Remediation of PAH-Contaminated Soils and Sediments: A ...

� Stimulatory effects, no effect, and inhibitory effects have been reported
(Kim et al., 2001; Kim and Weber, 2003). Some of the negative effects of
surfactants on biodegradation may be due to toxicity to microorganisms,
prevention of bacterial access to contaminants through sequestration of
micellar solubilized organics, or preferential biodegradation of the
surfactant rather than the contaminant. For instance, phenanthrene
solubilized by the surfactant Tween-80 was found to be unavailable for
biodegradation by Sphingomonas paucinobilis because this
microorganism preferred to use the hydrophobic portion of the surfactant
as a carbon source rather than destabilizing the micelles.
Because of the above interactions, the determination of a critical micelle
concentration for a surfactant in a complex medium such as soil can be difficult.
Desorption occurs at surfactant concentrations greater than the critical micelle
concentration, but at lower concentrations, admicelles (surface aggregates of
surfactants, also called hemimicelles) may form, sorb onto soil, and act as
additional sorption sites that can enhance PAH sorption instead of reducing it
(Doong et al., 1996, Santharam et al., 1997).
Anionic and nonionic surfactants are more commonly used in remediation
because they are less likely to sorb onto soil surfaces (Mulligan et al., 2001).
Nonionic surfactants are also advantageous because they have a low critical
micelle concentration, high cold water solubility, and low microbial toxicity (Kim
and Weber, 2003; Zhao et al., 2005). Cuypers et al. (2002) and Conte et al.
(2005) have shown that PAHs can be effectively desorbed using nonionic
surfactants like dodecylbenzene sulfonate.
Biosurfactants: Some synthetic surfactants can be toxic to microorganisms,
which may decrease the number of degraders in the soil (Sandbacka et al.,
2000). The addition of biosurfactants (either surfactant producing
microorganisms or natural compounds that act as surfactants) is one way to get
around the problem of toxicity. Biosurfactant-producing microbes have been
proposed as an alternative to chemical surfactants to enhance availability of
hydrophobic compounds (Hunt et al., 1994; Oberbremer et al., 1990).
� Pseudomonas strains can synthesize biosurfactants called rhamnolipids
(Nitschke et al., 2005).
� Surfactants that are produced by microorganisms tend to have lower
toxicities and are effective at wider temperature, pH, and electrical
conductivity ranges (Bordas et al., 2005).
� Atlas (1993) found that biosurfactant-producing indigenous bacteria
achieved higher hydrocarbon degradation rates than those achieved by
nutrient addition alone.
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