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Bioremediation is a process used to treat contaminated media, including water, soil and subsurface material, by altering environmental conditions to stimulate growth of microorganisms and degrade the target pollutants. In many cases, bioremediation is less expensive and more sustainable than other remediation alternatives. Biological treatment is a similar approach used to treat wastes including wastewater, industrial waste and solid waste.

Bioremediation has become an increasingly popular solution to the environmental pollution caused by the rapid technological improvements that began in the twentieth century. The release of pollutants, including petroleum and pharmaceutical products, pesticides, and heavy metals, has implicated environmental quality as a result of increased industrialization and war. The environmental impact of these pollutants poses a risk to both native microflora and human health.

Immobilization
Immobilization methods are a more efficient and cost effective approach to bioremediation. Immobilization restricts the mobility of cells and their molecular components through attachment to specific defined surfaces. This approach takes advantage of the ability of microorganisms to degrade diverse pollutants, and improves degradation by providing protection from environmental insults to the bacteria, such as strong physical forces and changes in pH when exposed to a mixed group of pollutants, a common limitation of bioremediation.

The method of immobilization involves fixing microorganisms to a defined surface to inhibit mobility of cells without eliminating their viability. During immobilization, the catalytic functions of the microorganism are preserved. The process of bioremediation is improved through use of immobilization, which takes advantage of the cellular processes naturally carried out by the microorganisms while reducing the risk of genetic mutations, increased resistance to environmental insults and enhanced survival in areas where pollutants are highly concentrated.

There are currently five forms of immobilization including (1) adsorption through weak bonds with a carrier surface, (2) electrostatic and covalent binding to a surface, (3) cell aggregation in the presence of metals, salts, or polymers, (4) entrapment within a carrier, and (5) encapsulation within microcapsules. Carriers are chosen based on both the immobilization and bioremediation process being performed. For example, bioaugmentation processes should take advantage of carriers that are biodegradable. Carriers should be non-toxic, affordable, stable, and easy to handle.

Adsorption is the immobilization of enzymes on solid carriers. This is achieved through weak physical interactions of the enzyme with the carrier, either through physical adsorption or by drying the enzyme on an electrode surface. Selection of the carrier depends on the active groups of the carriers, which must be chosen to allow enzyme-carrier interactions without disturbing the active site of the enzyme. Adsorption carriers depend on the bioremediation process being performed, but can include chitin, porous glass, and resins. Carries of enzymes should be effective at protecting the enzyme from hydrophobic interactions in the environment, as well as aggregation and proteolysis. A limitation of this form of immobilization is physical nature of the enzyme-carrier interaction. Weak bonds are used to facilitate this reaction, which can lead to the enzyme leaking from the carrier.

Binding of an enzyme to a surface carrier is similar to physical adsorption. However, this method of immobilization relies on covalent bonding, a stronger physical interaction than those found in adsorption. The carrier is washed in a buffer solution that makes the surface of the carrier hydrophilic. The hydrophilic surface of the carrier is then able to strongly interact with the negatively charged enzymes and strengthen the enzyme-carrier bond. Covalent bonding of the enzyme and the carrier surface prevents the leaking limitation present in adsorption.

Entrapment involves the immobilization of cells or enzymes within a porous carrier. Entrapment of cells provides protection from the environment, thus allowing cells to remain viable and physiologically active as biocatalysts. Selecting an appropriate pore size of the carrier is crucial to the success of entrapment; if the pores are too large, the cells or enzymes will leak from the carrier into the environment. Entrapment of cells or enzymes is an efficient, cheap, and eco-friendly method of immobilization, and has versatility in numerous biological and chemical applications.

Encapsulation is similar to entrapment in that it involves cells or enzymes being contained inside of a porous carrier. Encapsulation, however, implements a capsule with a permeable membrane, adding an extra layer of protection from the environment while allowing nutrients to cross the membrane to support cell growth. One of the limitations of encapsulation is the weak structure of the capsules containing the cells or enzymes. Capsules can become weakened by growing cells, allowing the encapsulated material to diffuse into the environment.