**6. Conclusion**

growth of soil microbial indigenous population [16]. Biodegradation efficiency depends on the ratio of hydrocarbon-degrading microorganisms in soil, the composition and physical state of hydrocarbon mixture and oxygen availability, and the condition of water, temperature, pH, and inorganic nutrients. The physical state of the hydrocarbon can also affect biodegradation. In addition, the biodegradation of hydrocarbon in bioremediation might be enhanced by the addition of surfactant. For use in bioremediation procedures, biosurfactants are more prom‐ ising than synthetic surfactant because they are produced by microorganisms in soil and are

Bioremediation involves the acceleration of natural biodegradative processes in contaminated environments by improving the availability of materials (e.g., nutrients and oxygen), condi‐ tions (e.g., pH and moisture content), and prevailing microorganisms [58]. Biosurfactants can improve bioremediation effectiveness by the following two mechanisms. The first mechanism includes the increase of substrate bioavailability for microorganisms; for bacteria growing on hydrocarbons, the growth rate can be limited by the interfacial surface area between water and oil. When the surface area of microorganisms with hydrophilic solvents like water is limiting, biomass increases arithmetically rather than exponentially. The second mechanism involves interaction with the cell surface, which increases the hydrophobicity of the microbial cell wall, allowing hydrophobic substrates to associate more easily with bacteria [1, 19]. Microbial cell hydrophobicity can be described as the affinity to adhere to hydrophobic substrates, such as hydrocarbons. This capacity can give the microbial cells the ability to better degrade hydro‐ carbons, and it can be a factor to understand microbial biodegradation rate differences [54]. The increase of microbial adhesion to hydrocarbons is directly related to the ability of such microorganisms to grow in the medium where hydrocarbons or other hydrophobic substrates are present [56]. If the biosurfactant compound is bound to the microbial cell wall, the cell surface will be more hydrophobic. Microorganisms can use their biosurfactants to regulate their cell-surface properties to attach or detach from surfaces accordingly to their needs [1].

There are many research reports dealing with the degradation of hydrocarbons and production of biosurfactants by microorganisms, as stated in Section 3, and there are some *in field* reports on the use of bosurfactants for bioremediation. For example, Thavasir et al. [59] demonstrated the enhanced degradation of hydrocarbons by the addition of biosurfactants to the culture media, as well as the enhancement of degradation by the addition of mineral nutrients (fertilizers). There are also reports on the identification and characterization of biosurfactantproducing microorganisms, including some genera not usually related to bioremediation, such as *Staphylococcus*. Studies include the determination of functional characteristics of the biosurfactants produced and their potential use in bioremediation [60]. Also, there are reports on the production of biosurfactants by microorganisms isolated from particular environments, such as marine sediment, that could be helpful in the bioremediation of those particular sites [61]. Furthermore, the efficiency of different surfactant solutions in removing crude oil from contaminated soil has been tested. Urum et al. [62] demonstrated the efficiency of surfactant solutions used in a soil-washing process. The synthetic surfactant SDS (sodium dodecyl sulfate) was as efficient as a biosurfactant derived from bacteria (rhamnolipid), and both were

commonly considered as low- or nontoxic compounds [57, 49].

102 Advances in Bioremediation of Wastewater and Polluted Soil

more efficient than saponins.

This chapter presents a description of biosurfactants and their uses in bioremediation. Biosurfactants are molecules produced by microorganisms that help them on the absorption and degradation of hydrophobic compounds. Bacterial strains have been extensively studied for biosurfactant production, but recent studies have also reported production of biosurfac‐ tants by yeasts. Yeasts are considered as GRAS microorganisms and are often used in the food and pharmaceutical industries and also in bioremediation.

The addition of chemical surfactants to enhance biodegradation efficiency in bioremediation processes is not acceptable because of its toxicity and persistence in the environment; hence, it is better to use biosurfactants. However, to use biosurfactants in bioremediation, the optimization of large-scale production is needed, as well as studies on the use of alternative carbon sources derived from agroindustrial wastes. Also, it is important to evaluate the possible on-site production of biosurfactants in contaminated sites to expedite contaminated soil restoration.

It is then necessary to continue the isolation of biosurfactant-producing microorganisms on the characterization of their metabolites, on the identification of factors and conditions for production optimization, and on their use in field studies. As with any important subject in science, there are more questions than answers; research in biosurfactants for use in bioreme‐ diation of contaminated sites is still an innovative subject that needs more reliable scientific data.
