Microbial surfactants and their use in field studies of soil remediation

Abstract

Introduction, 915–916 Surfactants, 916–917 Microorganisms and characteristics of biosurfactants produced, 917–918 Isolation of xenobiotic-degrading microorganisms with biosurfactant-producing capabilities, 918–919 Xenobiotics in soil, 919 Sorption of xenobiotics, 919 Biodegradation, 919–920 Surfactants and bioremediation, 920 Biosurfactants and organic solubilization and degradation, 920–921 Biosurfactants and metal solubilization, 921–923 Laboratory and field studies using Rhodococcus biosurfactants, 923–925 Conclusions, 925–926 Acknowledgements, 926 References, 926–929 The degree of association of organic and inorganic pollutants is governed by the complex physico-chemical interactions at interfaces. This association involves sorption onto soil constituents, sequestration within the soil matrix (micropores) and partitioning in a nonaqueous phase liquid (NAPL) that will, ultimately, control the fate of the contaminants. NAPL represents organic substances that are relatively insoluble in water providing a long-term source of pollutant. They represent a continuous input into the water phase to replace degraded or dissipating concentrations. As a result the bioavailability of contaminants to biodegradation may be reduced (Ogram et al. 1985; Miller and Alexander 1991). The enhancement of natural biological degradation processes, in what is termed bioremediation, can be a preferred cost-effective method of removing contaminants from soil-contaminated and other contaminated environments. The role of microorganisms has been shown to be essential in the remediation of, at least, organic pollution, and the activity of microbial types naturally present can be enhanced by bioremediation techniques which include increased aeration of the polluted material and nutrient additions (Christofi et al. 1998; Ivshina et al. 1998). Supplementing the source of microorganisms capable of degrading particular pollutants (bioaugmentation) and the enhancement of the desorption of pollutants from particulates using surfactants can increase hydrocarbon degradation (Deschenes et al. 1995; Thibault et al. 1996; Christofi et al. 1998; Ivshina et al. 1998). It is only 10–15 years that the use of surfactants in increasing the availability of hydrophobic pollutants in soils and other environments has been considered (Vigon and Rubin 1989). Previous studies have revealed that surfactants can enhance pollutant desorption and availability (Oberbremer et al. 1990; Zang and Miller 1992; Volkering et al. 1995; Rahman et al. 2002). They have been applied in oil washing for secondary oil recovery and to clean oil pipes and oil reservoirs (Osipow 1962). In desorption, both synthetic and natural (biological) surfactants have been used. However, it is not always the case that increasing the bioavailability of nonpolar organic substances through the use of surfactants leads to increased degradation or removal. Synthetic surfactants have also been shown to decrease organic degradation (Bruheim et al. 1999) possibly through, for example, the toxic effects of the surfactant (Rouse et al. 1994). It can also be assumed that increasing bioavailability of a pollutant can lead to an increasing toxicity to degrading microorganisms. It has been shown that sorption of toxicants is one major factor controlling bioavailability and toxicity (Welp and Brümmer 1998). In addition, the incorporation of the substrate within surfactant micelles can decrease once again its bioavailability, and, hence, reduce degradation rates. The use of biosurfactants in the control of the bioavailability of toxicants in soils and other environments is an attractive option because of their biodegradability (Herman et al. 1995). These surface-active compounds can be diverse with novel chemical structures and characteristics; they can be produced from cheap raw materials and the organisms producing them can be modified genetically to overproduce or produce new compounds. Additionally, they are significantly less toxic than synthetic petroleum-based surfactants (Banat et al. 2000). This review deals with microbial surfactant research, placing emphasis on the use of biosurfactants in remediation of contaminated soils outside the laboratory. It does not attempt to review the general use of surfactants in soil remediation or surfactant production by microorganisms. Mulligan et al. (2001b) covers the former while Lang (2002) deals with the latter. Surfactants are SURFace ACTive AgeNTS with wide ranging properties including the lowering of surface and interfacial tensions of liquids. Surface tension is defined as the free surface enthalpy per unit area (OECD 1995) and is the force acting on the surface of a liquid leading to minimization of the area of that surface. Both synthetic and natural surfactants exist capable of reducing the surface tension of water from 72 mN m−1 to around 27 mN m−1(Table 1). Biosurfactants are biological compounds that exhibit high surface-active properties (Georgiou e