plant helps to volatilize the contaminant; and phytotransformation, in which the plant root and its enzymes actively transform the contaminant. For example, horseradish peroxidase is a plant that is used to oxidize and polymerize organic contaminants. The polymerized contaminants become insoluble and relatively unavailable.

Plants such as Indian mustard (Brassica juncea) and loco weed (Astragalus) are accumulators and remove selenium and lead from soil. The aboveground plant parts are harvested to dispose of the metals. Algae are used to accumulate dissolved selenium in some treatments. Poplar trees have even been genetically engineered to contain a bacterial methyl reductase that lets them methylate and volatilize arsenic, mercury, and selenium absorbed by their roots.

Examples

Efforts to clean up oil spills have arguably been the highest-profile example of successful bioremediation in practice. The process has been used in some of the largest and most notorious spills of the late twentieth and early twenty-first centuries.

In March 1989, the Exxon Valdez oil tanker spilled millions of gallons of in Prince William Sound, Alaska. On many beaches, the US Environmental Protection Agency (EPA) authorized the use of simple bioremediation techniques, such as stimulating the growth of indigenous oil-degrading bacteria by adding common inorganic fertilizers. Beaches cleaned by this method did as well as beaches cleaned by mechanical methods. In another instance of successful bioremediation, selenium-contaminated soil in the Kesterson National Wildlife Refuge in California was partially decontaminated in the 1980s through the method of supplying indigenous fungi with organic substrates such as casein and waste orange peels. This promoted as much as 60 percent selenium volatilization in less than two months.

A great deal of research into biodegradation and bioremediation of oil spills was conducted following the British Petroleum (BP) Deepwater Horizon oil spill in the Gulf of Mexico in 2010. Because the oil flowed from an offshore wellhead, affecting deepwater, surface water, and coastal areas alike, the cleanup conditions differed from those experienced in many previous spills. Moreover, oil-degrading bacteria were already prevalent at the spill location, feeding on naturally occurring oil seeps along the seafloor. Researchers proposed enhancing anaerobic degradation in affected marshes and introducing genetically modified bacteria. They also learned that the indigenous bacteria were not consuming polycyclic aromatic hydrocarbons (PAHs) and that certain microorganisms naturally inhabit water trapped within oil and feed off it, which may lead to improved methods of bioremediation.

Another area of research involves bioremediation to address plastic pollution. In the 2010s and 2020s, experts increasingly raised concerns about the mounting environmental impacts of plastics, including the proliferation of microplastics into virtually every environment on the planet. Scientists investigated various microorganisms capable of breaking down otherwise non-biodegradable plastics in soil or water.

Bibliography

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